Copea, 1990(3), pp. 867-871
1990 by the American Society of
Ichthyologists and Herpetologists
THE BEHAVIOR OF JUVENILE ALLIGA-
TOR MISSISSIPPIENSIS AND CAIMAN CROC-
ODILUS EXPOSED TO LOW TEMPERA-
TURE.-Most studies of crocodilian thermal
biology have concentrated on thermal :.. .-
ences (Colbert et al., 1946; : :. : :, 1 .,
Johnson et al., 1978), upper thermal tolerances
(Colbert et al., 1 :. and the physiology : ... : .
1979) and behavior of thermoregulation (Fish
and Cosgrove, 1987; Lang, 1979; .: :.
1974). Many of these have been concerned with
the responses ofanimals to heating trials (Smith,
1976; Smith and Adams, 1978; Smith et al.,
1984) or in gradients where only the .;
of body temperatures selected in the wild are
represented (Lang, 1987). Few studies have
concentrated on the responses to low temper-
atures, and these have focused on the American
*i: ..., Alligator mississippiensis. Winter be-
havior and activity patterns of adult .i. .: .
have been examined in South Carolina (Brisbin
et al., 1982), North Carolina . et al., 1 ,
and Louisiana (Ci i.' i 1 :- ... Adult .:i,
tors (> 1.8 m) bask on warmer winter days and,
in response to freezing weather exhibit a sub-
I breathing posture (Smith, 1979) in
which the tip of their snout is kept out of the
water while the rest of the body is extended
S;:... ;/warmer water i ... etal., 1983).
S.. adult .i. survive freezing condi-
tions by maintaining air holes in the ice of fro-
zen ponds, rather than by staying submerged
under the ice.
Few observations have been made on the win-
ter activity of juvenile .;'..- particularly
those <1 m, due to the difficulty in locating
them. It has been suggested that juvenile alli-
gators overwinter in dens with adults: ::.
1 ., 'i ii **, 1" however, appropriate
den sites are not always available :i .:'",
1981). Because small .1:",.:... are likely to be
frozen in or under the ice, they may not be able
to exhibit the same behavior as adults (i.e., sub-
merged breathing) Under freezing conditions.
The ability of small .-:.: -. to behaviorally
adjust to periods of low temperature may be an
important factor contributing to the ii: ,r :'s
survival in the northern portion of the range.
In addition to the effect of body size on ther-
mal behavior of crocodilians, there may be be-
havioral and physiological differences between
species with temperate and tropical ranges
(Lang, 1979, 1981). Lang (1979, 1987) has sug-
gested that crocodilians living in thermally eq-
uitable environments (i.e., the tropics) are ther-
moconformers while those which occur in
variable environments (temperate areas) are
thermoregulators. If these differences are
species specific (rather than simply related to
the animal's current thermal environment), then
a species with a natural tropical . .. ..
as Caiman crocodiles) should show different re-
sponses to low : ....: ,*'... than a species like
the American '::; .* that has a temperate dis-
Since at least the 1 :.:: C. crocodiles have
been released in canals in parts of south Florida
(Wilson and Porras, 1983). The largest known
I : !. '"": of caimans is at the Home-
stead Air Force Base (HAFB), Homestead, Dade
County, Florida :: i... 1980; pers. obs.). In ad-
dition, there are probably breeding populations
in western Dade County (J. Wasilewski, pers.
comm.), and both juvenile and adult caimans
have been captured in Ft. Lauderdale, Broward
County (pers. obs.). Because caimans are an ex-
otic species, there is some interest in determin-
ing the potential for caimans to colonize more
northern counties in Florida. The natural trop-
ical/subtropical distribution of C.
S that low temperatures (<15 C) may be a
factor that will limit how far north in Florida
S :.E .: : ..ofcaimans can occur. The
purpose of this study was to: 1) determine the
ability of small .: .. .: : to survive freezing con-
ditions; and 2) compare the responses of small
. ...: : (warm-temperate) and caimans (trop-
ical) to cold weather.
