1098
Reproductive investment by green turtles nesting on Ascension Island
C.
GRAEME
HAYSJ
Department of Zoology, Universitv of Aberdeen, Aberdeen, AB9 2TN, United Kingdom
Department of Aquaculture,
COUN
R. ADAMS
University of Stirling, Stirling, SK9 4LA, United Kingdom
AND
JOHN
R.
SPEAKMAN
Department of Zoology, University of Aberdeen, Aberdeen, AB9 2TN, United Kingdom
Received October 15, 1992
Accepted February 10, 1993
HAYS, G.c., ADAMS, C.R., and SPEAKMAN,J.R. 1993. Reproductive investment by green turtles nesting on Ascension
Island. Can. 1. Zool. 71: 1098 - 1103.
Clutch sizes and egg sizes were measured for green turtles (Chelonia mydas) nesting on Ascension Island in the South
Atlantic (7°57'S, 14°22'W) in 1992, The mean number of eggs per clutch was 127.5 (SD = 22.9; n = 46 clutches). The
number of eggs per clutch increased in larger turtles and declined as the nesting season progressed. Mean egg size was
45.5 mm (SD = 1.45 mm; n = 47 clutches). Three turtles laid significantly smaller eggs than the rest of the sample. When
these three clutches were removed from the analysis, mean egg size increased in larger turtles. When the effect of female
body size was removed there was no relationship between the number and size of the eggs in a clutch. The depth from the
top to the bottom of the egg chamber, the depth from the top of the egg chamber to the topmost egg, and the depth of the
body pit were all independent of adult size. Egg size varied systematically within clutches, the largest eggs being laid first
and the smallest eggs last. This intraclutch variation in egg size had important consequences for the calculated gradient of
the relationship between egg size and adult size.
HAYs, G.c., ADAMS, C.R., et SPEAKMAN,1.R. 1993, Reproductive
lsland. Can. 1. Zoo!. 71: 1098-1103.
investment by green turtles nesting on Ascension
Le nombre d'oeufs par couvee et la taille des oeufs ont ete mesures chez des Tortues venes (Chelonia mydas) au cours
de la saison de nidification de 1992. sur lile de lAscension , dans le sud de lAtlantique (7°57'S, 14°22'0). Le nombre
moyen d'oeufs par portee a ete evalue a 127,5 (ecart type = 22,9; n = 46 portees). Le nombre doeufs par portee augmentait
chez les grosses torrues et diminuait a mesure que la saison de ponte avancait. La taille moyenne dun oeuf etait de 45,5 mm
(ecart type = 1,45; n = 47 portees). Trois torrues ont pondu des oeufs significativernent plus petits que les autres torrues
de lechantillon. Apres elimination de ces trois portees dans les analyses. la taille moyenne des oeufs sest averee plus grande
chez les grosses torrues. Apres correction pour elirniner l'effet de la taille des femelles, il ny avait plus de relation entre
le nombre doeufs et leur taille dans une couvee. La profondeur du haut jusquau fond de lenceinte de ponte. la profondeur
du haut de lenceinte de ponte jusqua loeuf sirue sur le dessus dans lenceinre et la profondeur de la cavite occupee par
la tortue etaient routes independanies de la taille de ladulte. La taille des oeufs variait significativement au sein des couvees :
les oeufs les plus gros etaient pondus les premiers et les oeufs les plus petits. les derniers. Cene variation intragroupe de
la taille des oeufs a une grande importance lorsqu'on calcule le gradient de la relation entre la taille des oeufs et la taille
des adultes.
[Traduit par la redaction]
Introduction
There is considerable
interest 'in the ultimate factors that
influence egg size (Smith and Fretwell
1974; Brockelman
1975; Parker and Begon 1986). Models aiming to predict
optimal egg size assume that the survivorship of eggs is related
to their size, with smaller eggs having a lower chance of survival. Since the probability
of survival must have an upper
limit the relationship
between egg survivorship
and egg size
cannot be linear but must be a curve of decreasing gradient as
the upper asymptotic
limit of survivorship
is approached.
