Observations on Torpidity in Captive Chipmunks of the Genus Eutamias
Author(s): Tom J. Cade
Reviewed work(s):
Source: Ecology, Vol. 44, No. 2 (Apr., 1963), pp. 255-261
Published by: Ecological Society of America
Stable URL: http://www.jstor.org/stable/1932172 .
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Spring 1963
CAPTIVE
the red-legged frog (Rana aurora aurora).
EUTAMIAS
Herpeto-
logica 16:251-259.
Straw, R. M. 1958. Experimental notes on the Deep
Springs toad, Bufo exsul. Ecology 39:552-553.
Stuart, L. C. 1951. The distributional implications of
temperature tolerance and hemoglobin values in the
toads Bufo
marines
(linnaeus)
and Bufo
bocourti
Brocchi. Copeia 1951: 220-221.
Thorson, T. B. 1955. The relationship of water economy to terrestrialism in amphibians. Ecology 36:
100-116.
Volpe, E. P. 1953. Embryonic temperature adaptations
and relationships in toads. Physiol. Zol. 26:344-354.
. 1957. Embryonic temperature tolerance and rate
OBSERVATIONS
255
CHIPMUNKS
of development in Bufo vallicepts. Physiol. Zobl. 30:
164-176.
Wright, A. H. 1932. Life histories of the frogs of
Okefinokee Swamp, Georgia. New York: Macmillan.
Zeuthen, E. 1942. The ventilation of the respiratory
tract in birds. Danske Vidensselskab. Biol. Med. 17:
1-51.
Zweifel, R. G. 1955. Ecology, distribution, and systematics of frogs of the Rana boylei group. Univ. Calif.
Publ. Zo6l. 54:207-292.
. 1957. Studies on the critical thermal maxima
of salamanders. Ecology 38:64-69.
1959. Effect of temperature on call of the frog,
Bombania variegate. Copeia 1959: 322-327.
ON TORPIDITY IN CAPTIVE CHIPMUNKS
OF THE GENUS EUTAMIAS
TOMJ. CADE
Department of Zoology, Syracuse University, Syracuse, New York
INTRODUCTION
Hibernation is well known in a number of sciurid rodents in the tribe Marmotini, and there have
been many excellent studies dealing with various
species of marmots and ground squirrels. Some
work has also been done on hibernation in the
eastern chipmunk, Tamias striatus (Allen 1938;
Woodward and Condrin 1945; Engels 1951;
Yerger 1955; Lyman and Blinks 1959; Panuska
1959), but surprisingly little attention has been
given to patterns of activity and torpidity among
species of the related genus Eutamias. No physiological studies have been reported, and the brief
records by Svihla (1936) and by Broadbooks
(1958) on a few captive specimens of E. amoenus
seem to be the only laboratory observations.
The literature in natural history and ecology
also contains few references to torpidity in western chipmunks, a fact which has not changed
appreciably since Howell (1923) first called attention to it nearly 40 years ago. Only twice have
descriptions been published of naturally hibernating chipmunks (E. townsendii) found torpid
in their nests (Walker 1923; Anthony 1924).
Grinnell and Storer (1924: 81) mention a trapinjured individual of E. specious which became
torpid and subsequently aroused in captivity, and
Criddle (1943) gives some information on E.
minimum. Broadbooks (1958) has summarized a
few other field observations; otherwise little seems
to be known.
Western chipmunks occupy a diversity of basically chaparral-like habitats in North America.
Some live in the brushy understory and loglittered floor of humid coastal forests (E. townsendii); others in rock-bordered, alpine meadows
and talus slopes at timber line in the Sierra Nevada
(E. alpinus) ; and some kind of chipmunk lives in
the understory or in the brush fields of all the
forested zones lying between these extremes.
Other species have ventured into arid pin6njuniper scrub forests (E. panamintinus; E. quadrivittatus hopiensis), sage brush desert (E. minimus), and dry chaparral associations (E. merriami).
Such a "plastic" group, containing populations
readily modified to take advantage of subtle environmental opportunities, may well encompass a
wide spectrum of physiological adaptations related to periods of activity and torpidity. It is
this likelihood of divergence in the physiological
adaptations of closely related species that makes
the genus Eutamias such an attractive group for
study.
