Meeting Abstract
Testosterone (T) mediates many life history and behavioral trade-offs in vertebrates. Biologists interested in these effects often use implants to experimentally elevate T for prolonged periods of time. Alternatively, GnRH challenges, or injections with gonadotropin-releasing hormone, can be used to induce a transient rise in T and correlate this short-term T elevation to other aspects of an individual’s phenotype. To date, GnRH challenges have not been widely used to experimentally manipulate T-mediated traits. To begin to address the possibility that GnRH challenges induce changes beyond their short-term effects on T levels, we injected free-living male tree swallows (Tachycineta bicolor) with either GnRH (n=17) or saline (n=9) and measured plasma T levels 30 min later. Each male had a nest with chicks, and we also quantified the rate of nestling growth over the 24h post-injection, as a proxy for male provisioning effort. Based on prior studies that suggest T decreases paternal care in many songbirds, we predicted that chicks of GnRH-males would gain less mass than those of saline-injected males. Surprisingly, we found that offspring of GnRH-males gained more mass than those of controls, despite the fact that GnRH elevated male T levels. Even within GnRH-males, the amount of T elevation was positively correlated with chick mass growth. We hypothesize that these findings stem from (a) females over-compensating for diminished provisioning by GnRH-males, (b) negative feedback clearing T from the bloodstream, or (c) other GnRH-responsive hormones enhancing paternal care. Further research is needed to tease apart these alternatives, but this study nonetheless opens the door to the use of GnRH challenges as an experimental treatment.
