Herpetologica, 58(1), 2002, 67–74
䉷 2002 by The Herpetologists’ League, Inc.
AN EXPERIMENTAL STUDY OF THE INFLUENCE OF
EMBRYONIC WATER AVAILABILITY, BODY SIZE, AND CLUTCH
ON SURVIVORSHIP OF NEONATAL RED-EARED SLIDERS,
TRACHEMYS SCRIPTA ELEGANS
NIRVANA I. FILORAMO1,2
AND
FREDRIC J. JANZEN1
Department of Zoology and Genetics, Program in Ecology and Evolutionary Biology,
Iowa State University, Ames, IA 50011-3223, USA
1
ABSTRACT. The period during which neonatal aquatic turtles migrate from their nests to the
water is a critical stage, as mortality is high. Thus, the patterns and targets of natural selection
involving the turtles are important to investigate. To evaluate these questions, we incubated eggs of
red-eared slider turtles (Trachemys scripta elegans) and overwintered the resulting hatchlings under
ecologically-relevant, common-garden conditions in the laboratory. We then performed an experimental release of 358 neonates in the field to investigate the possible effects of (1) water potential
of the substrate on which the eggs were incubated (⫺60 and ⫺100 kPa), (2) body size at the time
of release, and (3) clutch of origin on short-term posthatching survivorship. Only clutch significantly
affected survivorship in this field study. The lack of an effect of body size on survival may be due
to substantially drier weather during the migration period than in previous and subsequent experiments, which may have led to atypical burying behavior by the turtles.
Key Words: Body size; Emydidae; Hatchling; Survivorship; Trachemys scripta elegans; Turtle;
Water potential
TURTLES are long-lived organisms that
exhibit low juvenile survivorship followed
by high adult survivorship (Congdon et al.,
1994; Frazer et al., 1991a,b; Iverson, 1991;
Tucker and Moll, 1997; Wilbur, 1975).
Furthermore, the percentage of eggs that
produces viable hatchlings is low (Congdon et al., 1994; Frazer et al., 1991a). Early stages of turtle life histories are the most
prone to mortality, thus these stages are
important time frames in which to investigate the mechanisms through which
mortality selection may act.
One factor that may be of particular importance in determining survivorship of
young turtles is body size. The prevailing
view for neonatal reptiles is that bigger is
better (reviewed in Packard and Packard,
1988). However, there have been relatively
few controlled field experiments to investigate whether or not larger neonates experience higher short-term survivorship
(Congdon et al., 1999; Ferguson and Fox,
1984; Janzen, 1993; Janzen et al., 2000a,b;
Sinervo et al., 1992; Sorci and Clobert,
1999; Tucker and Paukstis, 1999; Tucker,
2000a). In each of these controlled field
experiments (with the exception of the two
smaller scale experiments in the study by
Congdon et al. (1999)), larger animals had
higher survivorship, thus supporting the
‘‘bigger is better’’ hypothesis.
An important factor determining hatchling body size in many turtles is the hydric
condition of the nest (Packard, 1999). Numerous studies on a variety of turtle species have shown that hatchlings emerging
from eggs incubated on wetter substrates
are larger than turtles emerging from eggs
incubated on drier substrates. Also, the residual yolk is correspondingly smaller in
turtles from the former environments than
in those from the latter (Packard, 1991).
Hydric environment is also known to affect other aspects of neonatal physiology
such as greater hydration of tissues and increased locomotor performance in turtles
from eggs incubated on wetter substrates
and larger neutral lipid reserves in the carcasses of turtles from eggs incubated on
drier substrates (Miller et al., 1987; Packard et al., 1988).
In addition to environmental conditions
in nests, the effects of clutch, be they ge-
2
PRESENT ADDRESS: Department of Ecology and
Evolutionary Biology, University of Connecticut,
Storrs, CT 06269-3043, USA.
67
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HERPETOLOGICA
netic or nongenetic, are increasingly recognized as crucial contributors to offspring
phenotypes and fitness in turtles (e.g., Janzen et al., 1995). Statistically, clutch effects
can be the primary contributors to the variance in offspring phenotypes in experimental laboratory studies of turtles (e.g.,
Packard and Packard, 1993). The impact
and biological significance of these maternal effects in natural populations of most
organisms remain to be explored in depth
(Mousseau and Fox, 1998).
