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Incubation temperature in the wild influences
hatchling phenotype of two freshwater turtle species
Julia L. Riley1*, Steven Freedberg2 and Jacqueline D. Litzgus1
1Department of Biology, Laurentian University, Sudbury, Ontario, Canada and
2Biology Department, St. Olaf College, Northfield, Minnesota, USA
ABSTRACT
Background: The nest environment influences phenotypic traits of hatchling turtles. Female
turtles select nest sites that promote hatchling survival, and alter nesting behaviour in response
to changing environments. Differences in phenotype generated by incubation environment
could provide variation in traits that natural selection can act upon. The relationship between
incubation temperature in the laboratory and post-hatching phenotype is well documented, but
whether incubation in nature generates biologically meaningful levels of phenotypic variation is
less well studied.
Questions: (1) What are the effects of canopy cover, laying date, and nest depth on incubation
temperature? (2) What are the relationships between incubation temperature, egg mass,
and hatchling phenotype? (3) What are the sex-specific effects of incubation temperature on
phenotypic variation in two turtles with temperature-dependent sex determination?
Organisms: Painted turtle (Chrysemys picta) and snapping turtle (Chelydra serpentina) nests
and hatchlings from Algonquin Park, Ontario, Canada.
Methods: In 2010 and 2011, we measured canopy cover at nests and hourly temperatures
within nests throughout incubation. Post-parturition, we measured egg mass of each clutch.
After emergence, we measured hatchling righting response (time taken to flip from carapace to
plastron), carapace length, and mass.
Conclusions: Canopy cover and oviposition date did not affect nest temperature, but nest
depth influenced daily temperature variance in snapping turtle nests. However, limited variation
in environmental characteristics suggests that a female’s ability to select microhabitats that
adaptively affect offspring survivorship or phenotype is limited. Female painted turtles with
heavier eggs selected nest sites that were warmer. Nest incubation temperature was related to
multiple hatchling characteristics. Painted turtle hatchling carapace length was positively
related to mean incubation temperature, but snapping turtle hatchling size was not related
to incubation temperature. Painted turtle hatchling righting response was not related to
incubation temperature, but snapping turtle hatchlings from warmer nests righted themselves
more quickly and hatchlings from nests with greater temperature variance righted more slowly.
Our predicted nest sex ratios suggested that warmer nests with heavier eggs would produce
female hatchlings. Also, in both species, carapace length was greater for hatchlings from nests
Correspondence: J.D. Litzgus, Department of Biology, Laurentian University, 935 Ramsey Lake Road, Sudbury,
Ontario P3E 2C6, Canada. e-mail: jlitzgus@laurentian.ca
*Present address: Department of Biological Sciences, Macquarie University, 209 Culloden Road, Marsfield,
NSW 2122, Australia.
Consult the copyright statement on the inside front cover for non-commercial copying policies.
Evolutionary Ecology Research, 2014, 16: 397–416
© 2014 Jacqueline D. Litzgus
predicted to produce females than from nests predicted to produce males. These findings from a
natural setting help to inform future research on the adaptive significance of temperature-
dependent sex determination.
Keywords: righting response, oviposition date, temperature-dependent sex determination,
constant temperature equivalent, temperature-dependent differential fitness hypothesis,
Charnov-Bull hypothesis, maternal condition-dependent choice hypothesis.
INTRODUCTION
In reptiles, incubation temperature influences hatchling size, body condition, frequency of
deformities, and offspring sex (Bull, 1980; McKnight and Gutzke, 1993; Diaz-Paniagua et al., 1997; Hewavisenthi
and Parmenter, 2001; Booth et al., 2004; Refsnider, 2013; Riley and Litzgus, 2013). In many turtle, lizard, and
snake species, incubation temperature also affects post-hatching performance (e.g. righting
ability, swimming speed), and performance differences can be long-lasting and reflect future
survival (Burger, 1989; Janzen, 1993a, 1995; Shine et al., 1997; Freedberg et al., 2001, 2004). More broadly,
incubation temperature is related to hatchling survival, growth rates, metabolism,
behaviours, and habitat selection (Brooks et al., 1991; Van Damme et al., 1992; O’Steen, 1998; Rhen and Lang,
1998; Du and Ji, 2003). Thus, incubation temperature determines a substantial portion of an
individual reptile’s characteristics.
Female turtles may use the environmental characteristics surrounding nests, such as can-
opy and vegetation cover, to select nest location; canopy cover may, in turn, affect nest
temperature, and consequently hatchling phenotypes (Weisrock and Janzen, 1999; Cotter and Sheil, 2014).
Additionally, female painted turtles (Chrysemys picta) show plasticity in oviposition date
and nest depth in response to environmental cues, which can affect nest temperature and
thus hatchling phenotypes (Wilson, 1998; Morjan, 2003a; Shawanz and Janzen, 2008). Female nest selection
has important implications for maximizing hatchling fitness in light of environmental
change [e.g. nest sex ratio in a changing climate (Wilson, 1998; Schwanz and Janzen, 2008)].
Studies examining the effects of incubation conditions on post-hatching fitness in reptiles
have largely been based on incubation at constant temperatures in the laboratory (Paitz et al.,
2010; Neuwald and Valenzuela, 2011). Nest temperatures fluctuate naturally throughout incubation,
and laboratory studies that incubated eggs at programmed fluctuating temperatures found
differences in hatchling developmental time, sex ratio, mass, and locomotor performance
compared with eggs incubated under constant conditions (Schwarzkopf and Brooks, 1985; Doody, 1999;
Ashmore and Janzen, 2003; Du and Ji, 2003; Booth, 2006; Mullins and Janzen, 2006; Les et al., 2007; Paitz et al., 2010).
