The Influence of Maternal Size on the Eggs and Hatchlings
of Loggerhead Sea Turtles
Anne Marie LeBlanc, David C. Rostal, K. Kristina Drake, Kristina L. Williams, Michael G. Frick, John Robinette, and Debra E. Barnard-Keinath
Southeastern Naturalist, Volume 13, Issue 3 (2014): 587–599
Full-text pdf (Accessible only to subscribers.To subscribe click here.)
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22001144 SOUTHEASTERN NATURALIST 1V3o(3l.) :1538,7 N–5o9. 93
The Influence of Maternal Size on the Eggs and Hatchlings
of Loggerhead Sea Turtles
Anne Marie LeBlanc1,*, David C. Rostal1, K. Kristina Drake1, Kristina L. Williams2,
Michael G. Frick3, John Robinette4, and Debra E. Barnard-Keinath4
Abstract - Our study examined variation in and correlation of reproductive traits for a
population of Caretta caretta (Loggerhead Sea Turtle) nesting in Georgia and compared
the results with those of other studies. We assessed variability in reproductive traits (i.e.,
maternal length, clutch size, egg diameter, egg mass, hatchling length, and hatchling mass)
on the population level and individual level. At the population level, we investigated interannual
and intraseasonal variation of these traits for 810 Loggerhead Sea Turtle nests in
Georgia, on Wassaw National Wildlife Refuge (NWR) and Blackbeard Island NWR during
2000–2003 and 2001–2003, respectively. As the nesting season progressed, we observed
a decrease in clutch size, mean egg diameter, mean egg mass per clutch, mean hatchling
length per clutch, and mean hatchling mass per clutch. Further, we measured all previously
mentioned traits on a subset of the female turtles encountered on the beach (n = 24) and
used these data to examine the variability of the traits on the individual level. Generalized
linear modeling using this more refined individual-level data set indicated that 55% of the
variability in clutch size was explained by a combination of maternal length (47%) and
hatchling length (8%). This model suggested that clutch size was positively related with
maternal length (t = 4.79) and negatively related with hatchling length (t = -1.90). Greater
maternal length resulted in larger clutch size, but not larger egg size; thus, egg size was
relatively constant irrespective of maternal length. These results support optimal egg-size
theory, indicating a trade-off between clutch size and hatchling size to produce the optimum
maternal investment per offspring.
Introduction
Caretta caretta L. (Loggerhead Sea Turtle) is a circumtropical species (Bolten
2003, Dodd 1988), and its presence in multiple marine environments provides a
model to investigate geographic differences in reproductive characteristics (Tiwari
and Bjorndal 2000, Van Buskirk and Crowder 1994). Previous research has
suggested that the reproductive ecology of Loggerhead Sea Turtles varies among
geographic nesting assemblages as a result of behavioral, morphological, physiological,
or dietary differences among widely separated populations (reviewed by
Broderick et al. 2003, Stokes et al. 2006). However, whether such differences exist
1Department of Biology, Georgia Southern University, PO Box 8042, Statesboro, GA
30460. 2Caretta Research Project, PO Box 9841, Savannah, GA 31412. 3Archie Carr
Center for Sea Turtle Research, Department of Biology, University of Florida, Gainesville,
FL 32611. 4US Fish and Wildlife Service, Savannah Coastal Office-Wassaw and Tybee
National Wildlife Refuges, 1000 Business Center Drive, Suite 10, Savannah, GA 31405.
*Corresponding author - annemarielbeich@gmail.com.
Manuscript Editor: Ray Carthy and Glen Mittelhauser
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within the same population is unknown due to the difficulty in intercepting individual
turtles for every nesting event within a season (Schroeder et al. 2003).