This study presents some exploratory findings
on several aspects of torpidity in 3 species of
Eutamias, with the hope of stimulating others to
undertake the detailed physiological investigations
so interestingly posed by the ecological distribution of this genus. The work was carried out
during the tenure of a postdoctoral fellowship
from the National Science Foundation in conjunction with a program of study at the Museum of
Vertebrate .Zoology, University of California,
Berkeley.
MATERIALS
AND
METHODS
On 18 October 1958, 35 live traps were set in
5 localities on the west side of Lake Almanor,
Plumas County, California, between the Almanor
postoffice and the southwest corner of the lake.
Seven chipmunks were captured as follows: one
256
TOM
adult male E. quadrinaculatus,
J.
one adult male E.
speciosus, one adult male and 4 adult female E.
a-noenus. In addition to these captured individuals, at least 3 other E. speciosus and 20 E.
amnoenus were seen during the day. Tevis (1953)
also found E. townsendii around Lake Almanor.
That same day, the chipmunks were transported
down from the mountains to Lafayette, Contra
Costa County, California, where they were placed
together in an outdoor cage with a floor space of
36 square feet and a volume of 72 cubic feet. The
cage was provided with 6 sleeping boxes made of
one-inch thick, unfinished redwood boards with
inside dimensions of 6" by 12" by 5" high. Each
had a 2-inch square entrance near the top of one
end and a removable lid for convenience in examining the occupants. Cotton was provided at
intervals for nesting material. The cage was
always provided with water and food, which consisted of sunflower seed, cracked corn, milo maize,
barley, and occasionally pieces of apple and lumps
of peanut butter mixed with oatmeal.
Permanently fastened around the neck of each
chipmunk was a collar made of differently colored
porcelain beads strung on copper wire. General
observations on activity and behavior were made
at a distance of 15 feet from inside quarters by
looking through a glass door leading onto the
sundeck where the cage was located. Body temperatures and weights, unless otherwise indicated,
were all taken at least 2 hours after dark, usually
between 2200 and 2300 hours. The animals were
weighed on a triple beam balance accurate to one
tenth of a gram, and all temperatures were estimated to a tenth of a degree C, using a Schultheis
rapid registering thermometer inserted to a depth
of 15 mm rectally, orally, or into a cheek pouch.
After cautiously removing the lid of a sleeping
box, it was often possible to get resting or sleeping rectal temperatures by gently lifting up a
chipmunk's tail and inserting the thermometer.
The chipmunks showed remarkably little resistance to this procedure so long as they were left
inside their sleeping boxes. To obtain active
body temperatures, the chipmunks were allowed
to run freely in a small room for several minutes.
From time to time individuals were placed in
plastic boxes and kept for variable periods in a
refrigerator at 3 to 40C in attempts to induce
hibernation. Observations ended 7 February
1959.
RESULTS
GENERAL
BEHAVIOR
AND ACTIVITY
The chipmunks quickly adjusted to caged life
and settled into a predictable routine of activities
in the first 2 or 3 days. Feeding, hoarding seeds,
Ecology, Vol. 44, No. 2
CADE
sunning, and chasing each other were the main
activities. Typically a chipmunk would go to the
food tray soon after emerging from its sleeping
box, fill its cheek pouches with shelled seeds, and
then retire to the top of one of the sleeping boxes
to sit and eat at its leisure. Or if it showed a
tendency to hoard seeds, it would eat a few, then
enter one of the boxes, store the rest, and continue to make trips with filled cheek pouches from
the food tray to the sleeping box for a good part
of the day. Idle chipmunks usually perched atop
the sleeping boxes, where they often sat in the
typical upright posture on their haunches or
stretched out in various relaxed postures to doze
in the sun. At other times, particularly when
several chipmunks were trying to collect seeds at
the same time, they chased each other vigorously
all about the cage and in and out of the sleeping
boxes.
The animals were strictly diurnal, never emerging from the sleeping boxes until the sun was well
up and always retiring for the night before sundown. They were reluctant to move about in the
dark inside their sleeping boxes and seldom attempted to escape from a box at night even when
disturbed.