Since there must also be a limit to the resources an individual
can partition to reproduction,
if individuals are operating at the
limit of their resource availability
there may be a trade-off
between the number and size of eggs that a female can produce. Hence, when female body size is held constant a negative relationship
between egg size and number is predicted.
'Present address: The Sir Alister Hardy Foundation For Ocean
Science, c/o Plymouth Marine Laboratory, Prospect Place, The Hoe,
Plymouth, PLl 3DH, United Kingdom.
Printed in Canada
I
Imprime
This negative relationship
has been found both intraspecifically in oviparous and viviparous snakes (Ford and Killibrew
1983; Ford and Seigel 1989), the yellow mud turtle (Kinostern on flavescens) (Iverson 1991), and lizards (Stewart 1979;
Nussbaum
1981), and interspecifically
across fishes (Elgar
1990), mammals (Read and Harvey 1989), birds (Rohwer
1988), and turtles (Elgar and Heaphy 1989).
Several egg size models predict a single optimal egg size
that maximises the number of offspring surviving; this optimal
size corresponds to the point where for further increases in egg
size the marginal increase in survival is offset by the marginal
decrease in the total number of eggs produced (Smith and
Fretwell 1974; Brockelman
1975; Parker and Begon 1986). If
the relationship between egg size and survivorship
is constant,
both spatially and temporally,
these models predict a spatiotemporally stable optimal egg size and, hence, changes in total
investment
are expected to be achieved by changes in egg
number. If, however, the nature of the relationship
between
egg size and egg survivorship
varies, in space and (or) time,
there may be spatio-ternporal
variability
in the optimal egg
au Canada
I
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.
HAYS ET AL.
size. leading to more complex trade-offs in the apportioning
of total investment between egg size and clutch size (McGinley
et al. 1987).
For loggerhead
sea turtles (Caretta carenai nesting in
Greece, egg size is independent
of adult size, but changes
seasonally, while egg number is positively related to adult size
(Hays and Speakrnan 1991, 1992). These data therefore suggest a temporally changing optimal egg size, with larger turtles
increasing their investment
per clutch, at anyone
time, by
increasing clutch size rather than egg size. Similarly, for some
species of freshwater
turtle (e.g., Pseudemys jloridana and
Sternotherus odoratusi, increases in clutch volume in larger
females are mediated through increases in egg number and not
egg size (Congdon and Gibbons 1985). In some other species
of freshwater
turtle (e.g., Kinosternon [lavescens, K. subrubrum, Deirochelys reticularia, Chrysemys picta, and Pseudemys scripta) both egg number and egg size increase in larger
females (Congdon and Gibbons 1985, 1987; Congdon et al.
1983. 1987; Iverson 1991). Such differences in egg size between
individual females at a given time are not anticipated from the
simple models of optimal egg size and require explanation.
In
two of these species (C picra and D. reticul.aria) the mean egg
diameter per clutch has been shown to increase with adult size
with the same gradient as pelvic width. This has led to the suggestion that in these species. pelvic width may be a constraint
on egg size; that is. small turtle
with a narrower pelvis lay
smaller eggs than those that optimise the number of offspring
surviving (Congdon and Gibbons 1987).
There would therefore appear to be a dichotomy between
species of turtles in which egg size is independent of the adult
size (suggesting that egg size may be set at an optimum) and
species in which egg size may be constrained by pelvic width.
However. the situation is complicated
further by other species
in which egg size increases
with adult size but where the
gradient of this relationship
is significantly
less than that
between pelvic width and adult size (Congdon and Gibbons
1987: Iverson 1991). For example. in the yellow mud turtle
iKinostemon jlavescens) mean egg diameter per clutch increases
with adult size with a gradient of 0.119, whereas pelvic width
increases with adult size with a significantly
higher gradient,
O. J 68 (Iverson 1991), Similarly, in Pseudemys scripta mean
egg diameter per clutch increases with adult size with a gradient of 0.026. while pelvic width increases with adult size
with a significantly
higher gradient of 0.096 (Congdon and
Gibbons 1987). In these previous studies examining the relationship between egg size and female size, mean egg size for
a clutch has been used as a measure of egg size (Congdon and
Gibbons 1987; Iverson 1991). However, if there is systematic
intraclutch variation in egg size, this may influence the interpretation of egg size - adult size relationships.
