Sexual Dimorphism and Dietary Divergence: Differences in Trophic Morphology between Male and Female Snakes

Abstract

Although the evolution of sexual dimorphism is most commonly ascribed to sexual selection, ecological divergence between the sexes may also be important. Significant differences between the sexes in head size relative to body length are common among snakes, and apparently are not attributable to sexual selection. If head-size dimorphism has evolved because it enables males and females to eat different prey sizes, then one might expect that: 1) heads of male and female snakes should differ in shape as well as size; 2) this difference should be greatest in feeding structures, especially those that constrain maximum ingestible prey size; and 3) such trophic structures should be disproportionately larger in the sex with the larger head. We examined relative proportions of different components of the head in cleared-and-stained skulls of four snake species showing sexual dimorphism: Acrochordus arafurae (Acrochordidae), Dendrelaphis punctulatus (Colubridae), Pseudechis porphyriacus (Elapidae), and Laticauda colubrina (Laticaudidae). Discriminant Function Analysis could distinguish >95% successfully between male and female heads in all four species. This dimorphism in head shape was primarily due to sex differences in trophic structures in three out of the four species. In each case, the sex with the larger head had disproportionately larger trophic structures, as predicted from the hypothesis that ecological divergence between the sexes has been important in the evolution of sexual dimorphism in snakes.