Experimental analysis of an early life-history stage: avian predation
selects for larger body size of hatchling turtles
F. J. JANZEN,* J. K. TUCKER & G. L. PAUKSTISà
*Department of Zoology and Genetics, Iowa State University, Ames, Iowa 50011±3223, USA
Long-term Resource Monitoring Program Pool 26, Illinois Natural History Survey, 8450 Montclair Avenue, Brighton, Illinois 62012±2032, USA
à1404±143rd Place NE, Bellevue, Washington 98007, USA
Keywords:
Abstract
body size;
life-history evolution;
natural selection;
predation;
survivorship;
Trachemys scripta;
turtles.
One common life-history pattern involves an elevated rate and nonrandom
distribution of neonatal mortality. However, the mechanisms causing this
pattern and the speci®c traits that confer a survival bene®t are not always
evident. We conducted a manipulative ®eld experiment using red-eared slider
turtles to test the hypothesis that diurnal avian predators are a primary cause
of size-speci®c neonatal mortality. Body size was a signi®cant predictor of
recapturing hatchlings alive and of ®nding hatchlings dead under natural
conditions, but was unimportant when diurnal predators were excluded from
the ®eld site. Overall recapture rates also more than doubled when predators
were excluded compared to natural conditions (72.4 vs. 34.9%). We conclude
that birds are an important cause of size-speci®c mortality of recently emerged
hatchling turtles and that `bigger is better' in this system, which has important
implications for life-history evolution in organisms that experience sizespeci®c neonatal mortality.
Introduction
Life-history research is a dynamic area of inquiry in
evolutionary biology (Caswell, 1989; Roff, 1992; Stearns,
1992; Charnov, 1993; Charlesworth, 1994; Tuljapurkar &
Caswell, 1997). Indeed, describing how organisms live,
reproduce, and die is fundamental to understanding
population biology, and there is an extensive and growing
literature as a consequence (reviewed by Roff, 1992;
Stearns, 1992; P®ster, 1998; Benton & Grant, 1999). A
signi®cant portion of this research has focused on analysing causes of natural selection on life-history traits with
experimental approaches in the ®eld, especially phenotypic and environmental manipulations (Wade & Kalisz,
1990; Sinervo, 1998; Travis & Reznick, 1998).
One fundamental life-history stage is that of neonates
or seedlings. This stage is typically characterized by high
mortality (e.g. summarized for turtles in Wilbur & Morin,
1988) that can dramatically alter population demography
Correspondence: F. J. Janzen, Department of Zoology and Genetics, Iowa
State University, Ames, IA. 50011, USA.
Fax: +1 515 294 8457; e-mail: fjanzen@iastate.edu
J. EVOL. BIOL. 13 (2000) 947±954 ã 2000 BLACKWELL SCIENCE LTD
(e.g. Thomson et al., 1997). Thus, any trait that confers a
survival advantage to certain neonates or seedlings
should be strongly favoured by selection. One key
neonate attribute that has received much attention is
offspring size. Most research has suggested that `bigger is
better' (e.g. Einum & Fleming, 2000; Janzen et al., 2000),
but these experimental studies have not manipulated the
system to document the cause(s) of selection favouring
larger offspring.
To evaluate a potential cause of substantial mortality
selection on body size during a critical life-history stage,
we conducted a ®eld experiment using offspring of the
red-eared slider turtle (Trachemys scripta elegans, Wied,
1838). We wanted to determine whether diurnal avian
predation is a cause of size-speci®c survivorship of
hatchling turtles during their migration from terrestrial
nests to water. Our previous experiments documented
signi®cant directional selection favouring larger offspring
during the post-emergence migration to water (Janzen,
1993a; Tucker & Paukstis, 1999; Janzen et al., 2000;
Tucker, 2000a), but were not designed to address the
cause(s) of this pattern. Circumstantial evidence from
these experiments and other studies (e.g. Janzen, 1995)
947
948
F. J. JANZEN ET AL.
suggests that predation is the likely mechanism behind
the observed size-speci®c mortality or recapture probabilities. Thus, using hatchling turtles produced from
eggs incubated in a common-garden environment, we
tested the predation hypothesis by leaving the ®eld site
unmanipulated during one experimental release of
hatchling turtles and by excluding birds from the ®eld
site during a subsequent experimental release.
