BIOLOGY
OF
REPRODUCTION
35,
1289-1297
(1986)
The Influence
of Hibernation
on Testis Growth
and Spermatogenesis
Golden-Mantled
Ground Squirrel, Sperm ophilus’
BRIAN
M. BARNES,2’3’4
MARIA
KRETZMANN,3
and IRVING
Departments
LICHT,4
ZUCKER3
of Psychology3
University
Berkeley,
PAUL
in the
and
Zoology4
of California
California
94720
ABSTRACT
Testis size and spermatogenesis
during,
and after the hibernation
were monitored
season.
During
peratures
of 3-4#{176}C. Torpor
bouts
which
animals
were
normothermic.
April
(when
torpor
1100
mg,
hibernation
mg
and
was
31
In
spontaneously
but spermatogenesis
began.
In contrast,
after
of
days,
serially
in individual
hibernation,
animals
6 days
duration
were
the 5 mo between
terminated),
the
did not advance
during
the month
spermatozoa
were
beyond
after
found
in the
estimated
epididymides.
squirrels
in spring,
hibernacula
have
regained
these
findings
in the spring.
the
suggest
normothermic
that
ground
squirrels
lateralis),
like other
hibernating
fall,
winter
and much
of the
1976)
and conserve
energy
peratures
at 1-2#{176}C above
surrounding
their
burrows.
torpor
rupted
by
mammals,
spend
the
spring
each
year
in
intervals
of
June
18, 1986.
April
28, 1986.
by Grants
NSF
fellowship
information
in this
Cold,”
1986.
Heller
New York.
2Repnint
requests:
University
of Alaska,
are
spontaneous
arousal
paper
HC,
M.
to Paul
to
is published
Mussachia
Brian
Fairbanks,
returns
to
episodes
8406751
(HD-06495)
arousal
J,
Brian
HD-14595,
Barnes.
Some
in a chapter
in “Living
and Wang
LCH (eds.),
Barnes,
Institute
AK 99775.
of
Arctic
gradually
body
annual
that
the
limited
several
500
to
before
to 3500
testis
weeks
temperatures
cycle
reproductive
system
during
seminiferous
tubules
and the
togonia
and
and
from
which
were present
testes
increased
rapidly
for
inter-
of
during
growth
before
of
they
and
thus
ground
of the hibernation
below
heterothermy
5#{176}C(Wang,
imposes
poten-
tially
important
constraints
on the endocrine
system
and
especially
on
the
reproductive
function
of
hibernating
mammals.
Ground
squirrels
are seasonal
breeders
with a 1- to 2-wk mating
season
each spring.
At the time
of entry
into
hibernation
in fall,
the
normothermic
are relatively
Licht,
M.
torpor
hibernation)
during which squirrels
were aroused
from
in the laboratory,
and presumably
in the
state.
Since males are reproductively
mature
at
1978).
This
6- to 8-mo
fat accumu(Mrosovsky,
periodically
from
entered
increased
normothermic
season
by maintaining
body
temthe temperature
of the soil
During
hibernation,
bouts
hypothermia)
body
temperature
(>35#{176}C); these
Accepted
Received
Supported
postdoctoral
(deep
are
arousals
brief
(hours)
and
infrequent,
squirrels
typically
spend
80-90%
(Spermophilus
underground
hibernacula.
During
this
hibernation
season,
they
subsist
on body
lated
during
the
preceding
summer
males
of testes
We suggest
after
ground
brief
animals
spermatocytes,
terminated,
field,
Golden-mantled
which
levels
mass
during
the hibernation
season was restricted
to intervals
major portion
of gonadal
growth
and spermatogenesis
occurs
with
(when
pachytene
torpor
was
INTRODUCTION
of
interspersed
December
that occurred
torpor.
The
when
first
trapped
emerge
from
their
golden-mantled
ground
squirrels
before,
spent 81% of days in torpor at body tem-
a
the
in the
Elsevier,
Biology,
males
are
is completely
small
and
germinal
epithelium
consists
and Sertoli
cells (McKeever,
regressed:
show
no
lumen,
of only sperma1966).
However,
at initial
capture
have
spermatogenic
after
emergence
in spring,
males
testes
and
developed
accessory
glands
1966;
(McKeever,
Bronson,
1979;
Kenagy
Barnes,
growth
occur
submitted
for
publication)
implying
and maturation
of the reproductive
during
the hibernation
season.
