No modular shape changes in the mandible of Andean white-eared opossum (Didelphis pernigra, Linnaeus, 1758)

Abstract

Morphological integration and modularity are concepts that refer to the covariation level between the components of a structure. Species of the opossums, genus Didelphis, have been the object of several taxonomic and morphometric analyses but no study has so far analysed mandibular morphological integration and modularity at a species-level. The aim of this work was to check whether the body (corpus mandibulae, mandibular corpus) and the ramus (ramus mandibulae, ascending mandibular ramus) are separate modules in Didelphis pernigra using a two-dimensional geometric morphometric approach. For this purpose, a sample of hemimandibles from 36 D. pernigra (13 males and 23 females) was analysed using 17 landmarks in lateral view. The modularity hypothesis based on different developmental origins was tested, by using the RV coefficient. Later, the integration level was assessed applying a partial least-squares analysis (PLS). The underlying aim was to know whether the traditional division between mandibular body and ramus has a modular basis, as well as the morphological integration level between these two structures. Results reflected that landmarks integration was not uniform throughout the mandible but structured into two distinct modules: ramus and body. Results allow to conclude that allometry plays an important role in shape variation in this species, and that the hypotheses of two-module organization in males cannot be confirmed. Models that accurately represent the biting mechanics will strengthen our understanding of which variables are functionally relevant and how they are relevant to performances, not only masticatories.