Optimal egg-size theory predicts that, in a given environment, there is an optimal
amount of maternal investment per offspring that results in the maximum level
of maternal fitness apart from maternal body size (Smith and Fretwell 1974). This
theory leads to a maternal energy trade-off between clutch size and offspring size,
regardless of maternal body size, resulting in optimum maternal fitness (Smith and
Fretwell 1974). However, physical constraints have also been reported to affect
egg size. For example, the relationship between egg size and clutch size reported
in two freshwater turtle species did not conform to optimal egg-size theory because
the pelvic-aperture width limited maximum egg size (Congdon and Gibbons
1987). The optimal egg-size theory assumes that increased energy expenditure per
individual will result in larger offspring with increased fitness, and that maternal
reproductive capacity is limited, either due to space or available energy resources
(Smith and Fretwell 1974). Optimal egg-size theory predicts that allocation to individual
offspring occurs relatively evenly within clutches (Wilkinson and Gibbons
2005). Previous studies have surmised that reproductive variation should manifest
in clutch size (Rollinson and Brooks 2008), and numerous studies have examined
reproductive variation in freshwater turtles (Congdon and Gibbons 1987, Congdon
and van Loben Sels 1993, Congdon et al. 1994, Janzen et al. 2009).
We examined variation in and correlation of reproductive traits for a population
of Loggerhead Sea Turtles nesting in Georgia and compared our results with those
of other studies. We investigated reproductive traits (maternal length, clutch size,
egg diameter, egg mass, hatchling length, and hatchling mass) from Loggerhead
Sea Turtles nesting north of Florida—a population currently in decline and for
which information regarding their reproductive ecology is lacking (TEWG 2000).
We investigated these reproductive traits at the population level and at the individual
level to determine from an evolutionary perspective which of them was being
selected for. We also tested optimal egg-size theory and predicted that we would
find a trade-off between clutch size and hatchling size.
Field-Site Description
We conducted this study on two barrier islands: Wassaw National Wildlife Refuge
(WI), off the coast of Richmond Hill, GA (31°87'N, 80°97'W), and Blackbeard
Island National Wildlife Refuge (BBI), off the coast of Valona, GA (31°30'N,
81°12'W). Female Loggerhead Sea Turtles use both of these barrier islands for egg
deposition from mid-May until early August, and eggs hatch from late June until
early October (Williams and Frick 2000). Both islands experience high daily tidal
fluctuations (sometimes >2.4 m), and are largely composed of Holocene deposits
derived from adjacent riverine systems and the continental shelf (Johnson et al.
1974). Historically, the highest nesting density of Loggerhead Sea Turtles on the
Georgia coast occurred on BBI, where Loggerhead Sea Turtle nest-protection programs
have been in place since 1965 (Dodd and MacKinnon 2003). WI is among
the few islands along the Georgia coast experiencing an increase in Loggerhead Sea
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Turtle nesting activity since monitoring and nest protection began in 1973 (Dodd
and MacKinnon 2003).
Methods
Population-level data
We conducted nightly patrols on WI (2000–2003) and BBI (2001–2003)
throughout the nesting season. We tagged, measured, and monitored nesting females
for return nesting events. We identified individual females using an inconel
front-flipper tag (National Band and Tag Co., Newport, KY) and a subcutaneous
passive integrated transponder (PIT) tag (Biomark, Boise, ID) in the front right flipper.
For each nesting female, we recorded the curved carapace length (CCL)—from
nuchal notch to the tip of the posterior marginal.
We relocated nests that were deposited in areas with a high probability (>80%)
of seawater inundation to higher parts of the beach to prevent nest loss, according
to United States Fish and Wildlife Service (USFWS) protocol on WI and BBI (as
discussed in National Marine Fisheries Service and USFWS 2008). Relocation
occurred within 6 hours after egg deposition to reduce the possibility of movementinduced
mortality (Limpus et al. 1979). As a predator deterrent, we covered all
nests with wire screens for the duration of their incubation. Tuttle and Rostal (2010)
found that relocated and in situ nest temperatures were not significantly different
and thus, we combined data from relocated and in situ nests for our analyses.
