. ETHOECOLOGI OF THE ROUND-TAILED GROUND
SQUIRREL, SPERMOPHILUS TERETICAUDUS
bgr
Charles Martin Drahek
A Dissertation Submitted to the Faculty of the
DEPARTMENT OF BIOLOGICAL SCIENCES
In Partial Fulfillment of the Requirements
For the Degree of
DOCTOR OF PHILOSOPHY
WITH A MAJOR IN ZOOLOGY
In the Graduate College
THE UNIVERSITY OF ARIZONA
1970
THE UNIVERSITY OF ARIZONA
GRADUATE COLLEGE
I hereby recommend that this dissertation prepared under my
direction by
entitled
CHARLES MARTIH D M B E K _____________
Ethoecology of the Round-tailed ground squirrel.
Spermophilns tereticaudua,__________________________
be accepted as fulfilling the dissertation requirement of the
degree of ________ Doctor of Philosophy______
(y-MW-i
Dissertation Director
.
_____________
1 7 d
iA/yv\-
Date
After inspection of the final copy of the dissertation, the
following members of the Final Examination Committee concur in
its approval and recommend its acceptance:*
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This approval and acceptance is contingent on the candidate's
adequate performance and defense of this dissertation at the
final oral examination. The inclusion of this sheet bound into
the library copy of the dissertation is evidence of satisfactory
performance at the final examination.
STATEMENT BY AUTHOR
,
This dissertation has been submitted in partial fulfillment of
requirements for an advanced degree at The University of Arizona and
is deposited in the University Library to be made available to borrowers
under rules of the Library„
Brief quotations from this dissertation are allowable without
special permission, provided that accurate acknowledgment of source is
made* Requests for permission for extended quotation from or repro­
duction of this manuscript in whole or in part may be granted by the
head of the major department or the Dean of the Graduate College when
in his judgment the proposed use of the material is in the interests of
scholarship. In all other instances, however,.permission must be ob­
tained from the author.
j
ACKNOWLEDGMENTS
I wish to thank Dr® E® Lendell Cockrum for his advice and
contributions throughout ray graduate work.
I thank Drs® Stephen M®
Russell $ Albert Re .Meads Everett H. Lindsay and Donald L. Bryan t who
critically read the manuscript and offered many helpful suggestions.
Most of the field work was conducted while I resided at the
University of Arizona-Boyce Thompson Southwestern Arboretum.
I greatly
acknowledge the financial assistance provided through University of
Arizona~NSF institutional grants
.
5701” 003=»8ll~b2 and 5701'=0Q3=-155-ay
in 1967 and 1968 respectively, as well as the staff at the Arboretum.
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Finally, I wish to acknowledge my wife, Jane, who by aiding me
with all phases of this study, as well as maintaining a full time
teaching job, deserves more credit than the average wife.
ill
TABLE OF CONTENTS
Page
LIST OF TABLES
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LIST OF ILLUSTRATIONS,
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ABSTRACT
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INTRODUCTION
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DESCRIPTION OF THE STUDY AREA.
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METHODS AND MATERIALS.
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Study Area . . . . .
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Squirrel Enclosure .
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Trapping and Marking Techniques.
Behavioral Observations.
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Movement Analysis. .
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' Burrow Excavation. .
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Climatological Data.
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MAINTENANCE BEHAVIOR . . .
Locomotion . . „ „ .
Grooming . . . . . .
Exploration. . . . .
Ingestion. . . . . .
Elimination. , . . .
Alertness. . . . . .
Digging. . . . . . .
Grass Gathering. . .
Tail-flicking. . . .
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^OOD HABITS. . . . . . . .
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ENVIRONMENTAL ADAPTATIONS.
Burrows. . . . . . .
Torpor. . . . . . . .
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TABLE OF CONTENTS
(Continued)
Page
COMMUNICATION
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Olfactory .
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Population -Structure and Density.
Sex Ratios © © © © © © © © © © © ©
Population Turnover and Dispersal
Home Range© © © © © © © © © © © ©
Familiarity of Home Range
Intraspecific Intolerance
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INTRASPECIFIC BEHAVIOR O
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Daily Activity© © © © © © ©
POPULATION DYNAMICS
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BEHAVIOR;IN THE ENCLOSURE © © © ©
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Predators © o o o » ® © o @
Competitors © © © ® ® . © «
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COMPETITIVE AND-PREDATORY FACTORS
ACTIVITY PATTERNS
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SUMMARY AND CONCLUSIONS © © © © © © © ©
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REFERENCES CITED 9
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Agonistic Behavior. © © © © © © © 9 6
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Epimeletic Behavior © © © © © © ©
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LIST OF TABLES
Table
1 „ Flora of the study area « , e . .=
••• . „ * . .
2 0 Ninety of the dye marks used to identify individual
ground scjumrelSo © © © © © © © © © © © © ©-© © © ©
3©
Mean burrow temperatures at various distances from
the entrance, and depths and lengths of excavated
burrow systems (N in parentheses) © © « © © © » © «
4©
Food preferences noted during 166 observations of
feeding round-*tail s © © © © © © © © © © © © © © © ©
5©
Vertebrate fauna occurring or observed in the
study area© © © © © © © © © © © © © © © © © ©
6. The population structure of the Round-tailed ground
squirrels captured in the study area in June, July
and August of 1967 and 1968 * © © © © © © . . . © ©
7©
Population structure of resident ground squirrels ©
8= The percentage of the 1967 resident squirrels found
in the study area in 1968 © » . * © © . © © © © « ©
9©
10.
The estimated home range size, standard error of the
mean and range in acres for 1967 and 1968 resident
squirrels © © © © © © © © © © © © © © © © © © © © ©
The frequency at which the home ranges of the 1968
resident squirrels overlapped with intraspecific
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Comparative behavioral traits and responses to
predators for various species of Soermophilus .
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LIST OF ILLUSTRATIONS
Figure
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Page
Location of the study area and vieinity in
'
south-central Arizona * * , , * « . . , . * , , « * * a »
4
Mean monthly precipitation for the Station,and
Florence, Arizona «
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Some common postures associated with the egocentric
behavior patterns of the Round-tailed ground squirrel
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Temperature ranges of the Station and Florence,
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Mean number of round-tails observed above ground and
mean ambient: temperatures during June, July and \
A U g U S t
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6„ Home ranges of an adult male, squirrel 1-100,
during the 1968 summer months 0 . a « = = , a
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Home ranges of an adult female, squirrel 3-60, before
and after its four young left the home burrow „ . a » e a
76
80 Home ranges of a juvenile female, squirrel 30-80,
before and after leaving its home burrow, , , 0 a a a 0 a
78
7a
vii
ABSTRACT
Bthoecological aspects of a natural population of Round-tailed
ground squirrels, Spermonhllus tereticaudus neglectus Merriam, v?ere
investigated during 1967 and 1968 in a Sonoran Desert Larrea zone,in
south-central Arizona.
The squirrel population in a 25 acre study
area was marked for permanent and observational identification to
elucidate adaptive behavior and population structure6
Maintenance behavior is described and its possible role in
communication is discussed.
Exploratory behavior is Shown to.be an
important feature in the establishment and familiarization of a home
range.
The utilization of burrows played an important role in the
round-tail’s adaptation to a desert environment.
Structure, mainten­
ance, temperatures and diurnal use of the subterranean systems are
discussed.
Although semi-colonial in behavior, the squirrels were
found to be solitary in their burrow habits.
Mesquite .trees were the primary food source, from which the
round-tails obtained a succulent diet, and was supplemented by the
fruits of creosote-bushes, perennial wildflowers, insects and carrion.
The vocal repertoire included numerous sounds, which were
associated with specific behavioral patterns.
Evidence Indicates that
a visual communicative system has evolved among the Round-tailed ground
squirrels, and an upright posture of alert behavior functions as a
signal to warn squirrels of predators.
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Ix
Sexually active adult males placed in a squirrel enclosure ex­
pressed sexual behavior toward adult females and intense, dominating
agonistic behavior toward juvenile maleSo
The daily activity of the squirrels was bimodal, with the onset
approximately an hour after sunrise, and cessation at sunset| thus, they
avoided the extreme ambient temperatures 0 The squirrels were inactive
above ground in October, November and December when periods of torpidity
were probably frequent0
An estimated resident population of 2B1 „Squirrels per acre was .
composed largely of adult and juvenile females both summers.
Forty-six
per cent of the 1967 residents.were present in 1968 with the adult fe­
males accountihg for the largest percentage and the juvenile males the
smallest.
Juvenile males left the home burrows earlier than juvenile
females and seldom established residency within the study, area.
Home .range estimates of 0.74 acres for adults and 0.77 acres for
juveniles were derived by the minimum area method and were found to
change throughout the summer months.
Influential factors were sex, age,
and food preferences.. Resident squirrels expressed familiarity with
their home ranges-.and
c h a s e d
transients attempting to-establish resi­
dency either from the study area or to sparsely populated areas therein.
Intraspecific behavior is described, and the role agonistic
tendencies and individual distances play in the social hierarchy and in
the spatial distribution of areas of activity is discussed.
INTRODUCTION
The limited knowledge of the North American ground squirrels
is in part the result of their geographic distribution.
The extant 23
North American species of Soermonhilus and five Ammospermophilus listed
by Hall and Kelson (1959) are limited almost entirely to the western
United States and Mexico, an area wherein relatively few zoologists live.
The Thirteen-lined ground squirrel, S. tridecemlineatus. has the largest
geographic range,, extends the farthest to the east and is readily ac­
cessible to many zoologists.
Consequently, the literature is the most
extensive for this species (Johnson,. 1917j Wade, 1927 5 Evans, 1951j
Rongstad, 1965; McCarley, 1966; Bridgwater and Penny, 1966; McCarley,
1970).
In contrast, the entire distribution of the Mohave ground
squirrel, S. mqhavensis, which singly shares the subgenus Xerospermo•philus with the round-tail, is limited to the Mohave Desert area Of
California (Hall and Kelson, 1959)» and the literature concerning this
species is nearly non-existent (Burt, 1930; Bartholomew and Hudson, I960)
Since Spermophilus is one of the few groups of mammals that is
strictly diurnal, not particularly secretive in Its habits, relatively
large in size and semi-colonial, it is surprising that more ethoecological studies have not. been made on members of the group.
The ability of
Spermophilus to undergo periods of torpor as a means of avoiding a
stressful environment has, in recent years, attracted the attention of
workers investigating the phenomenon of hibernation.
1
However, these
2
investigations have been conducted in the laboratory and have, not
required the investigators to remain in the field to study the animal
in its natural habitat (Bartholomew and Hudson, I960j Hudson, 1964;
Pengelley and Kelly, 1966).
The literature on S. tereticaudus indicates the scant knowledge
about this ground squirrel.
The distribution of the four subspecies,
restricted to sparsely populated desert areas of Arizona, Nevada,
California, Mexico and Baja California, is probably the reason more,
investigations have not been conducted®
There has been little concern
for this non-game animal with the exception of occasional pest control
measures to eradicate squirrels from agricultural lands0
As recently as 1964 the reproductive cycle of S. tereticaudus
■was determined (Neal,
1965b), and its disappearance during the fall and
winter months had.been the only evidence for hibernation until investi­
gations by Neal (1964), Hudson (1964) and Pengelley and Kelly (1966).
, Because the desert biome is of particular interest, due to the
stressful environmental conditions that are placed on the inhabiting
organisms, physiological adaptations of S. tereticaudus have been in­
vestigated under laboratory conditions? however, it is essential that
the adaptive behavioral characters displayed by this animal in its
natural habitat be studied to fully understand the implications of
laboratory results.
The objective of this study was to-investigate
some basic aspects of the ethoeoology of S, tereticaudus for a better
. 1
.
'
understanding of the role behavior plays in adaptation to a desert
environment and to elucidate evolutionary implications of population
dynamics and social organization.
. DESCRIPTION OP THE STUDY AREA
The study area vas situated in south-central Arizona in the
northeast portion of the Sonoran Desert designated by Shreve (1951) as
the Arizona Upland,
The Round-tailed ground squirrels in this area are
restricted to the Larrea zone of the plains and lower bajadas, being
replaced in the upper bajadas by Harris' antelope squirrel, Amraospermophilus:harrisi (Drabek, 1967),
The study site was located 12e0 miles
north and 1,0 mile east of Florence5 Pinal Coe$ Arizona (Fig, l)0
The Larrea zone is part of a physiographic unit of the Arizona
Upland that Shreve (1951) refers to as the Plains,
There the soil varies
from a fine alluvium to a coarse gravel overlying a calcareous hardpan,
caliche.
Creosote-bush, Larrea tridentata. is the dominant plant
species of the Plains, but the delimited 25 acres utilized as the study
area contained a large number of honey mesquite trees, Prosoois
iuliflbra.
Table 1 lists the flora found in the study area.
The peren­
nial plants were collected from the area as they bloomed and were
identified at the University of Arizona herbarium.
The elevation of the area was approximately 1850 feet, and the
area was bordered on the north and south by dry washes, varying from
2 to 8 feet in depth and 4 to 20 feet in width, which drained the area.
Besides the study area in the Larrea zone north of Florence,
in which a natural population of round-tails was observed, observations
were also conducted on captive squirrels introduced into an outdoor
squirrel enclosure.
This was located at the University of Arizona-Boyce
3
Phoenix 4 0 mi.
DESERT BIOLOGY STATION
FLORENCE
SUPERIOR
JCT
Picket Post Mfn
STUDY AREA
Arizona
Phoenix
Globe
• . Silver Bene Mine.'
Area of Map
.Tucson
N orth B u tte
South B u tte
Scale in Miles
FLORENCE
Fig, 1,
Location of the study area and vicinity in south-central Arizona
Table 1,
Flora of the study area0
Common and scientific names follow Kearney and Peebles (1964)»
'
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SPECIES
FAMILY
COMMON NAME
'
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Trees and shrubs
Ulmaceae
Celtis m l l i d a
Leguminosae
Prosoois juliflora
Honey mesquite
Leguminosae
Cercidium floridum
Blue palo-verde
Leguminosae
Olneya tesota
Ironwood
Zygophyllaceae
Larrea tridentata
Creosote-bush
Herbs and grasses
. .'
Desert hackberry
■
Gramineae
Schismus barbatus
Gramineae
Schismus arabicus
Portulacaceae
Calandrinia ciliata
-—
Gruciferae
Thelypodium lasiophyllum
— —
Cruciferae
Lepidium lasiocarpum
———
Cruciferae
Lepidium obloneum
Table 1
Continued
FAMILY
SPECIES
COMMON NAME
Herbs and grasses
Cruciferae
L e mdinm medium
Cruciferae
Sisymbrium irio
Leguminosae
Lutdnus snarsiflorus
Lupine
Legumihosae
Luninas c'oncinnus
Lupine
Euphorbiaoeae
Euphorbia sp»
Spurge
Boraginaceae
Pectocarya platycarpa
Boraginaceae
Cryptantha sp,
Boraginaceae
Plagiobothrys arizonicus
Compositae
Heterotheca subaxillaris
Compositae
Rayless goldenrod
Compositae
Franseria spe
Bur-sage.
Compositae
Pectis papposa
Chinchweed
7
Thompson Southwestern Arboretum, 4 o0 miles vest of Superior, Pinal Co.,
Arizona.
The Arboretum is utilized by the University as a Desert
Biology Station (Fig. 1).
Climatological data for the Biology Station and Florence,
Arizona (Figs. 2 and 3) show the annual precipitatidn to be biseasonal.
The weather data recorded in the study area corresponded to that of
Florence, which received an average annual precipitation of 9.59
inches, with peaks in August and December.