In this paper we examine the relationships
between egg
number, egg size, and adult size for green sea turtles (Chelonia mydas) nesting on Ascension Island in the South Atlantic
(7°57'S,
14°22'W).
We investigate whether a single optimal
egg size exists and whether there is a trade-off between egg
size and egg number. We examine intrac1utch variability
in
egg size and the effects of this variability on the calculated
gradient of the relationship
between egg size and adult size.
We also quantify the relationship
between adult size and nest
depth to see whether systematic variation in nest depth dependent on adult size may cause different optimal egg sizes for
different sized adults.
1099
Methods
STudy species and STudy site
The population of green turtles on Ascension Island has been the
subject of sporadic research over the last 30 years. There are approximately 5000 - 10000 nests on the island each year, an estimated
average of three nests in a season for each female; the females
return to lay successive clutches with a mean interval of 13.9 days
(Mortimer and Carr 1987). Green turtles usually nest at night, first
digging a body pit with all four flippers before excavating an egg
chamber using only their rear flippers. The green turtle rookery on
Ascension Island is an ideal location at which to make detailed
measurements of the variation of egg size with egg sequence within
a clutch. Firstly, this is because the nesting density is very high and,
hence, it is possible to encounter sufficient turtles at the start of egg
laying. Secondly, in green turtles, unlike some other species such as
the loggerhead turtle, the posterior of the carapace does not usually
extend over the whole of the egg chamber, so it is possible to observe
the eggs as they are laid and to record egg number and size within
a clutch.
Protocol
During February, March. and April 1992, Long Beach, one of the
major nesting beaches on Ascension Island. was patrolled at night to
locate nesting turtles. Turtles were first observed at a distance of
approximately 20 m through 8 x 50 binoculars to ascertain their
orientation and their stage of nesting. Turtles were then approached
from behind and watched closely until they started to lay their eggs.
When there were two observers in the field we then both \i) counted
the eggs as they were laid. and (ii) measured the maximum diameter
(to an accuracy of 0.1 mm) of approximately every 10th egg (the first
measurement usually being made of about the 20th to 30th egg laid)
using dial callipers (Rabone). We found it impossible for a single
observer to both count and measure the eggs as they were laid. Therefore, when there was only one observer either (i) the total number of
eggs was counted. or (ii) the diameter of approximately 10 eggs was
measured throughout egg laying. To maximise precision the dial
callipers were zeroed prior to each egg measurement. and to verify
their accuracy they were cross-calibrated with vernier callipers
(Carnlab).
For a sample of nests we used a semi-rigid tape measure (Stanley
Tools) 10 record the depth of the body pit, the depth from the top to
the bottom of the egg chamber. and the depth from the top of the egg
chamber to the topmost egg. We defined the top of the egg chamber
(and thus also the bottom of the body pit) as the posterior tip of the
postcentral scute at the carapace midline. When measuring the body
pit depth we defined the beach surface by placing an approximately
3 m long ruler over the nest with the ruler resting on the undisturbed
beach surface on either side. Body pit depth was then measured as the
vertical distance from the ruler to the top of the egg chamber. We
measured the depth of the egg chamber when excavation was complete, the depth of the topmost egg when egg laying had stopped, and
the depth of the body pit during egg laying.
When egg laying was complete and the turtle had started to cover
the nest we measured the curve carapace length (CCL) from the
anterior of the precentral scute to the posterior of the postcentral scute
using a flexible 1.5-m tape. To remove any variation caused by
different individuals making measurements, all egg diameters and
adult lengths were measured by one person (G.c.H.).
Data were analysed using Minitab Statistical Software (Minitab
Inc.). Least squared fit regressions are used throughout.