It is not
when
when
reproductive
animals
are
Lyman,
1289
of
1982;
and
that
system
known
development
occurs
relative
to
torpid
or aroused
(Wimsatt,
1969;
Wang,
1982).
In nonhibernating
mam-
1290
BARNES
mals,
greatly
cell growth
reduced
below
or
and gonadal
absent
at
20#{176}C(Davis
et
steroidogenesis
tissue
temperatures
1963;
al.,
Buyer
1966;
LeVier
and Spaziani,
1968).
hibernators
have evolved
mechanisms
compensation
genesis
and
that
permit
responsiveness
and
are
AL.
chamber
maintained
(lights
Davis,
It is possible
that
of temperature
development
of endocrine
ET
of gametotissues
at low
on
at
on
0900,
a
PST)
1OL:
and
the
were
subjected
course
of
to
1 wk.
Animals
similar
low
the
winter
temperature
born
in May
1983
temperatures
and
short
temperatures.
For example,
at low temperatures,
certain nerves
and muscles
of hibernators
remain
functional,
whereas
tissues
of nonhibernating
mammals
cease to function
(Lyman,
1982).
Alternatively,
gona-
photoperiods
dal
Animals
were
monitored
daily
for
placing
small
squares
of white
blotting
hibernation
paper on
dorsal
squares
maturation
could
mic periods
interval
of
be restricted
of spontaneous
normothermy
hibernation
but
above
ground;
to
the
normother-
arousal
or to an extended
that
follows
the end
of
precedes
in the
the
latter
appearance
of animals
case,
development
of
the reproductive
system
might
occur
while
ground
squirrels
are maintaining
high body
temperatures
but
are inaccessible
for study.
Delineation
of the relationships
between
reproductive
maturation,
thermy,
frequent
trolled
the
and
periodic
sampling
of
conditions
and
biopsies
permit
obtained
MATERIALS
General
concurrent
from
hetero-
monitoring
squirrels
individuals
endocrine
of gonadal
correlates
maturation
to
are
AND METHODS
Care
Golden-mantled
ground
squirrels
were trapped
in
late
May
1983
and
1984
in the vicinity
of Lake
Almador,
Lassen
Co., CA. Males
born
to pregnant
females
in 1983
and 1984
and adult
males
caught
in
1984
were
housed
singly
in plastic
cages
(45 X 24
X 20 cm) with wood
shavings
and burlap
squares
for
bedding.
Simonsen
Rat
Pellets
(Maintenance
Diet,
Simonsen
Laboratories,
Gilroy,
CA)
and
tap
water
were
provided
of birth
or capture
through
animals
were
kept
at room
and
105
in a 14L:
males
1OD
were
ad libitum.
November
temperature
photoperiod.
transferred
From
time
of each year,
(23 ± 2#{176}C)
In December
to
an
environmental
1984,
surface
of torpid
placed
during
their
presence
animals
during
spring
of 1984.
hibernation,
day indicates
torpid
or
h (c.f., Pengelley
24
males
also
(Mini-mitter
were
fitted
Model
by
the
are disand thus
whether
had
aroused
and Fisher,
with abdominal
V, Mini-mitter
Sunriver,
OR) that
broadcasted
frequencybody
temperature
to an automated
data
apparatus
that continuously
sampled
body
from each animal
at 20-mm
intervals.
Laparotomy,
At
These
from
each
continuously
preceding
1961).
Fifteen
radiotelemeters
Co.,
Inc.,
encoded
collection
temperature
animals.
an arousal
or absence
were
the
Biopsy,
beginning
and
Hibernation
via serial
hibernation
and ending
with
full
gonadal
competence
In a companion
paper
(Barnes
et al., in preparation),
this series
of observations
presented.
and
of
from
torpor
requires
animals
under
con-
in ground
before
their entrance
into
their
achievement
of
(epididymal
spermatozoa).
Animals
end
and reproductive
development
This study
characterizes
gonadal
development
of testes
and
arousals
individual
that
of body
temperature
(c.f.,
Lyman,
1982).
growth
beginning
during
photoperiod
2#{176}C
each day until
December.
