Yellow-bellied marmots (Marmota flaviventris) 'in the shape space' (Rodentia, Sciuridae): sexual dimorphism, growth and allometry of the mandible

Abstract.

Geometric morphometrics was applied in a quantitative analysis of the morphology of the yellow-bellied marmot mandible. Five age classes were recognised by premolar toothwear, and the size and shape of the lower jaw were compared between sexes and among ages. Although the social role of adult males and females is markedly distinct, with the former mainly engaged to defend a territory with a female harem and the latter taking care of the young, mandible sexual dimorphism was moderate. On the contrary, form differences among age classes were conspicuous, and ontogenetic scaling, aimed at increasing mandible robustness, accounted for the majority of the shape change during growth. At least one mandibular shape modification, however, was related to the different behavioural ecology of immature and adult marmots. The mechanical efficiency of the superficial masseter and, thus, the strength of the incisor bite become so much greater in the sexually mature individuals that it cannot be interpreted as size-related adjustment. The need for stronger incisive bites is not explained by diet change, as the yellow-bellied marmot adults feed on the same grasses and forbs as the young and yearlings. Increasing the strength of the incisor bite is likely to be adaptive in order to provide adults with more efficient 'weapons' for intraspecific conflicts, and self- and kin-defence from predators. Morphological remodelling in the yellow-bellied marmot mandible seems to be more concerned with changes in size and behaviour during growth than with ethological differences between adults of the two sexes.