Meeting Abstract
Male-biased size dimorphic mating systems are often characterized by strong male-male aggression and competition for mates. Thus, these mating systems are often classic study systems to test hypotheses of sexual selection. In contrast, it may be harder to delineate the role of sexual selection in size monomorphic mating systems as there are a multitude of selection processes that may be occurring concomitantly. Although body size is often one important variable in determining male mating success in many species, selection may favor other secondary morphological traits that manifest functionally into whole-animal performance, which could directly influence fitness. Thus, sexual selection on performance may allow for a way to detect if there is sexual selection for dimorphism even if no observable SSD exists. Here, we study a size-monomorphic mating system, the common chameleon from southern Spain. Because male chameleons fight each other by biting opponents, we tested the hypotheses that 1) males are dimorphic from females in this important performance ability and 2) because bite performance is often correlated with head morphology, that this trait would also be sexually dimorphic. We found that, when controlling for body size, male chameleons significantly differed in head morphology than females, most notably in the cranial casque (which is strongly correlated with bite force performance). In addition, males were sexually dimorphic in bite force, with significantly stronger bite forces than females for a given body size. We conclude that there is sexual selection on performance in this species despite the lack of selection that would generate differences in body size between the sexes.