All of the chipmunks showed a strong preference for sunflower seeds. Apple was readily
eaten, and so was the peanut butter-oatmeal mixture, but sunflower seeds were the only food
stored in the sleeping boxes. The stored seeds
were always shelled and placed in the bottom of
the cotton lining of the nest.
BODY TEMPERATURE
AND
TORPIDITY
A number of rectal temperatures taken under
various external and internal conditions are summarized in Figure 1. The body temperatures of
these nontorpid chipmunks were conspicuously
labile, ranging from a low of 310C during sleep
to a high of 40.6? C following vigorous activity.
After moderate to heavy activity, the body temperattures were typically above 380C; during
periods of rest, rectal temperatures were usually
in the range of 36 to 370C, but during sleep, temperatures were frequently down to 34WCand often
lower. No significant differences were noted between the resting or sleeping body temperatures
at ambient temperatures of 3 to 4VC in the refrigerator and those taken at 10 to 20'C outdoors, although the samples taken at the low range
of air temperature are small. Thus the body
temperatures of these nontorpid chipmunks fluctuated normally through a range of 9 to 100C
during their daily cycle of activities.
No chipmunk in the outdoor cage entered deep
hibernation under the moderate winter conditions
Spring 1963
CAPTIVE
EUTAMIAS
CHIPMUNKS
257
inactivity, although their body temperatures remained above 300C.
SLEEPING)
?OR
These changes in the activity of quadrimaculatuis
first became obvious 20 November, when he did
o 40
ACTIVE
not emerge from his sleeping box until 0945, a full
RANGE
z
2 hours after the other chipmunks had become
s~~~_
.-.
----:.....
- .- .'i! 8
!
active. His first appearances were noted as often
RESTING as possible thereafter. On 25 November, for
instance, he did not appear until 1300 and remained outside for only an hour. He was not
seen outside his sleeping box at all 2 December,
RANGE
although a check that night revealed that he still
held a rectal temperature of 340C. From this date
until 9 January quadrimaculatus often remained
30
inside his sleeping box all day, and if he came
out at all it was only for a brief period of 30 to
45 minutes in the middle of the afternoon, around
E. AMOENUS
E. QUADRIMACULATUS
1400 to 1500 hours. During these brief exits he
E. SPECIOSUS
seemed more interested in drinking than in colFIG. 1. Lability of body temperature in chipmunks
lecting seeds. All during this period he kept a
under various external conditions and different metabolic
large store of 200 to 300 g of shelled sunflower
states.
seeds in the bottom of his nest. After 9 January,
at W~~~~~~~
Lafayette. Furthermore, only 2 animals be- he began to appear regularly for short periods
came fat as most hibernating sciurids do (Table each afternoon, but never for more than an hour
I ). The weight of quadrimaculatus was quite or 2 at a time. Although he was observed freconstant with observed extremes of 85.1 and quently at night during this time, he was never
found in a torpid condition, and the lowest rectal
TABLE I. Body weights of captive chipmunks (Eutamias)
temperature recorded was 32.6?C.
A similar but less well-marked reduction in
ANIMALS
activity was also noted for amoenus-1, beginning
us amoenusamoenus 25 November when this animal first started to
Daequadriamoenusamoenusamoen
5
2
3
4
maculatusspeciosus 1
hoard seeds and to sleep in isolation. During the
49.1
58.1
60.3
81.6
56.8
52 .0
85.1
I Nov.
latter half of December there were a few days
51.5
69.5
65.5
87.5
58.7
55.2
11 Nov.
85.9
when this individual did not emerge from her
51.2
77.0
66.4
87.7
65.9
58.0
20 Nov.
88.6i
54.6
73 .3
60.7
88.3
95.3
60rO.0 57.1
1iDec.
sleeping box, but usually she did come out for a
87.6
_
_O
_
_
10Dec.
few
minutes each day. Like quadrimaculatus, she
91E.2
_
_
_
12 Dec.
was never found torpid at night with a low body
94.1
_
_c
14Dec.
