The evolution of social relationships in hyenas

Although most mammalian carnivores are solitary, roughly one quarter of carnivore species are at least somewhat gregarious. The smallest social carnivores, which include several mongoose species, live in highly cohesive groups, but larger carnivores live in fission-fusion societies. The carnivore family Hyaenidae contains four extant species that span a wide spectrum of sociality. Aardwolves are solitary except when breeding, striped hyenas live in tiny kin groups, brown hyenas live in small social groups called “clans,” and spotted hyenas live in complex, hierarchical primate-like societies containing up to 130 individuals. All members of the Hyaena family forage alone except spotted hyenas, which often hunt and feed in groups. Adults of all extant Hyaena species except aardwolves have skull morphologies specialized for cracking open large bones. However, only spotted hyenas exhibit extreme female aggressiveness and social dominance over males, and only spotted hyenas experience intensive feeding competition at fresh ungulate carcasses. Prolonged development of skulls specialized for bone-cracking handicaps young spotted hyenas during competitive feeding with adults such that they need maternal aid to feed on both soft and hard foods long after weaning. We propose that female aggressiveness and social dominance over males in spotted hyenas represent compensatory traits favored in mothers when skull development in offspring is refractory to selection pressure imposed by feeding competition. Thus both external selection pressures and developmental constraints appear to have shaped the evolution of extreme female aggressiveness and social dominance, traits unique to spotted hyenas among all mammalian carnivores.