The skull of Hydrodynastes gigas (Duméril, Bibron & Duméril, 1854) (Serpentes: Dipsadidae) as a model of snake ontogenetic allometry inferred by geometric morphometrics

Abstract

Allometric growth is one of the dominant factors of morphological variation, although this issue is still poorly explored. Using geometric morphometrics, we evaluated and described the ontogenetic allometry in the skull of the snake Hydrodynastes gigas. Furthermore, we described the skull morphology of the species and compared it with its congener, H. bicinctus. We found that 46.91 % of shape variation was explained by allometry: smaller centroid sizes were mostly correlated to short postorbital process, medial region of frontals elongated, shortened nasals and elongated parietal, and shortened/oblique supratemporals in relation to the anteroposterior axis of the skull; larger centroid sizes point mainly to postorbital process elongated, elongated nasals and shortened parietals, posterior region of parietal strongly tapered, and elongated/parallel supratemporals in relation to the anteroposterior axis of the skull. In general aspect, the skull of H. gigas differs from the skull of H. bicinctus by being less dorsoventrally compressed. Comparisons between our results and the results of other studies which considered allometry in specific bones of snakes’ skull showed some common patterns, although we found a different correlation between parietal and skull length. These results reinforce the importance of further studies to evaluate the existence of recurrent patterns of allometric growth in the skull of other representatives of this group of snakes. Moreover, the analyses presented herein revealed a significant ontogenetic allometry in the skull of H. gigas and represent the first approach of geometric morphometrics with this goal for snakes.