Dear Enemy Effect in Male Brown Anoles (Anolis sagrei)
Abstract
In the dear enemy effect, territory owners display more aggression towards unfamiliar strangers and less aggression towards familiar neighbors. This biological phenomenon is found in species that have territories that serve a breeding and feeding function. Glucocorticoids, like corticosterone, play an important role in mediating behavioral and physiological responses to stressors, such as increasing aggression in antagonistic encounters. I tested the possibility that corticosterone mediates aggressive behavior associated with the dear enemy effect in male brown anoles with two treatment groups: males with implants containing metyrapone, a glucocorticoid synthesis inhibitor, and males with control blank implants. Focal males in both treatment groups were exposed to stimulus neighbors for four days and subsequently their behavior was measured in trials with a familiar neighbor and an unfamiliar stranger. The aggressive behavior of focal males directed towards neighbors and strangers were recorded and assigned an overall aggression score. Behaviors that were recorded included dewlap extensions, head bob displays, sagittal expansions, dorsal crests, approaches, retreats, and attempted attacks. Overall, male brown anoles displayed more aggression towards strangers than towards neighbors, thus confirming the dear enemy effect. Furthermore, although males given metyrapone implants did not differ from control males in their aggression scores, there was an effect of corticosterone; males with higher plasma corticosterone concentrations exhibited lower aggression scores. These results are discussed within the context of corticosterone and aggression across social contexts.
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