Biological Journal of the Linnean Society, 2015, 116, 847–855. With 4 figures.
Do trade-offs between predation pressures on females
versus nests drive nest-site choice in painted turtles?
JEANINE M. REFSNIDER1*,†, AARON M. REEDY2, DANIEL A. WARNER3 and
FREDRIC J. JANZEN1
1
Department of Ecology, Evolution and Organismal Biology, Iowa State University, 251 Bessey Hall,
Ames, IA, 50010-1020, USA
2
Department of Biology, University of Virginia, PO Box 400328, Charlottesville, VA, 22904, USA
3
Department of Biological Sciences, Auburn University, 101 Rouse Life Sciences Building, Auburn,
AL, 36849, USA
†
Current address: Department of Environmental Sciences, University of Toledo, Wolfe Hall Ste. 1235,
2801 West Bancroft St., Mail Stop 604, Toledo, OH, 43606-3390, USA
Received 13 May 2015; revised 6 July 2015; accepted for publication 7 July 2015
Predation strongly influences reproductive behaviours because reproducing individuals must balance mortality
risks to themselves and to their offspring. In many freshwater turtles, the nest predation risk decreases with
nest distance from water, whereas the predation risk to females increases farther from water. To determine
whether predation pressure influences the distance from water at which female turtles nest, we measured
predation pressure on nesting females and on nests, as well as the distances of nests to water, in two populations
of painted turtles. Using models, we found that female survival in both populations was high and did not vary
with distance from water. Nest survival was also uncorrelated with nest distance to water, although it was
significantly lower than adult survival in both populations and was only 1.2% in one population. Our results
suggest that nest sites are not predictably safe from predators. Instead, turtles may hedge their bets by nesting
over a wide range of distances from water because any distance is risky for nests and no distance is particularly
risky for the nesting female. We suggest that other factors, such as suitable incubation conditions and/or postemergence hatchling survival, probably play a larger role than predation in driving nest-site choice in painted
turtles. © 2015 The Linnean Society of London, Biological Journal of the Linnean Society, 2015, 116, 847–855.
ADDITIONAL KEYWORDS: Chrysemys
picta – Illinois – Minnesota – nesting – predators – reproduc-
tion – reptiles.
INTRODUCTION
Predation can shape individuals’ decisions (Lima &
Dill, 1990), affect population dynamics (Krebs et al.,
1995; Korpim€
aki & Norrdahl, 1998), and drive evolution (Reznick, Bryga & Endler, 1990; Gorzelak, Salamon & Baumiller, 2012). Vulnerable life stages, such
as gravid females or inexperienced juveniles, may
experience particularly strong predation pressure
(Shine, 1980; Kullberg & Lind, 2002). The vulnerability of both juveniles and reproducing females to
predation pressure often leads to trade-offs in which
females must choose between minimizing predation
*Corresponding author. E-mail: jeanine.refsnider@utoledo.edu
risk to themselves or to their offspring (insects:
Courtney, 1981; Scheirs, De Bruyn & Verhagen,
2000; reptiles: Brodie, 1989; birds: Ghalambor &
Martin, 2001) or to minimizing predation risk on different stages of offspring development, such as eggs
vs. juveniles (insects: P€
oykk€
o, 2006; reptiles: Kolbe &
Janzen, 2001; birds: Streby et al., 2014).
In reptiles, females of many species do not care for
their eggs after oviposition but, instead, rely on the
micro-environment of the nest to provide adequate
incubation conditions for developing embryos.
Females must therefore choose nest sites that
achieve specific incubation regimes maximizing offspring survival, position newly-emerged juveniles in
close proximity to suitable habitat, and, in some
© 2015 The Linnean Society of London, Biological Journal of the Linnean Society, 2015, 116, 847–855
847
848
J. M. REFSNIDER ET AL.
cases, produce specific phenotypes (Refsnider & Janzen, 2010). In species that are primarily aquatic but
nest on land, the distance of a nest from water has
important implications for the micro-environment
experienced by developing embryos. Leatherback
turtles, for example, address the opposing pressures
of beach erosion risk close to the shoreline vs.
hatchling disorientation risk far from the shoreline
by nesting at intermediate distances from the shoreline (Kamel & Mrosovsky, 2004). In painted turtles,
females from southern populations nest much closer
to the water than do females from more northern
populations, probably because the soil closer to water
has higher moisture content and thereby provides
cooler incubation conditions in a warmer climate
(Morjan, 2003; Refsnider et al., 2014).
