JOURNAL OF EXPERIMENTAL ZOOLOGY 303A:872–879 (2005)
Endocrine and Behavioral Response to a Decline in
Habitat Quality: Effects of Pond Drying on the Slider
Turtle, Trachemys scripta
W. BEN CASH1 AND REBECCA L. HOLBERTON2
Department of Biology, University of Mississippi, MS 38677
ABSTRACT
The effect of the simulated drying of a pond on the behavior and corticosterone
secretion of Trachemys scripta was measured in a field situation. Slider turtles were held in
experimental and control ponds (12 15 m) enclosed with a drift fence integrated with springtriggered livetraps. The experimental pond water level was dropped 10 cm per day for 8 d, until water
was completely drained. Slider turtles responded to the draining of the pond by the emigration of the
majority (75%) of the experimental population. Emigrating turtles had significantly elevated
corticosterone at Time 0 (blood sample within 10 min of handling 5 4.48 ng/mL70.503SE) when
compared with turtles captured in a control pond (Time 0 5 0.954 ng/mL70.121SE), where
conditions were held constant. Turtles emigrated during the final 72 hr of pond draining when
ponds reached 30 cm depth and lower and water temperature was at least 30.81C or higher.
Additionally, the effect of trapping using spring-activated livetraps was tested. Turtles held in
livetraps (n 5 6) for 45–110 min showed a characteristic corticosterone response (Time 0 5 0.957 ng/
mL70.091SE; Time 30 5 2.85 ng/mL70.131SE), indicating that this trapping technique alone does
not stimulate corticosterone secretion. The findings of the study met our predictions that turtles
would respond to the draining of the pond behaviorally by emigrating from the habitat concurrent
with an elevated corticosterone concentration. This supports the view that corticosterone is involved
in stress avoidance mechanisms that allow organisms to respond to environmental perturbations.
r 2005 Wiley-Liss, Inc.
J. Exp. Zool. 303A:872– 879, 2005.
Animals respond in different ways to fluctuating
or adverse changes in habitat quality. Biologists
have described a continuum of physiological and
behavioral responses, including movement patterns, linked to some resource required for homeostasis (Dingle, ’84). While most intra- and extrapopulation movement can ultimately be related
to resource or mate acquisition (Greenwood and
Swingland, ’83), one category of movement that
may require a response by the entire population is
that of large-scale decline in habitat quality. From
an evolutionary sense, individuals develop mechanisms or strategies to respond spatially and/or
temporally to variable habitats, and movement is a
very basic part of these strategies (Rhodes and
Odum, ’96).
Many freshwater turtle species occupy a unique
niche in that they require aquatic habitats, but
have adapted the ability to leave the aquatic
habitat for short or extended periods of time
(often referred to as semi-aquatic). The majority
of such movements is linked to predictable life
history events (i.e., reproduction) or is the result
of seasonal changes in resource availability
r 2005 WILEY-LISS, INC.
(Gibbons et al., ’90). For example, the terrestrial
nesting excursions of females are part of the
normal reproductive life history for most freshwater turtle species (Kuchling, ’99). Similarly,
male slider turtles, Trachemys scripta, move
variable distances, both within and between
habitats, presumably in search of mating opportunities (Morreale et al., ’84; Thomas and Parker,
2000). However, in addition to predictable changes
in resource requirements, there are many unpredictable conditions that can arise that may require
turtles to emigrate from aquatic habitats.
Grant sponsor: Ralph Powe Research Award from the University of
Mississippi Field Station at Bay Springs, 001. NSFIBN 9873852 to RLH.
Correspondence to: Ben Cash, Department of Biology, Maryville
College, Maryville, TN 37804. E-mail: [email protected]
W. Ben Cash, Present address: Maryville College, Department of
Biology, Maryville College, 502 E. Lamar Alexander Parkway,
Maryville, TN 37804-5907.
Rebecca L. Holberton. Present address: 5751 Murray Hall, Department of Biological Sciences, University of Maine, Orono, ME 044695751.
