Dissertation Research: Endocrine Coupling of Sexually Dimorphic Display Morphology and Aggressive Display Behavior in Two Sister Species

Indiana State University, Terre Haute IN

Investigators

Abstract

During aggression males often behaviorally advertise signaling traits. Aggression and the structures used in signaling aggression usually are correlated evolutionarily, but among Sceloporus lizard species they can be decoupled. This study will determine how sex steroid hormones control the development of two such male-specific traits: 1) patches of abdominal color, and 2) stereotyped aggressive behavior used to display the patches to opponents. Work will study two Sceloporus sister-species, one with male patches and one in which males have evolutionarily lost the patches. Hatchling hormone levels will be manipulated, and both patch expression and aggression will be scored in adults to test the hypothesis that decoupling of the male patches trait and aggression arises from species differences in hormonal control of the two. Assays of hatchling blood samples will verify that the altered hatchling hormone levels are within natural levels. In many vertebrates, sex hormones (testosterone, estradiol) act during development and adulthood to produce differences between the sexes. Testosterone can be converted to estrogen, which influences development of brain regions involved in aggression. Testosterone also can be converted to 5a-dihydrotestosterone, which often affects development of male traits elsewhere in the body. Decoupling could occur if species vary in these hormonal levels. For example, males of these species may vary in the enzymes needed for these conversions of testosterone. Relatively little is known about the variation in the hormonal mechanisms controlling differences in aggression and in signaling structures in natural populations, as most research focuses on inbred laboratory strains of rodents and domesticated animal breeds.