Materials and methods
Experimental methods
Organism
Adult red-eared slider turtles (T. s. elegans) at the study
site in the Mississippi River State Fish and Wildlife
Refuge, Jersey County, Illinois, nest from mid-May to
early July, averaging ~14 eggs per clutch (Tucker et al.,
1998). Eggs average ~11 g and hatchling size is strongly
dependent on egg size (Tucker et al., 1998; Janzen et al.,
2000). Neonates hibernate in the nest before emerging to
migrate to aquatic habitats between mid-April and early
June the following spring (Tucker, 1997, 2000b).
Collection and incubation of eggs
In May and June 1996, 277 turtles were captured on their
nesting migrations at or near the ®eld site. These females
were induced to oviposit with an injection of oxytocin
(Ewert & Legler, 1978). Eggs were tamped dry, numbered
uniquely, weighed to the nearest 0.01 g, and incubated on
moist vermiculite (±150 kPa) in covered, plastic shoeboxes. Shoeboxes were rehydrated approximately once
weekly to maintain the initial level of substrate moisture
and then were rotated within the incubation room to
minimize the potential in¯uence of thermal gradients on
hatchling phenotypes. Incubation temperatures ¯uctuated as they do in natural nests (e.g. Weisrock & Janzen,
1999), averaging 27 °C with daily variation of 16±35 °C
(see Tucker & Warner, 1999). Because all eggs were
exposed to a similar thermal environment, this commongarden experimental design thereby eliminated the possible impact of phenotypic variation due to among-nest
variation in substrate moisture and incubation temperature (reviewed by Packard, 1991).
Hatchlings
Upon hatching, turtles were weighed to the nearest
0.01 g and carapace length was measured to the nearest
0.1 mm with vernier calipers. Hatchlings were kept in
plastic boxes with moist vermiculite (±150 kPa) placed in
a darkened room for hibernation, which was unheated
except when necessary to prevent freezing (mean
ˆ 6.7 °C) (Tucker & Paukstis, 1999). This ¯uctuating
thermal regime mimicked that observed in natural nests
at the ®eld site (Tucker & Packard, 1998). After hibernating until 6 May 1997, we divided hatchlings from 221
clutches into two groups of 525 turtles for this study. All
1050 hatchlings were reweighed and remeasured on
11±12 May and kept in a cool, dark room until the
experimental releases.
Field methods
Drift fence
We constructed a 260 m long drift fence with 17 pitfall
traps from 0.3 m high aluminium ¯ashing down slope
from the known nesting area (Tucker, 1997, 2000b).
Prior to the experimental releases, tall vegetation in
direct contact with the aluminium ¯ashing was removed
to prevent hatchlings from climbing over the fence.
Hatchlings that emerged from natural nests at the ®eld
site were caught in the drift fence between 30 April and
5 June (95 `natural' hatchlings) (see Tucker, 2000b),
indicating that the timing of the two experimental
releases described below was ecologically relevant.
Control treatment
Hatchlings were released at 0600 CDT on 14 May
following methods used in previous experiments (Tucker
& Paukstis, 1999; Janzen et al., 2000; Tucker, 2000a). The
release point was 40 m up slope from the drift fence,
centred perpendicularly to pit 9 on the drift fence. The
drift fence was monitored twice daily at 0600 and 1800
CDT through 19 May. Potential diurnal-avian predators
were noted, primarily red-winged blackbirds (Agelaius
phoeniceus, Linnaeus, 1766) and common grackles (Quiscalus quiscula, Linnaeus, 1758). Recaptured hatchlings
were identi®ed from photocopies of the plastron taken
prior to the releases and weighed to the nearest 0.01 g.
Searches for dead hatchlings were conducted intermittently when predator disturbance was desired (see below).
Predator-exclusion treatment
Hatchlings were released at 0600 CDT on 20 May in the
same location and manner as the turtles in the control
treatment. Once the hatchlings were released, an investigator was present continuously during daylight hours
until 24 May to actively exclude potential avian predators.
Because some birds were persistent and would attempt to
land in the release area despite our presence, we used
rocks and a slingshot to frighten away birds. This method
permitted us to exclude birds without entering the release
area and interfering with the turtles. No birds were struck
by projectiles. The drift fence was monitored every 30 min
on 20 and 21 May and hourly on 22 and 23 May.
