Publication Date


Advisor(s) - Committee Chair

Dr. Bruce A. Schulte (Director), Dr. Kenneth Crawford, Dr. Steve Huskey

Degree Program

Department of Biology

Degree Type

Master of Science


Animals that live in a social group are often organized in a hierarchy with rank determining access to resources. Maintaining a dominant position requires a high rate of energetically expensive aggressive displays and physical exertion. Lab based winnerloser studies, often conducted with individuals from non-social species, have shown that subordinates have higher stress hormone levels than dominant individuals (subordinatestress hypothesis). However, in carnivorous animals that are cooperative breeders, displays of aggression are associated with elevated stress hormone levels (dominancestress hypothesis). The effect of reproductive state on dominance and stress is not addressed within either of these hypotheses. The purpose of this study was to examine stress level in relation to dominance rank and reproductive state in a non-cooperative breeder and herbivore, the domestic horse. As rank and reproductive state can affect behavior, I examined activity budgets, behavioral patterns, and social interactions, as well as the proximity and identity of neighbors in the social group. At two facilities in Kentucky, I recorded the social interactions and measured fecal glucocorticoids in pastured, female horses that were either lactating or non-lactating. While fecal glucocorticoid level did not differ between reproductive state and rank, activity behavior demonstrated significant differences between reproductive states. Higher energetic requirements of lactation were reflected in significantly longer bouts of eating and significantly less time spent alert and socializing. The non-cooperative social nature of horses does not limit their reproduction or resource acquisition based upon rank, and therefore does not fit with the dominance-stress hypothesis or subordinate-stress hypothesis and instead offers the alternative of an independent-stress hypothesi


Animal Sciences