Ontogenetic allometry and non-size-dependent variation of head shape in the Mexican Dusky Rattlesnake

The evolutionary role of ontogenetic allometry is a topic of debate, as it can act as an evolutionary constraint or as a mechanism for functional optimization. We assessed the allometric changes in head shape of Crotalus triseriatus (Wagler, 1830) to understand how ontogenetic shifts in head morphology adapts to ecological demands as it grows. The sampling was conducted in Toluca de Lerdo, Estado de México, where dorsal-view photographs of the head were taken for the 22 collected specimens. We digitized 22 landmarks that represent the C. triseriatus head shape. We calculated the common allometric component and the residual shape components (RSC). Results showed statistically significant positive allometry, considering shape as a function of size, in the head shape of C. triseriatus. The main changes include a contraction along the sagittal plane, primarily in the snout region, and a marked expansion in the posterior part of the head. Size being the main source of variation appears to be a common pattern in snakes, though the functional source of the variation may relate to the natural history aspects of each species. In C. triseriatus, these changes seem to be associated with predatory efficiency.