We measured clutch sizes for all relocated nests (n = 582) when they were transferred
to their new locations. Maximum egg width and egg mass were measured
from a subset of 20 eggs per clutch for many of the relocated nests (n = 203 and
n = 37 nests, respectively); all eggs were brushed free of sand prior to measurement
using dial calipers or a digital scale. We determined clutch sizes for nests that were
left in situ (n = 228) by counting the number of eggshells left in the nest chamber after
hatchling emergence; any eggshell that constituted >50% of an egg was counted
(Broderick et al. 2003, Dodd and Mackinnon 2003). Following hatching, a subset
of 20 hatchlings (n = 82 nests) was brushed free of sand and measured using dial
calipers. We measured the straight carapace length (SCL)—from the nuchal notch
to the supracaudal notch—for each hatchling. Hatchlings were also weighed using
a Pesola spring scale in 2000 and a digital scale from 2001 to 2003.
Individual-level data
For a subset (n = 24) of the nesting female Loggerhead Sea Turtles encountered,
we measured all variables (maternal length, clutch size, egg diameter, egg mass,
hatchling length, and hatchling mass) for the same nest. We used this data set in
further analyses to examine individual variation of reproductive variables, as described
below.
Data analysis
Population-level data. We report averages as means (± SE). For all tests, we
defined significance as α ≤ 0.05. We tested the assumptions of normal distribution
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and equal variances prior to performing analysis of variance (ANOVA) and linear
regressions. If the assumption of a normal distribution was not met, we employed
the nonparametric Spearman’s signed ranks test. We calculated hatching success
as the number of hatched eggs divided by the total clutch size. If multiple clutches
were observed for a single female, we averaged the reproductive metrics measured
and used the means in our analyses. For nests where the female could not be identified,
we assigned a unique identifier so that each unknown was treated as a unique
individual. For turtles that had multiple nests per year, each individual measurement
of the nesting female’s length, clutch sizes, egg sizes, and hatchling sizes were
averaged so that unequal weighting would not occur. We used an ANOVA to test for
differences in mean maternal length, mean clutch size, mean egg diameter, mean
hatchling length, hatch success, and the date of oviposition among years.
We employed a t-test to examine differences in the dates of oviposition between
the two islands surveyed. Combining the data from both islands studied, the relationship
among the date of oviposition and mean clutch size, mean egg mass, mean
hatchling length, mean hatchling mass, and mean hatch success was analyzed using
parametric linear regression or, if necessary, nonparametric Spearman’s signed
ranks. We analyzed the relationship between the date of oviposition and mean egg
diameter for each island using nonparametric Spearman’s signed ranks. For these
analyses, the data were sorted by date of oviposition, and the daily mean for each
measurement was used for analyses to prevent unequal weighting.
To determine if there were any differences between the whole population data
set and its subset describing individual variation, we used t-tests to detect differences
in maternal CCL, clutch size, mean egg diameter, mean hatchling length,
hatch success, and the Julian date of oviposition.
Individual-level data. We examined variation of the same reproductive traits on
the individual level. Analyses were performed on a subset of turtles (n = 24) from the
2003 nesting season on BBI for which we measured all variables (maternal length,
clutch size, egg diameter, egg mass, hatchling length, and hatchling mass) for the
same female/clutch. If multiple clutches were observed for a particular female, only
data from the first observed nest were used for these analyses. We divided nesting
events into early (Julian date of oviposition on or before day 171) and late season
(Julian date of oviposition after day 171) categories. The data were analyzed using
generalized linear modeling (Proc GLMSELECT, SAS v. 9.2; SAS Institute, Inc.,
Cary, NC) where the final model selection was determined by Schwarz bayesian
information-criterion (SBC) statistics (Judge et al. 1985, Schwarz 1978, and described
in SAS version 9.2). We tested 2 models with this dataset. The first model
evaluated the impacts of multiple variables (e.g., season [early vs. late], maternal
size, egg diameter, egg mass, hatchling length, and hatchling mass) upon clutch
size. The second model evaluated the impacts of the same variables on hatchling
length. We used JMP statistical software package version 10.0.2 (2012©, SAS),
SYSTAT version 11, or SAS version 9.2 for all analyses.
