Social and ecological influences on survival and reproduction in the spotted hyena, Crocuta crocuta

Heather Elizabeth Watts

In this dissertation, I explore variation in survival and reproduction, incorporating molecular and behavioral analyses, in order to elucidate ecological and evolutionary processes in the spotted hyena (Crocuta crocuta). My approach was to examine variation within a single population of hyenas in the Masai Mara National Reserve (hereafter Mara), Kenya, for which longitudinal data are available, and to make comparisons between this population and a second in Amboseli National Park, Kenya, for which I collected corresponding cross-sectional data. Spotted hyenas live in complex social groups that are structured by linear dominance hierarchies. Contrary to the typical mammalian pattern, however, females are socially dominant to males. I examined patterns of survival to test the hypothesis that intense feeding competition, in conjunction with slow development of the feeding apparatus, favored the evolution of female dominance in spotted hyenas. As predicted by this hypothesis, weaning was a particularly challenging life history event for young hyenas, and both social rank and maternal aid influenced the probability of surviving after weaning. Despite the important role that top predators play in ecosystems, relatively little is known about the factors that influence large predator populations themselves. Therefore, I inquired how social and ecological variables affect survival and reproduction in spotted hyenas. First, based on longitudinal variation in the Mara, I found that prey availability, group size, and competition with lions all influenced birthrates, while competition with lions and rainfall influenced juvenile survival. I then tested alternative hypotheses to explain variation in fitness measures between the Amboseli and Mara populations. My results were most consistent with the hypothesis that interspecific competition with lions was the primary source of population differences. Population dynamics can have important influences on population genetic structure, which may in turn affect population persistence. For example, the genetic consequences of population bottlenecks are of particular concern to conservation biologists. While the Mara hyena population has remained large and stable in recent history, the Amboseli population has undergone a demographic bottleneck. Despite this bottleneck, however, the Amboseli population did not exhibit reduced genetic diversity relative to the Mara. Both the long generation time of hyenas and the frequency of migration may have influenced the genetic outcome of the Amboseli bottleneck. Finally, in light of the role that lions play in shaping reproduction and survival in spotted hyenas, I inquired whether hyenas use avoidance to minimize potentially costly encounters with lions. Using audio playback experiments, I demonstrated that hyenas do not consistently use avoidance behavior with lions. While the relative costs and benefits of an interaction did not influence response to lion playbacks, I did find consistent individual differences in risk-taking and vigilance tendencies with respect to lions, independent of social rank.

Social and ecological influences on survival and reproduction in the spotted hyena, Crocuta crocuta

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