In mid-July
was increased
to 23#{176}C
that
was lowered
approximately
3 ± 1#{176}C
was reached
on 10
1985,
the chamber
temperature
over
14D
ambient
regular
and
intervals,
animals
were
withdrawn
(0.8
Once
each
in late spring,
cold
chamber,
if
torpid,
laparotomized,
anesthetized
and the
week
and
L
1979).
tained
snipping
In addition,
by puncturing
off with
the
tissue
sionally,
superglue.
cula within
an average
=
length
that
puncture
more
frequently
4-10
males were removed
allowed
to fully
arouse
with
methoxyflurane,
dimensions
of 1 testis
linear
measured.
Paired
testis
mass
was
the equation:
Mass (mg)
=
1.068
where
cardiac
and
methoxyflurane
anesthesia
(Metofane,
PitmanInc.,
Washington
Crossing,
NJ).
Torpid
squirrels
were
not anesthetized
before
blood
beginning
from
the
by
weighed,
under
Moore,
ground
obtained.
was
Sampling
blood
was
ml)
Blood
W
and
=
width,
estimated
by
(L X W2)
-
in
mm
using
0.78,
(Kenagy,
a biopsy
of testis
tissue
was obthe testis with a 201 needle
and
scissors
approximately
1 mm3
of
exuded
the wound
Animals
2 h of
of 3.8
from
in
were
the testis
returned
surgery
days
sampled
on successive
preserved
in buffered
ethyl
alcohol
(ETOH),
the
and
later.
puncture.
Occa-
was sealed
using
to their
hiberna-
reentered
Alternate
biopsies.
Biopsied
formalin,
dehydrated
and
embedded
hibernation
testes
were
tissue
was
in 70%
using
JB-4
HIBERNATION
#{149}
AND
GONADAL
1291
FUNCTION
I
.4
‘I.
-5.
L
-.
j
#{149}.#{149}#{149}.#{149} #{149}.iZ
;;
FIG.
1. Stages
of
spermatogenesis
.i;:bi
in
the
golden-mantled
ground
squirrel
spermatocytes
only,
lumen
enlarging.
(b) Stage
3 (first
week
posthibernation),
(c) Stage
4 (second
week
posthibernation),
elongating
spermatids,
residual
spermatozoa.
(e) Stage
6 (begins
fifth
week
posthibernation),
spermiation
togonia
and spermatocytes,
tubules
collapsed.
(a-f:
X540).
(a)
round
spermatids
cytoplasm
prominent.
and regression.
(f)
Stage
2 (hibernation),
spermatogonia
and
primary
with
pale
nuclei
and acrosomal
condensation.
(d) Stage 5 (third
week
posthibernation),
tailed
Stage
7 (12-16th
week
posthibernation),
sperms-
1292
BARNES
Embedding
Kits
Plastic
blocks
were
and
hematoxylin
sections
interstitial
Mean
measured
cross
(Polysciences,
at
This
PA).
and
with
squirrels
alternated
deep
hypothermia
cross
brief
periods
body
temperature
returned
(Fig.
2).
minimally
3 pm
procedure
stained
provided
5-20
seminiferous
tubules
and associated
cells.
seminiferous
tubule
diameter
in microns
was
in 70 biopsies
from
13 animals;
at least 4
of
sections
round
tubules
were
After
the termination
biopsies
were obtained
scopically
Six
motile
for
Histological
and
sliced
eosin.
Warrington,
of
sample.
didymal
follows
Inc.,
ET AL.
measured
of hibernation,
and examined
of
(Fig.
primary
per
epimicro-
arousal;
doubled
spermatids
Elongated
with
and
Stage
were
2. Only
present.
acrosome
granule
round
spermatids
recognized
Stage
nuclei
one
absence
tubule.
Stage
7. Complete
regression
and
package,
±
the
on
18 December
(± 2 days)
During
respectively.
this
ended
and
interval,
h
minimum
of torpor
over
1985
than
did
the
yearling
Growth
and
rewarming
than
(Fig.
from
body
temperatures
bouts
(Fig.
3C)
course
of the
hiberna-
males
(caught
about
born
as adults
in
3 wk earlier
in spring
1984
Spermatogenesis
The
estimated
than
500 mg to
the 200 days
preceding
5). The rate of growth
to and during
hibernation:
preceding
and
during
(± 4 days),
individual
±
0.3
each
more
season
and
SE.
heterothermy
19 April
3.6
Neither
duration
significantly
Growth.
from less
and
during
(29 March
± 13 days
vs. 20 April
± 3 days;
p<0.05).