Although temperature variation has a marked effect on the development of reptiles,
potentially other less well examined, naturally varying environmental variables may play
important roles in determining post-hatching phenotype and fitness.
In many reptiles, offspring sex is also determined by the embryonic developmental
environment, a system known as ‘environmental sex determination’ [ESD (Bull, 1980;
Schwarzkopf and Brooks, 1985; Hanson et al., 1998; Pieau et al., 1999; Valenzuela, 2004)]. Temperature-dependent
sex determination (TSD), a form of ESD, is particularly common in long-lived reptiles
(turtles, crocodilians, tuatara). Many researchers have hypothesized about the adaptive
nature of TSD because of this commonality. Perhaps the most widely studied hypothesis is
the ‘temperature-dependent differential fitness’ or ‘Charnov-Bull’ hypothesis, which asserts
that TSD is adaptive if developmental environment differentially impacts sex-specific fitness
Riley et al.398
(Charnov and Bull, 1977; Valenzuela, 2004). It predicts that TSD maximizes offspring fitness by
matching offspring sex with the incubation environment that optimizes the lifetime fitness
of that sex. Warner and Shine (2008) showed support for this hypothesis by demonstrating
that reproductive success for each sex in a short-lived lizard (Amphibolurus muricatus)
is maximized at the incubation temperature producing that sex. A less widely examined
hypothesis to explain the adaptive nature of TSD is the ‘maternal condition-dependent
choice’ hypothesis, which proposes that mothers select developmental environments to pro-
duce the sex that would benefit most from their maternal effects, favouring TSD (Roosenburg,
1996). This hypothesis predicts that mothers carrying larger eggs will preferentially lay nests
in environments that produce the sex that benefits most from greater egg mass. This model is
predicated on the assumption that larger eggs produce larger hatchlings, and one sex may
benefit more from larger hatchling size. Although larger egg (and hatchling) mass may be
disproportionately favourable to female lifetime fitness in species where females grow larger
(Roosenburg and Kelley, 1996), the evidence does not consistently show that females lay larger eggs
in female-producing environments (Roosenburg, 1996; Morjan, 2003b).
To determine if natural incubation environments generate biologically meaningful
phenotypic variation in reptiles, the effects of incubation environment on hatchling pheno-
types must be studied under natural conditions (Shine, 1999). First, we examined whether nest
incubation temperatures were affected by nest depth, canopy cover, and nest lay date for
snapping turtles (Chelydra serpentina) and painted turtles (Chrysemys picta marginata) at
their northern range limits. Second, we examined whether natural incubation temperature
had an effect on egg mass and hatchling phenotype (righting response and body size) in
these two turtle species. Third, we predicted clutch sex ratios and interpreted whether sex-
specific phenotypic differences exist in nature that could differentially impact fitness of the
sexes (in accordance with Charnov and Bull, 1977).
METHODS
Study area
The study was conducted at two sites in western Algonquin Park, Ontario, Canada. The
first study site is located at the Wildlife Research Station (45⬚35⬘N, 7 8⬚30⬘W) within the
North Madawaska watershed. Nesting sites around the Wildlife Research Station vary from
natural sand dunes beside lakes to gravel embankments along access roads and Highway 60.
The second study site, the Arowhon area (west of the Wildlife Research Station), is bisected
by a gravel railway embankment built in 1895, now decommissioned as a rail-line but used
as a public hiking trail (Mizzy Lake Trail); the sparsely vegetated gravel embankment is the
main oviposition site for both turtle species (Schwarzkopf and Brooks, 1985; Hughes, 2003).
Our study area is contained within the Algonquin-Lake Nipissing ecoregion, and is a
rugged landscape underlain by Precambrian Shield outcrops (Ontario Ministry of Natural Resources,
1998). Elevations on the west side of Algonquin Park (370–570 m above sea level) are higher
than the surrounding landscape, and this creates a colder and wetter climate (Ontario Ministry of
Natural Resources, 1998). This climate is representative of the northern range limits of both turtle
species.
Incubation temperature and hatchling phenotype in freshwater turtles 399
Field methodology
In 2010, nest site monitoring began on 20 May and ended on 20 June. In 2011, nest site
monitoring occurred between 5 June and 4 July. Nest monitoring began when females
started to congregate in aquatic habitats adjacent to nest sites, and/or terrestrial nest search-
ing behaviour was observed. Researchers monitored nest sites visually on foot. Monitoring
occurred during peak nesting for both species (Ernst and Lovich, 2009): in the morning from dawn
(c. 5 am) to about 10 am, and in the afternoon from just prior to dusk (c. 5 pm) until after
dark, so long as there was nesting activity. Monitoring of nesting areas ceased when no
nesting activity was seen for three days in succession.
Nests were excavated after females had completed oviposition. Eggs were removed and
placed in plastic bins lined with moistened vermiculite (1 :1 ratio of vermiculite to water by
weight). As eggs were removed, they were numbered using a pencil to ensure they were
returned to the nest in the same order and orientation. The depth to the top and bottom
of the nest cavity was measured to the nearest 0.1 cm using a ruler. After the eggs
were removed, nests were filled with excavated soil to reduce desiccation. Nest locations
were marked with metal stakes and flagging tape. Eggs were transported to the laboratory
at the Wildlife Research Station, where they were weighed with a digital scale to the
nearest 0.1 g (SP202, Scout Pro, Ohaus Corporation, Pine Brook, NJ). Processing and
return of eggs to the nest occurred within 24 hours of oviposition, prior to adherence of
the vitelline membrane to the inner shell surface when a ‘white spot’ forms on the top of the
egg (Yntema, 1968; Rafferty and Reina, 2012); this ensured no trauma to developing embryos (Samson
et al., 2007).
