Sexual dimorphism, allometry, and interspecific variation in the cranial morphology of seven Meriones species (Gerbillinae, Rodentia)

Jirds (Meriones) are a mostly desert-adapted genus of gerbils, with a wide geographic range, through which it encounters various climatic conditions, which may influence their morphology. In this study, we quantified cranial morphometric variation both within and among seven jird species (M. meridianus, M. hurrianae, M. crassus, M. tristrami, M. persicus, M. libycus and M. vinogradovi), based on a two-dimensional landmark-based geometric morphometric analysis of 972 specimens, covering their entire geographic distribution. The aforementioned analysis was used to compare sexual dimorphism in size and shape within each species, as well as the relationship between size and shape (i.e. allometry) for each species. Despite greatly overlapping in morphospace (when examined visually), statistical analysis indicates significant differences in both size and shape among the seven examined jird species. UPGMA and CVA both show two main species clusters. Deformation grids indicate that these two clusters differ mostly in the relative size of the tympanic bulla, along with differences in the extent of nasal elongation, and the broadness of the zygomatic arch. Allometric changes in shape were analyzed in all species that show an allometric relationship. Sexual dimorphism in shape and size was detected in only three of the seven jird species. A visual inspection of the data indicates high overlap in shape space, and that male skulls are significantly larger than female skulls. When specimens were divided by sex, we found significant allometry in six of the seven species (for both sexes). A factorial multivariate analysis of covariance (MANCOVA) indicated that even when taking size variation into account, the residual shape variation was also significantly different among the sexes of the examined species. The outcome of this study confirms the presence of cranial variation in the examined jirds, and that the patterns of sexual dimorphism and allometry vary considerably among jird species.