The functions of male-male aggression in a female-dominated mammalian society

In most mammals, male-male contests over access to resources select for males to be larger, more aggressive and better armed than females. However, the functional significance of male-male aggression has been little studied in sex-role-reversed species, such as spotted hyaenas, Crocuta crocuta. This species has highly aggressive, socially dominant females, to which male access cannot be directly improved by fighting. Here, we first demonstrated that males nevertheless display intrasexual aggression at rates similar to rates of intrasexual aggression displayed by adult females. We then tested three hypotheses suggesting functions of this aggression among males. Males aggressed significantly less in the presence of an adult female than when adult females were absent, suggesting that they do not use aggression to prevent other males from remaining in close proximity to females. New immigrants received significantly more aggression from other immigrants than did established immigrants, but new and established immigrants were treated equally by adult natal males. These results supported a hypothesis suggesting that male-male aggression functions to restrict clan membership, although the data indicated that immigrants, not adult natal males, engage in aggression for this reason. Finally, a hypothesis suggesting that this behaviour functions to provide access to food was supported by data showing that male-male aggression occurred significantly more frequently, and at higher intensities, in the presence than the absence of food. In contrast to male-male contests in most mammals, those in spotted hyaenas appear to increase a male's access to females only indirectly, which is presumably due to the sex-role-reversed nature of social dominance in spotted hyaenas.