COPEIA, 1990, NO. 3
0900 1100 1500
(7) (9) (6)
Time of day
Fig. 1. The percentage of alligators a
baskingat different times,. .: : .. .::
periods between 15 Nov. 1985 and 2 Dec.
ber of days observations were made).
Materials and methods.-Five Caiman
.. 2035 g; SD = 535.1 839.0
cm TL; SD = 46.7 18.7 cm) wer
from the HAFB in Dec. 1984. They
trained in a 130 cm x 80 cm cattle t
30 C until Oct. 1985 when they were
a tank in an unheated ".....":":, at the
River i / Laboratory (SREL)
South Carolina, where air tempera
tuated between 15-30 C and water
tures between 1i C. Nine A. miss
(58- 'g, i' = 290.8 156.9,
TL, SD = 47.7 11.4 cm) were
either ... Par Pond, Savannah Ri
Aiken County, South Carolina (five
Kiawah Island, Charleston County, S
olina in mid-Oct. 1: and housed i
the unheated ..i ... until 14 Nov.
animals were fed chicken and beef h
a week and had access to land, water
On 14 Nov. 1985, all animals were
a 10 x 15 m clay-lined outdoor pone
Maximum pond depth was 150 ci
shoreline was available for basking. A
m :.. platform was placed in the
the pond to act as a supplemental ba
Air, water and substrate temperate
recorded at the pond's edge every ot
0900, 1100, 1500, 1700 h during fN
and every third day at 0900, 1100,
Jan.-Feb. Maximum and minimum air
temperatures were recorded daily
(Weksler max-min thermometer, 0
temperatures ranged from -7-33 C
temperatures from 0-25 C. December and Jan.
were the coldest months (mean temperature for
each month 7.4 C) and Nov. the warmest (mean
temperature 18.6 C). Animals were recorded as
basking (on shore or in .i' water with part
of the body out of water) or not basking (all
other positions). Cloacal temperatures were tak-
en on two occasions using a quick reading clo-
acal thermometer (Miller and Webber, Inc.).
S Results. '= i mid-late Nov., alligators and
caimans were often observed basking from mid-
1700 .. ..:. '1.: the afternoon 1). A
(4) greater percentage of caimans were observed
basking than i. ., .at ** "**. 1100, 1500, but
nd caimans this difference was .:. ..... only at 1500 h
observation (one tailed Wilcoxon signed rank test, P < 1 '.
1985(num- Fig. 1). Both species basked more often during
the mid-afternoon (i.e., 1 .-.. h). No :
interactions (i.e., :.. :.... or chasing) were ob-
Scrocodilus served within or between species: in fact, cai-
g, 37-80 mans and -i: ... were often observed bask-
e collected ing on top of one another.
vere main- In early Dec. air temperatures dropped rap-
ank at 25- idly. At 2030 h on 2 Dec., air temperature was
moved to 6 C and water temperature 15.6 C. All animals
Savannah were submerged at this time. A minimum air
in Aiken, temperature of 2.2 C was recorded at 0900 h
tures fluc- the next .,. -... Minimum water tempera-
tempera- tures ranged from 12.2 C (shallow water) to
issippiensis 12.9 C (deep water).
28-56 cm Three of the five caimans died as a result of
obtained exposure. One caiman was found dead on the
ver Plant, bank at 0900 h on 3 Dec. Three were found in
), or from shallow water and were lethargic but exhibited
iouth Car- strong gape response. The fourth was found in
n tanks in 8 cm of water in a hole 5 m from the pond.
1 All This animal's cloacal temperature was 8.8 C, it
heart twice had no righting response, and died 4 d later of
and a heat pneumonia (E. Howerth, pers, comm.). Another
caiman died of the same cause 7 d later. The
Placed in two surviving caimans were removed from the
Sat SREL. pond. One exhibited signs of pneumonia but
m. Ample recovered within a week. The other did not
1 x 0.75 exhibit signs of infection. Neither animal was
middle of returned to the pond. In contrast, all of the
asking site. .11.: :... were in the deepest portion of the
ures were pond, and none : : Location (deep vs shallow
her day at water) of the two species on 3 Dec. was :: ..-
jov.-Dec., icantly different (Fisher's exact test, P < 0.01)
1500 h in as was the proportion of each species surviving
and water the first cold front (one tailed Fisher's exact test,
at 0900 h P < (: :'.