We examined survivorship of neonates
in a population of red-eared slider turtles,
Trachemys scripta elegans, from west central Illinois. Trachemys scripta exhibits the
typical turtle pattern of high mortality in
the early stages of life and high adult survivorship (Frazer et al., 1991b; Tucker and
Moll, 1997). The beginning of life in T. s.
elegans can be broken down into four stages: (1) the period of embryonic development within the egg; (2) the fall and winter
months following hatching when the neonates remain in the nest; (3) the migration
from the nest to the water upon emerging
in late spring; and (4) the early years in
the aquatic environment. The objective of
this study was to tease out the factors influencing survivorship during the third
stage: migration of young red-eared slider
turtles from nest to water. Is it strictly
body size, or does the hydric environment
experienced during incubation affect posthatching survivorship in ways other than
through its typical influence on body size?
Furthermore, is there any clutch effect on
survivorship during posthatching migration from nest to water?
MATERIALS
AND
METHODS
Egg Collection
Fifty-eight female red-eared slider turtles were collected, from 2–7 June 1996
along the Illinois River in Jersey and Calhoun counties, Illinois during their nesting
forays (see Tucker, 1997, for a detailed description of the site). They were induced
to oviposit by injection of oxytocin within
one day of collection (Tucker et al., 1995).
Eggs laid by the turtles were patted dry,
weighed to the nearest 0.01 g, and then
[Vol. 58, No. 1
individually marked with a felt tip pen.
The eggs were stored in a substrate with a
0.9/1.0 g ratio of water to vermiculite in
Styrofoam boxes at ⬃19 C until they were
transported to Ames, Iowa, on 13 June.
Forty-two clutches (530 eggs) were used
in this experiment; the remainder were
employed in a companion laboratory experiment on posthatching metabolism of
residual yolk (Filoramo and Janzen, 1999).
Experimental Design
On 14 June (⫽day 0), 416 eggs were
placed in experimental boxes (n ⫽ 26 eggs
each) in the two different hydric treatments. The remaining 114 eggs were
placed in four extra boxes (n ⫽ 28 or 29
eggs each), which were not used in this
study. There were thus 16 experimental
boxes, eight ‘‘wet’’ [⫺60 kPa, 2.17 g H2O/
1 g vermiculite (Stronglite brand)] and
eight ‘‘dry’’ [⫺100 kPa, 0.84 g H2O/1 g vermiculite (Stronglite brand)] split evenly
between two incubators maintained at
28.5 C. These relatively wet water potentials, as determined by a Wescor dewpoint
hygrometer, were chosen to reflect water
potentials likely to occur most frequently
in nests of shallow-nesting turtles (sensu
Ratterman and Ackerman, 1989). The
same group of clutches was represented in
four wet and four dry boxes (two from
each treatment in each incubator). In this
manner, eggs from each clutch were represented in both treatments and both incubators. For females with clutch sizes
ⱖ16, two eggs from that clutch were
placed into each of the eight appropriate
boxes. For females with clutch sizes ⬍16
eggs, only one egg was represented in each
of the eight appropriate boxes. Eggs from
each clutch were randomly assigned to the
boxes, and were half buried in vermiculite;
this procedure is consistent with other
studies of this type (Filoramo and Janzen,
1999; Packard et al., 1988; Tucker et al.,
1998). The incubation substrate in the
boxes was rehydrated once weekly, and the
boxes were rotated within the incubators
regularly to eliminate the potential impact
of thermal gradients (Filoramo and Janzen, 1999).
March 2002]
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69
Data Collection
Bottomless paper cups were placed
around eggs on day 53 of incubation so
that newly-hatched individuals could be
identified (Janzen, 1993) during twice daily checks for pipped or hatched turtles
(Gutzke et al., 1984). Newly-hatched turtles were brushed free of adhering vermiculite and weighed to the nearest 0.01
g, and carapace length and width were
measured to the nearest 0.01 mm. After
all eggs had hatched, turtles were placed
into new boxes, all of which contained vermiculite at ⫺70 kPa. Turtles were then exposed to a gradual cooling phase from
September–November, to 4 C from December–February, and then to a gradual
warming phase from March–April (see Filoramo and Janzen, 1999, for details).
Temperatures used during this overwintering phase mimicked those measured in
nests for this population (Tucker and Packard, 1998; J. Tucker, unpublished). After
overwintering, the plastrons of the turtles,
which exhibit unique patterns (Tucker,
2000a), were photocopied and these
sheets were made into identification cards.
On 7 May, turtles were re-weighed and
carapace length and width were re-measured.
