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Abstract
Observations of feral horse stallions (Equus caballus) and other equids suggest these animals engage in assertive antipredator tactics. These are behaviors which increase an animal's exposure to a predator or which decrease the distance between the prey and the predator. Examples of assertive antipredator behaviors include sentinel behavior, predator inspection and approach, and predator harassment or mobbing. Many different taxa engage in these behaviors. Because assertive antipredator behaviors seemingly increase the risk to the performer, their widespread appearance in the taxonomic record needs to be explained. Chapter one of this dissertation reviews the major functional and evolutionary hypotheses for assertive antipredator behavior. This chapter brings together disparate research avenues to create a cohesive conceptual framework for future studies of assertive antipredator behavior. Chapter two investigates antipredator behavior in feral horses and challenges the popular notion that predation risk is not a significant determinant of feral horse behavior. For this study, I presented a realistic-looking model of a predatory mountain lion (Puma concolor) and a nonpredator control model of a pronghorn antelope (Antilocapra americana). The stallions were able to distinguish between the two models, identifying the predator model as more dangerous. Chapter two concludes that feral horse stallions engage in assertive antipredator behavior after model detection and disappearance, as part of a larger suite of antipredator tactics. It further concludes that the threat of predation is extremely relevant to behavioral decision-making in feral horses. Accordingly, I suggest the effects of predation need to be considered in any theories about the origins or maintenance of equid social organization. Finally, chapter three combines the work of the first two chapters, using feral horses as a model organism to test major hypotheses about the evolutionary maintenance of assertive antipredator behaviors. Specifically, I test the competing predictions of the kin-selection and individual-state hypotheses. By manipulating the perception of risk in stallions with different social and energetic constraints, I determine that there is tentative support for the kin-selection hypothesis, but almost no support for the individual-state hypothesis.