Functional Basis for Intersexual Differences in Bite Force in the Lizard Anolis carolinensis

In many species of lizards, males attain greater body size and have larger heads than females of the same size. Often this dimorphism in head size is paralleled by a dimorphism in bite force. However, the underlying functional basis for the dimorphism in bite force remains unclear. Here, we test whether males are larger and have larger heads and bite forces than females for a given body size in a large sample of green anoles (Anolis carolinensis). Next, we test if overall head shape differs between the sexes of A. carolinensis, or if instead, specific aspects of skull shape can explain differences in bite force and jaw adductor muscle mass. Our results show that A. carolinensis is indeed dimorphic in body and head size and that males bite harder than females. Geometric morphometric analyses show distinct differences in skull shape between males and females, principally reflecting an enlargement of the jaw adductor muscle chamber in males. Jaw adductor muscle mass data confirm this result and show that males have larger jaw adductors (but not jaw openers) for a given body and head size. Thus, the observed dimorphism in bite force in A. carolinensis is not merely the result of an increase in head size, but involves distinct morphological changes in skull structure and the associated jaw adductor musculature.