Importantly, locations that are suitable for nesting
may present direct costs to females, which can include
heat stress (Angilletta, Sears & Pringle, 2009) or an
increased risk of predation (Spencer, 2002; Spencer &
Thompson, 2003). Aquatic species that nest on land,
such as many turtles, exemplify the trade-off between
minimizing risk to the nesting female at the same
time as optimizing nest-site choice for the offspring
(the ‘selfish mother hypothesis’; Schwarzkopf &
Andrews, 2012). Predation on turtle nests often
increases with proximity to a wetland edge (Kolbe &
Janzen, 2002; Spencer, 2002; Strickland, Colbert &
Janzen, 2010), possibly as a result of the linear preysearching behaviour exhibited by many terrestrial
predators (Temple, 1987), which might be expected to
drive females to nest farther from water. However, if
predation risk to nesting females increases with distance from a wetland edge, females may favour their
own survival by nesting closer to the shoreline at the
expense of increased predation risk to the nest, as
seen in the Murray River turtle (Spencer, 2002). Moreover, in painted turtles, older females (i.e. those with
fewer future chances at successful reproduction) may
accept a greater predation risk by nesting farther from
water than younger females who have more future
opportunities to reproduce (Harms et al., 2005).
In the present study, we investigated whether predation pressure on either nests or nesting females
influences nest-site choice, in terms of nest distance
to water, in painted turtles. We placed models (sensu
Brodie, 1993) of nesting female turtles at different
distances from wetlands during the nesting season
in two populations, and measured the number of
predator attacks on the models to estimate predation
pressure on nesting females in each population. We
also measured shortest distances of naturally-constructed nests from wetlands, and whether nests
were depredated, in each population. We expected
that nest distance to water would predict survival of
either nests or nesting females, which would indicate
that predation pressure on nests or nesting females
represents a selective pressure that could influence
nest-site choice in painted turtles. Alternatively, if
nest distance to water does not predict nest or
female survival, then nest predation per se may not
be a strong driver of nest-site choice and, instead,
factors such as suitability of nest incubation conditions for embryonic development or survival of
emerging hatchlings may be more important.
MATERIAL AND METHODS
S TUDY
SITE AND SPECIES
We studied the western painted turtle, Chrysemys
picta bellii, a common freshwater species that occurs
primarily west of the Mississippi River from New
Mexico and Arizona (Lovich et al., 2014) to southern
Canada. Painted turtles live in a wide variety of
aquatic habitats, from which females emerge in May
and June to nest in sunny, open areas. When on
land, adult painted turtles, including nesting
females, are vulnerable to mammalian predators,
particularly raccoons (Procyon lotor), coyotes (Canis
latrans), red foxes (Vulpes vulpes), and river otters
(Lontra canadensis; Ernst & Lovich, 2009). Turtle
nests are depredated by a wide variety of predators,
with raccoons, striped skunks (Mephitis mephitis),
coyotes, thirteen-lined ground squirrels (Ictidomys
tridecemlineatus), and western hognose snakes (Heterodon nasicus) acting as primary nest predators in
many populations (Ernst & Lovich, 2009).
The present study was conducted within known
nesting areas at two localities during the nesting
season. In 2011, we collected data on nest and female
predation pressure at Thomson Causeway Recreation
Area, Carroll County, Illinois, located in the eastcentral portion of the subspecies’ range. In 2012, we
repeated the study at a northern site, Tamarac
National Wildlife Refuge, Becker County, Minnesota.
The Illinois study site is the location of long-term
research on western painted turtle nesting ecology
(Schwanz et al., 2009; Warner, Jorgensen & Janzen,
2010), including previous studies on nest predation
(Kolbe & Janzen, 2002; Morjan, 2003; Bowen & Janzen, 2008; Strickland & Janzen, 2010). At this site,
nest predation is highest near habitat edges (Strickland et al., 2010), particularly wetland edges (Kolbe
& Janzen, 2002; Spencer, 2002).
F EMALE
PREDATION PRESSURE EXPERIMENT
We used models to measure predator ‘attacks’ on
nesting female painted turtles (sensu Brodie, 1993).