On 1 April, a drift fence consisting of
aluminum flashing 0.3 m high and 285 m
long was installed along the Illinois River
between the water’s edge and the sites
where the nesting females were captured
the previous year (Tucker, 2000b). Twenty
13.25-l buckets were placed in the ground
every 15 m along the fence. To coincide
with the timing of hatchling emergence
from natural nests at the field site (Tucker,
1997), turtles (n ⫽ 358) were released simultaneously at 0900 h CST on 13 May,
40 m away from the drift fence directly
perpendicular to pit 11. This release point
was chosen because the slope of the field
was such that the turtles were more likely
to head towards one end of the fence (i.e.,
pit 1). Turtles were arranged so that they
were upright and not on top of one another, but they were not oriented in any single
direction. Pit traps were checked for recaptures twice daily at 0800 h and 2000 h
FIG. 1.—The relationship between recapture date
and the mass at release of the neonatal turtles. There
was no difference in body size of the turtles recaptured earlier compared to later during the migration
period.
CST. Turtles were identified upon capture
from the plastron photocopies. After daily
recapture rates dropped considerably (Fig.
1), the upslope section of the field east of
the drift fence (⬃300 m ⫻ ⬃300 m) (see
Tucker, 1997, for a diagram) was searched
thoroughly for dead turtles on 23 May by
two investigators for 5.3 h. The drift fence
continued to be monitored for recaptures
until 3 July (Tucker and Paukstis, 1999).
All recaptured turtles were released into
the Illinois River adjacent to the field site
upon completion of the experiment. These
methods differ only slightly from those of
prior studies where the drift fence and
study duration were sometimes shorter
(Janzen et al., 2000a,b; Tucker and Paukstis, 1999; Tucker, 2000a).
Statistical Analyses
All analyses were performed using JMP
3.1.5 statistical analysis software. Analysis
of covariance was used to assess potential
causes of variation in all three measures of
body size (mass, carapace length, and carapace width) recorded just prior to the experimental release. Initial egg mass was
designated as the covariate, with treatment
(⫺60 versus ⫺100 kPa) and incubator in
which the eggs/hatchlings were reared denoted as fixed effects and clutch of origin
considered to be a random effect. Logistic
regression was used to evaluate which factors had a significant impact on the prob-
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[Vol. 58, No. 1
TABLE 1.—Size of turtles just prior to experimental release as a function of incubation treatment and recapture
category. Values for incubation treatment are least squares means ⫾ one standard error (range) from analyses
of covariance and for recapture category are standard means ⫾ one standard error (range).
Incubation treatment
Trait
Mass (g)
Carapace
length (mm)
Carapace
width (mm)
Recapture category
⫺60 kPa
⫺100 kPa
Recaptured
Not found
7.16 ⫾ 0.03
(4.43–8.69)
31.78 ⫾ 0.07
(26.13–34.66)
31.20 ⫾ 0.07
(25.49–34.25)
6.99 ⫾ 0.03
(4.31–8.64)
31.50 ⫾ 0.06
(25.46–34.36)
30.92 ⫾ 0.07
(23.47–33.53)
7.10 ⫾ 0.11
(4.31–8.66)
31.66 ⫾ 0.20
(25.46–34.41)
31.00 ⫾ 0.24
(23.47–34.25)
7.13 ⫾ 0.05
(4.43–8.69)
31.72 ⫾ 0.08
(26.13–34.66)
31.12 ⫾ 0.08
(25.49–33.84)
ability of recapture (0 ⫽ no, 1 ⫽ yes). This
analysis was conducted three times, changing only the measurement of size used.
Additional predictor variables included in
the regression analyses were the same as
those used in the statistical analyses of
body size at release: treatment, clutch, and
incubator. All factors were considered to
be fixed effects in these analyses, because
treating clutch of origin as a random effect
(Stiratelli et al., 1984; H. Stern and A. H.
Jones, personal communication) did not
materially alter the conclusions.
RESULTS
A total of 383 of the 416 eggs hatched
(92%) (95% in the ‘‘dry’’ treatment and
88% in the ‘‘wet’’ treatment). Two of these
eggs contained twins, which were frozen
immediately for a future study, and three
hatchlings failed to survive overwintering.
An additional 20 turtles were unidentifiable after hibernation. These individuals
were not included in statistical analyses.
All three measures of body size at release were significantly affected by initial
egg mass, clutch, and incubation treatment
(P ⬍ 0.0004 in all nine cases); incubator
was not an important factor (P ⬎ 0.0871
in all three cases). Larger neonates derived
from larger eggs and this effect varied significantly among clutches. After accounting for these egg size and clutch effects,
heavier, longer, and wider turtles came
from the wetter (⫺60 kPa) incubation environment (Table 1).