50.9
56.1
52.1
53 .2
20 Dec.
86.7
temperature.
63.2
50.6
80.7
47.1
52.2
uJan.
85.3
Both these chipmunks were very gentle and easy
49.6
84.4
49.0
52.0
62.8
89.1
25 Jan.
51 .1
67.3
61.8
87.8
55.9
54.3
Mean
89.1
to handle during this time. When taken out of
their sleeping boxes at night and held in the hand,
89.1 g. Only specious and amoenns-3 gained they did not try to escape or move about very
appreciable weight. The heaviest weight of spe- much, and if left sitting undisturbed they would
cisus, 95.3 g, amounted to an increase of 13.8 g often go to sleep. If forced to move about, they
or a little better than 18% of its initial weight, quickly warmed up to rectal temperatures of 370C,
whereas the heaviest weight of ahmoenks-3, 77.0 g, after which they became more responsive, but if
amounted to an increase of 18.9 g or a little less left alone their temperatures soon dropped back
down to 340C or lower. None of the other chipthan 35%0 of its initial weight.
The patterns of daily activity revealed some munks showed clear-cut reductions in the amount
interesting features for qubadrimaculatuhs and of their activity as winter progressed, and none
amoenus- 1. These were the only 2 animals which of them acted lethargic, although their rectal temconsistently hoarded seeds and slept in isolation peratures, too, were frequently around 330 to
from the other chipmunks. Both of these chip- 340C at night.
munks became progressively less active as the
Figure 2 shows the change in body weight of
winter advanced, and they tehded to become lethar- several chipmunks held without food in a regic in behavior during the prolonged periods of frigerator at 30 to 40C. During a 9-day fast,
42 -
* AMBIENT TEMP 10?-20? C.
AMBIENT TEMP 3?-4?C. (ALL RESTING
-
-
-
-
258
Ecology, Vol. 44, No. 2
TOM J. CADE
900
90
0
s
sTORPID7
40
OR
CHEEK POUCH
OF
DEAD
ANIMAL
N
80
_70
35 -
SPECIOSUS
60N
W 30
(TORPID)
AMvENUS-3
AMOENUS-I
n50
w
AMOENUS-4
0
1
2
3
DAYS
,
Z
40
30
--
ESOPHAGUS
RECTAL
ESOPHAGUS
4
OF
(DEAD
5
FOOD
25
AT END)
6
7
DEPRIVATION
8
9
10
FIG. 2. Decline in body weights of chipmunks kept without food in a refrigerator at 30 to 40C.
a.1
2~~~~~~~~~~~
20
speciosus
lost a total of 43 g, starting at an iniw5
4
.
.
w
.
.
tially heavy weight of 94.1 g, or an average of
.*
4.8 g per day. The rate of weight loss was fairly
uniform during the 9 days, except for the 4th andl
5th days when the rate of loss was only one granm
per day.
the animal was not obUnfortunately,
I30
40
50
10
60
70
2
served on these 2 days.
On the 3d day, however,
MINUTES FROM BEGINNING OF AROUSAL
the animal had been observed curled up in a ball
FIG. 3. Rate of arousal of E. amoenus from deep hiberwith a respiratory rate of 10 breaths in 10 seconds
nation, as indicated by increase in rectal and oral temand a rectal temperature
of 26.6?C.
On the 6th
peratures, at a room temperatureof 200C.
day the animal was active with a rectal tempera3. For comparison, the passive increase in temture of 370C, and he was never again found with
a rectal temperature
lower than 30'C.
At the
perature, of a dead chipmunk, amoenus-4, is also
end of 9 days, speciosus
indicated.
weighed only 51.1 g, or
The initially slightly faster rate of
30.5 g less than his first recorded weight a few
increase for the dead animal may be explained by
days after capture.
the fact that the dead individual had a slightly
Since he was in a weak condition and clearly could not have survived much
smaller mass than did the live chipmunk. Cnlonger, the animal was returned to the outdoor
ceivably it might also reflect a physiological. difcage with the other chipmunks.
ferenlce resulting from some sort of circulatory
Seven days later
he again weighed 80.7 g, a gain of more than 4 g
shunting of the heat in the live animal to critical
per day.
organs. Rectal temperature lagged somewhat behind oral temperature, particularly during the
During their short periods in the refrigerator,
middle part of the arousal. The actual difference
amoenus-1
and amoenus-4
remained active-often
is probably greater than indicated by the measurethe lowest
hyperactive-and
rectal temperature
recorded for these individuals
menlts, because the rectal temperature was taken
was 340 C.
