Sexual dichromatism does not translate into sex‐based difference in morphology or diet for the African boomslang

Abstract

AbstractSelection for sexual dichromatism is thought to arise mainly from intersexual niche divergence or sexual selection, including mate‐choice and intrasexual competition. However, overt sexual dichromatism is rare in snakes, limiting inference regarding its origin and maintenance in these animals. We thus aimed to assess whether boomslang (Dispholidus typus) – a species of overtly sexually dichromatic African snake – exhibits evidence for intersexual niche divergence based on three ecologically relevant morphological variables (snout‐vent length, tail length and head length) and one ecological variable (diet). We measured morphological variables on 203 museum specimens, and characterized diets for male and female snakes on the basis of dissected specimen gut contents, supplemented with literature reports of feeding, as well as online photographs. Male and female boomslang show broadly similar gross morphology and do not differ in mean snout‐vent, relative tail length or head length. Moreover, male and female snakes do not differ in the frequency with which they consume different prey classes. Similarly, diets of adult and juvenile snakes did not differ significantly. The abundance of chameleons and bird chicks (36% and 54% of prey items respectively) in the diet of boomslang suggests dietary restriction due to arboreality rather than dietary specialization per se. Despite examining a range of morphological and ecological variables, we fail to detect evidence for intersexual niche differentiation. Rather, our results suggest that juvenile coloration is likely a result of selection for camouflage from visually oriented predators, and that the overt sexual dichromatism of adults following maturation represents a shift toward either intersexual mate‐choice or intrasexual competition. We therefore recommend that boomslang visual acuity and colour differentiation be directly examined in the future.