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Results
Population-level data
We tracked 301 nests on WI (2000–2003) and 509 nests on BBI (2001–2003),
and measured reproductive traits on each (Table 1). We found no significant difference
among years for maternal CCL (F3, 345 = 1.481, P = 0.219), mean egg
diameter per clutch (F3, 130 = 2.307, P = 0.080), mean hatchling length per clutch
(F3, 62 = 0.645, P = 0.589), or hatch success (F3, 445 = 0.644, P = 0.587). However,
we did find a significant difference in annual clutch size (F3, 433 = 2.628, P = 0.050),
with 2003 clutches (mean clutch size = 119 eggs) significantly larger than 2001
and 2002 clutches (mean clutch size = 112 eggs for both years). We did not find
any geographic variation in any of the measured reproductive traits, except egg
diameter, where BBI egg diameter was significantly larger than WI egg diameter
(t1, 79 = -2.836, P = 0.011). We combined data from all years and both study sites
for subsequent analyses unless otherwise noted. We observed 1–6 six nests per
year for individual nesting females. For this population-level data set, the number
of repeat samples per individual per year, as well as the data for all years and both
islands sampled is provided in Table 1. For the 4-year study, the dates of oviposition
ranged from 4 May to 19 August, with the median date falling on 20 June. We
found no significant difference in the dates of oviposition between BBI and WI (t =
-0.780, df = 628; P = 0.436) or among years sampled (F3, 805 = 2.214, P = 0.085). We
then examined each of the variables for seasonal variation. As date of oviposition
increased, there was a decrease in clutch size (n = 94, Rs
2 = -0.647; P < 0.001), mean
egg mass (df = 1, 20; R2 = 0.253, P = 0.017), mean hatchling length (df = 1, 46; R2 =
0.111; P = 0.021), and mean hatchling mass (df = 1, 19; R2 = 0.256; P = 0.019). As
Table 1. Summary data for the population-level and individual-level subsets used in this study.
Population-level data was collected on Blackbeard Island (BBI; 2001–2003) and Wassaw Island
(WI; 2000–2003). For each year (2000–2003), each individual nesting female’s length (CCL), clutch
sizes, egg sizes (egg diameter data are separated by island), and hatchling sizes were averaged so
that unequal weighting would not occur; the number of repeat samples per individual per year (n) are
provided. Individual-level data were collected on BBI (2003); data are from individual nests (n = 24)
laid throughout the duration of the study. Egg and hatchling sizes are expressed as means of each
clutch measured. For each data set, the mean (± SE) and sample size (n) are provided for each variable
measured.
Population Number of repeat samples Individual
level per individual per year level
mean ± SE (n) 1 2 3 4 5 6 mean ± SE (n)
Date of oviposition 170.5 ± 0.7 (809) na na na na na na 172.3 ± 3.4 (24)
Maternal length (cm) 100.0 ± 0.3 (349) 213 65 38 24 6 3 99.9 ± 1.1 (24)
Clutch size 114.6 ± 1.2 (437) 282 61 44 30 17 3 120.4 ± 3.8 (24)
Egg diameter (mm)
BBI 41.8 ± 0.2 (84) 58 10 3 7 4 2 41.9 ± 0.2 (24)
WI 41.0 ± 0.3 (50) 45 5 0 0 0 0
Egg mass (g) 39.1 ± 0.6 (30) 24 5 1 0 0 0 39.6 ± 0.6 (24)
Hatchling length (mm) 44.3 ± 0.2 (66) 61 4 1 0 0 0 44.2 ± 0.2 (24)
Hatchling mass (g) 18.7 ± 0.3 (28) 25 2 1 0 0 0 18.9 ± 0.3 (24)
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date of oviposition increased, there was a decrease in egg diameter on BBI (n = 64,
Rs
2 = -0.381; P = 0.002) and on WI (n = 44, Rs
2 = -0.419; P = 0.005).
Individual-level data
We found no significant differences between the population-level data set and
individual variation subset (Table 1) when comparing maternal length (t = 0.519,
df = 25; P = 0.608), clutch size (t = -1.030, df = 26; P = 0.313), mean egg diameter
(t = -0.906, df = 37; P = 0.371), mean hatchling length (t = 0.705, df = 51; P =
0.484), hatch success (t = -1.940, df = 26; P = 0.063), and Julian date of oviposition
(t = -0.515, df = 25; P = 0.611).
