Peramorphic males and extreme sexual dimorphism in Monodelphis dimidiata (Didelphidae)

Abstract

The southern short-tailed opossum, Monodelphis dimidiata, is a species known not only for its semelparous life cycle, but also for the extreme sexual dimorphism of adults, where males are not only larger, but also have distinctive morphological characters in their skull. Using geometric morphometrics and a suite of statistical tests, I analyzed the postweaning ontogenetic development of this species in order to evaluate the age-class where sexual dimorphism becomes significant and the amount of change exhibited by both sexes. My results showed that M. dimidiata partly follows the ontogenetic pattern described for didelphids by previous authors. The character that escapes the general pattern is rostral length, which becomes shorter instead of lengthening throughout the development. This change could be related to an increment in the bite force in the anterior part of the dentition. The amount of sexual dimorphism found for this species is larger than the reported previously for other American marsupials, and I also found a higher rate of growth in males at the attaining of sexual maturity. Based on my results and the information available for other didelphids, I can suggest that M. dimidiata males undergo through a process of hypermorphosis, resulting in a peramorphic condition. It is possible that the extreme sexual dimorphism present in this species is related to reproductive success, specially taking into account their semelparous life cycle.