Recaptured hatchlings were processed as in the control
treatment. We entered the release area on 23 May, and
again between 29 May and 6 June, to conduct extensive
searches for dead hatchlings from both treatments.
Statistical analyses
Hatchlings fell into one of the three categories at the
termination of the treatments: (1) recaptured alive
J. EVOL. BIOL. 13 (2000) 947±954 ã 2000 BLACKWELL SCIENCE LTD
Avian predation on hatchling turtles
(2) found dead, or (3) fate unknown. We employed
hatchling mass at release as a measure of body size for all
within- and between-treatment statistical analyses
among the three recapture classi®cations. Results using
carapace length instead of body mass as a measure of
hatchling size were similar and therefore are not presented. Unless stated otherwise, statistical analyses were
conducted using SAS (SAS Institute, 1988).
Between-treatment analyses
We compared recapture frequencies between the treatments using G-tests (Sokal & Rohlf, 1981). We also used
Kruskal±Wallis tests to compare mass at release between
the same hatchling recapture categories for the two
treatments.
Within-treatment analyses
Previous research showed that smaller hatchlings vanish
disproportionately during the post-emergence migration
(Janzen, 1993a; Tucker & Paukstis, 1999; Janzen, et al.,
2000; Tucker, 2000a), thus we began each treatment
with certain a priori expectations. For the control treatment where methods caused minimal disturbance of
diurnal predators, we expected that hatchlings recaptured alive would be larger than (1) all neonates not
recaptured (i.e. all hatchlings found dead or of unknown
fate) and (2) just hatchlings found dead. We evaluated
these two hypotheses using one-tailed Kruskal±Wallis
tests. For the predator exclusion treatment where
methods resulted in maximal disturbance of diurnal
predators, we expected that hatchlings recaptured alive
(1) would be neither larger nor smaller than all turtles
not recaptured and (2) would be larger than those found
dead, assuming that the dead hatchlings were killed by
predators despite our efforts. We evaluated the ®rst
hypothesis with a two-tailed Kruskal±Wallis test and the
second with a one-tailed Kruskal±Wallis test.
We also used random-effects logistic regression (Stiratelli et al., 1984) implemented in a Fortran programme
(Janzen et al., 2000; H. Stern and A. H. Jones, personal
communication) to determine whether hatchling mass
predicted the likelihood of recapturing hatchlings alive
(i.e. recaptured alive vs. found dead or of unknown fate).
Table 1 Descriptive statistics for 1050
hatchling T. s. elegans released in two
treatments.
Variable
Release mass (g)
Recapture mass (g)
Change in mass (g)
Recapture time (d)
Recapture time-actual (d)
Recapture location (pit)
Min. distance to pit (m)
949
This algorithm permitted us to account for maternal
identity properly as a random effect in the regression
analyses while we focused on our primary variable of
interest, hatchling mass.
To aid in visualizing the form of selection on hatchling
mass in greater detail, we used the cubic-spline technique, a nonparametrical regression method (Schluter,
1988; Janzen, 1993a; Brodie et al., 1995; Janzen et al.,
2000). We calculated standard errors for each spline
function by bootstrapping the data 100 times.
Results
Between-treatment analyses
Hatchling morphology and behaviour were similar in the
two treatments. Mass at release did not differ signi®cantly between the two treatments (KW ˆ 0.82, d.f. ˆ 1,
P ˆ 0.3650) nor did change in mass of hatchlings during
their migration to the drift fence (KW ˆ 1.83, d.f. ˆ 1,
P ˆ 0.1761) (Table 1). Overall, hatchlings in the two
treatments followed similar paths and took about the
same amount of time to reach the drift fence (Table 1).
The two treatments differed extensively, however, in
the number of hatchlings either recaptured, not found, or
found dead (G ˆ 188.05, d.f. ˆ 2, P < 0.0001). Of particular relevance, signi®cantly more hatchlings were (1)
recaptured alive in the predator-exclusion treatment
(G ˆ 46.29, d.f. ˆ 1, P < 0.0001) and (2) found dead in
the control treatment (G ˆ 70.12, d.f. ˆ 1, P < 0.0001)
(Table 2).