Models were constructed by covering the carapace of
empty shells of painted turtles with modelling clay
© 2015 The Linnean Society of London, Biological Journal of the Linnean Society, 2015, 116, 847–855
PREDATION PRESSURES AND NEST-SITE CHOICE
(Sargent Art) that appeared to be colour-matched to
a typical painted turtle carapace. Our use of actual
turtle shells ensured that models were the proper
size and shape to attract predators of turtles. Turtle
shells were collected in spring 2004 from the periphery of a marsh in Minnesota after a severe winterkill
of painted turtles, were dry, and did not retain scent
discernible to human observers. Modelling clay was
applied evenly, approximately 3 mm thick, over the
entire carapace and all margins of the empty shells.
Marks left by predators attempting to bite the models were preserved in the clay as a record of the
attack, and were used to determine the identity of
predators. The clay was unaffected by rain or heavy
dew, did not harden with exposure to heat or cold,
and could be molded and re-smoothed after each use
of an individual model. Five models of a range of
sizes were used at each study site.
We used a total of ten models, varying in size from
small to large adults. The same models were used at
both study sites, although the placement of individual models was randomized each evening. Models
were placed in the late afternoon, which is when the
majority of nesting by painted turtles occurs (Ernst
& Lovich, 2009), and were removed the following
morning. We set models flat on the ground in sparsely-vegetated areas so they were visible from
approximately 5 m away. The position and locations
at which we placed models in each population closely
resembled a painted turtle constructing a nest at
that study site. Models were placed at 10-m intervals
from the edge of a wetland starting at 0 m (i.e.
immediately at the water’s edge). At the Illinois site,
models were placed at 10, 20, 30 and 40 m from the
wetland; high water levels in 2011 meant that 40 m
was the maximum possible distance a model could be
placed from a wetland or other flooded area. In Minnesota, where wetlands were more isolated from one
another, we extended model placement out to 90 m,
at 10-m intervals, from a wetland edge. We placed
the models in different parts of the nesting area each
evening to avoid acclimatizing predators to the location of models. Each morning after model placement,
we scored models as ‘attacked’ if a predator’s bite
imprints were left in the clay or ‘not attacked’ if no
evidence of a predator attack was visible. We took
photographs of every model that showed evidence of
a predator attack. We also measured all incisor
imprints (individual tooth width and distance
between incisors) for subsequent identification of
predators, and then smoothed the modelling clay
over the bite imprint for reuse the next evening. We
compared dimensions of tooth marks with preserved
mammal skulls in the Iowa State University mammalogy collection to identify predator species.
N EST-SITE
849
CHOICE AND NEST PREDATION RATES
We monitored nesting by painted turtles during May
to June at each study site (2011 in Illinois and 2012
in Minnesota). Details of nesting observations are
provided in Schwanz et al. (2010). Briefly, nesting
females were observed from a distance to prevent
disturbance. After the completion of the nesting season, nest locations were plotted in ARCVIEW (ESRI)
using Cartesian coordinates established for each nest
using INTERPNT (Boose, Boose & Lezberg, 1998).
We used these location data to measure the distance
of each nest to the water’s edge (to the nearest
metre). At the Illinois site, nests were checked every
3 days for signs of predation, which generally
included desiccated eggshells around an excavated
hole larger and wider at the top than the hole excavated by the nesting female. In Minnesota, we covered most nests with predator-proof wire mesh but
left a randomly-selected subset uncovered to estimate predation rates at that site. These uncovered
nests were also checked every 3 days for signs of predation.
S TATISTICAL
ANALYSIS
All statistical analyses were conducted using SAS,
version 9.3 (SAS Institute, Cary, NC). We compared
the population mean distances from water at which
female turtles constructed nests using a t-test, and
we used chi-square tests of association to compare
nest predation rates and female predation rates
between the two populations. We modelled the survival of nesting females and nests in each population
using logistic regression, with distance to water as
the independent predictor variable. The response
variable in both models was survival, with depredated females (i.e. models receiving attacks by predators) and depredated nests coded as 0, and surviving
females (i.e. models not attacked by predators) and
surviving nests coded as 1.
Next, to quantify the strength of selection on nestsite choice, we calculated separate selection gradients
on nest distance to water for maximizing female survival and for maximizing nest survival. We first calculated absolute fitness (i.e. survival) of each female or
nest, as well as the probability of survival of each
female or nest (i.e. number of surviving females or
nests divided by the total number of females or nests).