Each rainy season was
followed by a bloom and heavy growth of the desert vegetation.
The
maximum temperatures occurred during, the summer months with meanmonthly-maximum temperatures remaining well oyer 100 F,
The squirrels
were inactive and not exposed to the lower temperatures of October,
November and December.
8
3.0
INCHES OF PRECIPITATION
2.5
2.0
1.5
1.0
0.5
FIG, 2. MEAN MONTHLY PRECIPITATION FOR THE
STATION AND FLORENCE, ARIZONA
9
D ESERT
---------
BIOLOGY STATION
FLORENCE
IOO H
90 H
DEGREES
F A H R E N H E IT
80 4
M E A N -M O N T H L Y -M A X IM U M
70 4
60
4
50
4
40
4
M E A N - M O N T H L Y - M IN IM U M
FIG. 3. TEMPERATURE RANGES
FLORENCE, ARIZONA
\
OF THE STATION AND
METHODS AND MATERIALS
Study Area
-
Rodnd-tailed ground squirrels are abundant in the alluvial
soil of the dense Larrea zone north of Florence, but the selected study
site was established in an area with a, sparse stand of creosote-bushes :
and mesquite trees.
This broken density allowed unobstructed viewing
of the ground surface for nearly 600 feet in all directions and offered
an ideal area to conduct an observational study on ground squirrels0
In May, 1967, the study area was surveyed by tape and compass,
measuring off 100 foot quadrants along ground contours with ten lines,
designated as rows one through ten, running north and south and
(A-M) running east and wests
13 rows
A 12 square inch aluminum tag attached to
a 2 foot wire was placed upright in the ground where each north-south
(l-10) row crossed the 13 (A-M) east-west rows®
The respective numbers
were painted on each tag (e6g6 A-l, E-6, etc.) to serve as reference
points.
At the beginning of the 1968 season additional markers were
placed at 50 foot intervals throughout the area,to afford the observers
intire accurate reference points.
Squirrel Enclosure
To study more intensely the intraspecific behavior between
different age groups and sexes of S e tereticaudus. a 70 by 80 foot out­
door, rodent enclosure was constructed at the Biology Station.
Sheets
of corrugated sheet metal, 26 inches wide, were riveted together with
10
11
a 1 inch overlap and placed in a trench 5 feet in depth.
This formed
the subsurface walls of the pen while one-eighth inch hardware cloth
wired to 6. foot steel posts placed 10 feet apart composed the above
surface walls.
hardware cloth.
A cement curb adhered the sheet metal to the posts and
The height of the pen was 45 inches.
Sheet metal
overlapped the top 1 foot to prevent the squirrels from climbing out.
Observations on squirrels released into the pen at the be- .
ginning of each summer were made several days a week at ground level
outside of the pen in 1967 and. from a tree seat built in a Chinese
pistachio tree located within the pen during the 1968 season.
The
presence of the observer from either position appeared undisturbing to
the squirrels.
Commercial chicken feed was contained in a ground level
feeder for the squirrels.
Other trees within the pen were a round leaf
eucalyptus, Eucalyptus nolvanthemos. two white gum eucalyptus, E.
camnaspe. and two red gum eucalyptus, E. rostrata.
Trapping and Marking Techniques
Squirrels were live trapped in non-commercial 4 by 4 by 12 inch
sheet metal traps which had one-half inch hardware cloth spot welded
on the tops and backs.
flower seeds.
The most suitable bait was found to be sun­
Ants made other baits of apple, melon, oats and peanut
butter mixed with oats unsatisfactory.
To mark the resident population in the study area, traps were
set at selected.sites.
Trapping was done only during the peak hours
of the squirrel activity,, and the traps were overturned when the ob­
servers were not present in the area.
After trapping a squirrel, the
12
trap site was located on the grid and recorded.
The squirrels were
then allowed to enter a 16 by 10 inch cloth bag* placed over the en­
trance of the trap, for weighing and handling.
Weights were obtained
to the nearest gram with a Chatillon"spring scale ranging from 0-250
grams.
Body temperatures were taken with a rectal probe of a Yellow
Springs telethermometer.
Before marking, each squirrel was sexed and aged.
As Neal
(1965a) pointed out, no satisfactory technique has been perfected to
separate Round-tailed ground squirrels older than 8 months into age
groups.
He found no reproductive characters which distinguished one
year olds from adults.
Thus, the squirrels in this study were con­
sidered either juveniles or adults, with a juvenile based on the follow­
ing characters:
small size, soft pelage, small unpigmented nipples and
undeveloped gonads (Neal, 1965a).
To provide a permanent identification, each squirrel was toe
clipped.
Two toes are usually clipped when permanently marking animals
and at least one-half of the digit is removed, as entire terminal
phalanges are occasionally lost under natural.conditions (Layne, 1954).
The chances of this happening are evidently not great in S, tereticaudus.
for of 170 squirrels captured, only one had lost a toe.
In this study
no more than one toe was clipped from each foot, and with the exception
of three individuals, no more than two toes were clipped.
Sciurids
have four toes on the fore feet and five toes on the hind feet.
Look­
ing at a ventral view of the animal the toes on the fore feet were
designated as units one through eight, counting from left to right,
while the hind toes were designated as units of ten, 10-100, counting
13
left to right (Davis, 1956)„
Thus, removing the lateral toe on the
right forefoot and medial toe on the left hind foot would designate a
squirrel, 1-60.
This system provided 139 permanent markings.
More
squirrels could have been marked by clipping a single toe or three
toes.
The removed toes showed no sign of subsequent infection nor
impairment to the digging, foraging or movement activities of the
squirrels.
To mark the squirrels for observational purposes Nyanzol D dye :
(Nyanza Color and Chemical Co., Lawrence, Mass.) was used following
the preparation described by Melchior and Iwen (1965).
fication was in the concentration of hydrogen peroxide.
..
The only modi­
It was found
that 25 cc of 3% hydrogen peroxide worked as satisfactory as 3.7 cc of
30% and was much easier to use.
The small size of the Round-tailed
ground squirrel did not make it possible to dye corresponding toe-dip
numbers on their sides, but a variety of symbols worked most satis­
factory in both application and individual recognition (Table 2).
The
symbols were applied with a one-quarter.inch brush, and then the
squirrel was returned to the trap for several minutes to allow the dye
to dry.
Upon the release of each squirrel, the release direction and
escape behavior were recorded including the distance and direction, moved
and the hole entered.
Trapping was conducted throughout the 1967 summer and the first
half of the 1968 summer.
Once the resident population was marked, sub­
sequent trapping allowed re-weighing and re-marking of marked squirrels
and marking of transient squirrels.
Table 2 «
Ninety of the dye marks used to identify individual ground squirrels.
The symbols along the abscissa Were painted on the sides of the squirrels while
the six body regions painted are listed on the left,. Each symbol and each body
region was used alone, then combinations of symbols and regions.
'
X
XX
•
1
2
3
■ 1
11
111
—
=
0
V
A
Hind legs
Fore legs
Head
Butt-tail
Neck ring
Hind-fore legs
87
88
89
90
15
Behavioral Observations
Observations began in the study area in June of 1967 and con­
tinued through July and August„ .Periodic observations were conducted
during the fall and spring indaths with daily observatibnb resumed in
June of 1968 and continued into Septembere
The observers sat at
ground level and used 7 x 35 binoculars and a Bausch and Bomb 15X-60X
zoom Balscope spotting scope,
A 35mm camera with a 400mm telephoto
lens was used to photograph the various squirrel postures and associated
behavioral patterns.
Observations were made from the same locality both
seasonsj, a site which afforded two observers a full 10 acre ground view
of the study area.
This observation station was at a large mesquite
tree which appeared to prevent the squirrels from recognizing sil­
houettes and most movements of the observers since our presence was
ignored by squirrels 100 feet away, and after several weeks squirrels
as close as 25 feet would forage and conduct daily activities.
Observations were conducted in the study area 6 days a week,
with 4 days devoted to the peak activity of the morning hours, while
afternoon and evening activity was observed.the remaining 2 days.
Each
day notes were taken on maintenance, predator, communicative, interand intraspecific behavior.
Periodically, mid-afternoon-activity v/as
observed to establish daily activity patterns of the squirrels.
In
1967, surveys were taken every half hour during the observation periods
to tally the, number of squirrels above ground.
16
Movement Analysis
Two observers spent 4 to 6 hours daily recording the squirrel
activity in the area®
The locality of each, squirrel seen was plotted
on a grid of the study area and its movements and behavior recorded*
The total spatial observations for each squirrel were then plotted on
graph paper„
Home range areas and shapes were estimated by connecting
the extreme points of observations with straight lines, according to
the minimum area method described by Gookrum (1962) and measured with
a compensating polar planimeterc
Burrow Excavation
Burrows were excavated in both the enclosure and study area at
the end of each season when the squirrels would be least disturbed.
An
8 foot long commercial plumberss snake was. used to locate the passage­
ways of the subterranean burrows before the surface was removed,. Each
excavated burrow was mapped and measured for depth and length.
Burrow
temperatures were recorded by placing the ambient temperature probe of
a telethermometer on the end of the plumber3s snake and slinking it into
the burrow as far as possible:,
Climatologlcal Data
Relative humidity and ambient temperature data in the pen and
study area were, obtained with a constant recording hygro-thermograph
(Fries Instrument Division),
In 1968 soil surface temperatures were
recorded with a telethermometer during the hours of observation.
MAINTENANCE BEHAVIOR
In analyzing the social organization of any animal, it is im­
portant to know what the animal W d s ,
As "Scott (1958:11) stated,
"oooothe primary phenomenon to be studied in the science of animal be­
havior is behavior itself."
The following descriptions of maintenance
behavior are based on patterns, observed in the enclosure and in the
study area.
The study of such egocentric behavior helps to understand
■
some of the patterns seen in social, behavior.
Locomotion
Ground squirrels were observed walking, running and jumping.
Squirrels walked only when they were investigating (cautious behavior)
and. when moving distances of a few feet.
During undisturbed foraging
activity, squirrels walked in a typical quadruped fashion by advancing
one forefoot followed by the opposite hind foot.
The head of the
squirrel was close to the ground and the.back was flexed.
In a cautious-
exploratory behavior the squirrel utilized a much slower walk.as it
approached the object being investigated.
Running was the means of locomotion most commonly observed in
found-tails.
Movement to creosote-bushes, mesquite trees and burrow
holes, further than a few feet away, was.a moderate running motion
characterized by a series of bounds.
They also moved in a faster,
galloping motion, observed only during intra-specific chases and es­
cape (Fig. 4)«
This running posture was an alternation of a stretched
17
Fig, 4»
Some common postures associated with the egocentric
behavior patterns of the Round-tailed ground squirrel
A 0 and BY are the post postures of alert behavior,
C, is a running locomotion.seen in intraspecific
chases and escape, D, is a foraging posture,
E, is the posture of a down-alert behavior.
Some common postures associated with the egocentric be­
havior patterns of the Round-tailed ground squirrel.
19
body with extended fore limbs and an arched back with, the fore limbs
pushing posteriorly while the hind limbs moved anterior, laterally«
A jumping movement was used when getting into and out of
trees and shrubsc
Squirrels were observed jumping up to 3 feet off
the ground into the low hanging limbs of mesquite trees,
A jump onto
the tree trunk at the termination of a run was also usede
capabilities were quite evident in the round-tail,
Arboreal
Movement within a
tree was accomplished by hanging on with the hind legs while the fore
legs extended and grasped an adjacent branch.
Often, squirrels were
seen, swaying on distal tips of mesquite and creosote branches, eating
the most recent growth.
Squirrels also jumped at low flying grass­
hoppers,
■
Grooming
Grooming behavior possessed by round-tails included dusting,
sunning and washing,. Each squirrel had a dusting area near its burrow
which consisted of a small depression of loose dirt.
The animal pawed
the area with several strokes then pushed its head and body into the
loose dust, sometimes in a rolling motion.
quick jerky motions.
Washing was performed in .
After licking the forepaws, the neck and head
region were rubbed, moving from the ears over the eyes and down the
nose*
The venter was mouthed, as well as rubbed with the forepaws,
while the tail was run through biting motions of the mouth.
Scratch­
ing the anterior portion of the body was accomplished with the hind
feet.
Squirrels emerging at the start of the day had a dark and
ruffled pelage, probably due to the high humidity of the burrow
20
(Vorhiesj 1945j Studier and Baca, 1968)„
Emerging round-tails nor­
mally sat sunning at the burrow entrance for 10 to 15 minutes, then
dusted and groomed=
A shiny, light pelage, slick in appearance was
the result of these activitiess
The dusting habits of S„ tereticaudus appestred to resemble
the comfort movements of sandbathing in heteromyids as described by
Eisenberg (1963)0
Such movements are a normal part of the behavioral
repertoire in most rodents that have adapted to arid habitats
(Eisenberg, 1967)>
This grooming behavior of round-tails may also
serve the dual function of dressing the pelage and marking as Eisenberg
has found in heteromyids.
Body scent left in the dusting depressions •
near the squirrel burrows may have communicatory value to other, in­
vestigating squirrelSo
Periodically, squirrels rubbed their heads and
necks against branches of creosote-bushes in scratching motions which
could also serve to leave a scent.
Another comfort movement possessed by round-tails was that of
stretching.
This was most normally accomplished by depressing the back
and fully extending the hind limbs posteriorly and fore limbs anteriorly.
The low-lying branches of creosote-bushes were sometimes grasped with
the forepaws while the hind feet moved distally to stretch the body.
All grooming behavior was conducted within the animal's home
range and most normally at or very near the home burrow entrance.
grooming was part of the beginning activities of the day.
Most
21
Exploration
Exploratory behavior was shown when a squirrel approached
strange objects and unfamiliar holes.
The posture of the body was a
low crouch with a slight arch in the back and a slightly bristled tail,
which often waved slowly.
As the unfamiliar object was approached,
the neck was fully extended while all four legs remained stationary.
This posture placed the visual, auditory and olfactory senses closest
to the object.
Cautious behavior was observed when a .squirrel in­
vestigated potential escape holes in the parts of its home range not
regularly utilised.
When foraging away from its home burrow, it in­
vestigated these proximal holes, then periodically ran and sat in them.
When startled it often used these holes rather than the home burrow.
Exploratory behavior, an important facet when gaining general
familiarity of a region, was seen in all round-tails observed.
arehs were repeatedly frequented.
Many
It is in these areas in which home
ranges evolve (Calhoun, 1963).
The escape behavior of captured squirrels, especially transients,
often included cautious movements.
Usually the squirrel ran directly
to the nearest burrow-, flattened the body, and slowly began investigate'
ing the hole.
In such instances the squirrel showed no concern for the
observers, which in some cases were only 5 feet away.
Escaping
squirrels sometimes cautiously investigated several holes before one
was entered and sometimes entered one momentarily then rapidly exited
to another.
. <
Traps set in the area appeared to act as novel attractants to
the squirrels, as seen in the Uinta ground squirrels, S. armatus.
22
studied by Balph (1968),. The adults gained confidence in approaching
the traps much more rapidly than did the juveniles, but both initially,
expressed a cautious behavior toward them0
In one instance, an adult
squirrel closed a trap by kicking 2 inches of dirt into it0
Ingestion
Most round-tail feeding activity occurred the first few hours
of the day.
However, when patches of chinchweed. Pectls m n n o s a . were,
in bloom, the last few hours before sunset were often spent foraging*
Two different feeding postures were observed.
Food was usually manipu­
lated with the fore feet while the animal hunched in a sitting position
(Fig, 4)o
The tail was extended on the ground and served as a sup­
portive "third leg".
