Social experience, hormones and aggressive behavior in the green anole lizard (Anolis carolinensis)
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Differences in individuals’ social experiences make an important contribution to individual differences in the expression of social behavior. Here I report the influence of recent social experience on aggressive behavior in the green anole lizard, Anolis carolinensis. Green anoles have distinct and highly stereotypical social displays that occur during territorial aggressive encounters. However, certain territorial owners act more aggressively than others. This dissertation reports a series of experiments based on the observation that a short period of aggressive social experience can facilitate subsequent aggressive behavior in this species, and the idea that gonadal hormones acting on limbic and hypothalamic nuclei in the brain mediate this experience-dependent facilitation of social behavior. I found that exposure to a video clip of an aggressively displaying conspecific male, for one ten-minute period each day for five days caused male anoles to become more aggressive toward a subsequent conspecific challenger, compared to animals exposed to a video clip showing two moving balls. During the video exposure period, animals showed a robust behavioral pattern of an initial rise followed by dramatic decreases in aggressive responses toward the aggressive video. Plasma testosterone concentrations measured using radioimmunoassays during the 5 days of social encounters showed a pattern similar to that of aggressive behavior. Using structural equation modeling to examine the causal relationships underlying this synchrony of endocrine and behavioral responses suggested that it was the experimental animals’ behavior in response to the stimulus that caused increased testosterone secretion, more probably than the contrary. Unlike testosterone or behavioral responses, plasma corticosterone levels in animals encountering aggressive conspecifics were high throughout the 5-day encounter period. When corticosterone synthesis was blocked temporarily during stimulus exposure, animals did not show decreases in behavioral responses to the stimulus in later exposures, suggesting that corticosterone is involved in behavioral habituation. Finally, I found that metabolic activity, indicated by levels of cytochrome oxidase activity, in Septum, Amygdala, Nucleus Accumbens, Preoptic area, and nucleus Rotundus was upregulated following exposure to the aggressive video for five days, suggesting that these areas are involved in processing this social experience, and in the observed experience-dependent facilitation of aggressive behavior.