Nineteen
percent
of all animals
were never observed
to be torpid;
these
animals
(termed
nonhibernators)
are treated
separately
in the following
analyses.
Patterns of Hibernation
began
temperatures
season.
Males
two
or more
yr old
spring
1984)
ended heterothermy
Testis
squirrels
surgery
tion
RESULTS
Ground
spent
days).
4) nor
changed
“Microstat,”
was used to perform
ANOVA,
two-tailed
t-tests,
linear
regressions.
Values
are reported
as means
body
duration
of this
interval
7.8 h to 16.2 h over the
the
from
Animals
1-10
(Fig.
in
statistics
by
4.
of
of spermatozoa.
microcomputer
levels
disturbed
3B).
Statistics
A
which
torpor
and 16.4 ± 0.8 h recooling
to minimum
body
temperatures.
The
minimum
body
temperature
attained
during
torpor
bouts
was 3.6 ± 0.1#{176}
C, and the
duration
of torpor
bouts
was 6.0 ± 0.7 days (range
spermatids
moving
apically
towards
the lumen.
Stage
5. Fully
formed
spermatozoa
present,
identified
by
free
sperm
heads
in the lumen
or a characteristic
“whorl”
of sperm
tail cross
sections.
Stage
6. Spermiation
with
cellular
debris
(regression)
present
in at
least
during
normothermic
as
spermatogonia
Stage
3. Round
identified.
present;
arousal
to
the
hibernation
season,
animals
with
spent
an average
of 11.8
± 1.0
h at
normothermic
spermatogenesis
spontaneous
Animals
torpor.
Over
radiotelemeters
sperm.
1, a-f):
spermatocytes
of
periods
of
3-4#{176}C) and
=
spent
81 ± 2% of the
hibernation
season
in deep
torpor
(Fig.
3A);
the remaining
time
was spent
in
periodic
arousals
and warming
from
and reentering
Classifications
stages
between
prolonged
(body
temperature
ground
mass
of
approximately
testes
increased
1100
mg in
the end of hibernation
(Fig.
(3.2 mg/day)
was similar
prior
i.e., slopes
of testis growth
the hibernation
season
were
40
0
28
118
§4
DEC
FIG.
2. Body
a radiotransmitter
temperature
implanted
JAN
of a male
ground
squirrel
in the abdomen.
Ambient
FEB
throughout
temperature
MAR
the
= 3
±
hibernation
1#{176}C.
-APR
season.
Temperature
MAY
was
recorded
remotely
every
20 mm
from
AND GONADAL
HIBERNATION
go-
FUNCTION
1293
A
80
36-
S
(14)
I-
(19)
(is)
(21)
4
j60
50
I
-
I
I
I
I
.
,
Dec
020
,
Feb
I
Mar
Apr
May
B
5
C
,
Jan
f
FIG.
4. Minimum
during
torpor
bouts
f
16
12
8
#{149}_
4
(6)
animals
number
(10
1
17).
______________________________________
I
I
I
I
end
I
temperature
(14)
(19)
(21)
-4
Months
3
Relative
-2
to End
mained
of Hibernation
termination
of
testis
growth
hibernation;
coincided
mean
testis
creased
from
1100
mg to a maximum
the month
after
the end of hibernation.
sustained
squirrels
these
nation
within
3500
Testis
mg in
mass
thereafter,
and eventually
reached
minimum
three
months
after
the termination
of hibernation.
Seminiferous
tubule
diameter
was significantly
correlated
with
testis
mass
(r=0.77,
p<O.OO1) and
2.5
testis
mass increased
Repeated
for
separately
surgery.
from
laparotomies
the rate of testis
surgery
is apparent
mass
140
animals
from
pm
to
250
pm
500
to 3500
mg.
and
biopsies
did
growth.
The lack of
in Figure
5 where
measured
that
for
for
the
animals
first
that
as mean
had
of testes
unaccompanied
prior
to the
by significant
During
hibernation,
in prophase
but there
unchanged
1). By the
(Table
had
(4 exceptions
are
and
histological
from
the
end of the
normothermy,
however,
round
spermatids.