We then used logistic regression (Janzen & Stern,
1998) to estimate the selection gradient on nest distance to water for separate models of female or nest
survival. For each population, we regressed absolute
fitness of females or nests against nest distance to
water, with the slopes of the regression lines representing the selection gradients (ß). Finally, for each
© 2015 The Linnean Society of London, Biological Journal of the Linnean Society, 2015, 116, 847–855
850
J. M. REFSNIDER ET AL.
population, we calculated the relative fitness for each
female or nest during the study year as absolute fitness divided by probability of survival (Lande &
Arnold, 1983). We then estimated the fitness of females
that construct nests at a given distance from water as
the product of female survival (based on survival of
female models) and nest survival (based on survival of
wild nests) at that distance in that study year. All values are reported as the mean SE, and statistical
tests were considered significant at a = 0.05.
A
RESULTS
The female predation pressure experiment was conducted for a total of 60 model-nights during 1–27 June
in Illinois and 180 model-nights during 8–26 June in
Minnesota. We observed seven predator attacks on
model females in each population, which is equivalent
to a predation frequency on nesting females of 11.7%
in Illinois and 3.9% in Minnesota (Pearson’s v2 = 5.0,
d.f. = 1, P = 0.026). All bite marks resulting from
predator attacks on models in Minnesota matched raccoon tooth dimensions (Fig. 1A). One model in Illinois
was attacked by a coyote; the other six predator
attacks were attributed to raccoons. In addition to the
bite marks left by predators, four models in Illinois
and five in Minnesota were flipped over during the
attempted predation event.
We observed 174 nests in Illinois and 50 in Minnesota; 15 of the nests in Minnesota were left unprotected to assess predation rates. All but two nests in
Illinois were depredated, with the two surviving nests
constructed at 31 and 42 m from water. Eight of the
15 unprotected nests in Minnesota were depredated,
for a nest predation frequency of 98.8% in Illinois and
53.3% in Minnesota (Pearson’s v2 = 63.1, d.f. = 1,
P < 0.0001). In one case, researchers observed a nest
in Minnesota being depredated by a thirteen-lined
ground squirrel (J. M. Refsnider, pers. observ.)
(Fig. 1B). On average, nests in Minnesota (mean
40.1 24.6 m) were constructed farther from water
than nests in Illinois (26.0 14.3 m; t = 3.86,
d.f. = 59, P = 0.0003) (Fig. 2), although this difference
could be driven by the greater availability of nest
sites located far from water in Minnesota compared to
Illinois. To prevent differences in habitat availability
from confounding results, subsequent analyses were
conducted for each population separately.
Distance from water was not a significant predictor
of survival of nesting females or nests in either population (all P-values > 0.30) (Fig. 3). Indeed, there was
very little variation in either female survival or nest
survival with respect to distance to water in either
population. Female survival was high regardless of
nest distance to water in both populations and,
B
Figure 1. Predation events. A, bite and claw marks left
by a raccoon (Procyon lotor) on a model nesting female
turtle. B, thirteen-lined ground squirrel (Ictidomys tridecemlineatus) eating eggs from a freshly constructed
painted turtle (Chrysemys picta) nest.
although nest survival rates also exhibited little variation with distance to water, overall nest survival
was only 1.2% in Illinois. Because there was so little
variation in either female or nest survival with distance to water, selection gradients on nest distance to
water were negligible (Illinois, female survival:
ß = 0.001, nest survival: ß = 0.059; Minnesota, female
survival: ß = 0.001, nest survival: ß = 0.003). Finally,
in Illinois, estimated female fitness increased with
nest distance to water, whereas, in Minnesota, the
relationship between female fitness and nest distance
to water was very weak (Fig. 4).
DISCUSSION
When different life stages experience differential predation pressures, selection may favour individuals
that optimize survival over all life stages, rather
than maximize the survival of a single life stage.
© 2015 The Linnean Society of London, Biological Journal of the Linnean Society, 2015, 116, 847–855
PREDATION PRESSURES AND NEST-SITE CHOICE
A 35
Number of nests
30
25
20
15
10
5
5
0–
6–
10
11
–1
5
16
–2
0
21
–2
5
26
–3
0
31
–3
5
36
–4
0
41
–4
5
46
–5
0
0
Distance to water (m)
9
8
7
6
5
4
3
2
1
0
0–
5
6–
10
11
–1
16 5
–2
21 0
–2
26 5
–3
31 0
–3
36 5
–4
41 0
–4
46 5
–5
51 0
–5
56 5
–6
61 0
–6
66 5
–7
71 0
–7
76 5
–8
81 0
–8
86 5
–9
0
Number of nests
B
Distance to water (m)
Figure 2. Distribution of painted turtle (Chrysemys
picta) nest distances to water in Illinois (A) and Minnesota (B) populations. Turtles in the Minnesota population (N = 50; 40.1 24.6 m) constructed nests farther
from water than turtles in the Illinois population
(N = 174; 26.0 14.3 m; t = 3.86, d.f. = 59, P = 0.0003).
