Morphological variation and sexual dimorphism in two sympatric dipsadine snakes from Southern Brazil

Abstract

Snakes have high morphological variation, including between sexes. Most of these variations are evidenced only by linear measurements, which are generally restricted to size traits. Using traditional and geometric morphometrics methods we analyze how body size attributes and head size and shape varies between sexes and species of two sympatric Tachymenini snakes, Thamnodynastes strigatus and Tomodon dorsatum. We took body size measurements from 87 specimens of T. strigatus and 83 specimens of T. dorsatum. We also extracted head shape and size of 146 specimens of T. strigatus and 156 specimens of T. dorsatum. Our results showed sexual differences in some body size attributes and in head shape and size. Females of both species had larger SVL, BM, HW, BW and heads than in males. But, T. strigatus is not sexually dimorphic in head size. Females of the T. strigatus also exhibited an enlargement of the parietal region compared to the males, while T. dorsatum females exhibited a wider snout than males. These sexual differences could be a result of fecundity selection and may also indicate that females consume larger preys than conspecific males. At the species level, T. strigatus is larger in body and head size than T. dorsatum. The head shape of T. strigatus is characterized by a more elongated head with larger frontal and prefrontal scales and slender snout compared to T. dorsatum. Shape differences between these two snakes probably evolved in association with divergences in habitat use e.g. terrestrial and arboreal and diet specialization e.g. piscivory, and malacophagy.