The squirrels walked slowly within the foraging
area then sat when something was picked up and eaten,
Mesquite beans
were handled with the forepaws to extract the seeds,
A second feeding posture was observed when the squirrels were
foraging in patches of abundant perennials such as chinchweed.
The
squirrels did not use their forepaws but just walked slowly and some­
times even lay prone on their venters, eating the tops of the flowers
in a grazing motion.
Since this type of foraging kept the squirrel *s
head down, it frequently lifted its head and peered around.
also the posture when squirrels fed on ants.
This was
When feeding on flowers
and fresh shoots in trees and shrubs.they again did not handle the food
but used their forepaws to sustain their precarious positions.
Other ingestive motions exhibited by the round-tails were the
gnawing on bark of mesquit© trees and the shredding of meat from a
carcass when carrion feeding.
Occasionally a squirrel placed some- .
thing iia its mouth with one forepaw while remaining in a walking
position.
Elimination
The Round-tailed ground squirrels did not normally interrupt
other activities to eliminatee
The posture differed from that of a
walking stance by a slight hunch in the back.
Fecal pallets were most
commonly observed around the periphery of the home burrox^ hole p and ..
once a squirrel was seen scratching the soil adjacent to its burrowp
defecating and covering.
This indicates a similarity to the elimina­
tive habits of the California ground squirrel, S, beechevi. reported by
Linsdale (1946).
The habit of elimination near the burrow entrance
may serve as another communicative sign to other squirrels, aiding'them
to recognise the presence of occupants during cautious-exploratory
behavior,
'
Alertness
Ralph and Stokes (1963) described alertness In ground squirrels,
as a.single continuum of activity that runs from rest to alarm, but at
rest the animal is less likely to escape than at alarm.
The posture
exhibited in an alert behavior, probably more than any other pattern,
characterises the Round™tailed ground squirrel for everyone who has
observed it in its'natural state.
The assumed position is a vertical
stance as exhibited, with variations, by most sciurids (King, 1955$
Ralph and Stakes, 1963$ Waring, 1966$ Wolfe, 1969)e
Due to the
24
possible resemblance
to
fence posts* the term ^pieket-pin18 has been
used to apply to many species of
ground
squirrels in an alert posture
(for example* Olin* 1954).
The forepays were placed in front of the chest* sometimes in
a crossed position.
The tail hung to the ground and the ^mek was often
extended* as well as the hind feet* placing the squirrel on its toes
(Pig.
4).
Squirrels were observed to hold this motionless* upright
"post" position* nearly 5 minutes.
The head was normally turned and
the squirrel often leaned in the direction of the stimulus* which may
have afforded binocular viewing and a directed reception for olfactory
and auditory cues.
The. round-tails reacted initially with a post position to all
potential predators* non-predators (jack rabbits and cattle)* observers
and to foreign objects (traps) placed in their home range.
The
squirrels assumed a down-alert position (Balph and Stokes* 1963) when
the stimulus was identified and there was no immediate danger (Fig. 4).
Visual and auditory cues of potential predators in the immediate vicin­
ity sent squirrels directly to the entrance of their home or escape
burrows where a post position was assumed.
Digging
Digging behavior was observed in both sexes often during the
. morning hours* tot excavating one-half hour before sunset was observed
on occasions.
Excavating of new burrows was seldom seen* as usually
older* unoccupied burrows were widened and utilized.
The digging motions
were sporadic* with the forepaws pushing the dirt under the body between
25
the hind legs, which then kicked the dirt farther back.
Only freshly
dug holes had dirt at the exit and then, only a shallow ridge e
One
squirrel spent 10 minute^ scattering a pile of dirt which had accumu­
lated at the hole entrance,
McCarley (1966) found this absence of
dirt around the burrow entrances also characteristic of the Thirteenlined ground squirrel, S„ tridecemlineatus.
Dirt was not observed being
moved with the nose as seen in the prairie dog by King (1955) <.
The digging behavior of the ground squirrels always included
frequent interruptions because they raised their heads to peer around.
This behavior afforded a periodic cue with the immediate surroundings
while in a vulnerable position to a predator.
Violent shaking motions
of the entire body after a period of digging eliminated loose dirt from
the pelage.
Burrow excavations showed that much loose dirt was always
left in the passageways$ probably to supply material for plugging the
entrances.
Grass Gathering
Grass gathering occurred at any time during the period of the
round-tail®s diurnal activity.
When collecting dried grasses, presum­
ably for nest material,, the squirrels combined running movements with
foraging postures, picking up dried grass stems with the forepaws and
placing them into their mouths.
This behavior was characterized by two
actions, 1) the rapidity at which the squirrel gathered, resulting in
a somewhat frantic behavior and 2) the little concern for anything else
while gathering.
On one occasion a juvenile female, squirrel 30-80,
was observed conducting only grass gathering activities for one and
26
one-half hours.
grass.
In 1 hour she made 4-0 trips to her burrow carrying
It took her approximately 1 minute to collect a mouthful of
grass and about 30 seconds were spent in the burrow after, each trip.
Some of her trips took her within 10 feet of another squirrel? but no
interaction resulted.
Activity was stopped shortly, however, when a
hawk passed over,
.
This behavior of removing materials from an area m y represent
signs,: as suggested by Calhoun (1963), by which an individual might
recognise the presence of neighbors.
The closer the home ranges, the
greater these signs will be.
Tail-flicking
Tail-flicking was a motion seen in round-tails in which the
tail was moved from side to side in an arc,
Balph and Stokes (1963)
'■ -v
reported the tail-flicking of the Uinta ground squirrel was not asso­
ciated with any specific movement or behavioral situation and suggested
it may be an intention movement of a shift in the position of the
animal.
However, in round-tails, tail-flicking was identified with
several behavioral patterns.
It was most accentuated when they were
confronting ground predators or as they established claims to newly
occupied burrows.
following pattern.
The latter behavioral expression consisted of the
The squirrel sat hunched forward at the newly
■
claimed hole and emitted a single peep.
This was followed by a drumming
of the hind legs .on the ground while the bristled tail was slashed from
side to side,: A peep was emitted about every 10 seconds, and the
process was repeated.
Occasionally, the hind leg drumming and tail-
27
flicking continued with peeps being emitted at a rate of one per
seeondc
After each drumming sequence, the squirrel turned slightly
to face squirrels in other directions.
This behavior sometimes con- .
tinned for an hour and extended over several days.
Tail-flicking was also part, of the behavioral pattern elicited
by ground predators, as observed during interactions with rattlesnakes
and gopher snakes.
Often .between drumming sequences the squirrel
dashed back and forth in.front of the snake to prevent it from enter­
ing the squirrel’s burrow.
Tail-flicking was also associated with the cautious and explora­
tory behavioral patterns.
slowly.
In.these instances the tail was waved very
EWIRQNMEffi’AL ADAPTATIONS
Adaptation to an arid environment requires a solution to
- '
problems posed by l) lack of water? 2) widely spaced food with, concomi­
tant loss of cover and 3) the extremes of heat and cold (Eisenberg,
1967)o
The physiological and morphological solutions to these problems
are varied among the vertebrates, as reviewed by Sehmidt-Mielsen (1964a),
but all point toward the maintenance of a constant body temperature,.at
least during the. active periods, and the conservation of body fluids«,
The adaptation of the heteromyid rodents to recover metabolic water in
the kidney and to live on dry food alone is a most striking example0
Burrows
Behaviorally, common adaptations in desert animals are nocturnal
activity and the occupancy and maintenance of subterranean burrow sys­
tems.
Since the round-tails apparently lack a kidney as efficient as
the heteromylds (Schmidt-Nielsen, 1964a) and are diurnally active, the
burrows represent a very important adaptation for their survivale
Yorhies (1945) showed the maximum temperature in the subterranean nest
of S. •tereticaudus during the summer was 29 G, -aven when the*, ambient
temperature exceeded 40 C and the soil surface temperature reached 75 C e
Burrow temperatures in the pen and the study area indicated there was
more than a 10 degree drop in temperature in the first 45*0 cm.(Table 3)«
All active burrows were found to be plugged at approximately 45*0 cm.
28
■which would help maintain constant reduced, temperatures in the sub­
terranean systems*.
Table 3„
Mean burrow, temperatures at various distances from the
entrance and depths and lengths of excavated burrow
systems (N in parentheses).
Entrance
30e0 cm
45 e0 cm
45.0 cm
■
(10) ^6,4 G
(10): 37.1 G
I'feximujn depth
(14) 37.0 cm
(25.0— 53.0)
(10) 34»2 G
plugged
Maximum length
___ ' : .. > ___
■.
(14) 179.0 cm
(30.0— 419 cm)
The 15 burrows excavated had a mean depth and length of 37,0 cm
and 179 cm respectively.
The declination in all burrows, was very gentle
The borrow systems usually had two entrances although as many as four
were found.
The mean diameter'of 23 burrow entrances was 57 mm, and
conqpas's readings indicated entrances vjere random (M^O) in relation to
the direction of the opening.
Kot all entrances■(16=25) were located at
the base of$ or even hearp creosote-bushes or other shrubs.
Some open­
ings $ especially those in the abandoned mounds of the Banner-tailed
kangaroo rat. Mpodomvs s-peotabilis. dropped straight down 3 to 6
inches with three smaller burrows entering laterally.
.
30
No squirrels were found in any of the burrows exeavated* but
all were known to have been used during the summer.
The absenee of
an occupant during excavations Was also experienced by Rongstad .
(1965) while investigating S. trideeemlineatus burrows.
No nests or
excrement caches were seen in the burrow systems of the round-tail
as described for S. trideeemlineatus by Rongstad (1965).
Edwards
(1946) reported seldom finding dung in nests or passageways in the
burrow systems of S. mexicanus. which had outside defecation areas..
Defecation by round-tails was observed outside of the burrows^ but
excrement was never piled.
The only excavated burrow which resembled
a nesting burrow, according to depth and length as classified by
Rongstad (1965) for the Thirteen-lined ground squirrel, was that of a
juvenile male, squirrel 5, whose burrow had a depth of 91.0 cm and a
length of 244 cm.
and burrows.
It was superpositioned by another series of holes
Probably the most striking thing about the burrow struc­
ture of round-tails Was the large number of branches and lateral
burrows, most ending as 1 foot cul-de-sacs.
Some of these could have
been plugged, hibernating burrows as found in S. tridecemlineatus by
Johnson (191?) and corroborated by Rongstad (1965).
There was no evidence suggesting that round-tails shared burrow
systems with such sympatric, fossorial species, comparable in size
(diameter), as the kangaroo rats, D. merriami and D. soectabilis.
On
one occasion a released round-tail entered a D. snectabilis burrow to
-
escape.
'
'
'
-:
'
After about 30 seconds the squirrel re-emerged from the burrow,
pursued by a Banner-tailed kangaroo rat.
Both re-entered another hole
of the same mound upon seeing the observers.
31
Many paths were noted connecting.groups o£ holes and often ex.
'
.
'
tended to mesquite trees and creosote-bushes® . Captured squirrels
.
.
released on these paths usually followed them to burrow entrances6
Such.paths may furnish the squirrels the quickest route to an escape
"
or home burrow when confronted with a predator while feeding®
.
-
-
Evans
(I95I) observed paths, linking the burrow entrances of S® tridecemlineatus®
To. determine the number of refuges available to a squirrel for
escape from a predator, the active squirrel holes in 12 randomly se­
lected 100 foot square quadrants were counted®
A mean of 29 squirrel
holes per acre indicated that each squirrel would have access to almost
20 holes®
During the summer months the round-tails exhibited a bimodal
daily activity pattern .and thus avoided the highest temperatures of the
day®
The few squirrels seen above ground during this hot period re­
stricted their movements to shaded areas, sat at the burrow openings
with frequent entries and foraged in the shade of creosote-bushes or
mesquite trees.
Hyperthermia tolerance for short periods of time was evident in
S. tereticauduSo
The maximum body temperature recorded was 43.05 G
(IO9.5 P) from a captured juvenile female, squirrel 8-20®
Neal (1965c,)
reported body temperatures as high a s .41 •9 C in active adults and
40®9 0 in active juveniles in June and July respectively.
Captured
animals often demonstrated excessive salivation, sometimes wetting the
entire face and neck®
This behavior acts to cool the anterior parts
sufficiently to protect the most sensitive parts of the central nervous
32
system and keep these below the body temperature of the remaining body
parts (Schmidt-Nielsen,
1964b).
When a squirrel became hyperthermic it had to unload the accum­
ulated heat and probably did this by entering and plugging its burrow.
The behavior at this point was probably similar to that described by
Bartholomew (1968) for Ammospermophilus leucurus. with the squirrel
flattening itself .against the cooler burrow floor to lose heat to the
substratum by conduction and to. the walls by radiation until the body
temperature approached the usual resting level.
Often squirrels active
at mid-day demonstrated a flattening of the body, stretching all four
limbs laterally to expose the complete venter to the soil, in a shaded
area above ground.
This may have been a response of a slightly over­
heated squirrel in an attempt to lose heat by conduction to the ground
surface.
Thus, the utilization of a burrow by 3. tereticaudus appears to
play an important role in the maintenance of body temperature and the
conservation of body fluids.
By allowing some fluctuation of body
temperature the round-tail can integrate the surface and subterranean
temperatures to an average where active heat regulation through expendi­
ture of water is unnecessary (Schmidt-Nielsen, 1964b).
Torpor
A physiological adaptation most species of Spermophilus have
made to,the environment is the ability to undergo hibernation or esti­
vation.
Estivation was defined by MaeMillen (1965) as torpor occurring
under conditions of moderate ambient temperatures irrespective of season
33
and duration, while hibernation.is torpor, occurring under conditions
of low ambient temperature and usually over a period of several days
to weeks.
Such periods of dormancy function as a means of avoiding
the most severe aspects of the environment and vary in duration.in
different species.
The period of dormancy in the Thirteen-lihed
ground squirrel,. S. tridseemlineatus, is approximately 135 days in
Texas (MeCarley, 1966) while the Mohave ground squirrel. S. mohavensis
has been reported to be active from April to August then dormant for
7 months (Ingles., 1965).
The latter is of especial interest because it
is the nearest relative of S. tereticaudus. being the only other living
member of the subgenus Xerosoer-moohilus (Hall and Kelson, 1959).
S. mohavensis is confined to the Mohave Desert where seasonal environ­
mental changes are not as marked and precise as in boreal zones, but
during winter food is scarce and there is an intermittent drought
(Pengelley and Kelly, 1966).
Pengelley and Kelly term S. mohavensis
an bbligant hibernator, having long periods of hibernation alternating
with a short period of the homothermic active state.
Bartholomew and
Hudson (I960) have demonstrated under laboratory conditions that 3.
mohavensis can spontaneously, become, torpid, during, the summer, even in
the presence of food and water, and consider it an estivator as well as
a hibernator.
S. tereticaudus possesses most of the typical features of a
hibernator (Hudson,
1964) and has demonstrated capabilities of torpid­
ity at different months of the year under Varying laboratory conditions
(Hudson, 1964; Hudson and Bartholomew, 1964; Neal, 1964; Pengelley and
Kelly, 1966).
Hudson (1964) considered the round-*tail to be only an
estivator after finding occasional incidences of torpor in late spring,
summer and early fall.
He felt that activity above ground observed
during..the summer months'did not lend credence to the possibility of
routine and prolonged periods of dormancy in any pattern other than a
nightly shallow torpore
Hudson worked with 8* t e chloims and S. t e
teretieaudua while the subspecies of this, study was S, t. negleetus.
Pengelley and Kelly (1966) found S» teretieaudua has almost abandoned a period of obligatory hibernation, although it hibernated for short
periods under laboratory conditions during October, November and
December.,.
