Sexual Selection and Sexual Dimorphism in Some Herbivorous Lizards

Abstract

I tested the hypothesis that sexual selection affects body and especially head size in lizards by exploring patterns of sexual dimorphism and social organization among nine species of herbivorous lizard. Because these species do not partition food with respect to body and head size (as do insectivorous species), elimination of this factor allows a test of the effects of sexual selection on male lizards (these morphological features are important in fighting ability). As predicted by the sexual selection hypothesis, the three low male aggression species have little or no head size dimorphism, while the eight polygynous species show significant dimorphism of their head and body size. In the latter species male head size increases allometrically with body size, while decreasing head size in females is the general case. An analysis of dimorphism patterns within the subfamily Iguaninae suggests that dimorphism is a highly variable condition that is not constrained in its distribution by historical factors. Sexual selection is the likely cause of the head dimorphism, and along with natural selection upon differential reproductive energetic investment, may influence body size. While the cause-effect relation is clear for the evolution of head size dimorphism, this is not necessarily true of body size and social structure.