Dynamics and Outcome of Aggressive Contests:
Mechanisms and Evolutionary Perspectives

Ryan L. Earley

Department of Biological Sciences
University of Alabama

Animals must make critical decisions during aggressive contests to avoid accruing too much in the way of time, energy, or injury costs while battling for a resource that, perhaps, they cannot win. We study how animals decide to give up or to persist during contests, and we have revealed that animals can toggle between strategies in a context-dependent fashion—for example, animals use different rules when engaged in intense fights versus relatively benign fights. We also have explored the physiological mechanisms that might underlie these decisions, focusing particularly on metabolic costs (e.g., accumulation of lactate, depletion of energy reserves) and hormonal states (e.g., how titers of testosterone might influence aggressive motivation). Once a contest is settled, winners and losers tend to behave differently when encountered by future opponents—previous winners often are more likely to start fights and they tend to win again while previous losers often are behaviorally depressed and go on to lose again. The specific mechanisms underlying experience-dependent changes in contest behavior continue to elude us but I will discuss some candidate mechanisms. Importantly, we still do not fully understand the fitness consequences of these so-called "winner and loser effects." In some respects, it makes sense to explore how prior wins or losses impact future wins or losses—that is, to keep the context in which we view "winner and loser effects" (i.e., contest behavior) constant in order to evaluate fitness effects. It is also possible that selection for "winner and loser effects" has more to do with how past fighting experiences impact other aspects of behavior such as reproduction and dispersal, and how these types of response influence fitness. We have no empirical data to support these claims but I hope to present some supporting evidence and to encourage some discussion on this topic.

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