Most nests were fitted with predator-exclusion cages to prevent depredation of eggs and
hatchlings. A variety of nest cage types were used to test the effects of nest caging on
hatchling fitness for another study (Riley and Litzgus, 2013). Each nest was randomly assigned to
a nest caging treatment (above-ground or below-ground hardware cloth, uncaged control)
and cages were deployed during egg reburial. These cage types did not affect incubation
temperature in either species (Riley and Litzgus, 2013). The eggs were reburied in the original
nest cavity, at the original depths and in the original order, with a temperature data logger
in the centre of the clutch. The temperature data logger used was either a waterproofed
iButton (accuracy of ±1⬚C or ±0.5⬚C; Thermochron DS1921G, Dallas Semiconductor,
Sunnyvale, CA) or a HOBO StowAway (accuracy of ±0.2⬚C; TidbiT TBI32-05 +37,
Onset Computer Corp., Bourne, CA). Temperature readings from data loggers of different
types, and with and without waterproofing, did not differ [our data: F3,2480 =2.01, P=0.94;
Roznik and Alford (2012)]. Data loggers recorded temperature hourly. In late August 2010
and 2011, close to the date when emergence of hatchlings was anticipated, above-ground
cages were fitted to the uncaged nests to trap emerging hatchlings.
Canopy cover (%) was measured using a densiometer positioned 30 cm above the nest
(Lemmon, 1957). In 2010, once per month from oviposition to October, and on the day of
emergence, canopy cover at nests was measured. In 2011, canopy cover was measured once
a fortnight from oviposition to October, and on the day of emergence.
Daily monitoring for hatchling emergence began a few days before the predicted
emergence period: 63 days post-oviposition for snapping turtles, and 89 days post-
oviposition for painted turtles (Ernst and Lovich, 2009). The first snapping and painted turtles
both emerged on 25 August in 2010, and on 27 August and 1 September for painted turtles
and snapping turtles respectively in 2011. When hatchlings emerged from their nests, all
Riley et al.400
the hatchlings and unhatched eggs were collected and transported to the laboratory at the
Wildlife Research Station for processing. Hatchlings ceased emerging on 30 September 2010
and 28 September 2011.
Midline carapace length of each hatchling was measured to the nearest 0.01 mm using
digital calipers (3148, Traceable Digital Calipers, Control Company, Friendswood, TX).
Hatchling mass was measured to the nearest 0.1 g using a digital scale (SP202, Scout Pro,
Ohaus Corporation, Pine Brook, NJ).
After exiting the nest, hatchlings often disperse overland to their overwintering habitat
(Paterson et al., 2014; Riley et al., 2014). Efficient locomotion allows hatchlings to escape predation,
desiccation, and drowning. A turtle that cannot quickly right itself is more likely to
succumb to these mortality risks (Finkler and Claussen, 1997). Thus, hatchling performance in
righting response trials indirectly relates to future survival (Delmas et al., 2007). Our righting
response test consisted of placing each hatchling on its carapace on a flat cloth-covered
board (30 ×15 cm) and waiting for the turtle to flip onto its plastron. Two variables were
timed to the nearest 0.01 s using a digital stopwatch: (1) latency period, which is the time
from placement on the turtle’s carapace until the first righting attempt; and (2) righting
period, which is the time from first righting attempt until successful righting (Freedberg et al.,
2004; Delmas et al., 2007; Rasmussen and Litzgus, 2010; Riley and Litzgus, 2013; Riley et al., 2014). To account for any
differences in laboratory temperature during righting trials, the ambient temperature of the
room was recorded for each righting trial. Trials were recorded with a digital camera
(Photosmart R742, HewPackard Development Company, Mississauga, ON), and latency
period and righting period were scored from recordings. If a turtle did not right itself within
15 minutes, it was removed from analysis. Measurements and righting trials occurred
within 24 hours of emergence, after which hatchlings were released back at their natal nests.
Analyses
Hourly incubation temperatures were extracted from data loggers and summarized by
calculating both average daily nest temperature and average daily temperature variance
from 48 hours post-oviposition until 24 hours pre-emergence for each nest for both species.
Daily temperature variance describes the degree to which temperatures fluctuate around
the mean within 24 hours (Paitz et al., 2010; Neuwald and Valenzuela, 2011). Temperature variance has
been shown to influence hatchling development (Schwarzkopf and Brooks, 1985; Doody, 1999; Ashmore and
Janzen, 2003; Mullins and Janzen, 2006; Les et al., 2007; Paitz et al., 2010).
Percentage canopy cover was averaged across incubation for each nest. Data were then
transformed using log10 (y + 1) to ensure normality. Nest lay date was coded in annual
numeric sequence (Wilimovsky, 1990). Hatchling righting response variables (latency period and
righting period) were averaged for each clutch, and were transformed using log10 (y + 1) to
ensure normality before statistical analysis. Finally, hatchling carapace length and mass,
and egg mass were averaged for each clutch.
Multiple linear regressions were constructed to determine whether nest depth, canopy
cover or lay date affected incubation temperature (daily mean and daily variance). In
addition to the main variables, these models also included the additional factor of year.
