Evaluating drivers of female dominance in the spotted hyena

Abstract

IntroductionDominance relationships in which females dominate males are rare among mammals. Mechanistic hypotheses explaining the occurrence of female dominance suggest that females dominate males because (1) they are intrinsically more aggressive or less submissive than males, and/or (2) they have access to more social support than males.MethodsHere, we examine the determinants of female dominance across ontogenetic development in spotted hyenas (Crocuta crocuta) using 30 years of detailed behavioral observations from the Mara Hyena Project to evaluate these two hypotheses.ResultsAmong adult hyenas, we find that females spontaneously aggress at higher rates than males, whereas males spontaneously submit at higher rates than females. Once an aggressive interaction has been initiated, adult females are more likely than immigrant males to elicit submission from members of the opposite sex, and both adult natal and immigrant males are more likely than adult females to offer submission in response to an aggressive act. We also find that adult male aggressors are more likely to receive social support than are adult female aggressors, and that both adult natal and immigrant males are 2–3 times more likely to receive support when attacking a female than when attacking another male. Across all age classes, females are more likely than males to be targets of aggressive acts that occur with support. Further, receiving social support does slightly help immigrant males elicit submission from adult females compared to immigrant males acting alone, and it also helps females elicit submission from other females. However, adult females can dominate immigrant males with or without support far more often than immigrant males can dominate females, even when the immigrants are supported against females.DiscussionOverall, we find evidence for both mechanisms hypothesized to mediate female dominance in this species: (1) male and female hyenas clearly differ in their aggressive and submissive tendencies, and (2) realized social support plays an important role in shaping dominance relationships within a clan. Nevertheless, our results suggest that social support alone cannot explain sex-biased dominance in spotted hyenas. Although realized social support can certainly influence fight outcomes among females, adult females can easily dominate immigrant males without any support at all.

Evolutionary Patterns of Intersexual Power

“…Intersexual power in vervet monkeys (Chlorocebus pygerythrus) is dynamic and varies with sex ratio [103,104]. In spotted hyenas (Crocuta crocuta), patterns of sex-biased power partly emerge from sex differences in coalitionary support [15], that in turn derive from sex-biased dispersal [16]. Future research should explore the explanatory value of these and other social factors in primate power structures.…”

“…In mammals and birds, intersexual power has been theorized to have a reciprocal relationship with sexual dimorphism, in which sexual dimorphism influences, and in some cases may be influenced by, species-typical patterns of intersexual power [6][7][8][9][10]. Specific patterns of intersexual power have also been attributed to sex ratio [3,9], seasonality [9,11], energetic costs [12], selection for deferential mates [10,11,13,14], intersexual differences in social support [15,16], winner-loser effects [17], and frequent intersexual interactions [18]. Intersexual power can also arise due to female control over reproduction [19,20] and the supply and demand (i.e., market effects [21]) of reproductive opportunities [2,3].…”

“…Multiple authors have discussed what is now termed the "lemur syndrome" of femalebiased power structures, monomorphism, and even adult sex ratios [9,30,31] and attributed this suite of characteristics to the unique ecology of Madagascar (e.g., [12]). Nevertheless, female-biased power structures occur in mammals not inhabiting Madagascar (e.g., spotted hyenas: [15,16]; review: [32]) and are the norm in some vertebrate taxa (e.g., raptors [10]). Consequently, a broader explanation for the evolution of female-biased power structures is required.…”