Sexual selection theory predicts that when females have the limiting reproductive rate, males should compete for access to those females via one of several mechanisms, such as male–male combat, sperm competition, or endurance rivalry. In this dissertation, I investigated the nature of intrasexual male competition in a sex role–reversed species, the spotted hyena (Crocuta crocuta), by examining each of those mechanisms. First, I tested hypotheses regarding the function of male–male combat. In most mammals, male–male combat functions to provide immediate access to females, but in spotted hyenas, females have complete control over copulation, so the benefits of intrasexual male aggression are less obvious. The results suggested that although males cannot control copulation, they use aggression to influence clan membership by restricting male immigration, which in turn affects their likelihood of siring cubs. The data likewise suggested that male aggression functions to provide access to food during competition with other males. Second, I took steps toward elucidating the importance of sperm competition in this species by describing ejaculate quality in wild male spotted hyenas and demonstrating temporal repeatability within individuals. Additionally, I found that immigrant males had significantly higher quality ejaculates than adult natal males, suggesting that adult natal males might experience reproductive suppression prior to dispersing. Third, I explored the notion that male spotted hyenas compete via an endurance rivalry by examining the relationship between tenure in the clan and annual reproductive success, and then investigated additional factors that might influence the outcome of this contest. The results indicated that immigrant males do indeed compete via an endurance rivalry, and for an immigrant to compete most effectively, he must remain in the clan and associate closely with females. However, pieces remain missing from the puzzle of male reproductive success in this species, because I found a striking quadratic effect of tenure on annual reproductive success, and the eventual decline in reproductive success remains unexplained. Finally, in addition to investigating male competition, I asked what evolutionary forces promote cooperative behavior among males. Kin selection and reciprocal altruism appeared to play no role in male–male cooperation; rather, males appeared to cooperate with one another to receive a variety of direct benefits. Overall, these results help illuminate the selective forces shaping competition and cooperation among males in a role–reversed species, and suggest how these forces might influence a male's reproductive output.
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