Biometry and pholidosis of <em>Thamnophis scaliger</em>: an atypical example of sexual dimorphism in a natricine snake

Abstract

Natural-history traits of Thamnophis scaliger (Mesa Central blotched garter snake), a Mexican endemism recently separated from Thamnophis scalaris, are almost unknown. We provide information on biometric and pholidotic traits according to sex for a large sample of individuals, and compare these morphological data with those available for T. scalaris, in order to place the species within the morphological context of the highly diverse genus Thamnophis. Moreover, we examine the adaptive value of sexually dimorphic characters from an evolutionary approach. Thamnophis scaliger appears to have fewer subcaudal and ventral scales than T. scalaris. Our sample also suggests that T. scaliger females have larger snout-vent lengths, masses, and body condition indexes (traits related to fecundity) than males, but that males have larger ventral and subcaudal scale numbers, and higher absolute and relative tail lengths (traits related to sexual selection) than females. Ventral and subcaudal scale number is a surrogate of vertebrae number (somites). If females are longer and relatively heavier than males but exhibit a lower number of somites, we suggest that this sexual dimorphism might be driven by the necessity for females of having larger and consequently more robust vertebrae to anchor muscles that have to move a heavier body.