Sexual dimorphism in the Neotropical snakes genus Chironius (Serpentes: Colubridae)

Abstract

Sexual dimorphism is a common phenomenon among snakes, with female snakes being larger than male snakes as a recurrent pattern . However, species that show male-male combat behaviour may impose a selective advantage by developing larger bodies in male specimens, like the diurnal Chironius snakes, which display courtship and male-male combat behaviours. In this study, we analysed sexual dimorphism in body, tail, and head size, and skull size and shape in twelve species of Chironius. We investigated whether sexual dimorphism patterns could be a result of allometric growth. The absence of sexual dimorphism regarding body and head length was the main pattern revealed by our analyses. Sexual dimorphism characterized by larger body and head sizes in males was observed in Chironius bicarinatus, Chironius foveatus, and Chironius fuscus. Only females of Chironius exoletus exhibited larger body and head sizes than males. Regarding the shape of the head, six species showed sexual divergences, with enlarged or robust heads. Sexual dimorphism in skull shape seems related to selection in Chironius flavolineatus, with no allometric influences. Larger tails in males of C. fuscus, C. flavolineatus, and Chironius quadricarinatus may represent an advantageous defensive strategy. Finally, the lack of divergence in tail length in the remaining species probably evolved due to arboreal habits in Chironius.