Geographic Variation in Sexual Size Dimorphism in Painted Turtles (Chrysemys picta) (original) (raw)
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Ecological, evolutionary, and allometric patterns of sexual size dimorphism in turtles
Macroevolutionary patterns of sexual size dimorphism (SSD) indicate how sexual selection, natural selection, and genetic and developmental constraints mold sex differences in body size. One putative pattern, known as Rensch's rule, posits that, among species with female-larger SSD, the relative degree of SSD declines with species' body size, whereas, among male-larger SSD species, relative SSD increases with size. Using a dataset of 196 chelonian species from all fourteen families, we investigated the correlation in body size evolution between male and female Chelonia and the validity of Rensch's rule for the taxon and within its major clades. We conclude that male-female correlations in body size evolution are high, although these correlations differ among chelonian families. Overall, SSD scales isometrically with body size; Rensch's rule is valid for only one family, Testudinidae (tortoises). Because macroevolutionary patterns of SSD can vary markedly among clades, even in a taxon as morphologically conservative as Testudines, one must guard against inappropriately pooling clades in comparative studies of SSD. The results of the present study also indicate that regression models that assume the x-variable (e.g. male body size) is measured without statistical error, although frequently reported, will result in erroneous conclusions about phylogenetic trends in sexual size dimorphism.
Copeia, 2008
Painted Turtles (Chrysemys picta) are often used to test life-history theory. However, within populations, the factors that contribute to among-individual variation in egg size and clutch size are poorly understood, and an understanding of the biotic and abiotic parameters that contribute to this variation is important when framing patterns of maternal investment in a life-history context. We examined proximate sources of reproductive variation in a northern population of Painted Turtles, we attempted to frame these sources of variation in a lifehistory context, and we evaluate which optimality model most adequately explains patterns of reproductive allocation in populations of small-bodied turtles. We used multiple linear regression on data from 168 first clutches of marked females that nested at a long-term study site in Algonquin Park, Ontario, Canada, in 2004. We found that mean egg mass was positively related to maximum plastron length (MPL) and female age, and negatively related to clutch size and water temperature prior to oviposition. Clutch size was positively related to MPL and carapace height, and negatively related to mean egg mass, and the number of clutches laid in the season. Body size (MPL) was the most important predictor of each reproductive parameter, and residual analysis indicated that egg mass was more conserved than clutch size across the range of female body sizes sampled in this study. Thus, egg size may be optimized as a body size-specific function, and in light of this, we suggest that 'phenotype-habitat matching' may occur in C. picta. If a female's phenotype (e.g., body size) influences the selective environment of her eggs and hatchlings (e.g., if larger females generally nest farther away from water), then the optimal strategy of maternal investment should vary among maternal phenotypes. The positive correlation between egg mass and body size that was observed in the present study can be explained in adaptive terms under hypotheses based on the concept of phenotype-habitat matching.
Biological Journal of the Linnean Society, 2014
The attainment of sexual maturity has been shown to affect measures of sexual size dimorphism (SSD) and adult sex ratios in several groups of vertebrates. Using data for turtles, we tested the model that sex ratios are expected to be male-biased when females are larger than males and female-biased when males are larger than females because of the relationship of each with the attainment of maturity. Our model is based on the premise that the earlier-maturing sex remains smaller, on average throughout life, and predominates numerically unless the sexes are strongly affected by differential mortality, differential emigration, and immigration, or biased primary sex ratios. Based on data for 24 species in seven families, SSD and sex ratios were significantly negatively correlated for most analyses, even after the effect of phylogenetic bias was removed. The analyses provide support for the model that SSD and adult sex ratios are correlated in turtles as a result of simultaneous correlation of each with sexual differences in attainment of maturity (bimaturism). Environmental sex determination provides a possible mechanism for the phenomenon in turtles and some other organisms.
Geographic variation in body size among French populations of the European pond turtle
Amphibia-Reptilia, 2020
The processes underlying macroecological gradients in body size are widely debated, in part because their intraspecific variability remains poorly described even in well-studied taxa such as vertebrates. In this study, we investigated how climate, habitat, genetic lineage and sex explain body size variations in French populations of the European pond turtle (Emys orbicularis). We measured 7016 adult individuals captured in 41 populations, covering most of the species’ distribution in metropolitan France, including Corsica. Body size variation in our sample was wide and comparable to that found across the species’ worldwide range. Variation was similar in magnitude at regional and local levels, suggesting that body size is influenced by local factors as much as by regional factors such as climate or genetic lineage. Smaller sizes were associated with Mediterranean or altered oceanic climates, and with two lineages (E. o. galloitalica and E. o. galloitalica/E. o. orbicularis), while l...