Because our analyses detected significant seasonal differences for the variables
measured for the population-level data set, we examined variation of the same
reproductive traits on the individual level using two generalized linear models to
determine which of these traits varied the least and thus was being selected for,
from an evolutionary perspective. The first model indicated that 54.8% of the variability
in clutch size was explained by a combination of maternal size (47.0%;
positive relation, t = 4.79) and hatchling length (7.7%; negative relation, t = -1.90).
The second model indicated that 62.0% (positive relation, t = 5.99) of the variability
in hatchling length was explained by hatchling mass.
Discussion
Overall, the values for the reproductive parameters we measured in this study
were within the range of those measured for other studies conducted in the same
location and around the world (for comparison to other studies see Table 2 and
Supplemental File 1 [available online at https://www.eaglehill.us/SENAonline/
suppl-files/s13-3-S975-LeBlanc-s1, and, for BioOne subscribers, at http://dx.doi.
org/10.1656/S975.s1.]; Baptistotte et al. 2003, Broderick et al. 2003, Dodd 1988,
Table 2: Summary of Loggerhead Sea Turtle measurements from other studies conducted worldwide;
(detailed data provided in Supplemental File 1,Tables 1–6 [available online at http://www.eaglehill.
us/SENAonline/suppl-files/s13-1-S975-LeBlanc-s1, and, for BioOne subscribers, at http://dx.doi.
org/10.1656/S975.s1]): range of mean maternal length (CCL), clutch size, egg diameter, egg mass,
hatchling length (SCL), and hatchling mass. Sample size (n) provided for the range’s minimum and
maximum study. Only mean samples with n ≥ 10 were used; ranges were excluded. For comparison,
population-level data from this study are shown for each individual nesting female’s length (CCL),
clutch sizes, egg sizes (egg diameter data are separated by island), and hatchling sizes.
This study (population level) Range of mean measurements
mean ± SE (n) worldwide, (n)
Maternal length, CCL (cm) 100.0 ± 0.3 (349) 71.9 (11)–105.1 (25)
Clutch size 114.6 ± 1.2 (437) 70.4 (128)–149 (26)
Egg diameter (mm) 37.61 (23)–49.9 (260)
Blackbeard Island 41.8 ± 0.2 (84)
Wassaw Island 41.0 ± 0.3 (50)
Egg mass (g) 39.1 ± 0.6 (30) 20.3 (500)–42.0 (4840)
Hatchling length (mm) 44.3 ± 0.2 (66) 40.0 (221)–45.8 (60)
Hatchling mass (g) 18.7 ± 0.3 (28) 18.7 (28)–22.0 (58)
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Ehrhart 1979, Ehrhart and Yoder 1978, Frazer and Richardson 1985a, Godley
et al. 2001, Kamezaki 2003, Margaritoulis et al. 2003, Stoneburner 1980, Van
Buskirk and Crowder 1994). The data are also comparable to historical data from
1973–2000 that have been collected on WI as part of a long-term monitoring project
(Williams and Frick 2000), and thus these morphological parameters have not
varied over time.
Seasonal variation
We found significant seasonal variation in reproductive traits for Loggerhead
Sea Turtles nesting in Georgia. In this study, mean values for all measured clutch,
egg, and hatchling variables decreased later in the nesting season (i.e., as date of
oviposition increased). Late-season clutch size decreases have also been reported
in other freshwater turtles and sea turtles (reviewed by Wilkinson and Gibbons
2005). Wilkinson and Gibbons (2005) also reported a seasonal egg-size decrease
in a freshwater turtle; however, the clutch-size decrease was only apparent in the
last clutch. Seasonal variation in these traits may be due to reduced reproductive
investment available for allocation in later clutches as maternal body fat reserves
are depleted (Wilkinson and Gibbons 2005). The decrease in egg size later in the
season may be related to reduced reproductive maternal investment or it could be
related to the different environmental conditions experienced by eggs developing
later in the season (e.g., colder mean temperatures, increased temperature variability,
altered hydric environment; Wilkinson and Gibbons 2005). For the latter, this
may be an adaptive explanation for temporal variation in optimal egg size (Rollinson
and Brooks 2008).
