SWETT, M.B.*; BREUNER, C.W.; Univ. of TX at Austin; same : Behavioral Polymorphism Correlates with Testosterone in the White-Throated Sparrow

The white-throated sparrow exhibits a unique genetic polymorphism affecting plumage and behavior in both sexes. Birds possessing a chromosomal inversion have brighter white stripes on the crown, are more aggressive and provide less parental care than tan-striped birds lacking the inversion. We are interested in investigating hormonal mechanisms that may underlie these differences in behavior. Prior studies failed to find differences between morphs in testosterone (T), or other androgens associated with aggression. In 2003, we found robust differences in plasma T levels between white-striped (WS) and tan-striped (TS) male morphs in both free-living (breeding) and captive (long day) birds. Data collected in 2004 from free-living birds reconfirms this finding. To our knowledge, this is the first evidence in this species of a difference in T that correlates with observed differences in aggression between morphs. Although female morphs also differ in aggression, we did not find a corresponding difference in T levels. However, the majority of female samples were collected during incubation and nestling feeding, stages when female T levels may be particularly low and differences undetectable. The release of T is controlled by the Hypothalamus- Pituitary-Gonadal (HPG) axis. We performed a gonadatropin releasing hormone (GnRH) challenge to help determine where in the HPG axis differences between morphs occur. When stimulated with GnRH, WS males tend toward a greater increase in T levels than did TS males suggesting that differences between morphs occur downstream of the hypothalamus. Future experiments will also examine female T levels early in the breeding season and the role of prolactin in morph-specific differences in parental care.