.
Observational evidence of torpidity in a natural population is
virtually impossible to obtain, especially when dealing, with a fossorial
animal, but repeated daily observations would indicate any continuous
absence of individuals or a group of individualse
In this study> dur­
ing the summer of 1968, the home ranges of 13 squirrels, were within;
unobstructed view o f •the observers»
The daily activity of these
squirrels was tallied to determine if any were absent for prolonged
periods.
The observers were confident that these fpiar adult males,
three adult females, three juvenile males and three juvenile females
were seen when abroad.
The summer observation period in 1968 extended
from 11 June to 28 September and included 37 days of observation with
each observation period occupying 4 to 6 hours.
One adult female,
squirrel 3-60, was seen each of the 37 days of observation.
'
The adult-
females and the juvenile males and females were observed 82,7 per cent
of the days with a range of 62-100 per cent while the adult males were
seen only 31®5 per.cent of the days (23-41%).
Some daily absences
.
35
could have been due to squirrel activity at times of the day other
than when the observers were present since the absences in the adult
females and juveniles ;
vfere. of only 1 and 2 day periods«
The: absences
of the adult males* however, extended from 2 to 9 days, after which
they were observed within the same home range.
All the adult males
in this study showed a marked absentia in the observational records
compared with adult females and juveniles.
This indicates that some squirrels, especially adult males,
may undergo some extended durations of torpidity in the summer months*
In the Thirt.een-lined ground squirrel, S„ trldeoemlineatus. all adult
squirrels became dormant in August but sometimes reappear in September
for a few days, and all adult, males are hibernating before all adult
females (McCarley, 1966),
Thus, the prolonged absences in the adult
male round-tails may have been due to an early entry into hibernation,
A habitat selected by dominant adult males, affording favorable forag"
ing areas, may allow them to build up the necessary fat deposits to
enter states of torpidity sooner than other squirrels.
Such entries
into torpidity would be highly adaptive for summer conditions since the
reduction in body temperature reduces pulmonary water loss (Hudson,
1964)$ a savings especially useful during, the summer months of dry
vegetation.
Other adaptations to an arid environment possessed by the round"
tails have teen suggested by Dice and Blossom. (1937) and Brown (1968),
These are the extremely light pelage which protects from solar radia™ "
tion and a thick epidermis modified to reduce water loss.
FOOD HABITS
With the exception of a few incidental observations reported by
Vorhies (1945)# the food habits’of S. teretie.andua were virtually un­
known,,
The Round-tailed ground squirrel’s maintenance of a subterran­
ean burrow system plays an important role in the conservation of body
fluids and temperature regulation* but diet# especially, a succulent
one* is also of great importance in maintaining water balance.
The .
utilization of succulent foods and rainwater, when available are two
more adaptations which enable the round-tail to exist in the arid en­
vironment of the southwesto
The following description of feeding behavior is based on 173
observations and indicates the food preferences of the round-tails
varied during the summer months (Table 4),
Although food preferences
related closely to food availability# the honey mesquite tree# Prosopis
juliflora, served as a continuous food.source in the study area.
In
68 instances (41.0%) squirrels were observed feeding on some part of
mesquite trees.
The parts eaten included the leaves# flowers and bark
but most frequently the beans.
The legumes were eaten both while on
the tree and after they had dried and fallen to the ground®
were opened and. the seeds were removed and eaten.
of food being cached.
The pods
There was no evidence
The few livestock that periodically traversed the
study area Xtiere also observed'edtihg; the mesquite pods.
On several
occasions round-tails were seen breaking pieces of cow manure* sometimes
dragging large chunks 10 feet to their burrow entrance* then extracting
36
-
Table 4 0
Food preferences noted during 166 observations of feeding round-tails.
(N in parentheses)
Food
June (N=3l)
July (N=?6)
August (H=59)
Total B3.66
12.0*
1,6*
6.0*
41.9%
51.3*
28.8*
41.0*
6.4*
9.3*
6.7*
7.8*
33.8*
20.0*
15.2*
21.8%
Trees and Shrubs
Celtis pallida
Prosopis .luliflora
Olneya tesota
Larrea tridentata
' -s'
■ ' "■... ■; ■’
Herbs and.Grasses
Leniditm sp.
2,6*
. Euphorbia sp.
Plaffibbothrys arisonicus
. 1.8*
9.7*
. 1.6*
1,8*
1.3*
. 1.2*
Aplopappus heterophrllus
1.3*
.0.6*
Franseria sp.
1,3*
0.6*
Feetis papposa
1.3%
27.1*
10.0*
1.3*
18,6*
.7.2*
3,2*
Animal matter
Insects
and eating the seeds which had been passed by the cows„
The squirrels
sometimes ate small strips of mesquite bark that were removed from the
trunk and branches.
The removal resulted in some bare patches measur­
ing 4 by 10 inches.
The ubiquitous creosote-bush was the second most preferred food
source in the study area.
observed eaten,
Only the fuSssy-white globular fruits were
Vorhies (1945) felt this was the only edible part of
the shrub for the squirrels.
of food for two reasons,
The creosote-bushes were a major source '
l) Although it bloomed most profusely during
the spring, it flowered throughout the year, offering a continuous
food source.
Often a creosote-bush completely lacking any yellow
flowers or fruits would be within 10 or 20 feet of another which had
numerous fruits or was beginning a fresh bloom,
2) The dense Larrea
afforded the squirrel population an abundance of creosote-bushes.
Following the onset of the summer rains in mid-July, the food
preference shifted markedly.
The study area then supported patches of
blooming chinchweed, Pectis papposa, resulting in reduced utilization
of the mesquite trees and creosote-bushes as food sources.
By the
middle of September all the chinchweed was dried, and during August
the squirrels again fed on mesquite.
The chinchweed bloom also altered
the normal behavior of the squirrels by causing a disregard for the
individual distance of other squirrels.
As many as eight squirrels were
observed feeding within several feet of one another on a patch of Pectis
Mo agonistic behavior was observed as the squirrels voraciously ate the
flower heads.
39
Observations on the consumption.of animal matter* .shown as
insect material in Table 4$ are attributed primarily to ants and ter­
mites.
On one occasion a juvenile female* squirrel 20-70* uncovered
and fed on what appeared to be a eoleopteran insect* and.in several
instances squirrels were seen jumping at grasshoppers in flight.
As
when foraging on lush patches of chinchweed* several squirrels would
feed closely together when insect larvae were found in cow manure pads6
On the morning■of August $* 1968* a group of seven squirrels consist­
ing of adult males* adult females and juvenile females were feeding
within several feet of one another at the base of an ironwood tree.
An investigation of the area showed hundreds of winged insects'in the
bottom sides of overturned manure pads.
Fecal pellets of captured
squirrels often consisted almost entirely of ant remains.
In reporting the omnivorous food habits of the White-tailed
antelope squirrel* Ammospermonhllus leueurus, in southern Nevada*
Bradley (1968a) found vertebrate remains in stomach samples throughout
the year.
Included were.species of the following rodents:
Dipodomys. Peromyseus and Qnyohomys.
Perognathus.
Species representing the above
genera were also sympatrie in the study area with romd-tailsy but due
to their nocturnal activity no interactions were observed.
The liter­
ature indicates that many ground squirrels possess carnivorous tenden­
cies (Johnson* 1922; Edwards* 1946* Packard* 1958; Bridgwater and Penny*
1966; Clark*
1968),
S. tereticaudus exhibited such behavior- on three
different occasions when squirrels which succumbed in the traps* were
left in the study area.
In one instance.a juvenile female, squirrel
8-50, pulled an entire carcass into the burrow she occupied after
40
eating on the neat of the shoulder and neck regione
The carcass of
a Mojave rattle snake. Crotalus scutulatus. was also eaten when left
near the entrance of a burrow.
Such carnivorous habits probably pro- .
vided the opportunistic squirrels more moisture than vegetative
matter.
Lizards comprised a large part of the stomach contents of
Ammospermoohilus analyzed by Bradley* but in this study no predation
was observed on the lizards (Cnemidophorus tieris, Scelooorus magister
and Dipsosaurus dorsalis) that -occurred in the area.
Much of the
vertebrate remains found in the antelope ground squirrels may have
resulted from squirrels finding torpid reptiles and carrion.
S, tereticaudus. in all probability* would be as .opportunistic in
similar situations* as indicated by observations of round-tails stalk­
ing and killing English Sparrows and eating portions of them (Bradley*
1968a).
A similar observation was made on the Mexican ground squirrel*
S. mexieanus by Edwards (1946)„
gommmcatxon
“Central to the notion of communication is the reception of
information through a stimulus that an organism perceives from the
external environment11 (Marler, 1967 $769).
With communication based on
the transmission and reception of a various number of stimuli, the most
important stimuli to which the Round™tailed ground squirrels responded
were vocal, olfactory$ tactile and visual."'
Vocal
Round™tails had a vocal repertoire that included growls, hisses,
squeals, peeps and trills, each associated with a different behavioral
pattern.
A low, guttural growl, much like that, of -a dog, was emitted by
some individuals when in the confinement of a trap and especially when
handled.
A “reptile-like” hissing sometimes accompanied the growling
of trapped squirrels.
A curling of the upper lip, to produce a snarl,
indicated this vocalization may play a role in threat behavior in intraspecific situations as observed in the Uinta ground squirrel by Balph
and Balph (1966).
However, the growl was of such low volume that it
would not have been perceived by us during the observed agonistic inter­
actions which were at least 75 feet away.
On. one occasion, growling was
heard after a. captured adult male, squirrel 1-100, entered a burrow on
the periphery of his home range.
Three minutes later another squirrel
surfaced from a partially plugged hole three and one-half feet from
41
where 1=100 had entered.
This growl was probably a result of the
thwarting situation* as Eisenberg (1963) sometimes witnessed in
heteroymids when animals were harassed by oonspeeifies or were d@=
fending a nest,
A squeal was sometimes produced by captured squirrels when
first approached by the observers.
Such a squealing vocalization
continued and intensified as the squirrel was handled for weighing$
dyeing and toe clipping.
Squeals were also heard from squirrels @n~
*
gaging in intraspecific fighting* especially when a subordinate indi~
vidual was being held down or bitten on the haunches by a dominant
squirrel.
Often the occurrence of such encounters was located by the
observers when squeals were heard from fighting squirrels over 100
feet away.
The subordinate squirrel only squealed during contact and
not during the escape run.
The primary motif of the vocal repertoire of the Rouzid-tailed
ground squirrel was a sharp* high frequency bird-like peep.
It was
produced by a sudden contraction of the abdominal muscles forcing air
through the larynx.
Variations were heard with several different be­
havioral patterns, but a single-peep was the roost common of all the
vocalisations utilised by the squirrels.
It appeared to indicate the
presence of a foreign object in the vicinity.
For example* some
squirrels sat in their burrow entrances* with only their heads out*
oriented toward a recently placed trap and emitted a single peep approxi­
mately every 30 seconds.
The duration of this behavior varied from . '
squirrel to squirrel but normally did not last longer than 10 minutes.
Neighboring squirrels* after responding in alert behavior* appeared to
become habituated to the vocal squirrel and continued ■with their
■
-
•
.
- .
*
'
■
grooming or foraging activities.
Peeps, eight to fifteen per second, produced a sound similar
to the "churr" of the Uinta ground squirrel as described by Balph and
Balph (1966) and the trill of the Thirteen-lined ground squirrel de­
scribed by McCarley (1966)»
Observations of the interactions between
ground predators and adult female round-tails with young did not in­
dicate that the trill was used as a distress call as McCarley (1966)
showed for S« trideeemlineatus.
Occasionally trills were emitted when
squirrels were observed tail-flicking near newly claimed holese
Thus,
it appeared to be used intraspecifieally as Balph and Balph (1966)
found in 3. armatus.
Trills were also heard from captured animals
while in the trapse
; v. •
••.
When a released squirrel entered a nearby hole, often several
peeps were heard.
Even when the squirrel was out of sight underground
these., sounds sometimes continued, faintly audible.
The literature indicates how varied the vocal responses of the
ground-dwelling sciurids are to predators.
Fitch (1946) found S.
beecheyi to have distinctive chirps that were given in response to the
sight of hawks and other large birds flying in the vicinity, including
Turkey Vultures.
A different call was elicited by the sight of rattle­
snakes, a 11snake-warning" chirp that could be heard 700 yards away 1
Waring (1966) reported that the vocal repertoire of Marmots, Marmota
flaviventris. did not seem- to include an aerial predator warning.
This
corroborated the earlier finding by Armitage (1962) that M. flaviventris
emits no alarm calls to Ospreys.
The Arctic ground squirrel, S.
44
uadulatns. emits a single v/histle upon the approach of an aerial pre­
dator and repeats it at 6 to 8 second intervals if the predator
alights nearby (Melchior, 1968)„
A trill is used by the Thirteen-
lined ground squirrel$ S„ tridecemlineatus. during predator inter­
actions (McCarley$ 1966), and the W n t a ground squirrel S. armatUSc
■which possesses a varied repertoire of sounds (Balph and Balph, 1966),
responds to airborne predators with 11chirps11 and ground predators with
"ehurrs”.
The Columbian ground squirrel, S, columbianus, voices an
alarm call when potential enemies- approach, which is then passed on
by other squirrels.
The calls become louder and faster until the
squirrel takes refuge in a burrow (Manville, 1959).
With only two exceptions (N=5l), there were no vocalizations
by round-tails as a warning for ground predators, and no vocaliza­
tions were ever heard when squirrels responded to aerial predators
(N"23)«
Visual communication by way of the post posture in alert be­
havior appeared to be the primary means by which squirrels were warned
of aerial and ground predators.
As the 11snake-warning" chirps of the
California groimd squirrel aided Fitch .(1948) and field workers to
locate rattlesnakes, the post posture of the round-tails forewarned
the observers in this study; to the presence of hawks and snakes.
Ap­
pearing almost as a simultaneous movement, all of the squirrels above
ground would assume a post posture and peer in the direction of the
predator.
The same response was seen when Coyotes were heard in the
distance.
Squirrels appeared to differentiate between Turkey Vultures
and hawks,.as the former elicited no response as they circled the study
area.
On.one occasion a vulture alighted on the ground, an action
45
which would have sent all the proximal squirrels underground had it
been a hawk# however, they appeared undisturbed.
Since observations were conducted near ground level, it was
easy to see the effective uses of the alert response as a communi­
cative signal.
Squirrels in the immediate vicinity of a ground
predator retained their post posture, sometimes emitting periodic
peeps, whereas the more distant squirrels resumed their previous
activities but frequently looked in the direction of the stimulus.
No sound communication was observed in epimeletic behavior
(parent-young) of round-tails nor was any tooth chatter heard as de­
scribed for Marmots by Waring (1966),
Olfactory
Frequent.nose-mouth and nose-rear contacts between scrotal
males and adult females in the enclosure were observed.
This was prob­
ably olfactory testing as seen in the courtship behavior of the
Thirteen-lined ground squirrel described by MeGarley (1966) and the
Uinta ground squirrel seen by Balph and Stokes (1963),
During sexual
behavior in the spring, olfactory cues probably play a very important
role.
In cautious and exploratory behavior the squirrels did a great
deal of sniffing, especially at the entrances of unfamiliar burrows.
Through maintenance behavior the squirrels left various signs in the
form of fecal pellets, dusting areas and branches which they had rubbed,
which could serve as olfactory stimuli to indicate their presence.
Ol­
faction may be an important means of communication in the burrow system.
