In this dissertation, I used data from almost two decades of behavioral observations of wild spotted hyenas (Crocuta crocuta), coupled with non-invasive techniques to measure steroid hormone concentrations from these same individuals, to investigate the behavioral endocrinology of this unique species. Female spotted hyenas are morphologically and behaviorally masculinized. These traits make this species particularly interesting subjects to investigate the interaction between hormones and behavior, and how this interaction is influenced by an individual's social or ecological environment.
Our lab has measured concentrations of androgens, glucocorticoids, and now, estrogens present in fecal samples collected from free-living, individually recognized hyenas. I first use these data to investigate both naturally occurring and anthropogenic influences on stress physiology in spotted hyenas. From our longitudinal study, I demonstrated that spotted hyenas are well adapted to variation in their natural ecology, including seasonal fluctuations in prey and rainfall. However, unpredictable events, such as sudden periods of instability in the social hierarchy, do influence spotted hyena stress physiology. Interestingly, adult male hyenas experienced an impact of increasing human disturbance on their stress physiology. This result was strengthened by a cross-sectional comparison that demonstrated that this population of hyenas, which is now exposed to high levels of human disturbance, had elevated stress hormone concentrations when compared to hyenas exposed to relatively low levels of human disturbance.
I next evaluated behavior likely to be mediated by steroid hormones in this species, focusing on aggressive behavior. Using careful behavioral rate calculations that controlled for variation in opportunity to behave aggressively, I showed that rates of aggression emitted by adult females toward all conspecifics are higher than those emitted by adult males, which is opposite mammalian norms. While rates of intrasexual aggression did not differ significantly between the two sexes, the quality of this aggression did, and attacks among females were of higher-intensity than those seen among males. My results also demonstrated that rates of aggression emitted by females as adults were positively correlated with variation in their prenatal androgen exposure, supporting the organizational hypothesis of steroid hormone control of behavior.
Finally, we investigated two functional hypotheses to explain a particular type of aggression observed among adult female hyenas. Unprovoked aggression describes attacks between individuals that are not related to maternal defense or control of resources. My results suggest that females use unprovoked aggression to test existing rank relationships. However, we did not find evidence supporting the notion that aggression is preferentially directed at females to suppress their reproductive efforts.
|Adviser||Kay E. Holekamp|
|School||MICHIGAN STATE UNIVERSITY|
|Subjects||Animal Physiology; Zoology|
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