Results
The number of eggs laid was recorded for 46 clutches. The
mean number of eggs per clutch was 127.5 (SD = 22.9,
range = 83 -170).
Clutch size increased in larger turtles and
decreased during the season. Of the total variation in the number of eggs laid per clutch, 14.5 % was explained by the size
I
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1100
J. ZOOL.
VOL. 71. 1993
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of the turtle and 13.9% by the date. Defining the date as the
number of days since March 1, the complete equation was
=
1.9 CCl - 0.95 date - 36.3
r2
=
0.28)
The depth of the body pit averaged 33.2 cm (SD
=
11.1;
(Fi2.43)
= 8.6.
p
< 0.01,
40
80
120
160
Egg number from start of clutch
(cm)
1. Relationship between mean egg diameter and the curve
carapace length (Cf.L. cm) of the adult turtle. Ignoring the three data
points marked as open squares, mean egg diameter was 47.74 mm
(SD = 1.13: /1 = 44 clutches) and increased significantly in
larger turtles: mean egg diameter (mm) = 0.102 eeL (cm) + '33.8
(fj1.42) = 11.8, P < 0.01, r2 = 0.22). None of the residual variation in egg size was explained by the date on which the clutch was
laid (stepwise regression, p > 0.05). The three clutches with small
eggs were significantly smaller than predicted on the basis of the adult
length (Z = 5.3,4.3.2.8,
p <ii 0.01).
number of eggs per clutch
o
130
FIG.
[1]
•
'2~--~-'--~--,---~~--~~
n = 83), the depth from the top to the bottom of the egg chamber averaged 58.5 cm (SO = 2.2; n = 48) and the depth from
the top of the egg chamber to the topmost egg averaged
32.0 cm (SO = 6.2; n = 63). All three measures were independent of turtle size (CCl) (Fi1.46) = 2.5, Fil.61) = 1.0, and
Fil.81) = 2.1, P > 0.05).
'
Egg diameters were recorded for 47 clutches. Mean egg
diameter was 45.5 mm (SO = 1.45 mm; ranse = 41.048.5 mm: n = 47 clutches). When three clutches with very
small eggs were removed from the analysis (i.e., a restricted
sample of 44 clutches was used), mean egg size increased in
larger adults but did not change during the season (stepwise
regression, p > 0.05) (Fig. I). We calculated the deviation
(Z score) of the three clutches with anomalously small eggs
from the regression equation relating egg size to adult size for
the remaining 44 clutches (Fig. 1). For each of these three
clutches with small eggs, Z > 2.8, i.e .. the probability of getting even one clutch with eggs this small from the observed
distribution of increasing egg size with adult size was less than
one in a hundred.
Both the total number of eggs and mean egg diameter were
recorded for 24 clutches. Using the egg number and mean egg
diameter predicted on the basis of adult length we calculated
the residuals (actual value minus predicted value) of egg number and mean egg diameter for these 24 clutches. There was
no relationship between the residuals of egg number and mean
egg size (F[I.22] = 0.04, p > 0.05).
For the 22 clutches for which both the diameter and the
number of that egg within the clutch were recorded, egg diameter was not independent of the number of the egg. Instead
FIG. 2. Relationship between the relative egg size (egg size minus
mean egg size for that clutch) and the number of the egg within the
clutch (/1 = 22 clutches). Each value is the mean for a 1O-egg
interval. (Egg number 1 = the first egg laid.)
--
larger eggs tended to be laid at the start of a clutch and smaller
eggs at the end. Thus, eggs that were among the first 75 to be
laid had diameters, on average, slightly « 0.5 mm) larger
than the mean size for the clutch, while the l40th to 150th eggs
had diameters, on average, > 1.6 mm smaller than the mean
diameter for the clutch (Fig. 2).
the decline in egg size between the start of a clutch (the
mean diameter of the 20th to 40th eggs laid) and the end of a
clutch (the mean diameter of the last 20 eggs laid) averaged
1.21 mm (SE = 0.25; n = 22 clutches). This decline in egg
size within clutches was entered into a stepwise regression
against the number of eggs laid in the clutch, the size of the
turtle (CCl), and the mean egg diameter of the 20th to 40th
eggs laid. The only factor that entered the resulting regression
equation was the number of eggs laid. the largest decline in
egg size being found in the largest clutches (Fig. 3). None of
the residual variation in the intraclutch decline in egg size was
explained by the date on which each nest was laid (stepwise
regression, p > 0.05).
