Sexual dimorphism and allometry in malacophagus snakes (Dipsadidae: Dipsadinae)

Abstract

Sexual dimorphism in snakes is generally described in association with body or tail size and scale counts, with relatively few studies addressing intrasexual divergence in the skull. Here, we analyzed sexual dimorphism in the size and shape of skull and body in three malacophagous dipsadine snakes, Dipsas mikanii, Dipsas neuwiedi and Dipsas turgida, as well as allometric effect on these components. We used linear and geometric analysis to assess: (1) if there is sexual dimorphism in cranial components; (2) if there are differences between the sexes regarding body and tail size, number of ventral and subcaudal scales; (3) whether there is covariation between cranial components and body size; (4) if there are changes in cranial shape associated with increased size; and (5) whether there is an allometric relationship between body and tail size. Our results showed that all three species are dimorphic in cranial shape and size (except D. turgida for cranial size), with females having longer and thinner skulls than males. In the three species, the female skull was negatively allometric, whereas the male skull was isometric. Allometry related to cranial shape was significant only in males of D. turgida, which showed greater snout robustness and eye size associated with enlargement of the skull. Females of D. mikanii and D. neuwiedi were significantly larger than males. Only males of D. neuwiedi showed positive allometry for the tail, while dimorphism related to scale counts followed the pattern found in most snakes, with females having a greater number of ventrals and males subcaudals (except D. neuwiedi in the latter case). Based on our results, we hypothesize that patterns of sexual dimorphism and skull allometry in malacophagous snakes may be explained both by aspects related to diet and reproduction. Meanwhile, patterns associated with body size reflect advantages related to fecundity favoring greater reproductive success of females.