Multiple linear regressions were also used to determine if nest temperature variables
influenced egg mass, and hatchling righting response (latency period and righting period,
tested separately), mass, and carapace length. Linear regressions used to examine whether
daily nest temperature was related to egg mass also included the factor of year. We also
Incubation temperature and hatchling phenotype in freshwater turtles 401
examined whether hatchling size (carapace length and mass) was related to egg mass, using
a linear regression that also included year. Linear regressions used to examine the relation-
ship between incubation temperature (daily mean and daily variance, tested separately) and
righting period also included the factors of laboratory temperature during each righting
trial [which is known to affect ectotherm performance (Hutchison et al., 1966)] and year. Linear
regressions used to examine the relationships between incubation temperature variables and
hatchling carapace length and mass also included year.
We were also interested in examining the potential relationship between sex-determining
temperatures and post-hatching phenotype, so we calculated the constant temperature
equivalent [CTE (Georges, 1989)] for each nest. For each nest, mean daily nest temperature
and daily range of temperatures over the middle third of incubation were calculated,
coinciding with the temperature-sensitive period of sex determination. For C. serpentina,
we used a minimal developmental temperature (T0) of 16⬚C (Freedberg et al., 2011), whereas
for Ch. picta T0 was set at 14⬚C (Les et al., 2007). The aforementioned values were used to
parameterize equation (4) from Georges (1989), which was solved via iterative methods in
Excel. Calculated CTEs were used to designate individual nests as female- or male-
producing. Specifically, nests with CTEs above the estimated species-specific pivotal
temperature for sex determination for this geographic region [Ch. picta =27.5⬚C (Schwarzkopf
and Brooks, 1985); C. serpentina =27.1⬚C (Ewert et al., 1994)] were considered ‘female-producing’,
whereas nests with CTEs below the pivotal temperature were considered ‘male-producing’.
Although female C. serpentina are also produced below a lower sex-determining threshold
of 22⬚C (Ewert et al., 1994), no nests in our study had a CTE below this threshold.
We could not identify the sex of hatchlings from their gonads because our study was
conducted at a long-term study site for painted and snapping turtles, and sacrificing cohorts
of hatchlings for gender identification was not logistically viable. The CTE accounts for the
impact of variation in development rate associated with fluctuating daily nest temperature,
and has been shown to be an accurate predictor of nest sex ratios in turtles with environ-
mental sex determination (Georges et al., 1994, reviewed in Telemeco et al., 2013). While individual nests
near the pivotal temperature may theoretically produce mixed sex ratios, mixed-sex clutches
in the wild are rare in Chrysemys and Chelydra (Janzen, 1994; Kolbe and Janzen, 2002) and very
few nests fell in the predicted transitional range in our study (see Results). Because mixed-
sex nest sex ratios can only be determined from temperature data when precise data on
the sex-determining response curve are available for that geographic region, we did not
feel that we could reasonably predict specific nest sex ratios for turtles in our population. We
thus simply presumed nests to be male- or female-producing. Student’s unequal variance
t-tests were used to determine whether the sexes differed in egg mass and hatchling
phenotypic variables [carapace length, mass, and righting response transformed using
log10 (x + 1)].
In all statistical tests, assumptions of normality and homogeneity of variance were
verified. Data were transformed to ensure normality as needed (see above for specifics). No
significant interactions were found in the models with multiple factors, so only main
effects were tested and reported. All summary data are reported as the mean ±1 standard
error. The significance level of α=0.05 was used for all statistical tests. Linear models were
constructed in R (R Development Core Team, 2012).
Riley et al.402
RESULTS
Mean painted turtle daily nest temperature was 22.81 ±0.29⬚C (min: 19.55⬚C; max:
24.70⬚C). Mean painted turtle nest temperature variance within a 24 hour period was
11.67 ±0.53⬚C (min: 6.02⬚C; max: 16.37⬚C). Mean snapping turtle daily nest temperature
was 23.21 ±0.16⬚C (min: 20.83⬚C; max: 24.94⬚C). Mean snapping turtle daily nest tempera-
ture variance was 10.19 ±0.33⬚C (min: 6.16⬚C; max: 15.30⬚C).
Environmental effects on nest temperature
Canopy cover and lay date were not correlated with nest temperatures for either species. In
painted turtles, canopy cover was not related to mean daily nest temperature (F1,17 =0.14,
P=0.71) or mean nest temperature variance (F1,17 =1.58, P=0.23). In snapping turtles,
canopy cover was not related to mean daily nest temperature (F1,43 =1.11, P=0.30) or mean
nest temperature variance (F1,43 =0.20, P=0.65). In painted turtles, nest lay date was
not related to mean daily temperature (F1,17 =0.17, P=0.69) or mean daily temperature
variance (F1,17 =0.19, P=0.67). In snapping turtles, lay date was not related to mean daily
temperature (F1,43 =0.37, P=0.55) or mean daily temperature variance (F1,43 =0.12,
P=0.73).
Nest depth was negatively related to nest temperature variance in snapping turtle nests
(F1,40 =9.76, P=0.01), in that deeper nests had less daily variation in temperature (Fig. 1),
but nest depth was not related to mean nest temperatures (F1,40 =2.22, P=0.14). Painted
turtle nest depth was not related to mean nest temperature (F1,17 =0.36, P=0.56) or nest
temperature variance (F1,17 =0.09, P=0.77).
Fig. 1. Nest depth and mean daily nest temperature variance were significantly, negatively related
in snapping turtles (Chelydra serpentina); deeper nests exhibited less temperature variance.
