Article

Order of oviposition and egg size in the red-eared slider turtle (Trachemys scripta elegans)

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Abstract

We studied intraclutch variation in egg size and shape among 432 eggs from 35 clutches of the red-eared slider turtle (Trachemys scripta elegans) from west-central Illinois. Egg shape varied systematically with respect to oviposition order. The first two and last two eggs laid were slightly but significantly longer than eggs laid between the first two and last two eggs. The last two eggs laid were slightly but significantly narrower than other eggs in a clutch. In contrast, egg mass did not vary systematically with oviposition order. Mean egg mass of the first two eggs laid was slightly but insignificantly greater than that of the last two eggs laid and that of eggs laid between the first two and last two eggs. In this species, follicles are apparently fairly uniform in mass when they are ovulated. Consequently, reproductive output can be adjusted by either ovulating more equal-sized follicles or adding material uniformly to all follicles ovulated in a particular reproductive bout. We suggest an adaptive explanation of differences between intraclutch variation in T. s. elegans and that reported in previous studies of intraclutch variation in sea turtles.

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G2 (N=20): with Class I (>5-10mm) follicles, with Class I and Class II (>10-fold) follicles, 25mm) and G3 (N=9) with Class I, Class II and Class III (>25mm) follicles. Analysis of variance, Scott-Knott's test, and Pearson's correlation analysis showed that there was no significant difference between the groups in body biometry; in the mean gonadosomatic index and gonadal morphometry, only the width of the oviducts in the right antimer and the mass and width in the left antimer were higher in G3, the only one that presented eggs. There was positive and harmonic development between body mass, carapace, and plastron, and gonadal growth occurred concomitantly with body growth, indicating a higher reproductive potential and a positive relationship between the size of the litter and the female litter. The gonadosomatic index proved to be an excellent reproductive indicator, and the ovarian evaluation was a better indicator of sexual maturity than the maximum carapace length. 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Bras. 40(2):141-154, February 2020 RESUMO.-[Biologia reprodutiva, morfologia e morfometriade ovários e ovidutos de Trachemys scripta elegans no Cerrado Brasileiro.] Embora Trachemys scripta elegans seja uma espécie exótica popular como animal de estimação no Brasil, estudos sobre biologia e capacidade reprodutivas são inexistentes no Cerrado brasileiro. Este estudo analisou características ovarianas e do oviduto e a capacidade de produção de ovos em T. scripta elegans criadas neste bioma, correlacionando estes achados ao tamanho dos espécimes e a maturidade sexual, visando comparações com populações nativas e exóticas, bem como interespecíficas e contribuir para a compreensão de seu impacto nos ecossistemas invadidos e com o estabelecimento de programas de erradicação. Assim, 39 fêmeas tiveram avaliadas a biometria corporal e a morfologia e morfometria dos ovários e ovidutos. 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Recent papers opposing ratio dependence focus on four main criticisms: (1) the empirical evidence we present is insufficient or biased, (2) ratio-dependent models exhibit pathological behavior, (3) ratio dependence lacks a logical or mechanistic base, and (4) more general models incorporate both prey and ratio dependence and there is no need for either of the two simplifications. We review these arguments in the light of empirical evidence from field and experimental studies. We argue that (1) empirical evidence shows that most natural systems are closer to ratio dependence than to prey dependence, (2) ''pathological'' dynamics in a mathematical sense is not only realistic, but the lack of such dynamics in prey-dependent models actually makes them pathological in a biological sense, (3) the mechanistic base of ratio dependence is (direct and indirect) interference and resource sharing, and (4) although more general models (with extra parameters) can never fit natural patterns worse than either prey- or ratio-dependent models, there are theoretical, practical, and pedagogical reasons for attempting to find simpler models that can capture the essential dynamics of natural systems.
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Clutch sizes and egg sizes were measured for green turtles (Chelonia mydas) nesting on Ascension Island in the South Atlantic (7-degrees-57'S, 14-degrees-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.
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Respond to criticisms of ratio-dependence (see Ecology 75(6): 1834-1850) by explaining in detail the origins and justification of the approach.