Such cases are expected to result in stabilizing selection, whereby an individual’s total fitness would be
optimized, whereas particular components of fitness
(i.e. fitness of specific life stages) would fail to be
maximized (Streby et al., 2014). In oviparous animals, a female’s choice of nest site often exemplifies
the trade-off between predation pressures on different life stages because females must balance the risk
to themselves, their eggs, and their hatchlings. In
the present study, we measured predation pressure
on two populations of nesting female painted turtles
and on their nests to determine whether predation
pressure on either life stage influenced nest-site
choice in terms of nest distance to water. We found
that nest distance to water did not predict predation
risk on either females or nests in either population.
Instead, female survival was high regardless of nest
distance to water. By contrast, nest survival was significantly lower, regardless of nest distance to water;
notably, nest survival was only 1.2% in the Illinois
population.
High survival of nesting females in both populations, regardless of nest distance to water, suggests
that predation pressure on nesting females was not a
strong driver of nest-site choice in the present study.
851
Instead, females may be at equal risk of predation
over the range of nest-to-water distances tested in
the present study. The Illinois population had higher
rates of predator attacks on nesting females than the
Minnesota population, which may be a result of differences in habitat characteristics affecting the
predator community. The Illinois study site is a
heavily-used campground, where predators such as
raccoons and skunks may be attracted to human food
left at campsites, dumpsters, and fish-cleaning stations (Strickland & Janzen, 2010). Such anthropogenic attractants may increase the density of
predators at the Illinois study site and, when combined with the high detectability of nesting turtles
(as a result of regular mowing and removal of
shrubby vegetation) and the unusually high density
of nesting turtles, probably contribute to a higher
predation rate of nesting turtles in Illinois. Indeed,
at the Illinois study site, we have observed female
painted turtles killed by predators when constructing
nests (F. J. Janzen and D. A. Warner, pers. observ.).
By contrast, the Minnesota site receives very little
public use and therefore has fewer anthropogenic
influences to attract large numbers of predators, the
painted turtle population nests at much lower density, and the areas used for nesting are in more natural habitat where long grass and other vegetation
more effectively conceal nesting females. It is possible that our experimental design underestimated
predation pressure on female turtles because we
were only able to measure predator attacks that
involved bites on the models’ shells. For example, at
the Illinois site, predation of nesting turtles has occasionally been observed where females are decapitated by a predator but sustain no damage to their
shells (F.J. Janzen and D. A. Warner, pers. observ.).
However, it is also possible that nesting females
could survive a predator attack if they are bitten on
the shell but do not sustain injury to soft tissue, in
which case we may have overestimated mortality on
nesting females as a result of predation.
In both populations, nest predation rates were considerably higher than female predation rates. Indeed,
long-lived species such as turtles rely on high adult
survival to outweigh low or unpredictable survival of
juvenile stages and facilitate population persistence
(Congdon et al., 2001). In 2011, nest predation rates
in the Illinois population were almost 100%. Even
taking into account the wide inter-annual variation
in nest predation rates in the Illinois population (0–
95%, mean 55%; Schwanz et al., 2010), the nest predation rate observed during the present study was
uncharacteristically high. One explanation for this
unusually high frequency of nest predation is that,
in 2011, the campground in which the nesting beach
was located was closed because of damage from a
© 2015 The Linnean Society of London, Biological Journal of the Linnean Society, 2015, 116, 847–855
852
J. M. REFSNIDER ET AL.
Population-specific relative fitness
Figure 3. Survival of painted turtle (Chrysemys picta) nests (A, C) and nesting females (B, D) regressed on nest distance from water in Illinois (A, B) and Minnesota (C, D). Distance from water was not a significant predictor of survival
of nests or nesting females in either population (all P-values > 0.30; 95% confidence intervals shown).
