Functional and ecological correlates of ecologically-based dimorphisms in squamate reptiles

Abstract

Sexual dimorphism in phenotypic traits associated with the use of resources is a widespread phenomenon throughout the animal kingdom. While ecological dimorphisms are often initially generated by sexual selection operating on an animal's size, natural selection is believed to maintain, or even amplify, these dimorphisms in certain ecological settings. The trophic apparatus of snakes has proven to be a model system for testing the adaptive nature of ecological dimorphisms because head size is rarely under sexual selection and it limits the maximum ingestible size of prey in these gape-limited predators. Significantly less attention has been paid to the evolution of ecological dimorphisms in lizards, however, which may be due to the fact that lizards' feeding apparatus can be under both sexual and natural selection simultaneously, making it difficult to formulate clear-cut hypotheses to distinguish between the influences of natural and sexual selection. In order to tease apart the respective influences of natural selection and sexual selection on the feeding apparatus of squamates, we take an integrative approach to formulate two hypotheses for snakes and lizards, respectively: (1) For gape-limited snakes, we predict that natural selection will act to generate differences in maximum gape, which will translate into differences in maximum ingestible prey size between the sexes. (2) For lizards which mechanically reduce their prey, we predict that the degree of dimorphism in head size should be positively correlated to the degree of dimorphism in bite force which, in turn, should be correlated to dimorphism in aspects of size or hardness of prey. Finally, we predict that functional differences in the feeding apparatus of these animals will also be linked with differences in sex-based feeding behavior and with selection of prey.