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We re-evaluated information presented in Bergerud et al. (1983) and suggest that no firm conclusions can be drawn with respect to factors limiting moose populations at Pukaskwa National Park (PNP), Ontario, and that predation by wolves alone does not limit moose at Isle Royale, Michigan. We comment on 3 aspects of their paper: methods, other limiting factors, and their model. Three alternative hypotheses are presented based on our discussion of effects of weather, food, cohort vulnerability, and other sources of mortality.
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(1) Examples of geometric growth patterns in large-mammal populations are examined, and used to assess a 'ratio' method for estimating growth rate. (2) Simple difference equation models for populations subjected to extensive removals are used to estimate potential rates of growth in the presence of removals, using the ratio method and a minimum chi-square (χ2) fitting procedure. Ten examples demonstrate use of the models and methods of estimating growth rates. (3) Density-dependence is evident in some examples, and complicates fitting and projecting into the future. (4) The approach requires absolute population estimates when removals occur, and is unsatisfactory if there is a substantial unmeasured loss associated with the known removals.
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Calf mortality is a major component of caribou (Rangifer tarandus) population dynamics, but little is known about the timing or causes of calf losses, or of characteristics that predispose calves to mortality. During 1984-87, we radiocollared 226 calves (≤3 days old) in the Denali Caribou Herd (DCH), an unhunted population utilized by a natural complement of predators, to determine the extent, timing, and causes of calf mortality and to evaluate influences of year, sex, birthdate, and birth mass on those losses. Overall, 39% of radio-collared calves died as neonates (≤15 days old), and 98% of those deaths were attributed to predation. Most neonatal deaths (85%) occurred within 8 days of birth. Few deaths occurred after the neonatal period (5, 10, and 0% of calves instrumented died during 16-30, 31-150, and >150 days of age, respectively). Survival of neonates was lower (P = 0.038) in 1985, following a severe winter, than during the other 3 years. In years other than 1985, calves born during the peak of calving (approx 50% of the total, born 5-8 days after calving onset) experienced higher (P < 0.001) neonatal survival than did other calves. Grizzly bears (Ursus arctos), wolves (Canis lupus), and unknown large predators (i.e., grizzly bears or wolves) accounted for 49, 29, and 16% of the neonatal deaths, respectively. The rate of bear-caused mortalities declined (P < 0.001) with calf age, and bears killed few calves >10 days old. Wolf predation was not related (P > 0.05) to calf age and peaked 10 days after onset of calving. Grizzly bear and wolf predation on neonates during the calving season was a limiting factor for the Denali Caribou Herd.
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We studied correlations among traits related to body size and reproductive behavior in marine turtles, using data from 96 different populations representing seven species. Our analyses focused on patterns of phenotypic covariation among species and among populations within species. At the species level, body size correlated positively with several reproductive traits, including egg size and overall reproductive effort. A trade-off between clutch size and egg size was confirmed for marine turtles, after factoring out the effects of body size. Patterns of variation within species were different from those among species. For example, in five out of six species there was a positive relationship between adult body size and clutch size, although this correlation was not found at the interspecific level. We also found important differences among species in the way life-history traits correlated with one another. Four species having a sufficient number of samples exhibited congruent worldwide patterns of body size variation. A comparative approach may prove useful for extending demographic models developed for loggerhead turtles to less well-known species, even though many of the model parameters have not been estimated for other species.
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In 1984, 94 complete three-egg clutches of Ring-billed Gull (Larus delawarensis) eggs were collected on Granite Island, northern Lake Superior. The fresh eggs (i.e., collected within 12 hr of being laid) were weighed, measured and frozen for later chemical analysis to determine if relationships existed between egg size, composition, and sequence. Egg size and mass decreased through the laying sequence. The average composition of fresh eggs was 35% yolk, 56% albumen, and 9% shell. Water accounted for 69% of fresh egg mass. Approximately 8% of fresh egg mass was yolk lipid. Dry yolk consisted of approximately 40% protein and 60% lipid. Caloric content averaged 1.55 kcal/g fresh egg mass. Within clutches, third-laid eggs contained absolutely less albumen and nutrients than earlier laid eggs. Albumen increased in proportion to the 1.2 power of egg mass, and the yolk in proportion slightly greater than the 0.66 power of egg mass. Results agreed well with egg composition studies of other semiprecocial species.