4
Illinois
Minnesota
3.5
3
2.5
2
1.5
1
0.5
0
1
11
21
31
41
51
61
71
81
Distance from water (m)
Figure 4. Population-specific relative fitness of female
painted turtles (Chrysemys picta) nesting at varying distances to water in Illinois and Minnesota.
spring flood. The lack of human activity during the
2011 nesting season may have attracted more nest
predators than are present in other years. Nevertheless, nest predation in the present study was not predicted by the distance to water in either population,
which suggests that no nest site is predictably safe
from predation. Therefore, turtles may be hedging
their bets by nesting over a wide range of distances
from water (Fig. 2) because there is no particular
distance that has a consistently lower probability of
predation at which to concentrate nesting. Previous
research at the Illinois study site also shows that
distance to water does not always predict a nest’s
predation risk. In some years, nest survival
increased with distance to water, although this relationship did not hold in all years (Kolbe & Janzen,
2002). Interestingly, Illinois nests were less likely to
be depredated if they were constructed near anthropogenic structures, regardless of distance to water
(Strickland & Janzen, 2010). If nest survival is only
correlated with distance to water in some years, then
selection on females to construct nests at particular
distances is likely relatively weak, as we observed in
the present study.
Our results suggest that painted turtles do not
select nest sites based on perceived predation risk
© 2015 The Linnean Society of London, Biological Journal of the Linnean Society, 2015, 116, 847–855
PREDATION PRESSURES AND NEST-SITE CHOICE
but, instead, may base nest-site choice primarily on
other factors. For example, a nest site’s future incubation regime is a critical consideration for nesting
females because incubation conditions affect offspring survival (Kolbe & Janzen, 2001; Warner
et al., 2010), hatchling condition and performance
(Refsnider, 2013; Refsnider et al., 2013), and sex
ratio in this species (Janzen, 1994; Mitchell, Warner
& Janzen, 2013). Nest-site choice may also be based
on survival of an additional life stage: the hatchling,
which must travel from the nest site to suitable habitat for development (Refsnider & Janzen, 2010).
Indeed, nest-site choice affects the direction that
emerging hatchlings travel to suitable habitat in the
study species (Warner & Mitchell, 2013). An experimental design similar to the one used in the present
study, but which also measures hatchling survival as
a function of nest distance to water (sensu Paitz
et al., 2007), would clarify the extent to which survival of the hatchling stage influences nest-site
choice in turtles, particularly because the survival of
the juvenile stage is known to influence nest-site
choice in other taxa (Resetarits & Wilbur, 1989;
Charteris, Allibone & Death, 2003; P€oykk€o, 2006;
Streby et al., 2014).
In both study populations, on average, turtles
nested at intermediate distances from water, which
were similar to nest distances measured in the Illinois population in other years (Harms et al., 2005).
Although this result supports our prediction that
females would nest at intermediate distance to
water, the reason may differ from what we initially
hypothesized. The lack of a relationship between
either female survival or nest survival and nest distance to water suggests that females cannot always
predict the safety of a particular nest site, either in
terms of their own predation risk or that of their
nest. In Emydura macquarii, nesting females
adjusted the distance from water at which they constructed nests in response to an increase in the
predator population (Spencer, 2002). Birds can also
adjust their nesting behaviour in response to perceived predation risk (Eggers et al., 2006; Fontaine
& Martin, 2006). An important avenue for future
research would be to monitor the complement and
relative abundance of the predator communities at
our study sites during the turtle nesting season to
determine whether other turtles also adjust their
nesting behaviour in response to actual predation
risk. If so, management of predator populations, particularly where they are artificially increased by
anthropogenic activity (Hoffman & Gottschang, 1977;
Congdon, Dunham & van Loben Sels, 1993; Gerht,
Huber & Ellis, 2002), may be an important conservation strategy for declining freshwater turtle populations.
853
ACKNOWLEDGEMENTS
This study was funded by the William Clark Graduate Student Award in Ecology and Evolutionary Biology (J.M.R.) and DEB-064932 (F.J.J.). We thank the
2011 Turtle Camp Research Crew, particularly members of the Turtle Camp Research and Education in
Ecology team, for dedicated data collection and the
US Army Corps of Engineers for access to the Illinois
site; W. Brininger, N. Powers, and H. Streby for
access to and accommodation at the Minnesota site;
and two anonymous reviewers for helpful comments
on earlier drafts of the manuscript. Research was
conducted in accordance with Institutional Animal
Care and Use Committee protocols #12-03-5570-J
and 6-08-6583-J (Iowa State University), Scientific
Research Permits NH10.0073 (Illinois Department of
Natural Resources) and 17839 (Minnesota Department of Natural Resources), and Special Use Permits
32576-OA022 and 32560-12-025 (US Fish and Wildlife Service).
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