Of 358 turtles released in the field, 63
(17.6%) were recaptured along the drift
fence. There is a possibility that some animals bypassed the fence; recaptures were
skewed towards one end of the fence (pits
1 and 2) (Fig. 2). However, no turtles were
ever found in the area adjacent to that side
of the fence even though it was searched
each morning after checking the drift
fence. Furthermore, the turtles would
have had to travel up a sharp rise in topography to circumvent the fence, as this
end of the fence was bordered by an elevated gravel road. It therefore seems unlikely that many turtles missed the fence,
but rather that they were concentrated in
pits 1 and 2 because of a funneling effect
of the elevated road. Only one dead neonate was found in the field even though
the study area was searched thoroughly
(see Materials and Methods).
No measurement of body size (carapace
length, carapace width, or mass) at release
was a significant factor in probability of recapture (e.g., P ⱖ 0.5487 in all three cases)
(Table 1). This result is also supported by
a t-test comparing the distributions of, for
example, body mass for turtles recaptured
and those not found (t ⫽ 0.236, P ⫽
0.8134) (Fig. 3). Incubation treatment (P
⫽ 0.4046) and incubator (P ⫽ 0.0666) also
did not influence survivorship in the field,
but clutch of origin did (P ⫽ 0.0092).
DISCUSSION
No factors that we considered (other
than clutch) significantly influenced survivorship of neonatal red-eared slider turtles
in this experiment. This outcome is largely
inconsistent with prior and subsequent results of similar experiments involving T.
scripta at this field site (Janzen et al.,
2000a,b, unpublished; Tucker and Paukstis, 1999; Tucker, 2000a). These authors
March 2002]
HERPETOLOGICA
71
FIG. 2.—Distribution of turtles caught along the drift fence. The pits were 15 m apart along the drift fence
and were numbered ascendingly from north to south.
found that body size of the animals affected the probability of recapture, whereas
we detected no influence of size on survivorship. Janzen et al. (2000a) first proposed that larger neonates had higher survivorship because they reached the fence
more quickly than smaller turtles. Thus
the exposure time of larger individuals to
avian predators, such as grackles (Quisca-
FIG. 3.—Frequency distributions for body masses
at release of turtles not recaptured (open bars) and
recaptured (solid bars).
lus quiscula) and red-winged blackbirds
(Agelaius phoeniceus), would be shorter.
In support of this hypothesis, Janzen et al.
(2000b) found that excluding birds from
the release site greatly increased the recapture rate from 34.9% to 72.4% and that
body size was no longer an important factor influencing survivorship in the absence
of birds. Furthermore, the authors discovered that larger turtles were recaptured
earlier than smaller ones.
May 1997 was dry compared to weather
during other studies at this site (Janzen et
al., 2000a,b; Tucker and Paukstis, 1999;
Tucker, 2000a) (Table 2), and this climatic
variation may have influenced our results.
The conditions after release were so dry
(Fig. 4) that the majority of the neonates
apparently buried themselves, thereby reducing desiccation and hindering detection by avian predators. This explanation is
supported by the fact that turtles were
found at the fence on days following an
intensive search of the release area on 23
May (see Materials and Methods for details). On that day, only one active turtle
72
HERPETOLOGICA
TABLE 2.—Rainfall (mm) for May in Jersey County,
Illinois during a 15-yr period. The star (夝) represents
the year this release was conducted. The X (^) marks
the years that the Janzen et al. (2000a,b, and unpublished) and Tucker (2000a) releases were conducted.
All data from the National Weather Service Cooperative Station located in Jerseyville, Illinois were provided by the Midwestern Regional Climate Center
located at the Illinois State Water Survey, Champaign, Illinois.
Year
1986
1987
1988
1989
1990
1991
1992
1993
1994
1995^
1996^
1997夝
1998^
1999
2000^
Rainfall for
May (mm)
170
498
323
734
2454
1394
483
1148
297
2068
1839
691
754
729
1521
was observed and another was found hidden under a rock used to mark the release
point. That turtles were subsequently captured at the fence after this search suggests that they almost certainly were under
the soil surface, and thus hidden from
view.
Another result of this hypothesized
[Vol. 58, No. 1
burying behavior was to lengthen the time
during which turtles were found along the
fence. Individuals from this study were
found at the fence ⬎1 mo after their release (see also Tucker and Paukstis, 1999).