. For the 22 clutches for which we had complete data on egg
diameters and egg number within a clutch, we investisated
how the decline in egg size within a clutch influ'~nced the
gradient of the relationship between mean egg size per clutch
and adult size. Firstly. we calculated this gradient when only
the last 40 eggs laid in a clutch were used in the analysis. the
last 50 eggs laid, the last 60 eggs, and so on. Secondly, we calculated this gradient using only the first 20 eggs laid in a
clutch, the first 30, and so on, until all the measured eggs were
included in the analysis. The gradient of the relationship
between mean egg diameter per clutch and adult size was low
«0.07)
when only eggs laid at the end of a clutch were
included in the analysis, high (> 0.135) when only eggs laid
at the start of a clutch were included, and O. J08 when all the
measured eggs were included (Fig. 4).
Discussion
In a previous study of clutch sizes for green turtles on
Ascension Island, Simon and Parkes (1976) reported that the
mean number of eggs per clutch was 116.3 in 1973 (n = 169
nests) and 127.9 in 1974 (n = 163 nests). These figures are
close to the value we recorded (127.5 eggs; n = 46 nests). As
in our study, Simon and Parkes (1976) found that there
was a significant positive relationship between the number of
j
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•
HAYS ET AL.
1101
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80
100
120
140
160
180
Number of eggs laid
3. Decline in egg size between the start of a clutch (mean
diameter of the 20th to 40th eggs laid) and the end of a clutch (mean
diameter of the last 20 eggs laid) ploned against the total number of
eggs laid in the clutch. Egg decline (mm) = 0.0334 (number of eggs
laid) - 2.83 (Fjuol = 13.0, P < 0.01, r2 = 0.39).
•
FIG.
eggs in a clutch and the size of the adult turtle. By using numbered flipper tags to allow individual turtles to be identified,
Mortirner and Can (1987) showed that the number of eggs per
clutch tended to decrease as a turtle laid successive clutches.
This is consistent with the seasonal decline in the number of
eggs per clutch that we found (eq. I), since turtles would have
been laying successive clutches as the season progressed.
It has been suggested that the larger clutches laid by larger
turtles may be accommodated
by larger turtles laying deeper
nests (Bjorndal
and Carr 1989). The relationship
between
adult size and egg chamber depth has not, however. been previously quantified.
We found that the depth to the bottom of
the egg chamber showed little variation and was independent
of the size of the turtle. The depth to the topmost egg in the
egg chamber was more variable but was again independent
of
turtle size. Thus. larger turtles did not dig deeper nests. nor
did they fill their nests fuller with eggs. However.
larger
turtles lay more eggs (this study; Simon and Parkes 1976) and
this would suggest that these larger clutches are accommodated by increasing the width of the nest.
The mean egg size for several spatially separated
green
turtle populations
has been reviewed by Hirth (1980). Means
include 45.7 mm at Tortuguero,
Costa Rica: 45.0 mm at Suriname; 44.7 mm at Europa Island, Indian Ocean: 44.6 mm at
Tromelin Island, Indian Ocean; 46.3 mm at Aldabra Atoll;
42.3 mm at Sharma Beach, South Yemen; 46.0 mm at Heron
Island. Australia: and 44.0 mm at Hawaii. The variation in the
mean egg size for different green turtle rookeries may indicate
spatial variability
in the relationship
between egg size and
survivorship
and, hence, selection for different optimal egg
sizes. Whereas the maximum reported mean egg diameter for
a population away from Ascension Island is 46.3 mm for green
turtles nesting on AJdabra Atoll, the previously reported mean
egg diameter
for green turtles on Ascension
Island was
54.6 mm (n = 10 nests, with JO eggs measured in each nest)
(Carr and Hirth 1962), which is substantially
larger than the
mean egg diameter reported here (45.5 mm) and for other
green turtle rookeries. Yet while the mean egg size measured
by Carr and Hirth is > 8 mm larger than that for any other
population,
the mean hatchling size they measured is in the
middle of the range reported for various green turtle populations (reviewed
by Hirth 1980). Our callipers were zeroed
o
0.06
20
40
60
80
100
120
140
160
Egg number
FIG. 4. Gradient of the relationship between egg diameter and adult
size when different parts of clutches were included in the analysis.
