Stress and aggression in agonistic encounters

Establishment of dominant/subordinate relationships is a complex agonistic process, mediated by multiple physiological systems. Aggression levels are often correlated with testosterone, the primary gonadal steroid in most vertebrate males. However, the way an animal responds to stress can also regulate the outcome of an agonistic encounter. My goal in this research is to understand how an animal’s response to stress can alter success or failure in an agonistic interaction, and how the interaction itself then alters the stress response. I was led to this work by an interesting trait of avian corticosteroid binding globulin: in birds, there is no sex-steroid binding globulin, and it was thought that androgens circulated in the blood without a chaperone. However, during my postdoc I discovered that corticosteroid binding globulins in birds also bind androgens, and serve as physiologically relevant androgen binding globulins in the avian system. This characteristic has broad implications for the interaction of testosterone and corticosterone, as increases in one hormone will affect tissue availability of the other. To approach this issue, I have begun working in the white-throated sparrow (Zonotrichia albicollis), which shows a genetically based behavioral polymorphism. This species is fascinating for a number of reasons: it is one of only a few polymorphic species in which both males and females show morph differences; interestingly, the morphs mate disassortatively, with each morph-type supposedly having equal fitness despite significant behavioral differences during breeding. For my work, the most important difference between morphs is in aggression. White-striped birds are more aggressive than their tan-striped conspecifics. I am currently examining the physiological mechanisms underlying these differences, including responsivity of glucocorticoid secretion, the effect of stress on androgen and aggression levels, the interaction of glucocorticoids and androgens at the binding globulin level, as well as differences in glucocorticoid receptor number in brain areas associated with aggression.