Incubation temperature and hatchling phenotype in freshwater turtles 403
Relationships between nest temperature, egg mass, and hatchling phenotype
Painted turtles
In painted turtles, average daily nest temperature was marginally, positively related to
egg mass (F1,17 =4.41, P=0.05; Fig. 2). Hatchling carapace length was not related to egg
mass (F1,17 =1.58, P=0.22), but hatchling mass was significantly, positively related to
egg mass (F1,17 =17.33, P<0.01).
On average, painted turtle latency period was 82.0 ±20.4 s, and righting period was
4.7 ±1.4 s. Neither latency (LP) nor righting period (RP) was related to mean daily nest
temperature (LP: F1,17 =0.09, P=0.77; RP: F1,17 =0.79, P=0.42). The righting response
variables were also not significantly related to daily nest temperature variance (LP:
F1,17 =1.92, P=0.18; RP: F1,17 =0.19, P=0.67). Latency period exhibited a non-significant,
negative trend with daily temperature variance, whereas righting period exhibited a
non-significant, positive trend.
Painted turtle carapace length was positively related to mean daily nest temperature
(F1,17 =4.81, P=0.04; Fig. 3), but no relationship was detected between carapace length
and daily nest temperature variance (F1,17 =0.08, P=0.78). Hatchling mass was not related
to mean daily nest temperature (F1,17 =0.06, P=0.81) or daily nest temperature variance
(F1,17 =0.19, P=0.67).
Snapping turtles
Average daily nest temperature over the course of incubation was not related to egg mass
(F1,43 =1.16, P=0.29). However, both hatchling carapace length (F1,43 =12.79, P<0.01)
and hatchling mass (F1,43 =17.34, P<0.01) were positively related to egg mass.
Fig. 2. Egg mass and mean daily nest temperature were significantly, positively related in painted
turtles (Chrysemys picta); nests in which females laid larger eggs experienced warmer nest
temperatures.
Riley et al.404
Snapping turtle latency period averaged 83.1 ±8.5 s, and righting period averaged
8.0 ±1.9 s. Latency period was not related to nest temperature variables (mean: F1,43 =0.75,
P=0.39, Fig. 4A; variance: F1,43 =1.01, P=0.32, Fig. 3B). Latency period exhibited a
non-significant, negative trend with mean nest temperature and a non-significant, positive
trend with nest temperature variance. Righting period was negatively related to daily
mean nest temperature (F1,43 =5.47, P=0.02, Fig. 3A) and was positively related to
daily temperature variance (F1,43 =7.75, P=0.008, Fig. 4B).
Snapping turtle carapace length was not related to mean daily nest temperature
(F1,43 =2.82, P=0.10) or daily nest temperature variance (F1,43 =0.21, P=0.65). Similarly,
hatchling mass was not related to mean daily nest temperature (F1,43 =1.16, P=0.69) or
daily nest temperature variance (F1,43 =0.12, P=0.73).
Sex determination of nests
Based on the predicted constant temperature equivalents, 32% (n=15/47) of snapping
turtle clutches were predicted to be predominately male (evolutionary-ecology.com/
data/2863Appendix.pdf, Table A1). Similarly, 30% (n=6/20) of painted turtle clutches
were predicted to be predominately male (2863Appendix.pdf, Table A2). In painted turtles,
neither latency period (t11 =–1.67, P=0.12) nor righting period (t17 =–0.47, P=0.65) was
significantly different between individuals from predominately male- and female-producing
nests (Table 1). In painted turtles, female-producing nests were characterized by heavier
eggs with an average mass of 6.76 ±0.15 g, and male-producing nests were characterized
by lighter eggs with an average mass of 5.93 ±0.20 g (t11 =3.25, P<0.01; Table 1). Painted
turtle hatchling carapace length was significantly larger for individuals from female-
producing nests than male-producing nests (t6=5.26, P=0.05; Table 1). Painted turtle
hatchling mass was not significantly different between nests predicted to be female- and
male-producing (t6=0.56, P=0.60; Table 1).
Fig. 3. Mean daily nest temperature and painted turtle (Chrysemys picta) hatchling carapace length
were positively related; colder nests produced hatchlings with smaller carapace lengths.
Incubation temperature and hatchling phenotype in freshwater turtles 405
In snapping turtles, neither latency period (t41 =−0.10, P=0.92) nor righting period
(t24 =0.39, P=0.70) was different between individuals from predominately male- and
female-producing nests. Egg mass was not related to whether a nest was predicted to
be male- or female-producing (t27 =0.69, P=0.50). Snapping turtle hatchling carapace
length was significantly larger in female-producing nests relative to male-producing
nests (t34 =3.75, P<0.05; Table 1). Hatchling mass did not differ between female- and male-
producing nests (t30 =1.86, P=0.07).
Fig. 4. The relationship between mean daily nest temperature (A) and daily temperature variance (B)
and righting response, which is divided into latency period (grey circles and dashed trend line) and
righting period (black circles and solid trend line) for snapping turtle (Chelydra serpentina) clutches.