Received 1 July 2005; Accepted 14 July 2005
Published online in Wiley InterScience (www.interscience.wiley.
com). DOI: 10.1002/jez.a.217.
SLIDER TURTLE RESPONSE TO POND DRYING
Many of the previously noted conditions tend to
affect distinct groups within a particular population (i.e., female nesting, male mate searching,
juvenile dispersal). However, broad unpredictable
changes in habitat quality (e.g., food base, water
availability) may affect the entire population.
Observations of T. scripta responding to changes
in habitat quality have been documented. For
example, Parker (’84) noted the emigration of
T. scripta from a pond that had been recently
subjected to algaecide application. Presumably,
this affected the food base, forcing turtles to
search for better-quality habitat elsewhere. Similarly, habitat loss in the form of reduced water
levels due to pond drying may lead to emigration
of a large percentage of the population, as noted in
several earlier studies on freshwater turtles
(Cagle, ’44; Gibbons et al., ’83; Moll, ’90).
In addition to various behavioral responses to
habitat change, animals must rely on physiological
responses to help maintain homeostasis. In the
few reptile species studied thus far, the major
glucocorticoid, corticosterone, is secreted in response to a stressor, which can either be acute
(e.g., immediate predation risk) or chronic (e.g.,
long-term decline in food resource) (Guillette
et al., ’95; Cash et al., ’97). The function of the
adrenocortical response to regulate behavioral and
physiological responses is context dependent. For
example, an acute increase in plasma corticosterone concentration, usually considered a ‘‘stress
response’’, has been referred to as a ‘‘stress
avoidance’’ mechanism by which the increase in
the hormone acts to redirect an individual’s
behavior away from regular activities and towards
life-saving ones such as increased foraging when
faced with potentially energetically challenging
conditions such as storms (cf. Wingfield, ’94;
Orchinik, ’98). In this context, plasma corticosterone levels would return to normal if the individual
is able to meet its new energy demand or if the
perturbation disappears. However, high glucocorticoid concentrations can promote the production
of energy from non-carbohydrate sources, such as
protein (gluconeogenesis) (Harvey et al., ’84), and
may also suppress immune function, particularly
when chronically high concentrations are maintained (Guillette et al., ’95). In contrast, acute
secretions of glucocorticoids may actually heighten
immune function (Dhabhar et al., 2000). In this
way, the actions of glucocorticoids are context- and
time-dependent (Orchinik, ’98), often making it
difficult for researchers to tease apart the role that
glucocorticoids play in homeostasis and survival.
873
Behavioral responses to potential stressors,
while less well documented, have become part of
the definition of the stress response (Orchinik,
’98). Evidence suggests that behavioral changes
can be taxonomic- and/or context-dependent. For
example, while an increase in locomotor activity
(measured using various techniques) coincident
with a rise in glucocorticoid concentration has
been shown in most vertebrate taxa to date
(mammals, Challet et al., ’95; birds, Astheimer
et al., ’92; Belthoff and Dufty, ’95; Breuner et al.,
’98; amphibians, Moore and Miller, ’84; and
reptiles, Cash and Holberton, ’99), data likewise
indicate that a decrease in locomotor activity
observed at other times may also be a consequence
of increasing glucocorticoid concentration (Astheimer et al., ’92; Wingfield et al., ’98). The apparent
discrepancies may be based on the individual’s
ecological and energetic constraints and the
magnitude of the increase in plasma glucocorticosteroid; moderate increases in corticosterone
may stimulate locomotor activity while extremely
high concentrations may decrease or inhibit it (see
Breuner et al., ’98; Wingfield et al., ’98). Current
research is now focusing on the ecological aspects
of behavioral and physiological responses to
environmental stressors, particularly in systems
with conservation concerns.
