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Mammalian societies in which females dominate males are rare, and the factors favouring the evolution of female dominance have yet to be clearly identified. We propose a new hypothesis for the evolution of female dominance and test its predictions with empirical data from the spotted hyena ( Crocuta crocuta ), a well-studied species characterized by female dominance. We suggest that constraints imposed by the development of a feeding apparatus specialized for bone cracking, in combination with the intensive feeding competition characteristic of spotted hyenas, led to the evolution of female dominance. Specifically, we propose that protracted development of the feeding apparatus in young hyenas led to selection for increased aggressiveness in females as a compensatory mechanism for mothers to secure food access for their young after weaning. Our analyses yielded results consistent with this hypothesis. Morphological and behavioural measurements indicate that skull development is indeed protracted in this species; spotted hyenas do not achieve adult skull size or feeding performance capabilities until after sexual maturity. The period between weaning and completed skull development is particularly challenging, as indicated by high mortality. Finally, maternal presence between weaning and full skull maturity, as well as the relative ability of females to aggressively displace conspecifics from food, are important determinants of offspring survival.

The ability to break open large bones has evolved independently in only three groups of carnivorous mammals, all of which have robust teeth, vaulted foreheads, and pronounced sagittal crests. One unusual skull feature, present in bone-cracking members of the family Hyaenidae, is a caudally elongated frontal sinus, hypothesized to function in resistance to bending and stress dissipation during bone-cracking. In the present study, we used finite element (FE) analysis to examine patterns of stress distribution in the spotted hyena (Crocuta crocuta) skull during unilateral biting, and inquire about the functional role of the fronto-parietal sinus in stress dissipation. We constructed and compared three FE models: (1) a ‘normal’ model of an adult Crocuta skull; (2) a model in which the caudal portion of the fronto-parietal sinus was filled with bone; and (3) a model in which we flattened the sagittal crest to resemble the plate-like crests of other mammals. During biting, an arc of stress extends from the bite point up through the vaulted forehead and along the sagittal crest. Our results suggest that pneumatization of the hyena's skull both enhances its ability to resist bending and, together with the vaulted forehead, plays a critical role in evenly dissipating stress away from the facial region. © 2008 The Linnean Society of London, Biological Journal of the Linnean Society, 2008, 95, 246–255.

Weaning represents a challenging transition for young mammals, one particularly difficult for species coping with extreme conditions during feeding. Spotted hyenas (Crocuta crocuta) experience such extreme conditions imposed by intense feeding competition during which the ability to consume large quantities of food quickly is highly advantageous. As adult spotted hyenas have massive skulls specialized for durophagy and can feed very rapidly, young individuals are likely at a competitive disadvantage until that specialized morphology is completely developed. Here we document developmental changes in skull size, shape, and mechanical advantage of the jaws. Sampling an ontogenetic series of Crocuta skulls from individuals ranging in age from 2 months to 18 years, we use linear measurements and geometric morphometrics to test hypotheses suggesting that size, limited mechanical advantage of the jaws, and/or limited attachment sites for jaw muscles might constrain the feeding performance of juveniles. We also examine skull development in relation to key life history events, including weaning and reproductive maturity, to inquire whether ontogeny of the feeding apparatus is slower or more protracted in this species than in carnivores not specialized for durophagy. We find that, although mechanical advantage reaches maturity in hyenas at 22 months, adult skull size is not achieved until 29 months of age, and skull shape does not reach maturity until 35 months. The latter is nearly 2 years after mean weaning age, and more than 1 year after reproductive maturity. Thus, skull development in Crocuta is indeed protracted relative to that in most other carnivores. Based on the skull features that continue to change and to provide additional muscle attachment area, protracted development may be largely due to development of the massive musculature required by durophagy. These findings may ultimately shed light on the adaptive significance of the unusual "role-reversed" pattern of female dominance over males in this species.