.5 C). Air On the night of 28 Jan., air temperature
and water reached a low of -7 C, causing the formation
SHORTER CONTRIBUTIONS: HERPETOLOGY
of 2.5 cm of surface ice on the pond. Water
temperatures ranged from 0-4.5 C at 0800 h.
Three .ii: .. were frozen to a fence which
ran through the center of the pond; their bodies
were perpendicular to the surface with their
nostrils above the ice. Two :,. :, a sub-
merged breathing posture (Smith, 1 .; with
their nostrils exposed and four, including the
two .. :- animals, both less than 40 cm
(hatchlings) were frozen under the ice. Cloacal
temperatures (Tc) recorded from seven animals
ranged from 0-4.4 C (SD = 1.0 C 1.6). The
two smallest animals were observed pushing on
the ice, apparently trying to break through to
breathe. Both animals .1 .... :., died --.:
had Tc of 0.0 C); apparently they drowned. The
other animals were lethargic and appeared
bloated. However, none of the larger (>50 cm)
animals died, even though three of them were
trapped underneath the ice for at least 12 h.
A layer of ice remained on the pond until the
morning of 30 Jan. As the ice melted the sur-
S:.: seven alligators began to move to shore
to bask. All but one animal were basking by
1300 h that day (air temperature 12.4 C), and
continued to bask until 1700 h when the air
temperature : 1 to 7.0 C. The next day
they again basked for most of the day. By the
third day after the ice melted they returned to
a more normal basking behavior, basking main-
ly in the afternoon on bright sunny days.
Discussion. : -i. i.i. :, i. .: .. in this study
did not maintain air holes, and were therefore
trapped under the ice and drowned. : : .,
when the larger (>50 cm) i'- .. .. maintained
air holes, they survived temperatures as low as
0.4 C. Hagan et al. (1983) reported the survival
of an adult radio-collared .i .: in 2.0 C water
in North Carolina. There is also a report of a
0.5 m i. :. surviving (and growing to 1.25
m) six winters in Pennsylvania (Barton, 1955)
where the mean temperature for the coldest
months varied from -5.5-1.3 C. Clarke (1953)
; a 0.5 m alligator surviving three win-
ters in Virginia where an average winter has 40
d of minimum temperature at or below freez-
ing. Hence, it may not only be the physiological
tolerance to low temperature per se that limits
the .:::, ... 's .... .... northward, but also
an ... : of : .... .and ... .to sur-
vive even short periods of ice formation. Winter
freezes i -: several days or more may also
act as a means of population regulation by kill-
ing an entire cohort of young. : i ." in some
situations .... : ..'. remain in close as-
sociation with adults at least through the first
winter ''' 1979) and in some cases for up
to 18 mo (Chabreck, 1 *:. ). Hence, it may be
possible that, in some areas, the juveniles sur-
vive severe cold fronts by overwintering in dens
with adults, or by using air holes maintained by
There were noticeable differences in both the
basking patterns and the behavior of caimans
and .: .. x. when exposed to low tempera-
tures. These differences support the suggestion
by Lang (1979, 1981, 1987) that there may be
differences in thermal behavior between species
that have temperate and tropical ranges.
In this study, the caimans were observed bask-
ing more often than the .": ..- in the after-
noon, .. ...:, as a result of differences in heat-
ing rates, i.e., the caimans might heat slower
and therefore require longer basking to reach
the same temperature. It is also - .: that
even by basking throughout the afternoon, the
caimans were not able to reach a suitable tem-
perature to resist infection. The immune re-
sponse is often reduced in reptiles at lower tem-
peratures (Cooper et al., '- .;. Crocodylus aculus
and C. niloticus also die when exposed to low
temperatures (Barbour, 1923; Coulson and
Hernandez, 1 .- .. and the historical ...'
tion of C. aculus in Florida reflects this intol-
erance to low temperature, : .. very closely
the isocline for the mean low temperature (18
C) in Jan. (Kushlan and Mazzotti, 1989). Cai-
mans in the United States may also be : ...... :
in distribution by an intolerance to low tem-
The .i .:ir to withstand low temperature has
both physiological and behavioral components.