The freshwater turtle T. scripta experiences
dramatic predictable (seasonal) and unpredictable
(draining of aquatic habitat) changes in its
habitat. However, the endocrine basis of the
impact of these changes on survival and reproduction is little understood. In an earlier study we
demonstrated the ability of T. scripta to express an
adrenocortical response to handling similar rate
and magnitude to that found in other vertebrate
taxa (Cash et al., ’97). We also showed that this
species could increase locomotor activity in response to elevated plasma corticosterone (exogenous) concentrations (Cash and Holberton, ’99).
We wished to begin to uncover the functional
relationships between corticosterone secretion, as
a stress avoidance mechanism, and behavioral
responses to environmental change in an ecological context in a species that could benefit from a
signaling system for habitat-linked emigration
responses. Specifically, we experimentally manipulated the drying of an aquatic habitat and
measured (1) the behavioral response (emigration)
of turtles to the draining of the pond, and (2) the
plasma corticosterone concentration of individuals
who remained in or emigrated from the pond.
We predicted that if increased corticosterone
874
W.B. CASH AND R.L. HOLBERTON
concentration, as an endocrine response to environmental perturbation, facilitates locomotor activity associated with emigration behavior in freshwater turtles, those individuals captured as they
left the pond would have higher baseline corticosterone concentrations than turtles captured under
unchanging habitat conditions.
METHODS AND MATERIALS
The experiment was conducted at the University of Mississippi Field Station at Bay Springs
(UMFS), Lafayette County, Mississippi. Two
ponds of uniform size (approximately 15 12 m2)
and depth (maximum depth 5 1.5 m) immediately
adjacent to each other were completely enclosed
using aluminum flashing (51 cm width; vertically:
10–15 cm below ground, 35–40 cm above ground)
(Fig. 1). The fence was approximately 1–1.5 m
from the edge of each pond. At intervals averaging
6 m71.2SE, 14 Tomahawks Thumb livetraps
(81.3 22.9 22.9 cm) were integrated into the
fence such that the trap openings were flush with
TRAPS
FENCE
15 m
Experimental
Pond
12m
Standpipe
Standpipe
Water Line
Water Line
Control
Pond
Fig. 1. Diagram of the ponds and fence design used in the
experiment. The ponds and the fence surrounding them were
of uniform construction.
the interior of the fence enclosure (Fig. 1). On the
common stretch of fence that both ponds shared,
two double-ended Tomahawks Spring livetraps
(81.3 22.9 22.9 cm3) were placed on each side of
the fence (see Fig. 1). Plywood covers (approximately 80 40 1.5 cm3) were placed on the East
side of the traps to provide shade to the turtles in
the trap. Covers provided shade until the afternoon sun reached the tree line on the west side of
the ponds, allowing the trees to then provide
necessary shade. Grass cuttings were placed on
the sides of the trap not covered by the plywood to
provide further cover.
Turtles used in the trials were wild-caught from
two separate sites in Lafayette County, Mississippi.
Turtle hoop nets (2.54 cm nylon mesh; Memphis
Net and Twine Company, Memphis, TN) were
baited with canned fish (sardines or jack mackerel)
and set along the margins of the pond. Turtles
were collected for the experiment on two dates.
Turtles used for Trial 1 experimental pond and
the control pond were captured on June 3–4, 1999
(n 5 20: 13 male, 7 female [Trial 1: 8 male, 2
female; Control pond: 5 male, 5 female]) and for
Trial 2 on June 25, 1999 (n 5 10: 3 male, 7 female).
When animals were observed in the nets, blood
samples were obtained using the protocol similar
to that described in Cash et al. (’97). For our
study, turtles captured in the wild could have been
in nets from 4 to 8 hr before removal for sampling.
Turtles were removed from nets and an initial
baseline
corticosterone
sample
(r10 min;
100–150 mL blood, referred to as Time 0 in all
figures) was collected in heparinized micro-capillary tubes by venupuncture (23 or 26 gauge
needles) of the front limb. This initial sample
has been shown to be the best approximation of
pre-disturbance baseline corticosterone concentration (Cash et al., ’97). Each turtle was held
individually in a plastic bucket (18.9 L) until a
second blood sample was taken 30 min following
initial net disturbance. After the second sample
was obtained, each turtle was sexed based on
secondary sex characteristics, measured for total
carapace length (70.1 cm), weighed (70.1 g), and
marked by shell notching for individual identification. The reproductive status of the turtles was
unknown; however, males and females were
assumed to be post-reproductive based on known
reproductive cycles of slider turtles in Mississippi.
