Evolution of the relative width of the head and alveolar surfaces in map turtles (Testudines: Emydidae: Graptemys)

Abstract

Replicate adaptive radiations are dichotomous morphological patterns related to resource use and repeated in a series of isolated habitats. Biologists have long noted a dichotomy in relative head width and alveolar width among species of map turtles (Graptemys), particularly with regard to adult females, which are much larger than adult males in all 12 species. I measured plastron length (PL), head width (HW), and alveolar width (AW) of nearly 2300 specimens representing the 12 recognized species, and used allometric regressions to investigate this genus as an example of both sexual dimorphism in trophic morphology and replicate adaptive radiation. Sexual dimorphism was noted in HW after correction for PL, with females having wider heads than males of similar body sizes, but AW as corrected for HW was not dimorphic, thus absolute differences in AW are the simple result of differences in HW. Sexual differences in HW probably relate to the dimorphic niches of Graptemys, as females of three species have been reported to occupy deeper water further from shore than conspecific males. Based on predicted HW at maximum PL in adult females, species were categorized as mega-, meso-, or microcephalic. Differences in relative head width are related to differences in degree of molluscivory, with little molluscivory in microcephalic females, moderate to high molluscivory in mesocephalic females, and high molluscivory in megacephalic females. Distributions of Graptemys species in sympatry and allopatry reveal patterns consistent with structuring of distributions via competitive interactions. Superimposing data on relative HW on a phylogeny of the genus suggests there may have been one or two episodes of character displacement, with subsequent character assortment, in Graptemys evolution. This conclusion is similar to the current model for the evolution of body size during the radation of Anolis lizards in the northern Lesser Antilles.