Over
discussed
stage
re-
beginning
first week
most
Weeks
of
of
ground
2 and 3
spermatids
elongated;
18 days after
the term,of hibernation,
spermatozoa
were
observed
the testes.
It was not until
31 days after
the
of
hibernation
that
motile
spermatozoa
2.0
1.5
4
1.0
0.5
not
affect
an effect
of
mean
testis
time
=
4.0
3.0
about
9);
‘
values
from
14 (mean
6 to 29 (mean
in-
declined
increased
3 to
from
with
mass
of
growth
no animals
spermatids,
essentially
termination
rapid
was
from
ranged
mean
± SE)
achieved
season.
Number
of
1#{176}C.
The
were virtually
below)
with
1
FIG.
3. Percentage
of days
torpid
(A), duration
of normothermy
during
periodic
arousals
(B),
and
duration
of torpor
bouts
(C) of
ground
squirrels
in the 5 mo before
the end of hibernation.
Values
(mean
± SE)
in (A) and (C) are from
animals
that were minimally
disturbed
by surgeries,
and in (B) are from
animals
with
radiotelemeters.
Ambient
temperature
= 3 ± 1#{176}C.
Numbers
of animals
sampled
is mdicated
in parentheses.
More
±
ranged
point
of meiotic
prophase.
again were
seen
hibernation
equivalent.
= 3
of hibernation
tene stages
spermatocytes
2
-5
point
plotted
(Tb;
hibernation
changes
in the histological
state
of the germinal
epithelium.
At the beginning
of hibernation,
the only
germinal
cells within
the seminiferous
tubules were
spermatogonia
and primary
spermatocytes;
spermatocytes
were
commonly
seen in zygotene
and pachy-
6
‘)
Ambient
per plotted
bouts
per
Sperinatogenesis.
8
4
sampled
of torpor
body
temperatures
throughout
the
is plotted
previous
#{176}
-180
-150
-120
-90
Days
Relative
-60
-30
0
to the End of
30
60
90120
Hibernation
FIG.
5. Estimated
mass
of testes
(mean
± SE)
of ground
squirrels
relative
to the termination
of hibernation
(Day
0). Means
for animals
operated
on for the first
time
are plotted
separately
(---).
The downward arrow
represents
the date after
which
50% of animals
had initiated
hibernation.
Numbers
of animals
per point
are indicated
in Table
1.
150
1294
BARNES
TABLE
1. Mean
histological
mals with epididymal
sperm
stage
relative
of testes
and the proportion
to the day hibernation
ended
ET
of ani(0).
AL.
Nonbibernators.
growth
Day
Stage
-202
2.0
-161
-124
2:0
2.0
-105
-94
2:0
2.0
Sperm
n*
.
4
-
1
juveniles
appears
of
whether
growth
squirrels
animals
between
juveniles
groups
differ
development
to
gonadal
ground
indicates
that
significantly
in nonhiber-
advanced
be
hibernon-
by
approxi-
mately
Only
2 wk relative
to that of hibernators.
4 adults
did not hibernate,
and few
2.0
-
5
ments
were
2.1
-
8
sperm
were present
in testes of all four
animals
by
mid-March,
which
suggests
that,
compared
to hibernating
adults,
gonadal
growth
and
spermatogenesis
:
4
6
2:8
2.6
0/2
12
14
3.0
0/5
0/4
18
19
22
24
26
4.5
3.6
4.2
4.0
4.3
0/3
0/3
0/5
0/4
0/4
-
made
5
were
advanced
adults.
Interrelationships
dal maturation.
5
fluenced
2:0
2.1
-14
-6
slightly
by
of testis
and hibernating
early
April
did
Fig.
6).
Testis
nating
timing
4
ii
39
was affected
only
nated.
Comparison
hibernating
only
in
(p<O.02;
seasonal
golden-mantled
-
i
-65
The
in juvenile
males
9
6
6
4
torpid
during
onset
1 mo
between
amount
The
the
least
at
of
measure-
individuals;
however,
in nonhibernating
hibernation
of time
that
the
and gonaindividual
hibernation
season
spermatogenesis.
Four
inof
65
animals
developed
spermatids
during
the hibernation
season.