Blood samples were kept on ice until centrifuged
for 10 min within 2–5 hr after collection. Plasma
was then removed from each micro-capillary tube
using a 50 mL Hamiltons syringe and kept frozen
SLIDER TURTLE RESPONSE TO POND DRYING
( 51C) until assayed for corticosterone concentration by radioimmunoassay at the University of
Mississippi following the procedures of Wingfield
et al. (’92) and described in Cash and Holberton
(’99). Turtles were transported to the study ponds
at UMFS soon after blood sampling.
The experiment was performed in two separate
trials (1 and 2). Trial 1 began on June 6, 1999 and
Trial 2 began on July 10, 1999. Both treatment
ponds were drained prior to the trials (May 1) and
all attempts were made to remove resident turtles.
Ponds were then refilled (May 15) and allowed to
adjust to ambient temperatures for 10 d prior to
the initiation of Trial 1. Turtles were randomly
assigned to either a control pond or an experimental pond based on the results of a coin toss.
Turtles were released into the ponds (Trial 1: May
26; Trial 2: June 30) and allowed to acclimate for
9 d. After this period, the pond level was lowered
approximately 10 cm per day (measured at the
bottom edge of the standpipe to the water’s
surface) using a standpipe on the west side of
each pond. The depth of the ponds at each
standpipe was approximately 90 cm. Each standpipe could be turned at the bottom elbow so that
the top of the pipe could be lowered the prescribed
10 cm below the water’s surface. Ponds were
drained over a 7–8 d period in each trial.
The ponds were monitored frequently by walking slowly around the perimeter of the fence to
check traps and observe possible turtle movement.
If a turtle was observed exiting the pond or in the
Tomahawks livetraps, a blood sample (Time 0)
was immediately obtained (within 10 min) and a
second sample at 30 min. These turtles were then
removed from the experiment. The interval
between these checks ranged from 30 min to 5 hr
during daylight hours with a maximum 10 hr
interval overnight. Water temperature (1C) was
taken adjacent to the standpipe daily (15:00–17:00
CST), 15 cm below the water surface. Control
turtles were sampled using baited hoop nets on
three separate dates (Trail 1: June 12, 13, 14; Trial
2: July 16, 17, 18) during each dry-down trial and
returned to the pond after sampling.
To test the effect of capture in the Tomahawks
livetraps on an individual’s corticosterone concentration, turtles (n 5 6: 3 male, 3 female) were
captured at the conclusion of the experimental
trials (using the hoop net technique described
previously) from the control pond and immediately transferred to Tomahawks livetraps along
the fence. Turtles were allowed to remain in the
traps for a minimum of 45 min and a maximum of
875
110 min before blood sampling for baseline corticosterone. These times reflect the minimum and
maximum amount of time turtles were potentially in livetraps during the dry-down phase of
the study.
Radioimmunoassay
Steroids were extracted from plasma (50 mL
sample) with freshly distilled dichloromethane
(Fisher Scientific, Pittsburgh, PA) and treated
with 2,000 cpm of radiolabeled corticosterone
(New England Nuclear, Boston, MA) to determine
the recovery efficiency of the assay, and corticosterone antibody B21–42, corticosterone-21-succinate-bovine serum albumin (Endocrine Sciences,
Calabasas, CA) was used for the competitive
binding portion of the assay. Samples were run
as replicates. The sensitivity of the standard curve
used in the analysis was 7.8 pg, the average
recovery efficiency was 81%, and the intra-assay
coefficient of variation based on sample replicates
was 5.7%. All samples were analyzed within one
assay to eliminate inter-assay variation.