Both
about half a minute after each oral temperature.lost weight rapidly, and ctmoenus-4
died on the 3d
Other individuals of amoenuss and qua~drimacuday at a body weight of 39.3 g or after a loss of
latus were kept in the refrigerator at 3 to 4? C
16.7 g.
f rom- 2 to 5 days with food available. None of
The other chipmunk of this group, anOvenus-3,
these anii-nals became torpid, none lost weight, and
began fasting at the heavy weight of 69 g and
rectal temperatures below 30'C were not recorded
during a 4-day period lost 15.5 g. On the 3d day
this individual
became torpid;
at 2135 on that
ally case.
day it was found curled up in the typical hibernaDISCUSSION
tion posture with only 15 respirations
in 25 to
HIBERNATIONIN SPECIESOF Eutamias
30 seconds,
with short periods
interspersed
of
apnea.
On the 4th day this animal was artificially
Tile fact that so few western chipmunks have
aroused by placing it at a room temperature
of
b~eeti foul-id inl a dormant state raises a question
200C.
about the nature of hibernation in these squirrels.
The rate of arousal, as indicated by increase in
Tile question is further emphasized by the fact that
rectal and oral temperatures,
is shown in Figure
seNveral observers have reported species of Eulta-
Spring 1963
CAPTIVE
EUTAMIAS
mias to be active above ground at various times
during every month of the winter season (Walker
1923; Taylor and Shaw 1927; Criddle 1943).
In the San Bernardino and San Gabriel mountains
of southern California, E. speciosus has been observed around public camp grounds in every month
of the year; it appears to become inactive for
periods of several days only during times of relatively deep and sustained snow cover and is likely
to be active on bright sunny days even in December and January (Cade, unpublished). Tevis
(1955) is the only writer who has reported prolonged inactivity of chipmunks during the winter.
Despite a mild winter with much thawing of snow
and little frozen ground, he neither saw nor heard
a chipmunk from 29 November to 19 March in
the Lake Almanor region. Emergence was gradual, and chipmunks did not appear in conspicuous
numbers until the middle of April.
In the present series of observations, one individual of E. amoenus entered a state of "deep
hibernation" with an oral temperature of 4.60C
and subsequently aroused from that condition.
The performance of this animal during recovery
seemed typical of the arousal of a hibernator
rather than of the slow return to normalcy of a
nonhibernator forced into a hypothermic state.
The average rate of arousal, 0.50C per minute, is
quite fast for an animal of its size. By comparison, the average rate of arousal of a smaller hibernator, such as Perogncathuslongimembris weighing about 8 grams, is around 0.60C per minute
(Bartholomew and Cade 1957), and for a larger
one, Citellus mohavensis weighing about 250
grams. the rate varies between 0.4 and 0.10C per
minute (Bartholomew and Hudson 1960).
Other indications of some kind of hiberation in
this group are fat accumulation in the fall and the
tendency of captive individuals to become inactive
in the winter. Tevis (1955) found "hiberation
fat" in specimens of all species occurring around
Lake Almanor, except E. amoenus, of which no
specimens were collected at the right time of the
year. The average gain in weight of these chipmunks in the fall was 20%. In contrast to these
findings, Broadbooks (1958), studying E. a-moenmisin eastern Washington, observed no significant seasonal increase in the live weights of a
large series of animals. In the present study only
2 out of 7 captive chipmunks gained appreciable
weight. By comparison, other species of hibernators, such as Zapus spp. and some species of
Citellus, that depend primarily upon body fat as
their source of energy during hibernation, typically gain an amount of fat equal to their fat-free
body weight ( Morrison 1960).