These
animals
spent
relatively
fewer
days
(27 ± 6%) torpid
in the month
preceding
spermatid
.
development
tids (82
animals
compared
to
3%). For example,
±
hibernating
during
this
for
time,
animal
without
#2098
24 days
after
surgery
the animal
developed
spermastopped
on
round
3 March;
sperma-
33
4.7
8/9
39
43
5.2
5.6
5/6
6
tids. Regular
torpor
was resumed
on 27 March;
at the
next
biopsy
on 16 April,
spermatids
were no longer
observed.
Upon
terminating
hibernation
on 24 April,
52
57
4:7
5.3
4/6
6
this
animal’s
68
73
5:3
5.4
1/2
4/6
Of
after
had
the 23 animals
biopsied
during
the first week
termination
of hibernation,
15 (65%)
already
developed
spermatids
in their
germinal
epitheia;
the
remaining
118
143
6.0
6.3
Number
of animals
10
were
by
these
on
6
8
2/2
1/1
testes
underwent
animals
normal
still
maturation.
had only
spermatogonia
5
4
assessed.
3.53.0-
V
2.5-
E
first
were
Regression
observed
in
was apparent
spermiation
rids,
fragments
plasm
to
were
minimal
had
(Table
as soon
1).
as
1.5-
we,
1.0-.
occurred.
Disintegrating
spermaof spermatozoa,
and residual
cyto-
cleared
size
the epididymides
in some tubules
2.0-
from
within
tubules,
2-3
hibernation.
At that time,
the
was composed
of spermatogonia
preleptotene
spermatocytes.
and
mo
testes
after
the
0.50-I
end
U
U
0
returned
U
ND
I
I
J
U
I
U
M
U
J
U
J
A
Months
of
germinal
epithelium
and relatively
few
I
MA
F
FIG.
6. Seasonal
changes
in testis
(closed
squares)
and
nonhibernating
squirrels.
Ambient
temperature
= 3
per point
ranged
from 6 to 18, mean
maas
±
=
(mean
±
(open
squares)
1#{176}C.
Number
9.
SE)
of hibernating
juvenile
ground
of animals
plotted
HIBERNATION
and
spermatocytes.
percentage
of
These
subgroups
spent
torpid
days
AND
differed
in the
in the
pre-
month
ceding the end of hibernation:
the animals
showing
accelerated
spermatogenesis
were torpid
60.3 ± 4.9%
of days vs. 82.4 ± 3.5% for animals
with delayed
development
(p<0.003).
GONADAL
and Barnes,
field study
Patterns
of Reproductive
Reproductive
season
dal
was
mass
Maturation
maturation
during
by modest
spermatogenic
characterized
but
not
by
the hibernation
increases
in gonachanges;
the in-
crease
in testis size prior
to the end
was less than
20% of eventual
growth,
of
hibernation
and develop-
ment
of germ
cells
the
stage
of meiotic
ended,
testis growth
appears
to have been arrested
at
prophase.
Once
heterothermy
was rapid
and complete
sperma-
togenesis
in less
developed
commonly
were
found
than
in testes
1 mo:
of
spermatozoa
squirrels
2-3
wk
after
hibernation
had ended,
and epididymal
sperm
were present
after
31 days.
The time course
for the
establishment
of spermatogenesis
in golden-mantled
ground
squirrels
nonhibernating
The
gonadal
remain
testis
is similar
(Clermont,
hydration.
that
1972).
The
functional
reported
for
that
occurred
during
specified.
Proliferation
their
development
preceded
hibernation
during
hibernation
changes
during
spermatogenesis
to
responsible
components
growth
to be
cytes
and
prophase
enlargement
to
thus
rodents
for
limited
the
hibernation
of spermato-
stages of meiotic
onset,
and
testis
might
be due solely
into
significance
hibernation
for
once hibernation
of gonadal
the completion
is terminated
of
is not
known.
Differing
patterns
of gonadal
growth
and development
in relation
to hibernation
have been described
for several
ground
squirrel
species.
On the basis
of
laboratory
studies,
Wells
(1935),
McKeever
(1966),
and Kenagy
(1980)
concluded
that
gonadal
maturation
is nearly
complete
by the end of hibernation
in
13-lined
and
golden-mantled
ground
squirrels,
whereas
Johnson
et al. (1933)
and Ellis et al. (1983)
reported
hibernation
little
gonadal
in 13-lined
growth
and
or development
Uinta
ground
during
squirrels.
