Date of Award

5-2015

Document Type

Thesis open access

Department

Biology

First Advisor

Troy G. Murphy

Abstract

In order to acquire and defend resources, individuals make decisions about the benefits of acquiring a resource weighed against the potential cost of injury and lost time and energy. Individual investment into aggression is expected to be proportional to the value of the contested resource, and that investment in defense of different categories of resources (e.g. food, mates) will vary depending on the value of each category of resource. Female Siamese fighting fish (Betta splendens) competed in dyadic trials with other female bettas in two experiments: one for food of either high or low value (small or large amount of food), and second, for males of either high or low value (large or small bodied male). Aggression was quantified as the number of behavioral displays and physical attacks performed during each competitive interaction. Within a dyad, female bettas expressed more physical aggression when competing over a small amount of food compared to a large amount of food, yet had fewer displays when competing over a small amount of food. Shorter display time and more attacks are consistent with the hypothesis that females perceive a small amount of food as more valuable because the resource may be depleted more quickly. In male stimulus trials, females increased aggression when they were presented with a small-bodied male compared to a large-bodied male. This is consistent with the hypothesis that females exhibit a choice for smaller males, which may pose less threat to females, as they have reduced aggression during courtship compared to larger males. To examine the type of selective pressures driving female aggression, I analyzed the difference between female aggression over food or males. Females showed more aggression when competing over food compared to competing over males, indicating stronger social selection for female aggression for obtaining food, while sexual selection for acquisition to mates has favored lower levels of aggression in mate-based competition. These results indicate that females modulate aggression depending on resource value, irrespective of the category of contested resource, and that a limitation of food resources appears to be a strong driving factor in female aggression, while competition for access to mates, although present, is less robust.

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