CHIPMUNKS
259
Although active chipmunks are likely to be seen
on occasions throughout the winter, there is no
question that these animals do undergo profound
seasonal changes in the amount of activity they
carry on above ground in their natural habitat.
Such a seasonal change was clearly reflected in
the pattern of activity shown by 2 of the captive
chipmunks,E. quadrimaculatus and E. anoenius,
but the inactive state was not accompanied by
entrance into deep hibernation in either case.
Svihla (1936), keeping animals in outdoor cages
near Pullman, Washington, and Broadbooks
(1958), keeping animals indoors at 680F, noted a
similar tendency for captive individuals of E.
amoenus to become less and less active as winter
progressed without ever entering deep hibernation.
Criddle (1943) observed the same gradual decrease in the above-ground activity of a wild population of E. minimus in Manitoba.
Available evidence from the field and from the
few captives studied supports the notion that species of Eutamias are not typical hibernators in the
sense that they easily and regularly allow deep
body temperature to drop down to values approaching low ambient temperatures for long
periods of time. The information available on
Tanias stratus indicates that it is similar to Eutamias in this characteristic (Woodward and Condrin 1945; Panuska 1959). According to Pearson's (1960) useful classification, these chipmunks
are "stubborn homeotherms," capable of maintaining a warm body temperature in a wide range
of environmental temperatures and resorting to
deep hibernation only when faced with severe
stresses such as exhaustion of the winter food
cache. Compared with some other hibernators
in this category, western chipmunks appear to be
much more stubborn than pocket mice (Bartholomew and Cade 1957), considerably more stubborn
than most species of ground squirrels that hibernate, and are perhaps about comparable to that
most stubborn of all hibernators, the domesticated
hamster (Lyman 1954), which is also a notable
food hoarder.
Since it is known that chipmunks store their
food in the lining of their nests in the hibernaculum
(Broadbooks 1958), it is consistent with the point
of view being developed here that the only chipmunks found in deep torpor in nature were dug
out of nests which contained no food (Walker
1923; Anthony 1924). A further point of consistency may relate to Tevis's (1955) observations
that no chipmunks were active above ground during the winter of his studies in the Lake Almanor
region. His studies were carried out during a
period of widespread failure in the seed crops most
260
TOM
commonly used by chipmunks for their winter
stores. In the fall of that year before their disappearance below ground, the chipmunks in this
area fed heavily on subterranean fungi, a bulky,
perishable food not usually stored but nutritious
enough to allow the chipmunks to build up considerable reserves of fat. With only residual stores
of seeds in their hibernacula but possibly more
body fat than is typical of Eutacmias when they
become inactive, it is possible that these chipmunks
switched over to a kind of physiological preparation that permitted many of them to spend the
winter season in a state of deep hibernation, since
it has now been shown that a western chipmunk
can survive deep hypothermia and arouse.
The kind of winter inactivity shown by captive
chipmunks, which become sluggish in behavior but
which permit their body temperatures to drop no
lower than 300 C, is reminiscent of the kind of
hibernation reported by Hock (1957, 1960) for
the black bear ( Ursus americanus).
Hock ( 1958)
has used the term "carnivorean lethargy" to designate the condition found in bears and some other
For the similar condition found in
carnivores.
Eutamias such a term is obviously inappropriate,
and a more general term such as "shallow hibernation," which contrasts nicely in meaning with the
flow well-recognized
term "deep hibernation"
made popular by Lyman and Chatfield (1955), is
preferable.
In short, then, as their main adaptation for survival during the winter, species of Eutamias seem
more dependent upon storing food sufficient for the
winter season in the hibernaculum than on drastically reducing metabolic rate and energy requirements by means of deep hibernation, although that
may be an alternative possibility for some species.
By greatly reducing their activity in the winter,
however, these chipmunks achieve a kind of adaptive compromise between the minimum energy
requirements of a deep hibernator and the high
energy requirements of a rodent that remains
active all winter. Thus they gain the advantage of
a relatively low energy expenditure during a
period when the food supply is short without
losing all the advantages of an active life, such as
escape from predators and from flooding of
burrows.