We do not know if the caimans would have sur-
vived the first cold snap if they had remained
in the deepest portion of the pond as the alli-
gators : All crocodilians use water as a means
of thermoregulating, but the patterns of move-
ment :.::. between species. Although A. mis-
sissippiensis move onto land to bask during the
day, C. porosus under similar conditions remain
submerged during the day to avoid rapid heat-
ing and move onto land at night ,: --. 1987).
Aii .... are observed basking in the winter
only on warm (air temperature >10 C) sunny
days, while tropical species may continue to bask
during cool weather (air temperature <10 C).
Neill (1971) observed a captive C. acutus in Palm
Beach County, Florida. ... : ,:.. unsuccess-
fully to raise its body temperature by basking
COPEIA, 1990, NO. 3
during cool winter weather: "On a cold day in
winter, the., !....: would move into a patch of
i: that struck the pool. t. would even move
out of the water to follow the patch of sunlight,
. becoming chilled and numbed by the
cold substratum and air." Neill pointed out that
this behavior may be ., :' i in the tropics
where morning temperatures rapidly increase,
but can be lethal in temperate areas. The cai-
mans in this study may have moved into the
shallow water and onto shore early in the morn-
ing to bask, and were unable to raise their body
temperature. They may also have been search-
ing for subterranean dens or sites for burrowing
i. from the low temperatures. Crocodylus
and C. sinensis are known to burrow in
response to low temperatures. If unable to bur-
row C. niloticus will die when exposed to freezing
temperatures (Pooley, 1969). Caiman crocodiles
are also known to burrow in litter or mud as
.. .. from drought and extreme tempera-
tures (Staton and Dixon, I ? Gorzula, 1978;
T ....: 1987), so it is not unreasonable to assume
that this behavior could also be exhibited in
response to low temperatures, and that if al-
lowed to burrow, caimans could survive even
lower temperatures than . in this study.
That the caimans in this study could not with-
stand low temperatures, and that two of five
caimans collected from the HAFB in 1988 also
died after 6 h exposure to 10 C (pers. obs.)
suggest that: 1) the caimans currently estab-
i. in south Florida will probably not move
north into areas that experience routine freez-
es; however, the ; .'.i.r of the introduction
of more cold tolerant populations exists; and 2)
there may be differences in thermal tolerances
and thermal strategies of .iL .. .. ... I caimans.
However, because of the unknown origin of the
caimans from IIAFB and the wide distribution
of caimans in South America, further study is
needed to determine whether the difference ob-
served between the i. .: and caimans in
this study (i.e., caimans basking more, moving
into shallow water instead of .. ... '.. the deep-
er/warmer portion of the pond, and having a
low tolerance to temperatures <10 C) are
species-specific or whether responses vary be-
tween populations exposed to different envi-
Acknowledgments.-We would like to thank the
..o people for help with various aspects
of this study: J. L. 'i-. ".. R. A. Seigel, F. Stone,
J. Garvin, D. Maffett, and P. Dixon. J. W. Gib-
bons, R. A. .:.. W. A. Dunson, and P. West
provided comments on the manuscript. J. Hin-
ton typed the manuscript. J. Coleman drafted
the .... This study was supported by contract
DE-AC09-76SROO-819 between the United
States Department of F.... and the Univer-
sity of Georgia. ( :- ofcaimans and travel
of FJM was supported by a contract between
Florida Power and '' .'" Co. and The Penn-
sylvania State University.
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LAURA A. BRANDT, Savan nah River Ecology Lab-
PO Drawer E, Aiken, South Carolina
29802, and FRANK J. MAZZOTTI, University of
Florida, Department of i. ; and Sci-
ences, Broward County Extension 3245
College Avenue, Davie, Florida 33314. Present
address (LAB): same as FJM. Accepted 5 Oct.