Sexual dimorphism is a common phenomenon among animals. The usual cause cited for sexual dimorphism in animals is sexual selection acting through female choice or male–male combat. Natural selection acting to reduce resource competition between the sexes, however, is an important alternative evolutionary scenario, but this possibility has received little empirical study. Here this issue is addressed by examining the relationships among body size, head shape and the functional aspects of diet in the adult male and female cottonmouth snake Agkistrodon piscivorus. In this species, males are larger in overall body and head size. Whereas an analysis of gross head measurements (simple linear head dimensions) shows little dimorphism in head shape, a more detailed analysis of head shape (using digital images of the snakes’ heads) revealed some subtle, yet functionally significant, differences in head shape between adult male and female cottonmouths. Specifically, male cottonmouths have longer quadrate bones, and have greater lateral surface areas than females. Male cottonmouths also consumed relatively taller prey (prey size relative to snake body size) than conspecific females, and the sexes consumed significantly different proportions of prey. Because the size of the quadrate bone is a strong determinant of maximum gape in snakes, we suggest that the observed shape differences may reflect functional differences in maximum gape between similarly sized male and female cottonmouths. In turn, such differences in maximum gape width may explain why males consume taller prey than similarly sized females.