BIOENERGETICS
AND SHALLOW
HIBERNATION
If this hypothesis is true, then it is worthwhile
to consider the energy requirements of a chipmunk
spending the winter in a state of shallow hibernation. How much food must such a chipmunk
store for the winter, and is this amount consistent
with the known capabilities of hoarding chipmunks? Unfortunately, metabolic rates of chip-
Ecology, Vol. 44, No. 2
J. CADE
munks in any physiological condition are unavailable, but an approximation can be made by
employing a number of reasonable assumptions.
Consider a small chipmunk such as E. amoenus
with a fat-free weight of 50 g. The calculated
standard or resting metabolic rate for such a mammal according to Kleiber's (1947) formula70 X W4, where W equals the body weight in
kilograms-is about 7.4 kcal per day. Assume
that the average metabolic rate of a chipmunk in
shallow hibernation is slightly substandard and,
further, that the chipmunk does not have to expend any extra energy to keep its body temperature above 30'C, a not unreasonable assumption
since the animal is curled up inside a well-insulated
nest (Broadbooks 1958).
The average daily expenditure of energy might
be about 6 kcal, or a total of 720 kcal during
a period of 120 days of hibernation. If one assumes further that the chipmunk is about 70%
efficient in using the total energy available in its
food (see Brody 1945), then its total energy requirement for a season of hibernation is equivalent
to about 294 g of graminecous seeds (see Spector
1956, for caloric values of various grains) or about
208 g of sunflower seeds or other highly fatty seed
such as pine nuts. It is also equivalent to 78 g
of animal fat.
Quite obviously no chipmunk can accumulate
enough body fat to survive a winter in a state of
shallow hibernation without supplemental food
stores, as a bear can (Hock 1960; Morrison 1960).
In his field studies of E. amoenus, Broadbooks
(1958) unearthed 3 November nests which contained, respectively, 70.0, 170.0, and 190.1 g of
food, mostly seeds. The latter 2 values are close
enough to the calculated need to permit the conclusion that a western chipmunk can pass the
winter in a state of shallow hibernation on the
energy available to it in 10 to 20 g of body fat
plus a single store of seeds placed in the lining
of its nest.
SUMMARY
The ecological distribution of species in the
genus Eutamias suggests adaptive differences
among these closely related populations with respect to periods of activity and torpidity. Seven
captive chipmunks representing 3 sympatric species, E. amoenus, E. quadrimaculatus, and E. speciosus, were studied from this point of view from
October, 1958, to February, 1959, in California.
The chipmunks were kept together in an outdoor
cage and were individually marked.
All chipmunks had labile body temperatures, and
one amoenus entered a state of deep hibernation
in a refrigerator at 30 to 40C without food and
Spring 1963
ECOLOGY OF SOIL ARACHNIDS
subsequently aroused at a room temperature of
20'C. One quadrimacuzlatusand one amoenus in
the outdoor cage became progressively less active
as winter advanced, but neither entered deep
hibernation. Evidence from the field and from
captives indicates that species of Eutacmiasdo not
necessarily enter a state of deep hibernation during the winter, although they may be inactive
above ground and lethargic in behavior with
slightly depressed body temperatures for weeks at
a time.
This kind of "shallow hibernation" is an adaptive compromise between the minimum energy
requirements of a deep hibernator and the high
energy expenditure of a rodent that remains active
during the winter. Energy is saved without losing
all the advantages of a wakeful life.
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STUDIES ON THE ECOLOGYOF SOIL ARACHNIDS
EDGAR C. GASDORF' AND CLARENCE J. GOODNIGHT
Department of Biological Sciences, Purdue University, Lafayette, Indiana
INTRODUCTION
In nearly every type of soil environment the
arachnids are well represented. In many soils,
they are the most numerous of the smaller animals
present. Of the arachnids, the mites are the most
1 Present address: Biology Department, Bradley University, Peoria, Illinois.
abundant forms encountered, and spiders, pseudoscorpions, opilionids, and other forms are found
in lesser numbers. In spite of the abundance of
arachnids in the soil, they have not been intensively
studied.
A number of investigators (Elliot 1930; Riha
1951; and Becker 1951) have found that soil