Riley et al.406
Table 1. Mean nest temperature, righting response variables, carapace length, mass, and egg mass for male- and female-producing nests as predicted
using constant temperature equivalents (CTEs) for painted turtle (Chrysemys picta) and snapping turtle (Chelydra serpentina) hatchlings from
Algonquin Park, Ontario, Canada
Predominant sex
ratio of nests
Daily mean
temperature
(⬚C)
Daily temperature
variance (⬚C)
Latency
period (s)
Righting
period (s)
Hatchling
carapace
length (mm)
Hatchling
mass (g)
Egg
mass (g)
Painted turtle
Male (n=6) 21.59 ±0.57 11.39 ±1.52 149.7 ±56.3 3.5 ±0.6 23.92 ±1.15 4.65 ±0.43 5.93 ±0.20
Female (n=14) 23.33 ±0.82 11.82 ±0.46 53.0 ±36.9 5.2 ±2.0 26.84 ±0.18 4.89 ±0.12 6.76 ±0.15
Snapping turtle
Male (n=15) 22.57 ±0.18 9.03 ±0.53 74.1 ±10.9 9.0 ±4.3 27.84 ±0.30 9.03 ±0.28 11.77 ±0.42
Female (n=31) 23.45 ±0.20 10.80 ±0.40 88.9 ±11.1 7.7 ±1.9 29.47 ±0.33 9.68 ±0.21 12.12 ±0.28
Note: Data are displayed as mean ±standard error.
DISCUSSION
Environmental effects on nest temperature
No significant relationships were found between canopy cover and nest temperature
variables for either species. In contrast to our results, other studies with painted turtles
have found that canopy cover was negatively related to mean nest incubation temperature
(Janzen, 1995; Weisrock and Janzen, 1999; Morjan, 2003a; St. Juliana et al., 2004; Cotter and Sheil, 2014). Also, we
observed no relationships between nest lay date and temperature variables for either species.
In contrast, Cotter and Sheil (2014) anecdotally observed a trend of increasing nest tem-
perature from the first to last nest laid at their study site in Ohio. Interestingly, snapping
turtle nest depth was negatively related to daily temperature variance. In New Mexico,
cooler painted turtle nests were also deeper and located closer to standing water than warm
nests (Morjan, 2003a). Thus, nest depth appears to be responsible for some of the variation we
observed in nest temperature in snapping turtles. Our study site in Algonquin Park is at the
northern range limit of the two study species, thus our sites may be cooler than those at
more southerly latitudes (Bobyn and Brooks, 1994). At northern latitudes, the active season for
ectotherms is often shorter than it is at southern latitudes (Christiansen and Moll, 1973). In Illinois,
the relationship between canopy cover and nest temperature was absent during two cooler
summers (Janzen, 1994), which reflects our result that in the cooler, northern limit of Ch. picta’s
range, there is no correlation between these two variables. At our study site, mean nest
temperature was similar to that in nests studied by Cotter and Sheil (2014) but the range of
nest canopy cover found at our study site was much more restricted than at theirs [nest
canopy cover ranged from 0 to 54% for painted turtle nests and from 0 to 37% for snapping
turtle nests in our study, and from 5 to 90% for painted turtle nests in that of Cotter and
Sheil (2014)]. Restricted variation in canopy cover, and the restricted active season at our
study site, may limit a female’s ability to select microhabitats to affect offspring survivorship
or size, but snapping turtles may be able to alter nest depth in order to positively influence
hatchling morphometrics.
Overall, our findings indicate that there is no substantial variation in environmental
characteristics (other than snapping turtle nest depth) at our study site, and that lay date,
canopy cover, and painted turtle nest depth may not be main drivers of nest temperature
variation. This is not to say that females select nests at random. Maternal nest-site choice
is well documented in snapping and painted turtles (Kolbe and Janzen, 2002; Mitchell et al., 2013).
Nest-site choice is a behavioural maternal effect that at other study sites has been found to
alter clutch phenotype [e.g. sex ratio (St. Juliana et al., 2004; Mitchell et al., 2013). It may simply be
that lay date and canopy cover at our study site do not vary enough to impact females’
ability to select nest environment in order to significantly affect offspring survivorship, size
or sex ratio. Other factors that we did not measure (e.g. soil hydration) may have stronger
influences on hatchling phenotype in our population.
Relationships between nest temperature, egg mass, and hatchling phenotype
The temperature–size rule (TSR), thought to be a general biological law for ecotherms,
states that individuals reared in warmer temperatures mature at a smaller body size than
those reared in cooler temperatures (Atkinson, 1994). In support of the TSR, smaller painted
and snapping turtle hatchlings were produced at higher temperatures when incubated in the
laboratory and under constant temperature regimes (Gutzke et al., 1987; Brooks et al., 1991). In
Riley et al.408
our study, in which we examined naturally varying nest temperatures, carapace length of
hatchling painted turtles was positively related to mean nest temperature; this is opposite to
the expectations of the TSR. Similarly, Mitchell et al. (2015) found a positive relationship
between hatchling size and natural nest temperatures in Ch. picta nests in Iowa. Also at
odds with the predictions of the TSR, we found that neither snapping turtle carapace length
nor mass was related to nest temperature characteristics. The unexpected nature of our
results and those of Mitchell et al. (2015) exemplifies the importance of examining ecological
phenomena in natural settings. In our study, we focused on the impact that natural nest
temperatures have on hatchling characteristics, but other environmental variables may
also affect hatchling phenotype. Moisture levels in nests affect Ch. picta and C. serpentina
hatchling phenotype: wetter nests produce heavier and larger hatchlings (Brooks et al., 1991;
Finkler, 1999; Packard and Packard, 2001). From studies on Ch. picta in the laboratory, size and mass
of hatchlings are more strongly impacted by water potential than temperature within nests
(Packard et al., 1989; Cagle et al., 1993). Thus, to truly understand how natural nests impact hatchling
characteristics, multiple environmental factors must be considered. Incubation temperature
exhibits a different relationship with hatchling size in the laboratory versus in the field, and
this difference points to a potentially important role of thermal variance on developmental
physiology.
