Sexual Size Dimorphism In Turtles Research Articles

Macroecological patterns of sexual size dimorphism in turtles of the world

Sexual size dimorphism (SSD) is a well-documented phenomenon in both plants and animals; however, the ecological and evolutionary mechanisms that drive and maintain SSD patterns across geographic space at regional and global scales are understudied, especially for reptiles. Our goal was to examine geographic variation of turtle SSD and to explore ecological and environmental correlates using phylogenetic comparative methods. We use published body size data on 135 species from nine turtle families to examine how geographic patterns and the evolution of SSD are influenced by habitat specialization, climate (annual mean temperature and annual precipitation) and climate variability, latitude, or a combination of these predictor variables. We found that geographic variation, magnitude and direction of turtle SSD are best explained by habitat association, annual temperature variance and annual precipitation. Use of semi-aquatic and terrestrial habitats was associated with male-biased SSD, whereas use of aquatic habitat was associated with female-biased SSD. Our results also suggest that greater temperature variability is associated with female-biased SSD. In contrast, wetter climates are associated with male-biased SSD compared with arid climates that are associated with female-biased SSD. We also show support for a global latitudinal trend in SSD, with females being larger than males towards the poles, especially in the families Emydidae and Geoemydidae. Estimates of phylogenetic signal for both SSD and habitat type indicate that closely related species occupy similar habitats and exhibit similar direction and magnitude of SSD. These global patterns of SSD may arise from sex-specific reproductive behaviour, fecundity and sex-specific responses to environmental factors that differ among habitats and vary systematically across latitude. Thus, this study adds to our current understanding that while SSD can vary dramatically across and within turtle species under phylogenetic constraints, it may be driven, maintained and exaggerated by habitat type, climate and geographic location.

Allometry of sexual size dimorphism in turtles: a comparison of mass and length data.

BackgroundThe macroevolutionary pattern of Rensch’s Rule (positive allometry of sexual size dimorphism) has had mixed support in turtles. Using the largest carapace length dataset and only large-scale body mass dataset assembled for this group, we determine (a) whether turtles conform to Rensch’s Rule at the order, suborder, and family levels, and (b) whether inferences regarding allometry of sexual size dimorphism differ based on choice of body size metric used for analyses.MethodsWe compiled databases of mean body mass and carapace length for males and females for as many populations and species of turtles as possible. We then determined scaling relationships between males and females for average body mass and straight carapace length using traditional and phylogenetic comparative methods. We also used regression analyses to evalutate sex-specific differences in the variance explained by carapace length on body mass.ResultsUsing traditional (non-phylogenetic) analyses, body mass supports Rensch’s Rule, whereas straight carapace length supports isometry. Using phylogenetic independent contrasts, both body mass and straight carapace length support Rensch’s Rule with strong congruence between metrics. At the family level, support for Rensch’s Rule is more frequent when mass is used and in phylogenetic comparative analyses. Turtles do not differ in slopes of sex-specific mass-to-length regressions and more variance in body size within each sex is explained by mass than by carapace length.DiscussionTurtles display Rensch’s Rule overall and within families of Cryptodires, but not within Pleurodire families. Mass and length are strongly congruent with respect to Rensch’s Rule across turtles, and discrepancies are observed mostly at the family level (the level where Rensch’s Rule is most often evaluated). At macroevolutionary scales, the purported advantages of length measurements over weight are not supported in turtles.

A Phylogenetic Analysis of Sexual Size Dimorphism in Turtles

Sexual size dimorphism (SSD) is widespread in a variety of vertebrate taxa, and its presence has been associated with a range of evolutionary and ecological factors. Turtles, as a lineage, have been shown to include species that exhibit both forms of SSD—male-biased (male larger) and female-biased (female larger)—as well as species that do not exhibit SSD (male size ≅ female size). Previous work has suggested that factors affecting the direction and magnitude of SSD in turtles may include habitat preference and fecundity; these correlations, however, have not been tested in a phylogenetic context. To provide a framework for evaluating potential correlates with the magnitude and direction of SSD in turtles, we examined the phylogenetic distribution of SSD in this clade. Published sources were surveyed for data on the direction and magnitude of SSD, carapace length, clutch size, egg size, and preferred habitat (terrestrial vs. nonterrestrial). Phylogenies were compiled from multiple published sources to develop a phylogeny for all taxa (n = 82) for which SSD data were assembled. The phylogenetic distribution of SSD was then evaluated using parsimony-based methods. These analyses indicated that the ancestral condition for extant turtles was female-biased SSD. Phylogenetic analyses conducted on subsets of the data (concentrated-changes tests, independent contrasts) indicated that changes in SSD state were not correlated with changes in habitat preference. Both male and female carapace length were found to be correlated with traits related to fecundity (egg size, clutch size), but the degree of dimorphism was not found to be correlated with these traits. A significant negative correlation between egg size and clutch size was also evident in turtles, as predicted by life-history theory.

Demography, sex ratio, and sexual size dimorphism in a northern population of common musk turtles (Sternotherus odoratus)

A population of common musk turtles (Sternotherus odoratus) was studied in Georgian Bay, Ontario, near the northern limit of the species' range, during the summers of 1991–1994. A total of 314 (229 adult males, 68 adult females, and 17 juveniles) turtles were captured, marked, measured, and released. Over the geographic range of S. odoratus there was a positive relationship between body size and latitude, the musk turtles of the Georgian Bay population being significantly larger than those in any of the more southerly populations. The sex ratio was significantly male-biased, and the study population was unique in having sexual size dimorphism, with significantly larger males. We have hypothesized that this dimorphism has arisen through sexual selection that is being driven by the male-biased sex ratio. Male–male competition for mates is intensified and thereby selects for larger males. Proximately, biased sex ratios and sexual size dimorphism in turtles have been attributed to differential ages and sizes at sexual maturity. However, we found no support for this hypothesis in our population. We have attributed sexual size dimorphism to either differential growth rates after maturity or differential mortality of adults. The male-biased sex ratio may be due to differential movement patterns, differential mortality of the sexes, or the effects of ambient temperature on sex determination.

Variation in Age and Size at Maturity of the Slider Turtle (Pseudemys scripta)

The adaptive strategy of the slider turtle, Pseudemys scripta, in terms of age and size at sexual maturity is discussed. Populations from a natural aquatic habitat and from a cooling reservoir on the US DOE Savannah River Plant are compared. (HCR)