Social behavior and relationships between conspecifics have led to the characteristic of obligate group living in some species. Benefits of these longer-term social groupings include assistance with predator defense, foraging, and infant care, which often outweigh the associated individual costs of group-living, such as increased competition for mates, food and other scarce resources. Social interactions between peers in such social groups include affiliative, aggressive, communicative, and/or cooperative behaviors. These behaviors are the result of both environmental factors and the individual's genetic, epigenetic, endocrine and neural mechanisms that affect fitness and that evolve through natural selection. My dissertation investigates the physiological mediation of these social behaviors and their relationship to the costs and benefits of group living in social carnivores. Chapter One reviews the endocrine mechanisms that mediate cooperative breeding in mammalian carnivores, focusing on reproductive suppression and alloparental care. My review indicates that breeding carnivores typically have higher levels of a suite of reproductive hormones than do non-breeders, while the effect of glucocorticoids on reproductive suppression appears to be sex specific. In my remaining chapters, I use the spotted hyena (Crocuta crocuta), a gregarious carnivore living in complex social groups, as my model organism in my study of the physiological mediation of complex social interactions. Chapter Two presents and validates a novel non-invasive method of collecting saliva from juvenile spotted hyenas that allows for detection of short-term fluctuations in glucocorticoid concentrations. I examine how social behaviors, such as aggression and play, affect glucocorticoid concentrations, and find that receiving aggression increases glucocorticoid levels while emitting aggression reduces them. Chapter Three investigates the social and physiological mechanisms that lead to hyenas' cooperative mobbing behavior, through exploration of when mobs occur, which hyenas participate, and what potential benefits might accrue. My findings indicate that individuals participate in this type of collective behavior primarily to obtain benefits for themselves, and that their decision to participate is a function of the characteristics and internal state of potential cooperators as well as their immediate social environment.
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