Additionally,
field studies
of arctic,
Belding’s,
roundtailed,
and golden-mantled
ground
squirrels
all describe
complete
or near complete
gonadal
growth
and
spermatogenesis
in males
upon
initial capture
after
emergence
from
hibernation
1964;
Morton
and Gallup,
(Hock,
1960; McKeever,
1975;
Neal,
1965;Kenagy
submitted
of arctic
newly
describes
small
cytes
testes
and
for
ground
publication);
squirrels
emerged
with
present.
only
males
In woodchucks,
in spring
diameter
emergence
than
ground
squirrels.
testicular
activity
begins
season,
and
described
duration
Saboureau,
growth
even
for
In the
during
of the
to be unchanged
(Saboureau
and
Dutourn#{233} and
ster, testis
hibernation
the
been
1983).
is delayed
(Smit-Vis
hibernaEuropean
the hiber-
spermatogenic
by the hypotherDutourn#{233}, 1981;
In the
golden
ham-
uninhibited
of spermatids
is described
as being
though
development
and
spermatozoa
hibernating
animals
activity
are greatest
at the
from hibernation
tors
other
hedgehog,
is reported
of
torpor
spermato-
spermatogenic
et al., 1972).
patterns
have
cycle
mia
as having
and
(Christian
Several
nation
in contrast,
a
(Mitchell,
1959)
spermatogonia
seminiferous
tubule
of first capture
after
time
DISCUSSION
1295
FUNCTION
in hibernating
vs.
and Akkerman-Bellaart,
by
non-
1967).
a
Interpretation
of
lack
of information
thermoregulatory
laboratory
studies,
these
studies
is complicated
about
torpor
frequency
history
of experimental
animals
often
were
specified
or
relatively
warm
(room
temperature)
wherein
have short
torpor
bouts
during
and
kept
show
maintained
ground
at room
mild
thus,
the
approximated
body
in the
squirrels,
timing
mass
of
from
(Kenagy,
and
1980;
animals
would
studies,
the
hibernation
have
Barnes,
only
rarely
during
winter;
season must
and troughs
1986;
Lee
be
in
et al.,
studies
used terminal
samnot allow reference
to when
hibernation.
ended
relationship
between
and an animal’s
first
ture
that
has not
animals
been established;
end heterothermy
first
ment
caught
occurs
in traps
during
and
this
experiment.
with
food
typically
lethargy
the hibernation
seasonal
peaks
1986).
In addition,
most
pling procedures
that do
In
un-
that may be more
than
the 3-4#{176}C
present
supplied
temperature,
hypothermia
animals.
held
in
ambient
conditions
hibernators
typically
which
body
tempera-
tures
decline
to intermediate
levels
conducive
to gonadal
development
temperatures
Golden-mantled
by
or
the
In
the termination
emergence
and
possibility
well before
field
of
cap-
remains
they
are
that
reproductive
developinterval.
The present
data
support
this hypothesis
(see below).
Age
differences.
Posthibernation
rates
of gonadal
development
did not differ
between
adult
and yearling males;
however,
since adults
ended
hibernation
earlier
than
did yearlings,
adults
generally
were reproductively
mature
in advance
of younger
animals.
This
BARNES
1296
ET
AL.
age difference
and the much
earlier
gonadal
development
in adult as compared
to yearling
nonhibernators
suggest
that
age is an important
factor
influencing
Ecological
timing
thermy
was terminated.
We suggest
that
in natural
populations
of golden-mantled
ground
squirrels
gonadal maturation
also
occurs
during
a “preemergence
of reproductive
Hypothermia
and
maturation.
Endogenous
Rhythms
Endocrine
function
Hypothermia.
growth
may
tures
of
typically
hibernators
from
be
inhibited
by
torpor.
Hormonal
temperature-dependent
do not appear
torpid
ground
dotropic
vitro
to
fail
to
turned
Growth
grow
until
to
normal
of gonads
during
the
Chatfield,
low
body
tempera-
growth
processes
are
in mammals,
and
be exceptions.
Testes
tested
at
low
Dempsey,
in hibernating
intervals
brief
and
1955),
of
arousal
it is possible
has re1954).
to prevail
(Lyman
and
the
frequency
during
the hibernation
season
was associated
with
more
rapid
testis
development
after
hibernation
had ended.