Temperature characteristics of painted turtle nests were not significantly related to
hatchling locomotor performance. In another field study, Cotter and Sheil (2014) similarly
found no relationship between incubation temperature and locomotor performance in
Ch. picta. In contrast, Refsnider (2013) found that in natural Ch. picta nests, increased
temperature variance resulted in increased righting response and swimming speed. Nest
temperature variance was substantially higher in our study than in that of Refsnider (2013):
more than half of the Ch. picta nests in our study experienced a temperature variance of
greater than 10⬚C, while none of the nests in Refsnider’s study did. It is likely that very high
temperature variance is associated with negative impacts on physiology, as it increases time
spent at thermal extremes that are suboptimal developmental environments (Freedberg and Wade,
2004). Interestingly, if we examine only the nests with lower variance (<12⬚C) in our study, a
negative association appears between temperature variance and righting time, although
reduced sample size precludes statistical confirmation of this trend. More research is
required to understand the relationship between temperature variance and hatchling
phenotype throughout the range of Ch. picta.
Snapping turtle latency period was not significantly related to nest temperature charac-
teristics. Conversely, righting period, the time it takes for the turtle to actively right itself,
was related to nest temperature characteristics. The righting period portion of the righting
response directly relates to motor function, and could have implications for juvenile survival
(Delmas et al., 2007). Hatchlings incubated in nests with lower mean daily temperatures had
faster righting periods. Laboratory studies that incubated eggs under constant temperatures
have also found a relationship between mean incubation temperature and locomotor
performance (Janzen, 1995; Freedberg et al., 2001, 2004; Booth et al., 2004). Janzen (1994) found that
C. serpentina hatchlings from cooler incubation temperatures swam faster than those
from warmer treatments, and C. serpentina hatchlings from nests incubated in median
temperatures ran faster on land than those incubated in extreme temperatures (Janzen, 1994);
however, because the results from different types of performance measures may be affected
by different levels of motivation to complete each task, it is difficult to directly compare
results among behavioural trials.
Incubation temperature and hatchling phenotype in freshwater turtles 409
In our study, higher nest temperature variance resulted in a slower righting period in
C. serpentina hatchlings. In contrast, laboratory studies have found that fluctuating
temperatures enhance locomotor performance (Ashmore and Janzen, 2003; Booth, 2006), and that
increased temperature variance increased mass, survival, and immune response (Du and Ji, 2003;
Les et al., 2007, 2009). These laboratory studies suggest that temperature fluctuations enhance
hatchling fitness, but the level of variance tested in these studies [±4⬚C in Ashmore and
Janzen (2003), ±3⬚C in Les et al. (2007, 2009), and ±2–3⬚C in Du and Ji (2003)] was far lower than
that we observed in most natural nests (mean of ±10–11⬚C) and presumably was not
sufficient to reach thermal extremes that are harmful to development. The high level of
variance characterizing nests of both species in our study may have caused development to
proceed at temperatures that had negative impacts on physiology.
Phenotype at hatching is affected by nest temperature, but once the hatchlings emerge
from the nest, does this phenotype remain with the individual over the long term, and does
it have fitness consequences? Survivorship of hatchling C. serpentina appears to be size-
dependent, with natural selection favouring larger hatchling size [aka the ‘bigger is better’
hypothesis (Janzen, 1993b)]. Similarly, larger Trachemys scripta elegans hatchlings released in an
area with avian predators had higher survivorship (Janzen et al., 2000; Myers et al., 2007). Support for
the ‘bigger is better’ hypothesis may be associated with the specific predator assemblage
within a geographic area, as support for this hypothesis is contested (Congdon et al., 1999; Paterson
et al., 2014). Hatchling size may benefit survival immediately after hatching during terrestrial
movements overland to overwintering sites, and selection is most likely strongest at this
life-stage due to high mortality (Ernst and Lovich, 2009; Paterson et al., 2012, 2014). In the laboratory,
incubation temperature has long-lasting effects on righting response in Graptemys ouachit-
ensis and T. scripta elegans: turtles incubated at warmer temperatures righted themselves
more quickly immediately post-hatching as well as after a year in captivity (Freedberg et al., 2004).
If hatching phenotype persists, then the differences in hatchling size and performance
ability that we observed may affect individual fitness later in life. A potential model system
to test this hypothesis would be a shorter-lived turtle (e.g. the chicken turtle, Deirochelys
reticularia).
Predicted sex ratio of nests
In snapping turtles, most viable constant incubation temperatures produce exclusively or
almost entirely (>90%) one sex, with a narrow range of approximately 1⬚C encompassing
the pivotal temperatures that produce a more even sex ratio (Ewert et al., 2005). Similarly, narrow
ranges for mixed-sex clutches are found in painted turtles (Bull, 1985). In our study, only 4 of
20 clutch CTEs fell in this 1⬚C range surrounding the pivotal temperature in Ch. picta, and
4 of 47 clutch CTEs fell in the 1⬚C range surrounding the upper pivotal temperature in
C. serpentina with no CTEs residing near the lower pivotal temperature. While within-nest
heterogeneity in temperature may create additional temperature variation, the large pro-
portion of nest CTEs that fall considerably out of the mixed-sex range (Table 1) suggests
that mixed-sex clutches were relatively rare at our site.
