NAVARA, K.J.*; HILL, G.E.; MENDON�A, M.T.; Auburn University, AL; Auburn University, AL; Auburn University, AL: Differential effects of exogenous yolk testosterone on growth and immune response in bluebird nestlings

Steroid hormones, such as testosterone (T) and dihydrotestosterone (DHT), deposited into the yolk by female birds and reptiles have been found to have a m�lange of positive and negative effects on offspring fitness including increased begging, feeding and growth rates as well as increased mortality rates. Testosterone has also been known to have immunosuppressive effects on adults. We examined potential positive and negative effects of yolk T on Eastern bluebird nestlings. Prior to development, bluebird eggs were injected with one of three treatments: high T, low T, or a control vehicle injection. Resulting offspring were measured every three days post-hatch for weight, tarsus, wing, and bill lengths. Just before fledging, each chick was challenged with phytohaemagglutinin (PHA) to measure T-cell immune response. Eggs injected with high T had significantly lower hatch rates than those in the other two groups. Chicks hatching from low T treated eggs were significantly larger on day 2 post-hatch than chicks in control and high T treatment groups, but by 14 days post-hatch (just prior to fledging), chicks in the high T group weighed significantly more than those in the other two groups. Further, chicks in the high T group showed a significantly smaller swelling response to PHA than those in the other two groups, indicating a less efficient T-cell immune response. These data suggest that yolk T has a variety of fitness effects that, taken together, determine the overall beneficial or detrimental outcome of yolk T deposition.