Does diet drive the evolution of head shape and bite force in chameleons of the genus Bradypodion?
Abstract The head is a complex integrated system that is implicated in many vital functions. As such, its morphology is impacted by different and sometimes conflicting demands. Consequently, head shape varies greatly depending on the environment and dietary ecology of an organism. Moreover, given its role in territory defence and mating in lizards, it is also subjected to strong sexual selection in these animals. We investigated the relationships between head shape, bite performance and diet in 14 of the 17 extant Bradypodion species to determine whether variation in diet can explain the observed diversity in bite force and head shape in this genus. We also evaluate differences between sexes in terms of the relationships between head shape, bite force and diet and predict tighter relationships in females given that the head in this sex is principally under natural selection. Our results show that there is indeed a correlation between head shape, diet and bite force, but the direction and magnitude are sex-dependent. Whereas we observed a correlation between absolute bite force and head shape in both sexes, size-corrected bite force was correlated with mandible and quadrate shape in females only. Despite strong correlations between bite force and prey hardness, and between prey hardness and head shape, we did not find any relationship between head shape and prey evasiveness. These data suggest that the cranial system in chameleons of the genus Bradypodion evolves under natural selection for the ability to eat large or hard prey. Moreover, significant differences in the ecomorphological relationships between the two sexes suggest that sexual selection plays a role in driving the evolution of bite force and head shape. These data suggest that ecomorphological relationships may be sex-dependent.