Body mass at release for the different survival categories of hatchlings also differed between the two treatments. Hatchlings recaptured in the control treatment
were signi®cantly heavier than those recaptured in the
predator-exclusion treatment (Table 2; KW ˆ 2.91,
d.f. ˆ 1, one-tailed P ˆ 0.0438), suggesting that exclusion of diurnal predators resulted in relatively more small
hatchlings surviving to reach the drift fence. Hatchlings
of unknown fate were lighter in the control treatment
than in the predator exclusion treatment (Table 2;
KW ˆ 4.82, d.f. ˆ 1, one-tailed P ˆ 0.0141), implying
that more small hatchlings survived to reach the fence,
Control
Mean (SD), Range
Predator exclusion
Mean (SD), Range
6.43
6.37
0.24
1.04
6.50
6.16
0.30
0.82
0.50
9.56
64.5
(1.01),
(1.02),
(0.41),
(1.04),
3.35±8.86
3.71±8.80
)0.49±2.29
0.5±7.5
9.93 (3.47), 1±17
70.4 (27.9), 40.0±151.6
(0.98),
(1.13),
(0.46),
(0.78),
(0.82),
(3.22),
(28.6),
3.98±9.43
3.40±8.83
)0.74±1.98
0.5±6.5
0.04±6.5
1±17
40.0±151.6
*n ˆ 525 for release mass for each treatment; for the remaining variables, n ˆ 183 for the
control treatment and n ˆ 380 for the predator-exclusion treatment.
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950
F. J. JANZEN ET AL.
Control
Predator exclusion
Status
n
Mean (SD), Range
n
Mean (SD), Range
Recaptured
Not found
Found dead
183
278
64
6.62 (0.99), 4.11±8.86
6.37 (1.00), 3.35±8.48
6.20 (1.10), 3.75±8.81
380
143
2
6.46 (1.00), 3.98±9.43
6.62 (0.95), 4.16±8.95
6.29 (0.76), 5.75±6.83
and thus fewer of them were among the hatchlings that
were not recovered, compared with the control treatment. Finally, hatchlings found dead in both treatments
were similar in mass at release (Table 2; KW ˆ 0.02,
d.f. ˆ 1, P ˆ 0.8811). These results are re¯ected in the
cubic-spline analyses, which illustrate linear selection
across the whole range of body mass in the control
treatment and essentially no selection when predators
were excluded (Fig. 1).
Table 2 Descriptive statistics for mass at
release for three survival categories of
hatchling T. s. elegans released in two
treatments.
Within-treatment analyses
Control treatment
We found 183 (34.9%) hatchlings alive and 64 (12.2%)
dead, with the remaining 278 (53.0%) turtles having an
unknown fate. We recaptured the ®rst hatchlings at the
drift fence at 1800 CDT on 14 May (i.e. 12 h after
release). The drift fence was monitored through 6 June,
but the last individual from this treatment recaptured
alive was found at 1800 CDT on 21 May. Live hatchlings in this control treatment were caught in every pit
along the drift fence, with the most being recaptured in
pits near the central portion of the fence (Table 1;
skewness ˆ ±0.39, kurtosis ˆ ±0.12, mean pit ˆ 9.93,
SD ˆ 3.47 pits).
Body mass of hatchling turtles at the time of release
varied systematically with the date of recapture at the
drift fence. Heavier hatchlings tended to be recaptured
sooner than lighter individuals (KW ˆ 21.56, d.f. ˆ 8,
P ˆ 0.0058), particularly in the ®rst 3 days of the
treatment when most turtles were recovered. Consistent
with these results, elapsed time from release to recapture was negatively correlated with mass at release
(r ˆ ± 0.27, P ˆ 0.0003).
Body mass of hatchling turtles at the time of release
also differed signi®cantly among the three categories
of recapture status (Table 2; KW ˆ 9.47, d.f. ˆ 2,
P ˆ 0.0088). Hatchlings recaptured alive were heavier
at the time of release than those not recaptured alive
(KW ˆ 7.98, d.f. ˆ 1, one-tailed P ˆ 0.0024) and heavier
than those just found dead (KW ˆ 6.89, d.f. ˆ 1, onetailed P ˆ 0.0044). These results were supported by
logistical-regression analysis, which found that mass of
hatchlings at the time of release signi®cantly predicted
the probability of recapturing them alive (v2 ˆ 8.85,
d.f. ˆ 1, P ˆ 0.0029). Larger hatchlings were more likely
to be recaptured alive than were smaller ones (Fig. 1).