Statistical analysis
All hormone data were log10 transformed before
analysis to correct for heteroscedasticity. A twofactor repeated measures analysis of variance
(ANOVA) was used to look for changes within
individuals over time (single-factor) and to compare these changes between treatment groups
(two-factor). Changes in hormone concentration
from Time 0 to Time 30, a measure of the acute
corticosterone response to handling stress, were
analyzed using a single-factor, repeated sampling
measures ANOVA. Comparisons of baseline hormone concentration from one sampling time (i.e.,
Time 0) of the two study groups were carried out
using a two-factor ANOVA. Comparison of change
in mass from initial capture to the time turtles
were captured emigrating from the pond were
analyzed using a single-factor, repeated sampling
measures ANOVA. All statistics were analyzed
using Statview version 4.5 (Abacus Concepts,
Berkeley, CA).
RESULTS
There was no significant difference in the
Time 0 corticosterone concentration between
turtles captured for use in both trials (Pretrial
groups) prior to the initiation of the experiments
(F1,28 5 0.265, P 5 0.611, n 5 30). Therefore, the
hormone data for both Trials 1 and 2 are pooled.
876
W.B. CASH AND R.L. HOLBERTON
Pre-trial turtles had a corticosterone stress response characterized by low Time 0 concentrations
which increased significantly by Time 30 (Time
05 0.95 ng/mL70.187SE, Time 30 5 5.7 ng/mL7
0.322SE; F1,29 5 58.02, Po0.0001, Fig. 2).
There was a significant difference between
corticosterone concentration and the treatment
groups (corticosterone stress response treatment group, F1,24 5 164.4, Po0.0001, Fig. 3).
Fifteen (8 male, 7 female) of the 20 experimental
turtles emigrated from the experimental pond.
Turtles emigrating from the pond did not exhibit a
significant corticosterone stress response (F1,14 5
1.21, P 5 0.290, Fig. 3), with Time 0 corticosterone
concentration (mean 5 4.88 ng/mL70.5SE) similar
corticosterone conc. (ng/ml)
7
6
5
4
3
2
1
0
0
time (min)
30
Fig. 2. Corticosterone stress profile (ng/mL; initial blood
sample followed by a 30 min sample) for slider turtles (n 5 30)
prior to the initiation of the experiment.
corticosterone conc. (ng/ml)
7
6
control
experimental
5
4
3
2
1
0
0
time (min)
30
Fig. 3. Corticosterone profile (ng/mL) for control (n 5 10)
and experimental (n 5 15) slider turtle groups. Control turtles
exhibited a characteristic response to capture, handling, and
sampling. Slider turtles captured while emigrating had high
initial corticosterone concentrations and no significant response at 30 min.
to that of the Time 30 concentration (mean 5
5.02 ng/mL70.5SE, Fig. 3). From this group of 15
emigrants, 11 were captured in the traps while
four were captured in situ along the fence.
Comparison of the increase in corticosterone
between these two sub-groups revealed no significant effect of capture mode on the corticosterone
response (F1,13 5 2.20, P 5 0.162). It appears that
five of the 20 turtles did not emigrate from the
experimental pond, but unfortunately this cannot
be proven at this time. No sign of predation was
apparent. Attempts to locate the remaining turtles
(n 5 5) in the pond yielded only one turtle buried
approximately 5 cm below the mud surface. Turtles captured emigrating from the pond did not
show a significant change in body mass during the
experimental period (F1,14 5 1.42, P 5 0.341).
The timing of emigration was similar in each of
the two dry-down trials. In Trial 1, turtles began
emigrating only after the pond reached 30 cm
depth and below. This was concurrent with a rise
in water temperature. The first turtles (n 5 3)
emigrated when the water temperature reached
30.91C. Five more turtles emigrated over the next
36 hr at water temperatures of 32.61C and 33.11C.
