A review of sexual dimorphism of eye size in Colubroidea snakes

Abstract

Eye size is interesting in snakes because in most species body length differs between the sexes, while the eye’s performance depends on its absolute size. So, does the smaller sex see less well? We hypothesized that eye sexual mensural dimorphism (SMD) would be smaller than Body SMD. We found among 26 snake populations that body length SMD was female biased in 47.6% and male biased in 38.1% of samples. Often the larger sex’s head was further enlarged but the SMD of absolute eye size was mitigated or annulled by the smaller sex’s eye being enlarged within the head, and the head enlarged relative to the body. Overall generally the SMD of eye size was smaller than body SMD. This accords with a hypothesis that eye size affects the evolution of head size and its SMD, both reflecting and emphasizing that absolute eye size is functionally important. Although Colubridae exceed Viperidae in length, Viperidae have larger eyes in absolute terms. In Colubridae the females have larger eyes and in Viperidae the males have larger eyes. Additionally we examine to what extent SMD in different characters is correlated, and briefly review other aspects of SMD, including some aspects of Rensch’s rule.