., maximum egg number from the end of the clutch that was
included in the analysis (ME). Gradient = 0.0809 log(ME) - 0.0636
(FjI.IOI = 317, P < 0.01, r2 = 0.97). 0, maximum egg number
from the start of the clutch that was included in the analysis (MS).
Gradient = (142176 - 0.OO0047(MS)4) x 10-6 (Fjl.l21 = 48.5.
P < 0.01, r2 = 0.80). The maximum measured egg number from
the end of a clutch was 144 (4th egg laid from a clutch of 148) while
the maximum measured egg number from the start of a clutch was
159 (l59th egg laid from a clutch of 161 eggs). Thus, the final point
on each plot represents all the measured eggs used in the analysis.
hence the calculated gradients for these two points are the same
(0.108). The regression equations from which the gradients were calculated were all significant at p < 0.05.
before every measurement,
and cross-calibrated.
hence we are
confident in both their accuracy and their precision, whereas
Carr and Hirth give no details concerning how their measurements were made. The discrepancy
with Carr and Hirth ' s
previous measurement
remains unresolved.
but may reflect
either a real temporal change in the mean egg size or simply
a methodological
difference between the two studies.
Eggs in 3 of a total of 48 clutches had signi ficantly smaller
mean diameters (Fig. I). Egg size in turtles typically shows a
consistent
pattern within populations.
either increasing
or
being independent of adult size (Congdon and Gibbon, 1985:
Hays and Speakrnan 1991). The finding that three turt les laid
significantly
smaller eggs than the rest of the sample was
therefore unexpected.
These clutches with small egus may
represent reproductive
senescence.
The turtles producing the
small eggs were at the large end of the size distribution.
However, marine turtles grow only very slowly, or not at all. once
they have reached sexual maturity (e.g .. 0.2 cm per year for
loggerhead
turtles in Australia
(Limpus
1985), and green
turtles in the Indian Ocean show no significant
increase
(Le Gall et a!. 1985)). Adult size is therefore probably not
influenced primarily by age but by the size at sexual maturity.
Thus, the large size of the three turtles that laid small eggs
does not necessarily mean that they were older than the rest
of the sample.
A second potential explanation
for these observations
is that
these three clutches were laid by turtles that had moved to
Ascension Island from another rookery characterised
by turtles
that laid smaller eggs. This hypothesis would require a major
breakdown
in nesting beach fidelity by these turtles, since
the nearest alternative
nesting beaches are > 2000 km from
Ascension Island. Such movement
between nesting beaches
has been documented.
For example,
a green turtle in the
I
1102
CAI'.
J. ZOOL.
Indian Ocean nested on beaches 2250 km apart (Le Gall and
Hughes 1987). Further evidence for poor nesting beach fidelity in some turtles is provided by observations
that every year
green turtles attempt to nest on Saint Helena (15°55'S, 5°43'W),
an isolated Atlantic Island 1100 km southeast of Ascension
Island, despite the fact that the rocky coastline and lack of
sandy beaches preclude successful
nest excavation
and egg
laying (Edwards 1990). If there are discrete subpopulations
of
turtles nesting on Ascension Island that represent turtles from
other rookeries,
this may be reflected in genetic heterogeneity
in the turtles. Bowen et al. (1992) sequenced the mitochondrial
DNA of hatchlings
from 35 different
nests on Ascension
Island and found that 34 of the 35 nests were characterised
by
one haplophyte,
while one nest was characterised
by a second
haplophyte,
supporting the suggestion that this population may
be genetically
heterogeneous.