In Trial 2, emigration (n 5 3) again began when
pond depth reached 30 cm and water temperature
was 30.81C. Four more turtles emigrated over the
next 48 hr at water temperatures of 32.81C and
33.41C. Mean water temperature was 28.11C7
0.7SE for Trial 1 and 29.91C70.6SE for Trial 2.
Considering both dry-down trials, all turtles
emigrated in the final 72 hr of the experiment
and only when the water temperature reached at
least 30.81C.
Turtles captured in the control pond showed a
characteristic stress response, with corticosterone
concentrations increasing significantly from Time 0
(mean 5 0.95 ng/mL70.12SE) to Time 30 (mean 5
5.4 ng/mL70.4SE; F1,10 5 126.7, Po0.0001, Fig. 3).
No control turtles were captured emigrating from
the pond during the observation period. The turtles
(n 5 6) tested for the effects of trapping on corticosterone secretion showed a significant increase in
corticosterone from Time 0 (mean 5 0.957 ng/mL7
0.009SE) to Time 30 (mean 5 2.85 ng/mL70.13SE;
F1,15 5 115.2, P 5 0.0001, Fig. 4), indicating that
restraint in the trap itself was not enough to
initiate the stress response.
DISCUSSION
This experiment tested the effects of rapid
decline in habitat quality on the physiological
SLIDER TURTLE RESPONSE TO POND DRYING
corticosterone conc. (ng/ml)
4
3
2
1
0
0
time (min)
30
Fig. 4. Plasma profiles of corticosterone secretion from
slider turtles (n 5 6) that were subjected to trap captivity for
45–110 min in livetraps. Turtles had characteristic baseline
corticosterone concentrations increasing significantly at
30 min.
and behavioral responses of the freshwater turtle,
T. scripta, by simulating the drying of the pond.
The decline in the water level in the experimental
pond resulted in the majority (75%) of the
experimental populations’ emigration, and these
emigrants had elevated corticosterone. These
results met the prediction that (1) turtles would
respond behaviorally by emigrating from the
pond, and (2) turtles would respond physiologically to a declining pond level with increased
plasma corticosterone. Earlier studies on this and
other vertebrate species suggest that corticosterone may serve as the internal proximate cue
facilitating this behavior (Breuner et al., ’98; Cash
and Holberton, ’99). In the only other known test
of the effects of simulated pond drying on freshwater turtles, Gibbons et al. (’90) similarly found
that the majority of slider turtles known to inhabit
the pond emigrated, while a small percentage
presumably stayed in the pond basin.
The movement strategy of T. scripta in the
experiment reveals possible intra-specific variation, with some individuals emigrating from the
declining habitat conditions and some individuals
remaining in the mud basin of the pond. Although
75% of the individuals from the experimental
population emigrated from the pond, one-quarter
of the turtles were believed to have remained in
the pond, although we cannot know this with
confidence. Only one of the five individuals was
found buried in the mud, and as there were no
signs of predation after searching up to a 100 m
radius around the pond, it was presumed that the
877
remaining four turtles were buried similarly. The
deep, soft sediment layer made it exceedingly
difficult to search the pond entirely. Based on the
limited data available, some freshwater turtles
have evolved two straightforward strategies for
dealing with fluctuating habitat conditions: emigrate or remain in a quiescent state (reviewed by
Gibbons et al., ’90). Slider turtles may possess a
behavioral strategy that allows them to respond to
environmental perturbations in a condition-dependent manner, perhaps based on their energy
reserves. The condition-dependent variables could
be related to some physiological measure like
energetic condition (e.g., fat stores) or to the
spatio-temporal dynamics of their habitat (e.g.,
geophysical characteristics of the surrounding
ecosystem). More detailed tests of the physiology
and behavior of T. scripta would be required to
further our knowledge of such strategies.
Studies have documented that the dynamic
movement characteristics of T. scripta and other
turtle species may be related to stress avoidance.