Predator-exclusion treatment
Fig. 1 Probability of survival of hatchling T. s. elegans in relation to
body mass during the control treatment (top panel) and the
predator-exclusion treatment (bottom panel). Hatchlings not recaptured were considered dead for these analyses. The surfaces were
derived using a cubic-spline algorithm and indicate (1) selection
favouring larger hatchlings in the control treatment and (2)
essentially no selection on hatchling size in the predator-exclusion
treatment. The SE lines ¯anking the selection surfaces were
generated by bootstrapping the data 100 times.
In contrast to the control treatment, we recaptured alive
more than double the number of experimental hatchlings (380 or 72.4%) and found only 2 (0.4%) dead, with
143 (27.2%) of the hatchlings unaccounted for. We
recaptured live hatchlings at the drift fence in this
treatment beginning at 0700 CDT on 20 May and ending
at 1800 CDT on 26 May (again, the drift fence was
monitored through 6 June). As in the control treatment,
live hatchlings were caught in every pit along the fence,
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Avian predation on hatchling turtles
centring around the middle (Table 1; skewness ˆ ±0.11,
kurtosis ˆ 0.39, mean pit ˆ 9.56, SD ˆ 3.22 pits).
Mass of hatchlings at release did not vary signi®cantly
with date of recapture (KW ˆ 13.89, d.f. ˆ 8, P ˆ 0.0848).
Even so, elapsed time from release to recapture (to the
nearest half day) was negatively correlated with hatchling
mass at the time of release (r ˆ ± 0.16, P ˆ 0.0013),
suggesting that heavier hatchlings were in fact reaching
the drift fence faster than lighter turtles. Focusing only on
the ®rst day of the predator-exclusion treatment, when
317 hatchlings were recaptured alive and the recapture
time was known to the nearest half hour, produced a
similar result (KW ˆ 27.50, d.f. ˆ 19, P ˆ 0.0935 and
r ˆ ±0.19, P ˆ 0.0009).
Body mass of hatchlings at the time of release did not
vary signi®cantly among the three categories of recapture
status (Table 2; KW ˆ 2.38, d.f. ˆ 2, P ˆ 0.3048). Hatchlings recaptured alive were similar in mass at the time of
release to those not recaptured (KW ˆ 2.09, d.f. ˆ 1,
two-tailed P ˆ 0.1486) and to only those found dead
(KW ˆ 0.10, d.f. ˆ 1, one-tailed P ˆ 0.3753). Results
from the logistical regression analysis con®rmed the
overall impression that mass of hatchlings at the time of
release in this treatment was not a signi®cant predictor of
the probability of recapturing those individuals alive
(v2 ˆ 2.48, d.f. ˆ 1, P ˆ 0.1154). Larger hatchlings were
just as likely to be recaptured alive as were smaller ones
when diurnal predators were excluded from the experimental area (Fig. 1).
Discussion
Mechanism behind patterns of offspring mortality
The results of this manipulative experimental study
document on avian predation as a primary cause of
size-dependent mortality of hatchling turtles between
the time they emerge from the nest until the time they
reach their future aquatic home. Larger neonates in the
control treatment were more likely to be recaptured alive
and less likely to be found dead than were smaller
individuals (Fig. 1, Table 2). This ®nding is consistent
with the results of previous experiments involving this
population of red-eared sliders (Tucker & Paukstis, 1999;
Janzen et al., 2000; Tucker, 2000a) and painted turtles
(Chrysemys picta, Schneider, 1783) (Tucker, 2000a), and a
different population of common snapping turtles (Chelydra serpentina, Linnaeus, 1758) (Janzen, 1993a). In
contrast, when birds were excluded from the ®eld site,
the probability of recapturing hatchlings alive more than
doubled and was independent of body size (Fig. 1,
Table 2). Birds have long been suspected of size-dependent predation on hatchlings of a wide variety of turtles
(reviewed in Janzen et al., 2000); our study suggests that
this is true.