How these genetic differences
relate to differences
in egg size between individuals remains
to be investigated.
Both egg size and egg number increased in clutches laid by
larger turtles on Ascension Island, while egg size' decreased
monotonically
from the start to the end of clutches. This would
suggest that if there is a single optimal egg size for this population, it is attained neither by all individuals nor for all the eggs
laid by the same individual. Similarly, for green turtles nesting
in Costa Rica, both egg number per clutch and mean egg diameter increased positively with adult carapace length (Bjorndal
and Carr 1989). This pattern for green turtles contrasts with
that found for loggerhead turtles, for which increases in reproductive investment per clutch in larger turtles were associated
only with increases in the number and not in the size of eggs
(Hays and Speakman 1991), but is similar to that reported for
some freshwater
turtles for which increases in both egg number and egg size occur in larger individuals
(Congdon and
Gibbons 1985, 1987; Iverson 1991). Parallel gradients of the
relationships
between (i) egg size and adult size and (ii) pelvic
width and adult size suggest that in some species of freshwater
turtle, egg size is constrained by the width of the pelvic canal
and hence individuals
cannot achieve the optimal egg size
(Congdon
and Gibbons 1987). This constraint
may also be
operating
in green turtles, although there are no data on the
relationship
between pelvic width and adult size for this species with which to test this hypothesis.
Variability in egg size
in relation to adult size among individuals does not necessarily
reflect constraint
by pelvic width. For example. if nest depth
were dependent upon the size of the turtle and optimal egg size
increased positively with depth, one would anticipate an increase
in optimal egg size with turtle size. In the current population
we can reject this interpretation,
since there was no relationship between nest depth and adult size.
There was no inverse relationship between the residual number of eggs in a clutch and the residual mean egg diameter, and
hence no evidence for the trade-off between egg size and egg
number predicted by egg size optimality models (e.g., Smith
and Fretwell
1974; Parker and Begon 1986). The absence of
this trade-off suggests that total clutch volume for green turtles
nesting on Ascension Island is not generally operating at the
limit of an individual's
resource availability (e.g., body cavity
volume).
Further evidence in support of this hypothesis
is
found in the large differences in egg number per clutch previously reported for nests on Ascension Island laid by the same
individuals
within a season. Thus for individuals
recorded
nesting 3 or more times, the number of eggs in the smallest
clutch averaged only 70.6% (range = 40.6-91.4%;
n = 12
VOL. 71. 1993
turtles) of the number in the largest clutch (Mortirner and Carr
1987).
The decrease in egg size within a clutch may have an adaptive
explanation, e.g., in terms of differing physicochemical
characteristics within the nest. Alternatively,
the decrease in egg size
within a clutch may simply reflect a constraint associated with
the physiology of egg production,
marine turtles being unable
to produce an entire clutch of eggs of the same size.
There has been no consideration
of how the gradient of the
relationship
between egg and adult size is affected by intraclutch variability in egg size. We found that this gradient was
. significantly
influenced
by which eggs within a clutch were
sampled (Fig. 4). Hence, when comparisons
are made with the
gradient of the relationship
between adult size and pelvic
width, it may be more insightful to select that part of the clutch
where the gradient between egg and adult size is maximal
(e.g., the start of each clutch). If only the mean egg diameter
for an entire clutch is considered,
the gradient of the relationship between egg size and adult length may be underestimated.
Acknowledgements
We thank the Administrator
of Ascension Island, Mr. Brian
Connelly, for permitting
this work to be conducted.
We are
indebted to Cable and Wireless, the Royal Air Force, and the
Office of the Administrator
for providing accommodation
on
Ascension Island, and to Steve and Tina Cox and Sam Turtle
for all their help when we were conducting fieldwork. G.C.H.
was supported by grant GR9/585 from the Natural Environmental Research Council.
We thank Jeanne Mortimer
and
Brian Bowen for stimulating discussions
concerning this work
and three anonymous reviewers for their constructive comments.
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