False map turtles (Graptemys pseudogeographica)
and slider turtles moved to a diversity of habitats
including agricultural fields and temporary wetlands during a study of two riverine populations
(Bodie and Semlitsch, 2000). These movements
were related to resource exploitation and drying of
habitat (Bodie and Semlitsch, 2000). Blanding’s
turtle (Emydoidea blandingii) was found to move
from relatively less vegetated ponds to ponds with
more available vegetation in northern Illinois
(Rowe and Moll, ’91). Conversely, desert tortoises
(Gopherus agassizii) respond to drought and
limited resources by significantly reducing their
activity levels, measured comprehensively as home
range size, burrow use, and distance traveled per
day (Duda et al., ’99). Dunlap (’95) found that
western fence lizards (Sceloporus occidentalis),
deprived of food and water and in poor energetic
condition, had high plasma corticosterone concentrations and increased activity. Unfortunately,
little is known of the physiological correlates of
movements like those described above. Whether
an increase in corticosterone has some relationship to these behavioral strategies requires
further investigation. Ott et al. (2000) found that
gopher tortoises (G. polyphemus) with high relative corticosterone had decreased burrow use
(interpreted as decreased activity), although the
results are speculative based on a small sample
size. The physiological and behavioral responses
associated with corticosterone and the slider turtle
in our study may be appropriately termed a stress
878
W.B. CASH AND R.L. HOLBERTON
avoidance response (see Wingfield and Kitaysky,
2002).
Another important finding was that turtles
captured in situ along the fence showed no
difference in their corticosterone concentration
from those captured in livetraps. Likewise, the
test of the effects of capture in the livetraps
revealed that the method did not cause a rise in
baseline corticosterone. These results are similar
to those obtained for gopher tortoise (G. polyphemus) subjected to similar tests of trap effects
(Ott et al., 2000). What stimulated the increase in
plasma corticosterone in those turtles emigrating
is unknown. Emigrating slider turtles did not lose
a significant amount of body mass during the
trials, so presumably a marked change in energetic condition was not responsible for the
behavior. The simulated dry-down of the aquatic
habitat was rapid when compared with most cases
of dry-down under natural conditions. However,
the physical changes in the pond habitat (e.g.,
decreasing pond level and increasing water temperature) are very similar to a natural dry-down
caused by drought conditions, simply temporally
condensed. The physical, and hence the physiological, cues should still be relevant, like increasing
water temperature as the habitat decreases in
depth. The increasing temperature may be the
proximate external cue that elicits the physiological and behavioral response. The timing of
emigration and similarity of temperature at the
onset of emigration suggests that increasing
temperature was in fact a significant contributor
to turtles leaving the pond. Further tests under
controlled laboratory conditions of increasing
temperature and its effect on both hormone
concentrations and behavior are warranted.
The importance of integrating the behavioral
ecology of organisms with landscape ecology has
been recognized (Lima and Zollner, ’96). Further
study of the behavioral and physiological ecology
of slider turtles may also help understand adaptations that have allowed this turtle to successfully
inhabit a wide variety of habitats over a large
geographic range. Slider turtles have experienced
a long evolutionary history (in terms of geographic
range and habitat types occupied) throughout the
Southeastern US, and as an introduced species in
other regions (Silva and Blasco, ’95). Predictions
about how individuals will respond to a stressor
(natural or anthropogenic) should be important
considerations when management decisions are
made. Understanding how individuals respond
behaviorally and physiologically during broad-
scale ecological perturbations must be taken into
account when considering aspects of conservation
biology for slider turtles and other organisms.
ACKNOWLEDGMENTS
Many thanks are due to Marjorie Holland,
Mark Baker, David Mathis, and the entire staff
at University of Mississippi Field Station at Bay
Springs for invaluable logistical support. Thanks
to USDA National Sedimentation Laboratory for
the use of equipment. The Ralph Powe Research
Award from the University of Mississippi
Field Station at Bay Springs and NSFIBN grant
9873852 to RLH supported this work in part. Tibor
Mikuska provided much needed assistance with
the fence installation. I thank R. Brent Thomas
and the 2002 Maryville College Herpetology class
for helpful comments on the manuscript.
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