Ontogeny of pileus shape in Natrix natrix and N. tessellata

Abstract

We examined the divergence in pileus shape and ontogenetic allometric changes in pileus shape in two closely related species of snakes - Natrix natrix and N. tessellata. These two species have similar ecology, but different microhabitat and diet preferences. We analysed the ontogenetic series from hatchlings to adults. Hatchlings of two species significantly diverged in pileus size and shape. At the adult stage, significant sexual dimorphism in pileus size and shape was found within both species. Adults of N. natrix and N. tessellata (females and males) diverged in pileus shape, but not in size. Allometric shape changes accounted for significant amount of variance in pileus shape among different age classes, and appear to be species- and sex-specific. Our results suggested that allometries of pileus shape are highly evolvable traits, which may result from natural selection for functional optimisation of head shape in Natricine species.