In contrast, all live turtles in other experiments at the site were recaptured within
nine days (Janzen et al., 2000a), eight days
(Janzen et al., 2000b), and 13 days (Tucker,
2000a). Because many turtles were apparently hidden under the soil surface and
were trickling into the fence slowly, avian
predators may have minimized their active
searching for migrating turtles. Reduction
of avian predation on released turtles is
supported by the fact that we found only
two dead animals (only one of which was
from our release (0.3%)), whereas Janzen
et al. (2000a) found 43 dead ones (12.1%).
In a predator exclusion study (Janzen et
al., 2000b), only two (0.4%) dead turtles
were found (in contrast to the 64 (12.2%)
found when diurnal predators were not excluded). Also in contrast with these and
other studies (e.g., Tucker, 2000a), we
found no correlation between date of recapture and body size at release (Fig. 1),
further supporting the burying hypothesis.
If the hypothesis of reduced exposure to
avian predators as the selective force for
larger body size in young turtles is correct,
then reducing the predation pressure of
birds should remove the effect of body size
FIG. 4.—Number of turtles recaptured per day (bars) and the amount of rainfall associated with each day
(diamonds).
March 2002]
HERPETOLOGICA
on survivorship. Such a size-independent
pattern of survivorship was indeed observed in this study. Another possible explanation for the lack of a body size effect
on survivorship in this study is that turtles
not recaptured died in a size-independent
way (e.g., through desiccation or nocturnal
predators). We can, however, reasonably
exclude insufficient variation in size as an
explanation for our results. Turtles in this
study exhibited comparable variation in
size (e.g., range of mass at release was
4.31–8.69 g; Table 1) to those used in prior
experiments wherein size significantly explained the probability of recapture (e.g.,
range of mass at release was 4.25–9.16 g
(Tucker and Paukstis, 1999)).
If the hypothesized burying behavior of
neonates neutralizes the survival effect of
body size, this experiment would have
been ideally suited to investigate the effect
of nest hydric conditions during embryonic development on offspring survivorship. During relatively dry spells such as
the one encountered in this study, hydration of tissues, which is influenced by hydric conditions experienced during incubation (e.g., Packard et al., 1988), may become important for survivorship. Unfortunately, the substrate water potentials
used in this experiment were apparently
too similarly ‘‘wet’’ to elicit possible hydric
effects on survivorship during migration
from the nests. In support of this contention, Tucker and Paukstis (1999) found
that T. scripta from wetter (⫺150 kPa) incubation conditions were more likely to be
recaptured alive and less likely to be found
dead than turtles from drier (⫺200 kPa)
incubation conditions during an experimental release. Additionally, our recapture
rate (17.6%) (eggs incubated at ⫺60 and
⫺100 kPa) was higher than their recapture
rate (11.3%), further supporting the idea
that wetter may be better for developing
turtle embryos (Packard, 1999).
It remains important to determine the
mechanisms of selection under ‘‘typical’’
environmental conditions. At the same
time, patterns of natural selection during
environmental extremes like drought can
vary dramatically from the ‘‘typical’’ patterns, substantially affecting crucial eco-
73
logical and evolutionary processes (e.g.,
Grant and Grant, 1989; Maad, 2000; Sumerford et al., 2000). For example, offspring recruitment in this study was sizeindependent and half the magnitude of
that detected in experiments conducted
during wetter springs (17.6% versus 34.9%
in Janzen et al., 2000a). Consequently, experimental studies like this one and others
(Congdon et al., 1999; Janzen, 1993; Janzen et al., 2000a,b, unpublished; Tucker
and Paukstis, 1999; Tucker, 2000a) will
continue to be significant not only for
gaining insight into early stages of turtle
life histories, but also for clarifying broader
issues in evolutionary ecology.
Acknowledgments.—We extend our sincerest gratitude to J. Tucker and D. Warner for their tremendous help in the field, to J. Anderson, M. Balk, N.
Notis, and L. Solberg for assistance in the laboratory,
to C. Anthony, B. Danielson, W. Gutzke, D. Vleck,
and an anonymous reviewer for critically reading the
manuscript, and to N. Booth and K. Postlewait of the
Illinois Department of Natural Resources for allowing us to construct the drift fence, even though it
interfered with their management duties. Eggs were
collected under Illinois Department of Natural Resources permit W-96-0302, and permission to use the
animals in this study was granted by the Iowa State
University Animal Care Committee (log no. 5-73570-1-J). This research was supported in part by National Science Foundation grant DEB-9629529 to F.
J. Janzen. This is Journal Paper No. J-19588 of the
Iowa Agriculture and Home Economics Experiment
Station, Ames, Iowa, Project No. 3369, and supported
by the Hatch Act and State of Iowa Funds.
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Accepted: 25 April 2001
Associate Editor: Carl Anthony