Drake (2001), who also worked at our study sites, found via ultrasound that
there was little variation in yolk size across the season for multiple females, and
that most variation in Loggerhead Sea Turtle egg size was the result of albumin
deposited in the oviduct. Wallace et al. (2006) found that Dermochelys coriacea
(Vandelli) (Leatherback) hatchling size was most correlated to yolk size and not the
amount of albumin, which created the most variation in egg size. Seasonal variation
has also been shown to affect growth rates in hatchling Loggerhead Sea Turtles,
thus potentially enhancing survivability for faster-growing hatchlings from the
earlier portion of the nesting season (Stokes et al. 2006).
Relationships between reproductive parameters
Our results support the optimal egg-size theory presented by Smith and Fretwell
(1974). In this study, clutch size increased with increasing maternal length, which
is expected since this has been observed in most sea and freshwater turtle species
(e.g., for review see Rollinson and Brooks 2008, Tiwari and Bjorndal 2000). This
trend has also been reported for Loggerhead Sea Turtles nesting in Greece (Hays and
Speakman 1991), Cyprus (Broderick et al. 2003), and throughout the Mediterranean
(Broderick and Godley 1996, reviewed by Margaritoulis et al. 2003), as well as for
Leatherbacks nesting in Costa Rica (Price et al. 2004) and some freshwater turtle
species (Wilkinson and Gibbons 2005). Larger female marine turtles can physically
hold more eggs than smaller females (Broderick et al. 2003). Even though body size
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may restrict clutch size due to turtles’ rigid body cavities, larger females still might
produce small clutches due to resource limitation or other constraints (Gibbons et al.
1982). We found that 47% of the variation in clutch size was explained by maternal
length, with larger females producing larger clutches than smaller females; 53% of
the variation in clutch size was due to unknown factors. Similar trends have been reported
in Georgia (Frazer and Richardson 1985a, b), Florida (Ehrhart 1995), Greece
(Hays and Speakman 1991), Cyprus (Broderick et al. 2003), and Brazil (Baptistotte
et al. 2003, Tiwari and Bjorndal 2000). In fact, positive correlations between these
reproductive parameters (e.g., clutch size and egg size) and maternal body size have
been reported for many turtle species (Bjorndal and Carr 1989, Hays et al. 1993, reviewed
by Wilkinson and Gibbons 2005). Caldwell (1959) found a negative relationship
between maternal length and egg diameter, but we found a positive linear relationship
between maternal size and egg diameter. Using our model, egg size did not
contribute a significant amount of variation to clutch size, indicating that this trait is
conserved among Loggerhead Sea Turtles. In our study, the majority of the variability
in clutch size was due to maternal size and hatchling size. Rollinson and Brooks
(2008) reported that within individuals, clutch size varied more than mean egg mass
across years. Indeed, many studies have found no relationship between egg size and
maternal body size; examples include Hays and Speakman (1991) for Loggerhead
Sea Turtles, Broderick et al. (2003) for Loggerhead Sea Turtles and Chelonia mydas
L. (Green Sea Turtles), and Wilkinson and Gibbons (2005) reviewed studies on many
species of freshwater turtles.
The egg measurements we obtained were similar to those reported for other Loggerhead
Sea Turtle populations around the world, suggesting that this trait may have
significant evolutionary and physiological implications (i.e., egg size is under strong
selection pressure). Tiwari and Bjorndal (2000) found that egg size decreased with
increasing latitude when looking at turtles in the Atlantic and the Mediterranean.
However, we observed the same range in egg size reported by Tiwari and Bjorndal
(2000) throughout one season for turtles on BBI and WI. In fact, a cautionary note for
researchers may be to maintain consistency in the time of season when the eggs are
measured (i.e., sample early, mid-, or late season or sample the entire season) so that
seasonal variation is not a factor when making statistical comparisons.
