Clinal and Allometric Variation in the Skull of Sexually Dimorphic Opossums

Abstract

Three species of sexually-dimorphic opossums are broadly distributed across South America: the habitat generalist Didelphis albiventris, the Atlantic forest-dweller D. aurita, and the Amazonian forest-dweller D. marsupialis. We used 2D geometric morphometrics to quantify skull size and shape variation in the three opossum species and test the hypothesis that degrees of sexual dimorphism and morphological variation should follow a cline across different South American environments. We first detected a strong impact of allometry on skull shape variation especially in males of the three species that tend to show stronger bite force, which is thought to be related to sexual selection. The degree of sexual dimorphism varies in relation to environmental seasonality. The skull of the plastic species, D. albiventris, showed the strongest ecogeographical pattern, showing conformity to Bergmann’s rule. In this species, size increase and shape changes are associated with colder climates and stronger bite force. Skulls of D. marsupialis are moderately impacted by climate, following productivity patterns of tropical regions associated with fruit availability. The most territorial species, D. aurita, has the strongest allometric effect and shows no clinal variation. Our results also support a degree of evolutionary constraint on the skull morphology of the three South American opossums. The black-eared opossums clade exhibits a weak (D. marsupialis) or nonexistent (D. aurita) association between skull morphology and climate. Skull shape changes of D. aurita are allometrically driven while those of the white-eared opossums clade (D. albiventris) vary in relation to the environment.