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The moose (Alces alces) population in Pukaskwa National Park, Ontario varied from 0.4 to 0.2 moose/km2 from 1975 to 1984 and was limited by wolf (Canis lupus) predation. Other noninsular moose populations coexisting with wolves, both hunted and unhunted, are also limited at densities of ≤0.4/km2 when recruitment (R) equals natural mortality (Mn) at a ratio of about 30 moose/wolf. When moose inhabit ranges without wolves, densities commonly reached 2-3 moose/km2 and food supplies become the limiting factor.
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The female reproduction of Chrysemys scripta elegans was compared in natural lakes and a lake receiving thermal effluent from a coal-fired power plant. Upon maturation turtles from the heated lake were younger and larger than those from the natural lake. The nesting season began earlier with mean clutch size being larger for the females in the heated lake. Reproductive potential was much greater in the heated lake, giving that population a much larger rate of natural increase.
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The relationship between the energy expended per offspring, fitness of offspring, and parental fitness is presented in a two-dimensional graphical model. The validity of the model in determining an optimal parental strategy is demonstrated analytically. The model applies under various conditions of parental care and sibling care for the offspring but is most useful for species that produce numerous small offspring which are given no parental care.
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Eggs of the turtle Chrysemys picta bellii from four western Nebraska populations were incubated under laboratory conditions to compare egg size and hatchling size as measures of offspring size among populations and in the context of maternal body size. Generally, hatchling size (mass and carapace length) was linearly related to egg size (mass, width, and length) and did not vary between the two years of the study or among populations after adjusting for egg size. However, hatchling carapace length adjusted for egg width was greatest in a population with the most elongate eggs indicating that linear egg measurements may not be useful for interpopulation comparisons. The smaller range of variation, but greater variability in hatchling size relative to egg size and a narrow range of maternal body sizes (carapace length and mass), seemed to preclude a correlation between maternal body size and hatchling size within two populations. In two other populations, hatchling carapace length, egg width, and egg length increased with maternal carapace length by a common slope, although egg wet mass had a steeper slope than did hatchling wet mass. It is probable that under natural incubation conditions (e.g., warmer, drier, and more variable), hatchling mass may increase only slightly or not at all with maternal body size. My results suggest that both hatchling and egg size should be considered in turtle life history studies, particularly for models that predict delayed sexual maturation when offspring size and survival increase with maternal body size and age.
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The theory is developed which relates the optimal division of parental resources among offspring (hence clutch size) to the nature of the intraspecific competitive process, the rate of mortality, and environmental predictability. It assumes that natural selection results in parents adjusting resource investment per offspring in a manner which achieves the highest ratio of offspring fitness per unit of parental resource expended, given a certain relationship between the expected fitness of individual offspring and the investment per offspring. This fitness-resource relation can be constructed from knowledge or models of the frequency distribution of parental resource utilized by individual offspring in the population and from the nature of the competitive process. Model examples are given based on assumptions concerning territorial quality, and on the rules of the game governing contests for resources. These models of interference lead to strong optimization for high expenditures of resource per offspring....
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The controversy over whether wolves (Canis lupus) can regulate ungulate numbers is difficult to assess, owing to data limitations, relaxation of predator control, and the fact that current predator – prey theory was developed from the study of invertebrate populations. A ratio-dependent predator – prey model appears to be supported by data on predation on ungulates, and the data indicate that wolves have a significant impact on numbers of moose (Alces alces), and thus can exert a regulatory effect on that species.
