Sexual dimorphism in neonate and adult snakes

Sex differences in body size and head dimensions are widespread in adult sn akes, but because data are scarce for neonates, it is unclear whether diffe rences are present from birth or arise post-natally. Here we analyse patter ns of sexual dimorphism in neonates and adults of four species of natricine snakes, Nerodia sipedon, Storeria dekayi, Thamnophis radix, and T. sirtali s. Two measures of body size (snout-vent length, mass), four measures of he ad morphology (head length, head width, jaw length, and interocular distanc e), and tail length were obtained from wild-caught adults and from offsprin g born to wild-caught females. Among neonates, significant sexual dimorphis m was found in body size for S. dekayi and T. sirtalis, in head dimensions for S. dekayi, T. radix, and T. sirtalis, and in tail length for all four s pecies. Among adults, significant sexual dimorphism was found in body size, head dimensions, and tail length for all four species. The degree of sexua l dimorphism in body size among adults greatly exceeded that among neonates . In contrast, the degree of sexual dimorphism in head dimensions was simil ar between neonates and adults. The presence of significant sexual dimorphi sm among neonates suggests that hypotheses regarding the evolutionary signi ficance of sexual dimorphism in snakes should consider newborns as well as adults.