Sex-specific growth, shape, and their impacts on the life history of a long-lived vertebrate

Individual growth rates both comprise and determine life-history phenotypes. Despite decades of interest in understanding the relationship between individual growth and life history, chelonian longevity has limited our ability to robustly estimate individual growth curves that span the life of both sexes.

(1) Do patterns of growth in size and shape differ between the sexes of the painted turtle, Chrysemys picta? (2) Does individual variation in size and shape affect female reproductive effort?

Using 30 years of field data on shell morphology of a single population of painted turtles, we used principal components analysis to summarize multivariate size and shape. We assessed the ability of three non-linear growth models - the logistic, Gompertz, and von Bertalanffy - to predict size-at-age and used model comparison to justify sex-specific model fits. We correlated age-specific size and shape of females with their reproductive efforts.

Model comparison supported separate fits of the von Bertalanffy growth function for each sex; non-overlapping confidence intervals imply differences in sex-specific asymptotic size, but not growth rate. Higher-order axes of variation in shell morphology described significant sexual dimorphism in shell shape related to the sphericity and curviness of the shell. Shell sphericity of females covaried with clutch size, mean egg mass, and total clutch mass. Irrespective of shell morphology, we found evidence of an egg number versus egg mass trade-off. Yet, females who matured at a larger size produced greater reproductive efforts.