46
Tactile
As with olfaction,, the tactile stimuli to which the ground
squirrels reacted were difficult to observe®
The tactile communica­
tion observed in the round—tails included the contact of wrestling
juveniles during play and intraspecific fights®
The squirrels un­
doubtedly made much greater use of tactile stimuli while underground«,
Tactile vibrissae would be important to the round-tail$ which is
virtually blind in the labyrinth of burrow passageways, due to the
possession of a highly adaptive all cone eye (Walls, 1942)„
Visual
Vision appeared.to play the most important role in communica­
tion among the Round-tailed ground squirrels, as it preceded or ac­
companied all the other forms of stimulation previously mentioned®
A
visual perception of other ground squirrels seemed to afford the indi­
vidual some sense of security after periods of disturbance by predators®
Sciurids, except Glaucomys. are distinguished from other ro­
dents by a pure cone.retina (Walls, 1942), and some have been shown to
possess color vision .(Creseitelli and Pollack, 196$) ®
Other visual
adaptations possessed by ground squirrels, as discussed by Walls
(1931, 1942), indicate the importance of vision in many behavioral
patterns,
A yellow, spherically shaped lens acts as an intra-ocular
filter providing an immunity to dazzlement and less involuntary eye
movement.
These characters allow a squirrel to tolerate strong light
intensities without blinking and to perceive moving objects while not
moving the head, when in a flattened posture or a signaling, post position®
47
It appears that the visual capabilities of round-tails serve
as the primary defense against both aerial and ground predators.
Un­
like many animals that use secretive:actions to escape predators* the
ground squirrels possess behavioral patterns that afford good visual,
coverage of the environment and neighboring squirrels.
This adapta­
tion in S. tereticaudus appears much like that seen in the Uinta .
ground squirrel as elaborated by Balph and Balph (1966),
The open
habitat of the Larrea zone promotes such behavioral patterns by pro­
viding unobstructed viewing at ground level.
COMPETITIVE.AND PREDATOR! FACTORS
The reptiles, birds and mammals that occurred or were observed
in the study area are those characteristic of the Lower. Sonoran Larrea
zone (Table $).
The marked exception was the Porcupine, Brethizon.
dorsaturn, which inhabited a desert wash on the northern boundary of the
area.
Occurrences of Porcupines in the desert-shrub of southern Ari­
zona are not uncommon, see Reynolds (1957)»
No predation on ground
squirrels was observed during the study, but the recognition of poten­
tial predators was based on the behavioral responses of.the squirrels
and on known instances of predation on other species of ground squirrels
or small mammals as reported in the literature (Fitch, 1948j King, 1955;
Peterson, 1961; Willis, 1963; McGarley, 1966; Stebbins, 1966),
Predators
Potential predators were observed in the area on 53 occasions,
40 per cent of the observations were on Red-tailed Hawk activity.
The
appearance of a hawk in the area, either circling above or passing low,
immediately elicited an alert response from every ground squirrel.
Alerted squirrels were sometimes over 500 feet apart.
After assuming
a post posture, each squirrel ran immediately to its nearest burrow and
posted again.
The post or down-alert posture was held by the squirrel
at the burrow entrance until the predator left,
A low flying aerial
predator in the immediate vicinity, however, sent the animals'-"diving
underground without a second look.
'
■ ■■
Squirrels foraging in trees and
43
.
Table 5<>
Vertebrate fauna occurring or observed in the study area0
Squirrel behavior and literature accounts are the basis for the denotation (*) of poten­
tial predators. Vernacular and'scientific names for reptiles* birds* and mammals follow
Stebbins (1966)* Phillips* Marshall and Monson (1964) and Gockrum (I960) respectively.
REPTILES
Dlpsosaurus dorsalis. Desert Iguana
.BIRDS
'
Cathartes aura-, Turkey Vulture
Sceloporus magister. Desert Spiny Lizard
*Buteq iamaieensis. Red-tailed Hawk
CEsmidophoroua tigris. Western Whiptail
*Buteo albonotatus. Zone-tailed Hawk
*Masticophis flagellum plceus. Western Black Racer
*Falco sparverius. Sparrow Hawk
*Pituophis melanolencus. Gopher snake
Lpphortyx gambelli, Gambsl’s Quail
*Crotalus scutulatus. Mojave rattlesnake
Zenaidura -macroura. Mourning' Dove
Zenaidura asiatica. White-winged.Dove
*Geococoyx: californicus. Roadruziner
Golaptes auratus, Gilded Flicker
Centurus uropygialis, Gila Woodpecker
^Toxostoma bendirei, Bendire's Thrasher'
Toxostoma curvirostre. Curve-billed Thrasher
Lanius ludovicianus. Loggerhead Shrike .
Table 5»
continued
MdJ'imLS
Lenng alien!» Allen8s antelope jack rabbit
L e m s californicus„ Black-tailed, jack rabbit
Sylvilaeus audobonl, fesert cottontail
SneriGoohilus tereticaudus. Round-tailed ground squirrel
Perognathus'ampins. Arizona pocket niouse
Perognathus baileyif Bailey8s pocket mouse
Perognathus penicillatus. Desert pocket mouse
Dipodomys speotabilis. Banner-tailed kangaroo rat
Dipodomys merriami. Jferriam's kangaroo rat
Onychomys torrldua. Southern grasshopper mouse
Peromyscus eremicus. Cactus mouse
Erethizon dorsatum. Porcupine
*Cania latrans. Coyote
*Tazldea taxus. Badger
!
K
.
shrubs also ran to their burrows.
51
This response to aerial predators
was always the same and invariably, no vocalisations were heard.
Usually the hawks only circled above, but on occasion they passed
through the area 1 to 2 feet above the ground, alighted in a tree
momentarily, then glided low for several hundred feet in another dir­
ection,
Squirrels that had to run greater distances to an escape
burrow sometimes flattened their bodies at the burrow entrances rather
than posted.
The light pelage of the round-tails .blended with the
background soil, and the posture was apparently an attempt to escape
detection.
As MeOarley (1966) observed in S. tridecemlineatus. the
flattened posture makes a motionless squirrel almost undetectable.
All ground predators elicited the same alert reaction of stand­
ing upright, then running to the burrow entrance.
When a snake was in
the immediate vicinity, squirrels closest to the snake violently
flicked their tails from side to side and drummed their hind legs at
their burrow entrances while facing the snake.
If the snake continued
to move in their d i r e c t i o n t h e squirrels dashed back and forth several
feet in front of it, as if to prevent it from entering a hole.
ran to within a foot of its tail but never touched it.
They
This behavior,
similar to that described for S„ beecheyj by Pitch (1948)$ continued
until the snake entered a burrow.
Often, several neighboring squirrels
were seen around a snake, indicating that the immediate danger repre­
sented by a predator pre-empted concern for individual distances.
One
female, squirrel 20-70, rapidly left her burrow within 20 feet of the
observers followed immediately by a Black Racer, MagtieotMs flagellum.
52
She proceeded to harass the snake -without any apparent concern for
our presence*
Potential reptilian predators were observed in the study area
on 14 occasions, 50 per cent of the observations were on Mojave rattle™
snake(a). Grotalus scutulatus.
Snakes were seen coiled near burrow
entrances early in the morning and a strike was elicited from a
Gopher snake when a caged squirrel was placed near it*
Sudden close™•
quarter, encounters with snakes in burrows must have often occurred,
putting the ground squirrel at a tremendous disadvantage*
A Bendire6s Thrasher was observed fighting with an adult
squirrel as it jumped in the air with wings spread and feet turned
toward the squirrel*
On another occasion an adult squirrel ran be­
tween a juvenile and a pursuing Thrasher*
The round-tails always demonstrated an alert behavior toward .
passing Roadrunners, and one chased a squirrel around the base of a
desert hackberry several times while making stabbing motions with its
bill.
MeCarley (1966) observed roadrunner predation on the Thirteen-
lined ground squirrel.
Coyotes and Badgers are also considered in predator roles,, as
their extensive diggings, seen periodically in the area,, indicated that
ground squirrels could have been caught in blind-ending burrows*
The squirrels did not appear to object to the presence of the
jack rabbits, cottontails and cattle which on occasion passed through
the area.
The nearest squirrels posted momentarily then resumed ac­
tivities once the object was recognized as harmless.
53
The round-tails were subject to predation at all times, and it
appeared that when above ground they guarded against it very effec­
tively,
Their population density and visual communication provided
a great number of eyes to perceive predators and alert neighboring
squirrels„
During above ground activity high numbers were probably a
basic element of survival*
This effective guard against aerial and
ground predators indicated that predation is probably greatest by
reptiles in the burrow systems.
Although, the amount of predation in the study area was not
known, observational evidence of very low reduction in the population
indicated that it must have been small while the study was underway.
Due to the numerous observations of predators in the area, especially
the hawk activity, it is felt that t h e .observer's presence did not
keep predators away.
Competitors
Due to the nocturnal activity of the syapatrie rodents in the
area, interspecific encounters were not observed.
No evidence was
found to indicate active round-tail burrow systems-were shared with
other rodents| however, all three, species of lizards occurring in the
area were observed utilizing active squirrel burrows.
Squirrels did
use burrows in the abandoned mounds of the Banner-tailsd kangaroo rat,
Dipodomys spectabilis. but they only retreated to active burrows of
this kangaroo rat out of the necessity to escape.
Holdenried (195?)
found the Spotted ground squirrel, S, spilosoma. to inhabit active as
well as old burrows of D. spectabilia.
54
Some food eompetit-ion would be expected^ but the speraopbilio
pocket mice and'kangaroo rats probably fed very little on the succulent
plants so important to the round-tail.
The food habits of D,
spectabilis and D, merrismi Indicate both kangaroo rats gather primarily
the seeds from grasses (Vorhies and Taylor, 1 9 2 % Reynolds, 1958)6
These studies also found that mesquite beans were highly preferred food,
but the lack of arboreal capacities prevented the rats from harvesting
the succulent beans,
Reynolds (1958) felt that the round-tail did not
seriously compete with D , merrxami for food.
BEHAVIOR IN THE ENCLOSURE
To observe more intensely the behavioral patterns associated
with the encounters of adult and juvenile male and female Roundstailed,
ground squirrelss a squirrel enclosure constructed at the University8s
Desert Biology Station was utilized.
Squirrels were placed in the
70 by 80 foot enclosure each summer, in June, and observations 3 days
a week, during the hours of peak activity, continued into August.
The
following behavioral patterns are based on observations of nine adult
males, seven adult females and three juvenile m l e s .
The preparation of the genital, tract for breeding in round-tails
apparently occurs during hibernation, as Neal (1965b) found motile sperma­
tozoa in the oauda epididymides of scrotal males in early January.
He
found live spermatozoa in squirrels until 8 April then the testes began
-
regressing about mid-April.
-
R-i-'
However, captivity delayed the regression
of the male reproductive organs and captive males remained sexually
active until the end of August.
It was not reported if females were
examined to see if this was also true.
Some of the squirrels that were to be placed in the enclosure
were collected in the spring, kept in captivity and released in the
enclosure in June,
At this time- some of the adult males had retained
a scrotal condition, with the testes in the scrota.
the level of gonadotrophins was relatively high.
This suggested that
They exhibited sexual
behavior toward adult females and an emphatic agonistic behavior toward
other squirrels, especially juvenile males.
55
56
The sexually active males consistently ran toward the femalea$
stopped short of them, then extended their necks and began sniffing,
the mouth region of the females«
Is a male approached# its tail yas
usually stiffened in a vertical position while the female slowly
flicked her tail in a vertical motion*
The male sniffed and licked
her mouth and at this point the female usually retreated# running to
the edge of the enclosure or into a burrow.
When the male approached
more slowly# a prolonged oral contact resulted before the male sniffed
and licked, the rear, of the female.
The male then grabbed the haunches
of the female with, his fore legs and attempted eopulatory movements#
but the females invariably broke away and ran.
This suggested that the round-tail copulation occurs above
ground as in the Thirteen-lineti ground squirrel (MoGarley, 1966) and
not below ground as is suspected for the Uinta ground squirrel by
Balph and Stokes (1963).
They also found that in the Uinta ground
squirrel a slow approach by the male and his associated posture appar­
ently has an "appeasing" function# with the female permitting contact.
It was not known if the unreceptive behavior of the females was
due to a low gonadotrophin level or the confining conditions of the
enclosure.
However# the labia of the genital opening were not enlarged
as Neal (1964.) observed during the breeding season.
The ground squirrels placed in the enclosure dug their burrows#
not differing in structure from those in the study area, and used sep­
arate holes, although some holes entered a common burrow system.
The
general pattern of the agonistic behavior was similar to that observed
in the study area but much more intense.
The adult males in a scrotal
57
condition -were dominant over the adult females and juveniles6
One
scrotal male, squirrel 1-40, dominated all the other squirrels sharing
the enclosure, including another scrotal adult male.
.
This male
harassed juvenile squirrels to the extent that it dug at the entrance
of a hole into which a juvenile had been chased, eliciting squeals from
the subordinate animal,
hole,
When the juvenile was caught away from the
1-40 prevented it from re-entering.
Encounters between
1-40 and.a juvenile male, squirrel 3-90,
were observed on June 25, 1968,
Intense agonistic behavior was.ex­
pressed by the dominant 1-40 throughout the day.
was observed being harassed by
attempted to enter a hole.
'
At 0950 hours 3-90
1-40 which chased and bit it whenever it
Each encounter brought loud squeals from
3-90 which then retreated to a shaded area in the pen or attempted to
climb the enclosing fence.
Periodic fighting continued throughout the
day, resulting whenever 3-90 attempted to enter a hole.
At 1927 hours,
3-90 was observed lying on its side near a wall of the enclosure, its
right hind leg quivering and its eyes half closed.
When I approached,
it crawled 4 feet, dragging its hind legs, then rolled on its back and
turned all four feet upward in a’defensive posture.
It-allowed.itself
to be handled and appeared to be in a state of complete exhaustion
demonstrating a very rapid, deep respiration.
This condition apparently
resulted from the constant dominating harassment from 1-40 and an ex­
posure to the high ambient temperatures which had reached a maximum of .
41.8 C.
This squirrel was removed from the pen, caged and fed lettuce -
leaves and sunflower seeds for several days until it recovered.
58
On June 28, 3-90 was again placed in the enclosure,,
This time
it was successful' in excavating and occupying an abandoned squirrel
hole^ as the adult male,
1-40i> was not seen until the following day0 .
At this time 1-40 was no longer in a scrotal condition, and no agonistic
behavior toward 3-90 was expressed®
Even when 3-90 remained abroad
foraging for 30 minutes it was not chased by
1- 40,
The above observations are probably indicative of the extreme
intolerance adult males have toward other squirrels, especially sub­
adult males, during the height of sexual activity.
Such behavior may
affect the breeding population by limiting the breeding role of in­
experienced males (Brown, 1966)*
ACTIVITY PATTERNS
Amual
Round"*tails were active above ground through. September, but
periodic observations in the study area during October, November and
December indicated the;squirrels were inactive these months.
Pengelley
and Kelly (1966) found that under laboratory conditions the. round-tails
went into a state of torpor (hibernation) in October, November and
December, but due to frequent interruptions of the hibernating state
they termed S. tereticaudus a poor hibernator.
They indicated that the
round™tail has almost abandoned a period of obligatory hibernation and
retains only traces of an endogenous circannian rhythm.
Because this
frequency of arousal is a function of the ambient temperature (Pengelley
and Kelly, 1966), some squirrels might be observed above ground during
unseasonably warm periods in October, November and December,
This is
corroborated by observations of S. tereticaudus active above ground in
his study area at the end of November and early January (Neal, 1964)«
Neal (1964) discussed the reproductive cycle of the round-tail
and found males were reproductively active in early January but breeding
did not begin until mid-February.
