Quantifying the age- and sex-dependent morphological variation in two syntopic mustelids: Martes martes and Martes foina

Abstract

Age determination is a prerequisite to establish reliable age and sex-specific patterns of morphological variation between individuals, populations and species. In this paper, we explored sex and age morphological variations in pine and stone martens in the same region of Bresse (France) during the same period. Using a large dataset of stone martens and pine martens genetically identified, we discriminated individuals using a set of morphological measurements (body mass, head-and-body length, tooth wear, tail length, neck circumference, posterior foot length, baculum length) and evaluated their power in correctly classifying individuals in three age classes: juveniles, subadults and adults. We then derived cross-sectional growth curves for the two species and estimated the age at the end of growth by adjusting sex and species-specific von Bertalanffy growth curves. Our results showed that sub-adults and adults are difficult to disentangle based on morphological measurements, especially because subadults reached adult size by the age of eight months. On the adults, the sexual size dimorphism in favor of males for all traits we measured was clearly more pronounced in pine martens than in stone martens. Stone martens showed significantly larger measurements than pine martens of the same sex, except for head-and-body length which was not significant and posterior foot length which was larger in pine martens. We discussed our results in the light of various hypotheses concerning biogeographical variation and long-term morphological divergence. Our results suggest that inter-specific differences in morphology might facilitate coexistence of both species in the same area.