Reproductive Allometry in the Common Map Turtle, Graptemys geographica
The American Midland Naturalist, 2007
According to optimal offspring size theory, natural selection pressures balance the egg size/clutch size trade-off at a point where increases in maternal body size result in increases in clutch size but not increases in egg size. However, many turtle species show increasing egg size with increasing maternal body size. The anatomical-constraints hypothesis explains this pattern by hypothesizing that smaller females lay smaller-than-optimal eggs because of morphological constraints; larger eggs simply would not pass through the pelvic aperture and caudal gap of the shell. We examined relationships among female body size (measured as plastron length), clutch size and egg size for a population of common map turtles (Graptemys geographica) at Presque Isle State Park in Erie, Pennsylvania. Correlation analyses were conducted using log-transformed data in order to address questions of isometry and allometry. Clutch mass increased isometrically with plastron length. Egg size and clutch size were both significantly negatively allometric in their relationship with plastron length. It appears that larger females split the increased reproductive allocation made possible by increased maternal volume devoted to eggs between increasing both clutch size and egg size, consistent with predictions of the anatomical-constraints hypothesis.
Understanding reproductive allometry in turtles: A slippery “slope”
Ecology and Evolution, 2019
Measures of reproductive output in turtles are generally positively correlated with female body size. However, a full understanding of reproductive allometry in turtles requires logarithmic transformation of reproductive and body size variables prior to regression analyses. This allows for slope comparisons with expected linear or cubic relationships for linear to linear and linear to volumetric variables, respectively. We compiled scaling data using this approach from published and unpublished turtle studies (46 populations of 25 species from eight families) to quantify patterns among taxa. Our results suggest that for log–log comparisons of clutch size, egg width, egg mass, clutch mass, and pelvic aperture width to shell length, all scale hypoallometrically despite theoretical predictions of isometry. Clutch size generally scaled at ~1.7 to 2.0 (compared to an isometric expectation of 3.0), egg width at ~0.5 (compared to an expectation of 1.0), egg mass at ~1.1 to 1.3 (3.0), clutc...
Climate and patterns of body size variation in the European pond turtle, Emys orbicularis
Biological Journal of the Linnean Society
It is generally assumed that ectothermic vertebrates show a reversed Bergmann's cline, but several studies suggest the opposite for turtles. Here, we assess this issue using the widely distributed European pond turtle (Emys orbicularis), which displays strong geographic size variation. We tested carapace lengths of more than 2000 individuals from the entire distribution range against latitude and climatic factors and estimated the effects of those on femalespecific traits such as clutch size and frequency. Also, we compared our data against Pleistocene (Eemian) E. orbicularis from Germany. Regional fluctuations of temperature and precipitation are better predictors of body size than latitudinal temperature clines, especially in females. In addition, female body size, activity period and nesting season might be correlated, as small southern females lay several smaller clutches per season, whereas larger-bodied northern females produce only a single large clutch. In conclusion, although climatic conditions appear to have an effect on the body size of E. orbicularis, the species does not follow Bergmann's rule. Moreover, comparison of extant and fossil E. orbicularis from Germany revealed that fossil pond turtles were generally smaller than extant conspecifics from the same region.
Authorea
Turtles have been prominent subjects of analyses of sexual size dimorphism (SSD) owing to their mating system and habitat diversity. In prior studies, marine turtles were grouped with non-marine aquatic turtles (NMAT). This is odd because it is well-established that the marine environment imposes a distinct selective milieu on body form of vagile vertebrates, driven by convergent adaptations for energy-efficient propulsion and drag reduction. We generated a comprehensive database of adult marine turtle body size (38,569 observations across all species), which we then used to evaluate both the magnitude of SSD in marine turtles and how it compares to SSD in NMAT. We find that marine turtles are not sexually size dimorphic, whereas NMAT typically exhibit female-biased SSD. We argue that the reason for this difference is the sustained long-distance swimming that characterises marine turtle ecology, which entails significant energetic costs incurred by both sexes. Hence, the ability of either sex to allocate proportionately more to growth than the other is likely constrained, meaning that sexual differences in growth and resultant body size are not possible. Consequently, lumping marine turtles with NMAT dilutes the statistical signature of different kinds of selection on SSD and should be avoided in future studies.