Based on the calculated CTEs, the sex ratio for both species was probably female-biased
(2863Appendix.pdf, Tables A1 and A2). The adult snapping turtle sex ratio is unknown
at our study site, while the sex ratio in the adult painted turtle population is 3.44 females
per male (Samson, 2003). This highly skewed sex ratio has been consistent within the adult
population for over 20 years (Samson, 2003 and unpublished data). Because females are under strong
Riley et al.410
selection to invest approximately equally in the sexes under temperature-dependent sex
determination (Bull, 1980), this skew suggests that painted turtles at our site do not select their
nests to influence the sex ratios as theorized by Janzen (1994). Natural nest temperatures that
were estimated to produce mainly males were colder and less variable for both species;
however, these sites may have been undesirable owing to factors associated with nest
construction. Although Schwanz and Janzen (2008) found that females adjust their nest
microhabitat selection to influence sex ratio of hatchlings, perhaps environmental con-
ditions limit the opportunity for the production of viable male-producing nests at our study
site. Morjan (2003b) modelled that there is likely low potential to adjust sex ratio through
nest-site selection by female painted turtles despite the presence of a significant relationship
between vegetation cover and nest sex ratio at her study site. Our failure to detect any
relationship between canopy cover and nest temperature suggests that the potential for sex
ratio adjustment may be even further limited at our site.
In both snapping and painted turtles, hatchling carapace lengths were larger for female-
producing nests than male-producing ones. The ‘temperature-dependent differential fitness’
or ‘Charnov-Bull’ hypothesis for the adaptive significance of temperature-dependent sex
determination (TSD) suggests that phenotypic differences associated with incubation at
male versus female temperatures may facilitate sex-specific differences in fitness (Rhen and Lang,
1998). Although it is logistically challenging to determine the impacts of hatchling size on
lifetime fitness in long-lived species, our findings suggest that the developmental environ-
ments producing each sex are associated with significant differences in the phenotypic traits
of offspring, which could potentially be acted upon by sex-specific selective pressures.
Spencer and Janzen (2014) suggested that sex-specific selection pressures may not only
occur later in life, but may act upon hatchlings during their first winter; overwintering
temperatures were found to affect Ch. picta male and female hatchling metabolic rates and
energy reserves differently. This suggests that sex differences in hatchling size caused by nest
temperature may immediately impact fitness of turtles. Yet, differences in size associated
with incubation temperature and egg mass were seen in 3-year-old diamondback terrapins
(Malaclemys terrapin), suggesting that mechanisms underlying size differences in turtles can
persist for several years (Roosenburg and Kelley, 1996). In snapping turtles, variation in hatchling
size is not correlated with post-hatching growth in the laboratory (Brooks et al., 1991; Bobyn and
Brooks, 1994); however, larger hatchlings are favoured in competitive interactions (Froese and
Burghardt, 1974), which suggests a positive feedback system may exist for hatchling size in the
field. Hatchling size is associated with several other functionally important traits in turtles,
including terrestrial movement (Janzen, 1993b; Tucker, 2000), competitive ability (Froese and Burghardt,
1974), swimming speed (Myers et al., 2007), and post-hatching survival (Janzen et al., 2000), providing
additional opportunities for sex-specific selection to act.
Painted turtle nests that were warmer and predicted to be female-producing were charac-
terized by a larger egg mass than those that were cooler and predicted to be male-producing.
If females select nest sites depending on the characteristics of their eggs, then this nest-site
choice may have an impact on hatchling phenotype. Similarly, Roosenburg (1996) found that
Malaclemys terrapin females selected warmer nest sites for larger eggs (but see Morjan, 2003b).
However, the mechanism a female turtle uses to assess her pre-laid egg size and how she
might use this knowledge during nesting selection is unknown, and thus more research is
required to fully interpret our findings. In M. terrapin, egg mass is correlated with juvenile
body mass in females, but not in males; this relationship is predicted to speed female
maturation by 2–3 years while having no effect on male maturation (Roosenburg and Kelley, 1996).
Incubation temperature and hatchling phenotype in freshwater turtles 411
If painted turtles similarly experience sex-specific benefits of egg size, larger eggs laid in
female-producing environments could produce sex-specific fitness consequences and could
provide support for the adaptive significance of TSD (Roosenburg, 1996). Alternatively, more
metabolic heat may be produced from larger developing embryos, and this may increase the
temperature within the nest cavity and affect TSD (Carr and Hirth, 1961; Standora and Spotila, 1985),
although this potential for heating has been regarded as too little or too late in incubation to
be significant (Maloney et al., 1990; Booth and Astill, 2001). Nevertheless, it is a potential avenue for
future research. In snapping turtles, there was no relationship between egg mass and nest
temperature. Two pivotal temperatures characterize TSD in this species, thus the relation-
ship between female nest-site choice and egg size may be more complex. Furthermore,
dimorphism in adult size shows considerable intraspecific variation in snapping turtles
[from larger males to no dimorphism (Galbraith et al., 1988)], so a clear relationship between egg
mass and nest incubation condition may not be expected.
ACKNOWLEDGEMENTS
Financial support was provided by the Natural Sciences and Engineering Research Council of
Canada (NSERC; CGS-M scholarship to J.L.R. and Discovery Grant to J.D.L.), Canadian Wildlife
Federation, Ontario Ministry of Natural Resources (OMNR), Toronto Zoo, and Laurentian
University. In-kind contributions were provided by Algonquin Provincial Park (OMNR) and the
University of Guelph. The following people assisted with fieldwork: M. Keevil, P. Moldowan, K. Hall,
H. McCurdy-Adams, and L. Monck-Whipp. Thanks to Dr. Ron Brooks for access to his long-term
site and turtles for data collection. All work was carried out under an approved Laurentian University
Animal Care Committee protocol (AUP # 2008-12-02) and was authorized by permits from OMNR.
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