Endogenous
cycles.
Annual
cycles
of body
mass,
and
reproductive
function
lated
by
a temperature-compensated,
rhythm
in golden-mantled
ground
ley and Asmundson,
1974;
Mrosovsky,
1980;
Licht
et
al.,
1982;
Barnes,
are
regu-
and
sexual
maturation
interval
after
Thus,
newly
for
of heterothermy
above
ground
emerged
condition
1979;
publication)
ochetero-
and
(c.f.,
males captured
in the field
(e.g.,
Kenagy
and Barnes,
may
have
ceased
torpor
earlier.
A lengthy
normothermic
period
with
the end of hibernation
would
be
expensive.
Such
an interval
may
be
to cost as much
again as the entire hiberna-
tion season (c.f., Wang,
a major
aspect
of the
1978),
and it would
represent
male ground
squirrel’s
annual
energy
stores
budget.
Food
the
hibernacula
Towns,
1981)
and
of adult
male
and Columbian
such
caches
been
have
arctic
(Shaw,
would
in
described
(McLean
and
1925)
ground
be of obvious
bene-
fit to males during
the preemergence
interval.
These
conclusions
are contingent
on there
being
a close
similarity
between
the
hibernatory
behavior
of
laboratory-housed
ground
squirrels
and that
of animals
under
been
only
shown
in
field study
(Wang,
Male
conditions;
natural
Richardson’s
of ground
1973).
golden-mantled
all
almost
other
a similarity
such
has
ground
squirrels
in the
squirrels
during
hibernation
ground
hibernating
squirrels,
like
species,
appear
males
the
to a need
attributed
of males
to establish
territoriality
timing
of reproductive
maturation
to follow
the end
of heterothermy
may be an expression
of the phase
relations
between
these
rhythms
rather
than
a conse-
or dominance
prior
to female
emergence
(Michener,
1983).
The emergence
of females
also could
reflect
a more
rapid
rate
of
reproductive
maturation
quence
of the
The
initiation
of hibernation.
in the
ground
and less
females
in the
in males.
squirrel
that
low body
temperatures
of spermatogenesis
interrupted
torpor
late
hiberna-
tion
season
suggests
that
the signal
to initiate
gonadal development
may
be generated
before
the end
of hibernation
but
response
to this
signal
may
be
temperature-sensitive.
ground
squirrels,
sive factor
that
interval
heterothermy.
that
Thus,
tissue
restricts
begins
temperature
gonadal
with
the
in
golden-mantled
may be a permisdevelopment
to the
final
termination
of
above
ground
earlier
each year than
do females
(Bronson,
1979;
Michener,
1983;
Kenagy
and Barnes,
submitted for publication).
This
sex differences
has been
circannual
squirrels
(Pengel1978;
Kenagy,
1986),
study,
month-long
reproductive
1964;
Bronson,
squirrels,
limited
increase
in testis
size during
the hibernation
season
was
restricted
to intervals
of arousal
from
torpor.
Although
the present
data do not permit
a definitive
conclusion
reparding
this point,
they provide
supporting circumstantial
evidence
since reduction
in torpor
hibernation,
1983).
in mature
McKeever,
tempera-
and
hamsters
that
Michener,
wk
coinciding
energetically
estimated
of
to
Implications
after
the termination
the emergence
of males
3-4
1951),
after
body
temperature
(Lyman
and
Fawcett,
has been hypothesized
interval”
before
submitted
accessories
insensitive
Energetic
our
laboratory
during
the
to gona-
respond
1984).
Sexual
are
similarly
(Lyman
and
grafts
implanted
testosterone
tumorous
tissue
and
do not
when
(Barnes,
hamsters
the
and
squirrels
stimulation
tures
in
hibernating
In
curred
and
time
required
to establish
at high
reproductive
body
temperatures
competence
in
than
ACKNOWLEDGMENTS
We thank
Anita
Pearson
for discussion,
Kathy
Moorhouse
for animal
care,
Pamela
ance,
and
Paul
Hailer
for photographic
Dr. E. B. Pivorun
for providing
the automated
photography,
and histology,
Enayati
for technical
assistservices.
We are grateful
to
data collecting
equipment.
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