The conclusion that avian predation caused the different recapture patterns is supported by an additional
J. EVOL. BIOL. 13 (2000) 947±954 ã 2000 BLACKWELL SCIENCE LTD
951
result. In the predator-exclusion treatment, the percentage of turtles of unknown fate was smaller than in the
control treatment and the percentage of turtles recaptured alive was larger (Table 2). This ®nding suggests that
many of the hatchlings whose fate cannot be determined
with certainty in the control treatment and in previous
experiments were probably killed by birds. The uncertainty of their fate is presumably due to transport by
predators out of the area, complete ingestion by predators, or our inability to ®nd their carcasses in the ®eld.
We were unable to perform replicates because the
large scale of the study made them unfeasible and
environmental conditions changed substantially as summer progressed. This was even true to some extent
between the two treatments that we did execute.
However, the results of the control treatment were
similar to those from comparable studies conducted at
the site in other years (Tucker & Paukstis, 1999; Janzen
et al., 2000; Tucker, 2000a).
Did we attract an unnaturally large number of predators to the area by releasing so many turtles simultaneously? Two lines of evidence suggest that we did not.
First, we collected a number of hatchlings that emerged
from natural nests, and have captured as many as 60
such `natural' turtles in a single day (Tucker, 1997),
which is about half the number recaptured in one day
during the control treatment. Furthermore, the survival
and mortality probabilities from the control treatment
were nearly identical to those obtained in an earlier
experiment where fewer turtles were released simultaneously (178) and recaptured in one day (31) (Janzen
et al., 2000). We also collected hundreds of females
during their nesting forays to obtain eggs for this
experiment. Had these females been permitted to nest,
even with nest predation the area would have been ®lled
with numerous hatchlings in late spring when our
experiment was conducted. The other important evidence in favour of the ecological relevance of our design
derives from an experiment in which equal numbers of
turtles (1) were released simultaneously as in prior
studies and (2) were placed in nest cavities to emerge
and migrate at their leisure (F. Janzen, J. Tucker &
G. Paukstis, unpublished work). For both sets of hatchlings, larger individuals were recaptured alive signi®cantly more often than smaller turtles.
We are left with the conclusion that birds killed smaller
hatchlings signi®cantly more often than larger individuals. The primary avian predators observed in the area
were red-winged blackbirds (A. phoeniceus) and common
grackles (Q. quiscula). These birds could conceivably have
gape limitations in the size range of hatchling T. scripta, a
mechanism that has been invoked by other researchers
(e.g. Swingland & Coe, 1979). Still, these two species
readily attack and kill young turtles (e.g. Janzen, 1995).
A more likely explanation is that larger neonates are
faster than smaller ones (Miller et al., 1987; Janzen,
1993b; Miller, 1993). Consequently, smaller hatchlings
952
F. J. JANZEN ET AL.
spent more time migrating and, thus, may have been
exposed to predators for a greater period than were larger
turtles. This scenario implies that predators exert selection favouring larger hatchlings simply because they
encounter smaller turtles more frequently. Future
experiments are required to test this hypothesis.
Although avian predation was most likely the mechanism behind the differences between the two treatments reported here, other factors could affect
survivorship of hatchling turtles. Low energy reserves
seem unlikely to be important because neonates of
aquatic turtles appear to have ample quantities of
residual yolk or fat bodies to sustain them until they
reach an aquatic habitat (reviewed in Packard & Packard,
1988; Vleck & Hoyt, 1991). Desiccation is more likely to
exert a signi®cant impact on hatchling mortality during
this period, because water reserves may constrain hatchling migration behaviour. Tucker & Paukstis (1999)
found that hatchling T. elegans from wetter incubation
substrates were more likely to be recaptured alive and
less likely to be found dead during the migration from
nest to water than neonates from drier incubation
substrates. Desiccation is unlikely to have been important in the present study, however, because all eggs
were incubated under identical wet moisture regimes
(±150 kPa) and precipitation was moderate prior to and
during the control (2 cm precipitation) and predator
exclusion (0.1 cm precipitation) treatments. Hatchlings
also exhibited similar activity patterns and change in
body mass between release and recapture in the two
treatments (Table 1). These results are inconsistent with
any explanation that would invoke differential dehydration as a source of hatchling mortality in this study.