A long-term study conducted by Rollinson and Brooks (2008) provides some
support to the claim that a positive correlation between egg mass and maternal body
size may exist in small-bodied turtles because of maternal physical constraints on
egg size coupled with selection towards an optimum value (Congdon and Gibbons
1987, Smith and Fretwell 1974). Rollinson and Brooks (2008) hypothesized that
egg size increases with female body size until females are large enough to produce
eggs of an optimal size (reviewed by Rollinson and Brooks 2008), as found in our
study with Loggerhead Sea Turtles.
In this study, hatchling length explained 7.74% of the variability in clutch size,
second to maternal size. Hatchling size increased with increasing maternal length
but was negatively correlated with clutch size, indicating the trade-off between
clutch size and hatchling size. A positive correlation between maternal size and
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offspring size (even within a population) has also been reported by Fox and Czesak
2000 ( reviewed by Roff 1992) and Rollinson and Brooks (2008), while others have
found no correlation (reviewed by Rollinson and Brooks 2008). In our study, over
half of the variability in hatchling length was explained by hatchling mass. Positive
correlations between egg size and hatchling size have been reported in numerous
species, including several species of freshwater turtles (reviewed by Wilkinson and
Gibbons 2005, Rollinson and Brooks 2008).
Ecological, evolutionary, and physiological implications
In summary, the nesting biology of Loggerhead Sea Turtles on WI and BBI in
Georgia, is similar in many ways to that previously described for Loggerhead Sea
Turtles in the southeastern US. Over half of the variability we saw in Loggerhead
Sea Turtle clutch size was attributable to maternal length and hatchling length,
while the other half of the variation was not explained in our models. In general,
larger females produce larger clutches up to a point, but there also seems to be
a trade-off between clutch size and hatchling size in this Loggerhead Sea Turtle
population. We found that egg size did not contribute significantly to clutch-size
determination. Therefore, it is conserved among females, and a certain size of egg
is being selected. The results of this study agree with the findings of Wallace et al.
(2007), who surmised that larger clutch sizes, rather than bigger eggs, are produced
due to the high mortality at early life stages. There is further support for this theory
in the results of modeling by Mazaris et al. (2005) to assess sea turtle population
viability, which concluded that producing many hatchlings could make up for
mortality in other life stages. However, increasing juvenile survivorship does not
offset the loss of adults in turtle populations; adults represent the most important
and critical life stage for conservation (Congdon et al. 1993, 1994).
The results of many studies suggest that larger hatchlings may have increased
fitness (Froese and Burghardt 1974, Janzen et al. 2000, Miller et al. 1987), especially
in a resource-poor environment (Noriyuki et al. 2010); however, Congdon et
al. (1999) was not able to detect any benefit of increased hatchling size. Rollinson
and Brooks (2008) offered the hypothesis that larger females have more energy to
allocate to reproduction and more space in which to hold their clutch. Larger females
also dig deeper nests to hold larger clutches, thus their hatchlings must have
greater energy reserves (i.e., better fitness) to emerge from a deeper nest cavity.
Future research should focus on whether energy allocation per egg differs between
smaller and larger Loggerhead Sea Turtles and if this translates into actual benefits
in terms of maternal or hatchling fitness.
Acknowledgments
We thank the Caretta Research Project on Wassaw NWR and the USFWS Loggerhead
Sea Turtle Program on Blackbeard Island NWR for providing personnel, equipment, and
funding to conduct this project. We also thank David Veljacic, Robert Cail, Peter Range,
Karen Pacheco, Darryl Woodard, Scott Gilge, Mark Dodd, and Adam Mackinnon, as well
as USFWS interns (Alison Brady, Laura Csoboth, Neil McIntosh, and Jake Tuttle), and
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Caretta Research Project volunteers. Logistical support was provided by Georgia Southern
University, Statesboro, GA; USFWS; Georgia Department of Natural Resources, Atlanta,
GA; and The University of Alabama at Birmingham. In addition, we thank Thane Wibbels
and Susan Jewell for their review of this manuscript as well as Jessica Stephen and Nick
Farmer for statistical assistance. Permits were provided by the Georgia Department of
Natural Resources (29-WMB-02112), USFWS (41620-00007; 41620-02018), and Georgia
Southern University IACUC. Additional funding was provided by Georgia Southern University
Competitive Research Grants and Academic Excellence Award.
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