Mlz AsMsiii
Daily activity patterns of the round-tail were studied during
the 1967 season.
A surveillance of 10 acres in the study area, un­
obstructed from the observer’s view, was made each one-half hour to
59
60
tally the number of squirrels above ground.
These surveys were con­
ducted on various days to include every daylight hour, and at the time
of each survey general weather conditions were noted.
These were later
combined with recorded relative humidity and ambient temperature
readings.
S. tereticaudus is a diurnal animal and may be above ground
during any daylight hour, but the number of active squirrels and the
amount of activity was influenced by the ambient temperature.
During
the summer months, there was a bimodal pattern of activity (Fig. 5),
whereas observations in the fall and spring indicate the activity peak
was at mid-day.
Figure 5 represents the general activity pattern of
the round-tails based on the mean number of squirrels active at the
time the surveys were taken on 23 random days from June 28 to August 5,
1967.
The ambient temperatures are the mean temperatures for the hours
the survey was taken.
During the summer the squirrels were not active at sunrise, the
coolest time of day, but rather activity began approximately 1 hour
after and peaked 3 to 4 hours later.
Mot all squirrels emerged from
their burrows at the same time, and even sibling juveniles sometimes
emerged in intervals separated by as much as 30 minutes.
uals consistently emerged earlier than others.
Some individ­
Upon emergence, most
squirrels spent 10 to 15 minutes at the entrance of their burrows,
either sitting outside or with just their heads out.
Once active, for­
aging was the main activity while grooming and burrow excavation were
secondary.
Squirrels often moved throughout their entire home ranges
during the morning activity, and they sometimes investigated the main
40
40
ambient temperature
Lumber of Squirrels
Temperature in Decrees Centigrade
squirrel number
TSSO
sunset
sunrise
Hours of Day
Fig. 5.
Mean number of round-tails observed above ground and mean ambient temperatures during
June, July and August surveys.
See text for discussion.
&
62
burrow entrance of inactive neighboring squirrels without consequence,
However, when neighboring squirrels were also above ground, such tres­
passing resulted in chases*
Squirrel activity began dropping off 3 to 4 hours after sunrise
and continued to decrease until late afternoon*
On most of the cloud­
less summer days the ambient shade temperature reached 38 G by 1200
hours while temperatures in the sun exceeded 42 0, and the relative
humidity dropped below
30%c
Surface temperatures of the soil recorded
as high as 62 G during the peak of the afternoon heat.
Through the
afternoon hours above ground activity all but completely ceased.
Squirrels seldom left the immediate vicinity of the main burrow entrance
during the afternoon activity, but an occasional animal was seen on a
short run from one shade source to another.
The squirrels that were out
during the afternoon hours consistently returned to their burrows with­
in 5 minutes and re-emerged after 10 to 15 minutes^
It is during these
hours that the squirrels would have to be most concerned with maintain­
ing an optimum body temperature as described in the section on burrows.
The highest temperatures of the day peaked in the summer months
at 1500 to 1600 hours.
An increment in squirrel activity became notice­
able 2 hours later and continued until dark.
Most squirrels had termi­
nated surface activity one half hour before sunset, but occasionally
some were active until dark, although they remained near their burrow
entrances.
This late cessation contrasted with the delayed onset of
morning activity.
The evening activity of a squirrel primarily in­
volved movement throughout the home range and foraging but seldom
grooming and- digging.
63
In late July and throughout August cloudy afternoons became
numerous and the squirrel activity increased.
However? this increment
in the morning, afternoon and evening activity correlated with the
ambient temperature and not the cloud cover.
On days following a heavy
rain, the activity was noticeably increased, even under clear skies,
when the temperatures were unusually low,
A strong wind delayed the onset of activity at the beginning of
the day, but a wind arising later did not appear to send the squirrels
underground.
Rainfall sent squirrels below ground, and the onset of
activity was delayed following a night of heavy rains and low tem­
peratures.
POPULATION DYNAMICS
Population Structure and tensity
"
' ""
One hundred and fifty-one ground, .squirrels were caught and marked
in the 25 acre study area from June 24$ 1967 to August 17, 1968 (Table 6)
v
'
Immigrants and juvenile squirrels born in the area were considered resi­
dents if they remained in the area at least 2 weeks to occupy and defend
a burrow other than a home burrow®
The entire resident population was estimated to have been marked
and consisted of 41 squirrels in 1967 and 64 individuals in 1968
(Table ?)*
The 1968 population is probably more correct for the area
because capturing techinques had been perfected®
As indicated by Table
7, the adult and juvenile females made .up the largest percentage of the
resident population while the juvenile males were the smallest category
because most of them were transient squirrels (Table 6),
The mean number of resident ground squirrels observed in the
study area during the 1967 and. 1968 summer months was 52.5 squirrels.
- From this an estimated population density of 2,1 resident squirrels per
acre was derived.
Although this represents a conservative estimate, by-
excluding the transient squirrels that passed directly through the area
or those that attempted to establish residency for periods of almost 2
weeks, it does indicate the density of the semi-colonial round-tails
that were resident in the area.
64
Table S„
The population structure of the Round-tailed ground squirrels captured in the
study area in June, July and August of 1967 and 1968.
The percentage that were resident (Res.) is given for each group (l in parentheses).
ADULT:MALES
ADULT FEMALES.
JUVENILE MALES
JUVENILE FEMiALES
1967 Population (?6)
21$
(16)
Res.
50$
(8)
20$
(15)
. 26$
. (20)
Res.
70$
(14)
Res. ■ 33$
(5)
23$
(22).
.16$
(15)
.33$
(25)
Rese
.56$
(14)
1968 Population (94)
. . 21$
(20)
Res.
60$
(12)
Res.
91$
(20)
ReSo
40$
(6)
40$
(37)
.Res*
70$ '
(26) ■ .
Table 7»
Population structure of resident ground squirrels.
(1 in parentheses).
.ADULT MALES.
ADULT FEMALES
JUVENILE MALES v
JUVENILE FEMALES •
1967 Residents (41)
20%
(8)
34%.
(14)
12%
(5)
34%
(U)
31%
(20)
9%
(6)
4.0%
(26)
1968 Residents (64)
20%
(12)
67
Sex ga^ioi
.
The resident male and female adults deviated from a 1:1 ratio
both summers (Table 6)e
In 1967 there was one resident male to l e8
resident females* and in 1968 the ratio was one resident male to 1„7
resident females,,
The proportion of resident juvenile males to resi«= .
dent juvenile females deviated even further with nearly three times.as
many juvenile females resident in 1967 and four times as many in 1968
(Table 6).
These ratios resulted because nearly twice as many juve­
nile males moved out of the area each summer than remained* while as
many as 70 per cent (in 1968) of the juvenile females marked in the
area established residency.
This same trend has been seen in the
Thirteen-lined ground squirrel* S. tridecemlineatus. by Rongstad (1965)
and McCarley (1966). ; The former suggested sex-specific differences in
mortality and hibernation dates as well as dispersal, as possible fac­
tors influencing the greater number of juvenile females.
.
Population Turnover and Dispersal
The resident population in 1968 included 46 per cent (19) of
the 1967 resident squirrels (Table 8).
Adult females represented the
largest percentage of these* followed by juvenile females.
The 1967
resident juvenile males were represented by only one individual in the
study area the following .year.
Observations indicated that most juve­
nile males had left their home burrows by June and probably even the
study area.
McCarley (1966) found, in the Thirteen-lined ground squirrel
that juvenile males consistently moved farther from the home burrow to
establish residency than did the juvenile females.
The juvenile males
Table 8„
The percentage of the 1967 resident squirrels' found in the study area in 19680
Below is the percentage represented by each age group of the remaining 19
squirrels (N. in parentheses).
ADULT MALES
ADULT FEMALES
j u v e n i Ie m a l e s
JUVENILE FEMALES
Totals 46% (19)
38%
(3)
57%
(8)
20%
(1)
16%
(3)
4-2%
(8)
5%
(1)
.
50%
(7)
37%
(7) .
:£
69
of Richardson's ground squirrel, S e richardsoni, were also found to.
move farther from the. home burrow than the juvenile females (Quanstromj,
1968),
By June (1968) only two litters, of four each, remained at the ;
home burrows, but one consisted of all. females and the other of threee
With the expected sex ratio of a 1:1 distribution, this indicated
that the juvenile males of the round-tail probably dispersed earlier
than the females.
In 1967, 67 per cent (10) of the juvenile males
failed to establish residency in the area, and the following summer
60 per cent (9) of the marked males disappeared (Table 6).
Home Range
When studying the ecological relationships of any animal
species, it is of primary importance to obtain information on its spa­
tial movements.
Seton's (1909) concept of wild animals not roaming at
random but possessing a /home region, followed by Burt's (1943) defi­
nition of a home range, provided the impetus for the development of
techniques to determine the movements of animals.
As Sanderson has pointed out in a review (1966), the techniques
for locating and observing mammals to determine their movements are
far ahead of the techniques for interpreting the data.
These former
techniques included, urine and feces dyes, radioactive materials,
capture-mark-release, photographic devices and radiotelernetry.
However,
in discussing the methods that have been used for analysing the move­
ment data and determining home range estimates, he listed
13 different
\
methods and implied it was incomplete,
These included everything from
the minimum area and the several boundary strip methods to the adjusted
70
range length, most of which are discussed by Stiekel (1954) and more
recently by Cockrum (1962)„
Sanderson (1966) suggested that the sizes
and shapes of a speciese home rang© have little significance in them­
selves and that researchers should concentrate on ecological .studies,
emphasizing the animal1s specific needs.
The latter would indicate
why an animal was at a particular place at a particular time.
When studying the population density, and structure, and the
territory and exploratory behavior of any mammalian'..species,, basic
information on the daily movements is essential,
larger (1953), how­
ever, pointed out that investigations can define but. not measure an
absolute or true home range as was defined by Burt (1943).
The home
range of an animal in its natural state is not a permanent plot of
ground with well defined fixed boundaries but a dynamic area continu­
ously changing in size and shape from month to month.
One of the
important reasons for attempting to determine a home range by calcula­
tions based on data obtained from biased trapping, is to determine a
size index.
This home range index can then be compared to the indices
of other species or subspecies.
Unfortunately, the indices of the
compared species have often been determined by different techniques
and analyzed by different methods.
In an attempt to determine a home range index for an animal,
the following conditions should be m e t : movements should not be
hampered by traps, handling should be minimized, the population should
not be killed, information should be obtained on where the animal is
and what it is doing at any particular time, and inter-and intraspecific actions should be observed.
These prerequisites can be
-
71
achieved only by the direct observation of a natural population*
The major disadvantages are that the technique is time consuming, and
only a small portion of a population can be studied at one time.
The movement data on S, tereticaudus in this study were ob­
tained by direct observation, .Traps were used for marking the animals
and were moved periodically to capture as. many different squirrels as
possible.
It was easy to see the biased movements that would have been
recorded in strictly trapping each quadrant simultaneously.
As Balph
(1968) saw in the Uinta ground squirrel, S, armatus. most squirrels
readily approached strange objects (traps) in their home ranges, and
many dominant animals ran from trap to trap, traversing ranges, to
obtain the bait.
In this study dominant squirrels warded off subordi­
nate animals which actually had home burrows very near.
These animals
would invariably be trapped but many times quite distant from their
normal area of daily activities.
Conversely, to some squirrels the
punishment of capture was apparently greater than the reward, especially
if they became over-heated in the trap, resulting in a subsequent
avoidance of traps.
Thus, if a squirrel was not recaptured, it did not
necessarily mean that it was no longer in the area.
Mohr (1947) felt that minimum home range estimates were funda­
mentally more comparable between species and individuals than any
estimate based upon grids, and they appeared to be more consistent and
reliable, as quadrants gave results varying directly with the size of
the quadrant,
Hayne (1949) favored the minimum home range method be­
cause the animal, without a doubt, was known to have been present, and
he felt the home range should be stated in a conservative manner.
72
When movement data is collected by direct observation, the
most accurate and conservative analysis to determine an index for the.
home range size, appears to be the minimum area method.
In this
method the extreme points of capture and observation of each individ­
ual are connected by straight lines (Cockrum, 1962) to delineate the
area.and shape of the home range.
trap records to be excluded.
Observation data allowed biased
Probably no one shape satisfactorily de­
scribes the home ranges of all species (Sanderson, 1966) and those of
the round-tails varied from nearly rectangular to circular and lineare
The movements of a resident round-tail were plotted only if six
or more observations were obtained.
than four different days.
In all eases these included more
Of all the individuals used for home range
calculations, 92 per cent (58) were observed more than ten times.
When it was found that the size of the calculated home range was inde­
pendent of the number of observations, it was felt that six observations
were not too few from which to derive an index for the home range.
Some
squirrels that were observed eight times had an estimated home range of
1.0 acres while other individuals seen twice as many times had an esti­
mated home range of 0.2 acres.
The occasional sallies outside of the home range that are to be
excluded according to Burt8s (1943) definition were obvious when the
home ranges were plotted, as well as during the time of the observation.
Such forays were not included in the calculations, as they would seem,
to introduce an additional, subjective element into the calculated
range, which is already only a crude obstruction of the actual pattern
of the animal8s movements (Bayne, 1954).
The importance of such trips,
73
however, should not be over-looked.
As emphasized by Stickel (1954)
they may represent a natural tendency, for exploration, important in
the invasion of depopulated areas and in the extension of species
ranges»
'
The mean home range size for adult and juvenile resident
squirrels in the study .area is shown in Table 9,
The summer ranges
of adult males were hot significantly larger than those of the adult
females (P < 0 e7), and the juvenile ranges were not significantly larger
than the adults (P <0*9) 8
Home ranges of juvenile females were not
significantly larger than those of juvenile males ( P < 0 ol)o
Home range sizes and shapes shift and vary with sex, age, sea­
son and population density (Burt, 1943) more or less continuously from
month to month, season to season and year to year.
In this study many
resident individuals were observed enough times to determine their
home range areas oh a monthly basis.
Figure 6 shows the estimated home
ranges occupied by an adult male, squirrel 1-100, during June, July and
August of 1968.
preferences.
These;differences ar© primarily the result of food
In August the small patches of ehinchweed in bloom were
scattered and attracted squirrels to different regions in the area.
An adult female, squirrel 3-60, had four juveniles at her home
burrow in June of 1968, and as the juveniles dispersed in July and
August she increased her home range (Fig, 7),
The adult females that
had young at the home burrow directed 'the majority of their attention
toward the juveniles when they were above ground.
At the beginning of
the summer the movements of 3-60 were restricted to the proximal vicin­
ity of the home burrow.
In July.and August her movements greatly
Table 9. The estimated home range size*, standard error of the mean and range in acres for
1967 and. 1968 resident squirrels.
(E in parentheses).
SQUIRRELS
1967
1968
.■COMBIMED
Adult males ■
(3) 0.97 + 0,32'
(0,40— 1.52)
(4) 0.71 + 0.32
(0.11— 1.28)
(7) 0.^2 ± 0.21
(0.11— 1,52)
Adult females
(10) 0.62 ± 0.14
(0.11— 1.53)
(6) 0.85 ± 0.24
(0.21— 1.74)
(16) 0.71 + 0.13
(0.11— 1,74)
All adults
(23) 0.74 - 0.11
(0.11— 1.74)
Juvenile males
Juvenile females
All juveniles
(6) 0.53 * 0.10
(0.06— 1,06)
(8) 0,90 ± o . a
(0.16— 1.87)
(24) 0.86 ± 0.10
(0.10— 2.04)
'
(32) 0.87 + 0.10
(0.10— 2.04) :
(40) 0.77 + 0.07
(0,06— 2.04)
'
75
June
July
August
25 feet
Fig. 6.