Life-history evolution
The biological importance of a particular stage or age (e.g.
Horvitz et al., 1997) and the underlying causes behind
variability in and values for vital rates characterizing a
particular stage or age (e.g. Travis & Reznick, 1998) are
crucial issues for understanding patterns of life-history
evolution. Manipulative experiments such as this study
provide an excellent approach for elucidating those very
questions (e.g. Wade & Kalisz, 1990; Travis & Reznick,
1998).
Natural populations of turtles are characterized by high
hatchling mortality (>70%) (e.g. Cunnington & Brooks,
1996). Lifetime ®tness is nonexistent unless an individual
survives to maturity, because the juvenile period is a
crucial life-history stage. Traits that confer a ®tness
bene®t to individuals by enhancing their probability of
maturing will therefore be strongly favoured by selection. For a variety of organisms (reviewed by Azevedo
et al., 1997) and for reptiles overall (reviewed by Packard
& Packard, 1988), larger body size is generally thought to
confer greater relative ®tness to offspring than smaller
body size, and this hypothesis has been con®rmed in
nearly all studies of young reptiles, including the present
investigation (Olsson & Shine, 1997; Elphick & Shine,
1998; Civantos et al., 1999; Sorci & Clobert, 1999; Tucker
& Paukstis, 1999; Bronikowski, 2000; Tucker, 2000a;
earlier studies reviewed in Janzen et al., 2000; but see
Madsen & Shine, 1998; Congdon et al., 1999).
The causes of this size-dependent mortality in most
systems have not been documented de®nitively. Furthermore, in turtles, most attention has focused on the nest
itself with relatively little experimental consideration for
the terrestrial migration period after emergence from the
nest (Janzen, 1993a; Tucker & Paukstis, 1999; Janzen
et al., 2000; Tucker, 2000a). An attractive feature of our
experiment is that we identify an important cause of
neonatal mortality during terrestrial migration and the
magnitude of its impact in the ®eld. Indeed, the considerable increase in hatchling survivorship when diurnal
avian predators were excluded from the site suggests a
possible selective factor in the evolution of nocturnal
emergence behaviour in some species such as sea turtles
(e.g. Bustard, 1967; Mrosovsky, 1968).
What does all this mean for the evolution of turtle life
histories? All else being equal, selection favouring larger
offspring should lead to the evolution of smaller clutch
sizes (because egg size largely determines offspring size
[Tucker et al., 1998; Janzen et al., 2000]) and delayed
maturity (because larger females can lay larger eggs
[Tucker et al., 1998]), among other life-history traits
(reviewed in Stearns, 1992; Charlesworth, 1994). However, several factors could inhibit the evolution of
these life-history traits, including heritability, maternal
effects, tradeoffs, morphological constraints, ontogenetic
changes in selection, and environmental stochasticity
(discussed in Janzen et al., 2000).
Our manipulative ®eld study has elucidated a primary
mechanism of selection causing size-speci®c survivorship
during a critical life-history stage in a freshwater turtle.
Larger body size of hatchling turtles led to decreased rates
of avian predation. Future experiments will be necessary
to identify the particular bene®t(s) of large body size (e.g.
improved locomotor performance) and the impact on the
life-history pattern of turtles.
Acknowledgments
We thank J. Hatcher, J. Uttley, and M. Tucker for
assistance in the ®eld. N. Booth and K. Postlewait
(Illinois Department of Natural Resources) at the Mississippi River State Fish and Wildlife Area in Rosedale,
Illinois allowed free access to the Stump Lake nesting
area. We greatly appreciate them allowing placement of
the drift fence at the site even though it complicated
their management duties. We also acknowledge the
Janzen Lab, B. Brodie III, A. Bronikowski, and two
anonymous reviewers for constructive comments on
various drafts of the manuscript. This study was partially
supported by the Illinois Natural History Survey and the
J. EVOL. BIOL. 13 (2000) 947±954 ã 2000 BLACKWELL SCIENCE LTD
Avian predation on hatchling turtles
Upper Mississippi River System Long-Term Resources
Monitoring Program; preparation of the manuscript was
partially supported by NSF grant DEB-9629529 to F.J.
Journal paper no. J-000 of the Iowa Agricultural 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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Received 10 August 2000; accepted 16 August 2000
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