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It has been hypothesized that in passerine birds the larger size of last-laid eggs is part of a brood-survival strategy. We examined the usefulness of the brood-survival hypothesis in explaining intraclutch variation in egg mass of Yellow Warblers (Dendroica petechia). In 4- and 5-egg clutches, egg mass increased significantly with laying order. Although last-hatched nestlings in broods of 4 had higher survival rates than their counterparts in broods of 5, there were no differences in the absolute or relative mass of last-laid eggs in clutches of 4 and 5 eggs. In addition, the mass of last-laid eggs that hatched but did not produce a fledgling was not significantly different from that of last-laid eggs that did produce a fledgling. Finally, the relative mass of last-laid eggs was also not correlated with hatch spread or with date of clutch initiation. The results of this study do not support the brood-survival hypothesis.
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I examined the components generating selectivity in wolves preying on five ungulate species in Banff National Park, Alberta. Overall selectivity for elk and deer species, and apparent avoidance of bighorn sheep and mountain goats, were due primarily to lower habitat overlap of wolves with the latter two species, and therefore lower encounter rates. For social ungulates, I argue that the herd should be considered the unit of encounter, with encounter rates proportional to the number of herds rather than the number of individuals. However, large herds predictably associated with certain areas may be visited intentionally by wolves, increasing effective encounter rates. Foraging theory suggests that all ungulate prey should be equally profitable to wolves upon encounter and therefore the factors affecting encounter rates are critical in determining prey selectivity. A simple model incorporating different habitat overlap, herd sizes, and predictable herds predicts qualitatively different functional responses of wolves to changes in density of the different prey types. The model also demonstrates how apparent selectivity for a prey type can result from the different ways in which prey are encountered.
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The author incubated eggs from 17 clutches of common snapping turtles Chelydra serpentina on wet and dry substrates, producing "large' and "small' hatchlings, respectively. Hatchlings were individually marked, measured, and released in a National Wildlife Refuge area, Whiteside County, Illinois from which the eggs were collected originally. Subsequently, hatchlings were recaptured. Survivorship was not related to clutch, incubation conditions, or locomotor performance, but was significantly size dependence. However, larger body size of hatchling turtles may not evolve rapidly because the strength of selection was moderate in magnitude and the heritability was relatively low. -from Author
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Classical predator-prey models do not predict the positive relationships between the abundances of organisms on adjacent trophic levels seen in natural communities. Modifications that can relieve this disparity can be categorized as mechanisms of @'predator damping,@' which can be expressed in a simple, analytical form as ratio dependence. It has been suggested that the strength of predator damping can be estimated from the shape of the relationship between prey and predator biomass using ratio-dependent models. I present a theoretical argument showing why this approach should only be applied when predator and prey are the top two trophic levels in an ecosystem. Using published data from temperate lakes that include large-scale fish manipulations, I also show that the relationship between phytoplankton biomass and Daphnia biomass suggests much weaker damping when higher trophic levels (fish) are absent than when fish are present. Thus, inferring the strength of predator damping using patterns of abundance in ecosystems is of dubious validity when interactions are imbedded within simple food chains.
Article
Criticises ratio-dependence models (see special feature in Ecology 73: 1539-1566).
Chapter
Reviews ways of modelling two interacting populations - either predator-prey, competition, or mutualism.
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1. We investigated the functional response of wolves (Canis lupus) to varying abundance of ungulate prey to test the hypothesis that switching from alternate prey to preferred prey results in regulation of a caribou (Rangifer tarandus) population at low densities. 2. We determined prey selection, kill rates, and prey abundance for four wolf packs during three 30-day periods in March 1989, March 1990 and November 1990, and created a simple discrete model to evaluate the potential for the expected numerical and observed functional responses of wolves to regulate caribou populations. 3. We observed a quickly decelerating type II functional response that, in the absence of a numerical response, implicates an anti-regulatory effect of wolf predation on barren-ground caribou dynamics. 4. There was little potential for regulation caused by the multiplicative effect of increasing functional and numerical responses because of the presence of alternative prey. This resulted in high wolf: caribou ratios at low prey densities which precluded the effects of an increasing functional response. 5. Inversely density-dependent predation by other predators, such as bears, reduces the potential for predators to regulate caribou populations at low densities, and small reductions in predation by one predator may have disproportionately large effects on the total predation rate.