Home ranges of an adult male, squirrel 1-100, during
the 1968 summer months.
“ ------June
July
August
S,r*sr.?f"..“"i1
77
increased in distance and frequency as the juveniles lefts
A
juvenile female, squirrel 30-80$ was on© of the four young
belonging to 3-60.
During June, 30-80 remained within 75 feet of the
home burrow entrance and usually close enough to not have to utilize
any other burrows for escape.
During July she began wandering farther,
from the home burrow site and utilized escape burrows when necessary.
By August, 30-80 established residency in a burrow within 100 feet of
the original (Fig. 8).
Observations did not indicate that 30-80 or
any juveniles was "driven” away from the home burrow by the parent as
is seen in some animals (Burt, 1949).
instead, 30-80 gradually in­
vestigated and frequented new areas while increasing the avoidance of
other regions.
The mean home range estimates of 26 juveniles (six males and
twenty females) in August of 1968 were slightly larger than in June,
but this difference was not significant (P<0.6),
The foregoing data represent the first home range estimates to
be reported for S. tereticaudus. and the only movements for any species
of Spermonhllus that have been observed directly and calculated by the
minimum area method.
MeCarley (1966) made direct observations on the
movements of S» trldecerolifteatus but used the inclusive boundary strip
method (Blair, 1940; Stickel,:1954) to calculate estimates.
Bradley
(1967) reviewed the known home range estimates for other ground squirrel
species and reported the 14®9 acres for Ammosnermonhilus leucurus as
the largest.
The home range of the Mohave ground squirrel, S. mohavensis
which singly shares the subgenus Xerospermophilus with S. tereticaudus. .
Was subjectively estimated to be in excess of 3 acres (Burt, 1930).
__________ June
___________ July
----------- August
25 feet
Fig. 8.
Home ranges of a juvenile female, squirrel 30-80
before and after leaving its home burrow.
79
Although the home range estimate of 0,74 acres derived In this study
for adult round-tails is conservativey it does not appear unusually
small when compared to the 0,36 and 0,59 acre estimates for adult males .
and females respectively of S. beechevi reported by Evans and Hbldenried
(1943).
Once a 1967 squirrel established residency and a home range
evolved* a fidelity for the area continued.
The 1968 home ranges w e r e .
coincident in part with 1967 ranges for 68,5 per cent (13) of the resirdent squirrels.
The remainder of the second year squirrels occupied
areas within 150 feet of their 1967 home ranges,
A similar permanency
in successive years has also been noted for the Thirteen-lined ground
squirrel* S. tridecemlineatus, by Evans (1951), Rongstad (1965) and
McCarley (1966),
The latter has found that in some eases the general
location of home ranges has remained the same for 3 years.
This is an
important factor in the breeding population* as McCarley (1970) reported
that 89 per cent of the 1967 squirrels in his study area were either
directly or indirectly descended from only 20 per cent (four individuals)
of the 1963 resident population.
Familiarity of Home Range
The round-tails spent a great deal of their time above ground
exploring.the environment of their home ranges.
These explorations
probably play an important part in their peripheral extensions and dis­
persal movements * which according to Brown (1966) are as important to
an animal as the restricted movements around the home site.
As seen
in the escape behavior of resident squirrels trapped for marking and
80
re-marking, the round-tails demonstrated familiarity with their indi­
vidual home rangesc
Escape behavior was observed in 68 resident
individuals on 263 occasions after the captured squirrels were re­
leased in random directions.
Squirrels released on the periphery of
their home ranges always ran farther into their home
ranges,
even when
the traps were pointing the opposite direction and they had to go
around the observers.
They ran rapidly, traveling in straight lines
for distances sometimes.greater than 100 feet, then usually stopped and
posted before entering a,hole.
The mean distance traveled before a
squirrel entered a burrow was 23 feet (H-263), and there was no dif­
ference between adults and juveniles.
On a few occasions resident
squirrels moved as far as 600 feet beyond their home ranges and subse­
quently returned.
These observations indicate that 5.' .teretieaudas
possesses familiarisation with its entire home range.
Transient squirrels and squirrels caught large distances from
their home ranges demonstrated a characteristic escape behavior.
They
appeared totally bewildered when released, moved cautiously to the
nearest burrow entrance and slowly entered.
When investigating strange
holes, the squirrels showed no concern for the observers and sometimes
entered three different ones momentarily before remaining underground.
Homing capabilities from distances greater than 1000 feet are probably
very poor, as indicated in homing studies conducted by Bradley (1968b).
Intraspecific Intolerance
.
Territoriality exists as a part of the behavioristic patterns
of many kinds of mammals, but it is only through direct observation
81
that one can be absolutely: certain of the presence of such behavior,
as it can not be inferred from overlapping home ranges that territori"■
ality is non-existent (Burt, 1943, 1949)„
Territoriality in the sense
of Noble (1939) and Scott (1958) is the defense of a delimited area
with attacks resulting when an intruder crosses!a line.
Burt (1943)
described two fundamental types of territories in mammals, l) a terri­
tory for breeding and rearing of the young and 2) a territory for
assuring a continual food supply and shelter.
The latter serves to
force the excess, of optimum numbers into marginal habitats and influ­
ence the maintenance of the population density.
Many seiurids have been shown to be territorial (Gordon, 1936;
Smith, 1968), but the observations on S. teretlcaudus in this study
indicate that this term, is not appropriate to describe their intra­
specific agonistic behavior,
Table 10 indicates that among the resi­
dent squirrels there was an abundant overlap in the home ranges.
The
home ranges of five adult males (resident) overlapped with no other
adult males and in only two instances overlapped with resident adult
females.
Occasional sallies of adult males and females also produced
situations of proximal distances between members of the same sex and
age.
Adjacent squirrels entered common escape burrows (not simultan­
eously) even when the neighboring animal was watching.
To elicit a
chase from a resident squirrel, an intruder had to be near the en­
trance of the home burrow and be seen by the resident.
After chasing
an intruder, the squirrel did not always return to the site where the chase began, and social rank,.rather than the locality within any
portion of the home range, determined the outcome of the intraspecific
Table 10«
The frequency at which the home ranges of the 1968 resident squirrels over­
lapped with intraspecific■groups.
(N in parentheses).
See text for discussion.
ADULT MALES
Home ranges overlapped:
■
ADULT FEMALES
JUVENILES
1968 residents
0
(5) Adult males
•($) Adult females
(6) Juvenile males
(24) Juvenile females
(2)
(2)
8,6%
. (1)
2.8%
(14)
11%
11.6%
(15)
88.4%
(21)
91.4%
13*8%
W:v
( 6)
83.4%
■ (16) 12.6%
(97)
76.4%
0
(5)
'
- ' *3
encounters6
Juveniles were chased away from mesquite trees and traps
(food source) by dominant adults ©yen when they were located within the
home range of many squirrels and were sometimes closer to the home
burrow of the subordinate.
Chases involving adjacent squirrels of
equal rank often reversed as the initial intruder approached his home
burrow.
The intolerance and concomitant avoidance observed in the roundtails appeared to be not unlike that seen in the Uinta ground squirrel,
S. armatus. by Balph and Stokes (1963).
They observed that neither home
ranges nor any specific delimited areas were defended, and the intoler­
ance was best characterized by the term^, individual distance, as used
by Hediger (1950).
This is seen in an animal that allows another to
approach only up to an arbitrary distance.
Physical contact is not
tolerated, apart from reproduction.
The intraspecific intolerance among the round-tails was a single
continuum that was the greatest toward transients and the least in cer­
tain foraging and predator situations.
Transient squirrels cautiously
investigated holes for refuge and remained longest near those that were
usually farthest from resident squirrels.
The residents were unhesitant
in their pursuit of the transients and would chase the intruder out of
their home range, sometimes traversing the ranges of one or two adjacent
squirrels.
Usually the adjacent resident squirrels, above ground at the
time, would join the chase as the transient passed nearby.
This resulted
in a "relay effort" sometimes involving three or four resident squirrels,
as they moved the transient out of the area.
feet were not uncommon.
Combined chases of 600
This, along with the fact that the squirrels
. 34
being chased seldom attempted to enter a hole, probably results in a
greater amount of predation upon transient squirrels by aerial pre­
dators,
Never did a resident squirrel chase another resident, but
always the transient,
A resident that had terminated its "leg" of the
chase at distances beyond its home range was not molested by other
residents as it returned.
Often a transient would temporarily enter a
hole in an area where adjacent squirrels were not above ground, only
to be chased from the area by the resident(s) later the same day or a
following day.
The chases involving several residents were, of greater
intensity than the lattere
Confrontation with ground predators and temporarily abundant
food sources were circumstances that resulted in the greatest amount of
tolerance between squirrels.
Adult and juvenile, male and female squir­
rels were observed feeding within a few feet of one another on patches
of ehinehweed, Peoti's papposa, and insect larvae.
Also, the presence
of a predator appeared to eliminate concern for the individual distance.
Although specific areas were not defended by the round-tails
in the traditional sense, their intolerance toward one another, spaced
the home burrows and. forced transient squirrels either out of the study
area or into sparsely inhabited sections therein.
IKTRASPEGIFIG BEHAVIOR
Since populations of animals do not exist in nature as randomly
distributed individuals but as socially organized groups, a knowledge
of the social organization is essential in understanding the general
ecology of a species (Scott, 1956)e
The reciprocal interactions of the
Round-tailed ground squirrels are evidence for social behavior as de­
fined by Etkin (1964) and are placed in the following categories;
epimeletic (care-giving) behavior, play behavior and agonistic behavior.
Epimeletic Behavior
Scott (1958) described epimeletic behavior as the attentive be­
havior of the parent toward the young.
This maternal, or parental
behavior includes the rearing, feeding and aiding the distressed indi­
vidual.
The adult female round-tails stayed close to their young when
they were abroad and constantly watched them as they foraged and ex­
plored the immediate vicinity of the home burrow.
The juveniles that
were still at the home burrow in June did not usually move over 15 feet
from the entrance.
Unlike the Thirteen-lined ground squirrel (McCarley,
1966), the inspection of the young by other squirrels was not tolerated.
During interactions with potential ground predators^ females were ob­
served chasing their young to the home burrows, then confronting the.
predators, while the young often sat at the burrow entrances with their
heads out observing the confrontation.
By July the adult females
directed much less attention toward the young which explored and foraged
85
86
further from the home burrows sometimes using nearby escape holes in
situations warranting a retreat.
The young that established their own
burrows within the home range of the adult female were later seen in
agonistic encounters with the parent, especially when they approached
the home burrow.
PISZ
An accepted theory for the function of play is practice for
adult activity (Loizos, 1966).
Juveniles that play, while the primary
needs are cared for by the parenty form behavioral patterns which ap­
pear in their adult lives and the practicing of such patterns gives a
selective advantage over the juveniles that do not play (Loiaos, 1966).
Sibling; juvenile round-tails were often seen in encounters which ap­
peared agonistic, such as boxing, wrestling and short chases; however,
they differed from agonistic encounters primarily because the juveniles
remained together after contact and returned to their pervious activi­
ties of foraging, grooming or exploring.
As Balph and Stokes (1963)
pointed out in observations on the Uinta ground squirrel, such en­
counters 'serve to develop proficiency in agonistic behavior but do not
function as such since a separation of the juveniles does not result.
Agonistic Behavior
Agonistic behavior in animals is considered any behavior asso­
ciated with conflict or fighting between two individuals and is a
continuum of the following tendencies: attack, threat, passivity and
escape (Scott, 1956).
The agonistic expressions observed in the
round-tails were primarily attack and escape.
These tendencies resulted
87
when the individual distance of a resident squirrel was trespassed by
another resident or when a transient appeared*
Fighting occurred most
often when a squirrel was attempting to establish residency? while
threat and passive behavior were usually seen among resident squirrels»
When transient Squirrels were attacked by residents# contact
was not mad© unless the attacker swiftly approached unseen,,
.
Even when
seen at the last moment# the transient squirrel jumped into the air as
the attacker passed underneath«
If fighting resulted# the squirrels
either stood, in an upright position and boxed with their fore legs or
rolled on the ground and wrestled with all four legs.
Grasping and
biting the haunches seemed to .be the primary concern and invariably
elicited saueals from the bitten individuale
When one resident squirrel encountered another# it approached
cautiously with:its back hunched and its erected tail waving slowly in
a horizontal motion.
The hairs on the back and tail bristled#
One
squirrel moved along side to place its tail near the other1s head# then
both moved in a circular pattern.
The curled upper lip produced a snarl#
but growling was not heard by the, observers;, as it was in the threat
behavior of the Uinta ground squirrel (Balph and Balph# 1966),
As dis­
cussed in the section on vocalizations# the growling heard from roundtails on other occasions was of low volume and may not have been audible
to us during the threat behavior.
If the. threat behavior did not re.
.
suit in passivity# contact was made as the squirrels jumped at each
other with all fours.
After the initial fight# the subordinate animal-
usually escaped and was chased.
The general pattern of threat and
attack behavior in the round-tails appeared to closely resemble the
■
88
stereotyped patterns seen in three species of the Spermonhilus sub”
genus, Ictidoimrs. described by Grubitz (1968)„
From their observations
on the Uinta ground squirrel, S„ armatug, Balph and Stokes (1963) felt
that the function of threat in ground squirrels was apparently the same
as fighting, as it resulted in keeping the animals at a distance.
They
stated that threat behavior was a more advanced form of agonistic be­
havior in that it involved less energy and risk of injury than fighting
but accomplished the same results.
When a squirrel attempted to establish residency at a newly
claimed burrow, it exhibited toward neighboring residents the tailflicking and hind leg drumming discussed under maintenance behavior.
This became more intense when another squirrel approached and may have
also been a threat behavior pattern, as often the approaching animal
avoided contact.
This behavior was observed in a juvenile male,
squirrel 5, that forced a juvenile female, squirrel 10-60, from a hole
at which the latter had previously resided.
Each time 10-60 approached
5, a lengthy fight resulted and terminated only when 10-60 ran to
escape.
Unsuccessful attempts by 10-60 to regain her burrow continued
for 2 hours.
The juvenile male occupied the contested area the rest of
the summer while the female took up residency in an adjacent area.
SUMMARY AMD CONCLUSIONS
Behavioral aspects, of the ecology of a natural population of
Round-tailed ground squirrelss S„ tereticaudus. were investigated during.
1967 and 1968.
The 25 acre study area, 12.0 miles north of Florence in
south-central Arizona, was sparsely dominated with mesquite trees and
creosote-bushes and biseasonally received an annual precipitation of less
than. 10 inches„
A squirrel enclosure.constructed at the University of
Arizbna-Boyee Thompson Southwestern Arboretum near Superior, Arizona,
was utilized to supplement observations on intraspecific behavioral
patterns.
The resident squirrel populations were live-trapped each sum­
mer, permanently toe-clipped, and dye-marked for observational identifi­
cation.
Observations during the summer months were conducted 4 to 6
hours daily in the study area to record the inter- and intraspecific
behavior and to elucidate movement patterns. .
The round-tail’s egocentric behavioral patterns indicated that
foraging had priority when above ground, and the activities of grooming
(dusting)elimination, and grass-gathering may have served a secondary
function of leaving signs, as shown for heteromyids by Eisenberg (1963),
to indicate the presence of one squirrel to another.
Exploratory be­
havior led to familiarity of the home range and the establishment of
escape burrows.
The utilisation of extensive subterranean burrow systems prob­
ably enabled the round-tails to minimize body water loss as well as
maintain optimum body temperatures.
