SWETT, M. B.; BREUNER, C. W.; Univ. of Texas, Austin; Univ. of Texas, Austin: Testosterone Correlates with Morph-type in the Behaviorally Polymorphic White-throated Sparrow (Zonotrichia albicollis)

The white-throated sparrow exhibits a unique genetic polymorphism that effects plumage and behavior in both sexes. Birds possessing a chromosomal inversion have brighter white stripes on their crown, are more aggressive and provide less parental care than tan-striped birds lacking the chromosomal inversion. Two prior studies failed to find significant differences in testosterone (T) or other androgens that correlated with morph type. In this study, we found robust differences in plasma T levels between white-striped and tan-striped male morphs in both free-living (breeding) and captive (long day) birds. To understand the biological relevance of these plasma differences, we need to calculate free levels of plasma reaching tissues. In birds, there is no sex steroid binding globulin to regulate free testosterone levels. However, corticosteroid binding globulin (CBG) binds T with relatively high affinity. To estimate free testosterone levels in these birds, it is necessary to quantify CBG levels, as well as baseline corticosterone (CORT) levels, as CORT will compete for binding sites. We have completed the CORT analysis (and see no morph differences in CORT levels), and are currently assessing CBG capacity. Contrary to our expectations, we did not find a significant difference in plasma testosterone between the female morphs. However, the majority of our female blood samples were collected during the incubation phase of the nesting cycle when testosterone levels in females are expected to drop. Testosterone levels in females may be higher, and differences more apparent, during territory establishment. In upcoming field seasons, we will investigate this possibility. Future plans also include evaluating the role of prolactin in morph specific differences in parental behavior.