.89
Each squirrel occupied a separate
90
burrow system^ ■which had been widened to a mean depth and length of
37e0 cm and 179,0 cm respectively.
When ambient temperatures exceeded :
body temperatures$ the squirrels retreated to the cooler burrows?
plugged the passageways about 45=0 dm from the entrance and normally
remained until the late afternoon hours.
The round-tails were in­
active in the late fall and early winter months, and observational
evidence indicated that the adult males may avoid high ambient tempera­
tures during the summer by undergoing periods of estivation or entering
hibernation early, as is done by the adult males of other species of
SnermophilUs (MeOarlev, 1966),
The mesquite trees were the primary food sources in the study
area, and the food preferences shifted to perennial wild flowers when
available.
The arboreal capabilities enabled the round-tails to obtain
the succulent seeds, flowers, leaves and bark of the mesquite trees.
The creosote-bush was the second most utilized food source.
Carnivorous
food habits were observed, indicating that.S, tereticaudus is as oppor­
tunistic as other spermophiles(reviewed by Bradley, 1968a) when insects
and carrion are available „
A succulent diet and the avoidance of ex­
treme ambient temperatures, are adaptations the Round-tailed ground
squirrel has made to an arid environment-,
A light pelage and a modified
epidermis are other adaptations.suggested by Bice and Blossom (1937) and
Brown (1968) respectively.
Vocal, olfactory, tactile and visual communication were observed
in the round-tail population.
Olfactory and tactile receptors probably
played the more important conmmnicative roles in the subterranean
passageways where the squirrels would have limited visual reception.
91
The vocal repertoire# including a sharp* high frequency peep as the
primary motif, indicated similarities to those of other ground squir­
rels reported by Balph and Balph (1966) and MeCarley (1966),
Vision appeared to play the most important communicative role
among the round-tails when above ground.
The upright postures accom­
panying alert behavior, and not vocalisations, warned the squirrels of
predators and intruders.
The sudden posting of one squirrel elicited
a specific response from other squirrels.
Table 11 indicates behavioral .
•
-
traits and responses to predators reported for various species of ground
squirrels.
All of those reported have vocal warnings for predators,
and S, beecheyi and S, armatus have aerial predator calls distinctive
from those calls warning of ground predators,
Balph and Balph (1966) found the system of sound communication
of the Uinta ground squirrel, S. armatus, to be unspecific and suggested
that this may have been promoted by habitat, social organization and
the reliance on visual communication, but no explanation was proposed
as to why S. armatus retains different calls for aerial and ground
predators,
The Barrow ground squirrel, S. parryi. is the northernmost species
of ground squirrel in North America, occurring in the arctic tundra in
northern Alaska, an area barren of woody vegetation.
However, it does
not inhabit the coastal flats and treeless plains, but instead, colonies
are located on ridges, sandbanks, hillocks and other raised areas that
afford good drainage and low permafrost, where subterranean burrows
can be constructed (Mayer, 1953).
Consequently, there is poor visual
coverage of the environment and of other squirrels, resulting in the
Table 11,
Comparative behavioral traits and responses to predators for various species of
Spermophilus,
Subgenera are in parentheses,
Species
*Vooal warning different from that of ground predator.
Post posture
Tail-flicking
Vocal warning*
aerial predator
Vocal warning*
ground predator
yes
Linsdale, 1946
yes
Linsdale* 1946
*yes
Fitch* 1943
yes
Fitch* 1948
yes
Harris* 1968
yea
Harris* 1968
no
this study
no
this study
(Otospermophilus)
S. beecheyi
(Callospermophilus)
S, lateralis
?
?
(Xerospermophilus)
S, tereticaudus
S. mohavensis
yes
this study
?
yes
this study
?
?
?
(Spermophilus)
S. armatus
S. undulatus
■ yes .
yes
Balph and Stokes* Balph and Stokes*
1963
1963
?
?
*yes
Balph and Balph*
1966
yes
Balph and Balph
1966
yes
Melchior* 1968
yes
Melchior* 1968
Table 11®
continued
Species
S. parryi
S. columbianus
Post posture
?
>*es
Manyille, 1959
■
Tail-flicking
Vocal warning,
aerial predator
Vocal warning,
ground predator
yes
Mayer? 1953 .
yes
Mayer, 1953
yes
Mayer, 1953
'yes
Manville? 1959
yes
Manville, 1959
yes
Manville, 1959
■
(tctidomys)
S® tridecem-.
lineatus
- yes
MoOarley, 1966
; yes ;
McCarley, 1966
yes
McCarley, 1966
■ yes
MeCarley, 1966
S„ mexicanus
?
?
yes
Harris, 1968
yes
Harris, 1968
S. spilbsoma
■?
■?'
yes
Harris, 1968
yes
Harris, 1968
94
need for a vocal rather than visual communicative system.
In fact;
Mayer (1953) observed that these squirrels have a poor visual sense.
If a system of visual communication is to have high survival,
value, good visual coverage of the environment, gregarious organiza­
tion and a behavioral response visible to other squirrels is obliga­
tory.
The round-tail appears to possess all of these, for it inhabits
the sparse, open habitat of the Larrea zone; it is -semi-colonial; and
predators elicit an upright body posture.
When squirrels were active
above ground and away from their home burrows, they were always active
in high numbers, whereas when few squirrels were above ground they re­
mained at or very near their home burrows.
High numbers would provide
many eyes to perceive predators and would be a basic element of sur­
vival.
Black (1963) has presented paleontological evidence to support
the theory that the various species of Soermonhilus evolved from the
parent stock. Miospermophilus. in the Miocene and would have had to
adapt to changes in the environment.
He visualized the ancestral
squirrels as chipmunk-like (Butamiss). as such would be well suited to
make shifts into an open grassland habitat as well as adapted for an
arboreal habit.
In the late Tertiary the contractions of humid forests, and an
expansion of more xeric communities, Madro-Tertiary geoflora, in the
southwest (Martin and Mehringer, 1965) accompanied a general trend
toward increasing dryness in the western United States, leading to the'
present arid deserts.
An increase in aridity gave rise to southwestern
95
deserts* the youngest environments existing today* chiefly during and
since the Pliocene (Lowe*
1964)®
As S. tereticaudus adapted to the desert environment in the
sparse Larrea zones of the Sonoran and Mohave Desert areas* it is
reasonable to assume that the. use of vision to frequently survey the
environment for intruders and predators would be advantageous„
Further­
more* standing upright in a post posture a squirrel would greatly
increase its exposure to the surrounding visual stimulic
Tinbergen (1964?.214) has stated* “many signaling movements
resemble incomplete versions of movements which themselves have another
function” and that various behavioral patterns may acquire signal value„
Thus* it is suggested that the posting posture seen in round-tails
serves the secondary function of signaling other squirrels within sight.
It is thought that the ancestral squirrels possessed a vocal system of
communication because'they probably occurred in habitats not affording
good visual coverage of the environment or were not gregariously organized
Those ground squirrel species that have specific warning calls for pre­
dators either have a greater survival with the utilization of a vocal,
system or are evolving a visual system of communication as their post
posture suggests.
Tail-flicking is another behavioral pattern that has been ob­
served in many species of ground squirrels (see Table 11)* but too few
quantitative investigations have analyzed its cause and function.
The
tail-flicking in the Uinta ground squirrel* S. armatus. is not assoc­
iated with any specific movement or behavioral situation and is
apparently only an intention movement that Indicates a shift in the
96
animal’s position (Balph and Stokes* 1963)«
Manville (1959) observed
it in the Columbian ground squirrel* S e, columbianua. and suggested it
may be a silent signal to other squirrels* a distraction to possible
predators or even an expression of the animal’s high-strung temperament0
In round-tail behavior tail-flicking appears to be a visual com­
municative signal displayed when a squirrel is confronting a ground
predator or establishing .claim to a burrow.
This may warn other squir­
rels not to approach as Linsdale (1946) proposed after observing tailflicking by S, beechevl.
Again* it is reasonable to assume that tail-flicking in S,
teretioandus, associated with specific behavioral patterns* has been
secondarily specialized as an increasingly effective signal* whereas the
unspecific role it plays in S. armatus (Balph and Stokes* 1963) is pos­
sibly due to a lesser dependence on a visual communicative system.
The
tail-flicking in other species of Spermophilus have been reported as
incidental observations and only quantitative studies will elucidate
its role.
No other ground squirrel species was sympatric with S„ tereticaudus in the Larrea zone although populations have teen reported to
overlap with the Harris antelope ground squirrel (Neal* 1964).
Active
burrows were not eo-inhablted with interspecific rodents in the area*
and the non-arboreal capacities and physiological adaptations of the
sympatric heteromyid. rodents probably made Competition for succulent,
foods negligible.
Adult males trapped in March and held captive, remained scrotal
and demonstrated sexual behavior toward adult females and intense
97
agonistic, behavior toward juvenile males when released in the squirrel
enclosure in June,
These scrotal males dominated and harassed juve­
niles to near death while in a sexually active condition but tolerated
them when their testes regressed.
This implies that during the spring
the sexually active.adult males may be very important in limiting the
role the inexperienced males have in the breeding population.
The daily activity of squirrels during the summer months was
bimodal and was inversely proportional to ambient temperatures.
How­
ever, the squirrels were inactive during the coolest hours of the day
because the onset of the activity was an hour after sunrise.
Round-
tails retreated to their burrows at mid-day (consequently avoiding the
hottest temperatures) then re-emerged in the late afternoon to remain
active above ground until sunset.
The behavior during the afternoon
activity was markedly different from that in the morning.
Rainfall
sent active squirrels underground and delayed the onset of daily activity.
One hundred fifty-one Round-tailed ground squirrels were caught
and marked in the study area.
The estimated population of resident
ground squirrels was 2.1 per acre, with the largest percentage repre­
sented by adult females and the smallest by juvenile males.
The sex ratio of resident juvenile males to resident juvenile
females was 1:2.8 in 1967 and 1:4.3 in 1968 and apparently reflected'the
time of year.the study was conducted.
This preponderance of females
after the young have left the home burrow has been reported for other
juvenile sciurids (Evans and Holdenried, 1943? Linsdale, 1946? Fitch, "
1948? King, 1955? Hawbecker, 1958? Rongstad, 1965? McCarley, 1966;
Quanstrom, 1968) and is usually thought to have resulted from sex-specific
98
emigration and. predation.
Most round-tail juveniles had left the home
burrow when marking and observation were initiated in June and the few
litters that remained were predominately females which established
residency within the home ranges of their maternal parents.
The larg­
est percentage of transient squirrels were juvenile males, a fact also
reported for the Thirteen-lined ground squirrel (Rongstad, 1965;
McCarley, 1966) and the California ground squirrel (Fitch, 1948).
These transient male round-tails were constantly in unfamiliar areas,
frequently chased by residents and hesitant to enter escape burrows,
and appeared to be extremely vulnerable to aerial predators and the
environmental adversities.
Unfamiliarity of burrow systems made con­
frontations with ground predators and intraspeoific conflicts more
likely.
Once a juvenile male established residency, however, the
chances of mortality were probably no greater than those of the females
If the juvenile males undergo periods of summer torpor or enter
hibernation earlier than the juvenile females, as found in the adult
males of the Thirteen-lined ground squirrel (McCarley, 1966) and im­
plied for the adult males by observations in this study, this could
account for the fact that more juvenile females than males are trapped.
However, absentia in juvenile males due to hibernation can not be
differentiated from disappearance by dispersal and mortality, and. in
view of the fat depositions that are acquired by a round-tail in pre­
paration for hibernation (Neal, 1964),' it is highly.unlikely that a
juvenile could enter an early hibernation.
The sex ratios in the resident adults were markedly different
from those of the juveniles.
The ratios were one male to 1,8 females
99
in 1967 and one male to 1,7 females in 1968,
These closely approxi- ,
mated those reported for the Thirteen-lined ground squirrel of 1:1,6
by Rongstad (1965) and 1:2.1 by McCarley (1966) and a ratio of one
adult male to 1,9 adult females for S. beechevi (Evans and Holdenried,
1943),
Rongstad (1965) has suggested over-wintering mortality to
account for the adult sex ratio, but it would have to be sex-specific,
as would disease and predation,,to greatly reduce the number of juve­
nile and adult females.
Possibly the males were more successful hibernators,.for their
dominant behavior would assure them the most desired foraging areas,
resulting in greater and faster deposition of fat to prepare them for
periods of dormancy.
A dominant behavior was also seen in juvenile
males, and in one instance a juvenile male was successful in driving a
resident juvenile female from her burrow to take possession.
If a large percentage of the adult and juvenile females do not
successfully hibernate, the adult sex ratios would change in favor of
the males.
However, when the annual survival rates are inspected for
S, teretlcaudus in this study (Table 8) and for S. tridecemlineatus as
reported by Rongstad (1965) and McCarley (1966), it is found that this
is not the case.
In fact, both adult and juvenile females had higher
survival rates than did adult and juvenile males,
Fitch (1948) re­
ported the survival rates of the California ground squirrel, S. beechevi.
over one winter, also to be higher, in the adult and juvenile, females.
Investigations by Linsdale (1946) on the California ground
squirrel extended over a period of 6 years, and he found that the number
of ground squirrels fluctuated widely throughout the year and from year
100
to year.
Further,, his results indicated that the sex ratios are not
consistent, therefore, studies of short duration, 2 years in this
investigation and Rongstad1s (1965) and 3 years in McGarley8s (1966)
and Pitch's (1948) are inadequate to determine if the sex ratios vary
seasonally.
A preponderance of males was a seasonal phenomenon in
S. beecheyi but this was only evident when sex ratios were determined
monthly on an annual basis over a period of 6 years.
An investigation
of population dynamics oyer a 2 year period indicates what the survival
rates are for only 1 year.
These may differ over a period of many
years as was indicated by Linsdale'.s (1946) investigations.
This is one of the few investigations to obtain movement data
for ground squirrels by direct observational methods and to calculate
home range estimates by the minimum area method.
Home ranges were con­
sidered dynamic areas, changing from month to month in size and shape,
and estimates were only approximations of the actual pattern of the
animal5s movements.
Conservative home range estimates of 0.74 acres
and 0.77 acres were derived for adults and juveniles respectively.
These estimated home ranges were much smaller than the 3 acres reported
for the closely related Mohave ground squirrel, S. mohavensis, sub­
jectively estimated by Burt (1930).
See Bradley (1967) for home range
estimates of other ground squirrels.
Fluctuations in the home ranges were influenced by changing food
sources, movement from home burrows by the young and less attention to
the young from the adult females.
Resident squirrels showed familiarity
with their home ranges when released on the periphery, while transients
and squirrels trapped several hundred feet from their familiar ranges
101
appeared bewildered and used caution when retreating into burrows.
When transients attempted to occupy burrows, they were chased from the
study area or into sparsely populated areas therein by residents.
This
limited the number of transient squirrels that established residency
and prevented extensive overlapping of home ranges, especially among
adult animals.
Epimeletic behavior between the adult females and juveniles was
observed until the young left the home burrow.
Play, among sibling .
juveniles, appeared to afford practice for the various patterns asso­
ciated with agonistic behavior.
Attack, threat, passivity and escape
tendencies were observed as a social heirarchy was established and main­
tained among the resident squirrels and as transients were prevented .
from establishing residency in densely populated areas.
Often, threat
behavior appeared to replace fighting, especially between residents.
Neither home ranges, nor delimited areas were defended by resi­
dents, .but squirrels maintained individual distances.
The intolerance
level in the individual distances was sufficient to space the home
burrows and influence home range size while still allowing the. squirrel
population to retain a high density, an important factor in the roundtail6s visual communicative system.
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