Meeting Abstract

23.2  Friday, Jan. 4  Hormone-trait relationships for experimentally enlarged clutches continue to challenge the prolactin-based model for clutch-size determination RYAN, C.P.*; DAWSON, A.S.; SHARP, P.J.; WILLIAMS, T.D.; Simon Fraser University, Canada; Centre for Ecology and Hydrology, Edinburgh, U.K.; The Roslin Institute, University of Edinburgh, U.K. ; Simon Fraser University calen_ryan@sfu.ca

Clutch-size is a chief predictor of avian lifetime reproductive success, with fitness ramifications for both females and their offspring. While adaptive variability in clutch-size in response to predation, seasonality, or condition-dependant cues has theoretical and empirical support, remarkably little is known about the proximate mechanisms that enable both plasticity and repeatability in this trait. The only formal mechanistic hypothesis for clutch-size determination in birds predicts an anti-gonadal effect of Prolactin, a peptide hormone commonly associated with incubation and chick rearing. Now over 20 years old, this model has become widely accepted despite a scarcity of experimental support. Correlational findings from captive-breeding zebra finches (Taeniopygia guttata) in our lab do not substantiate a relationship between prolactin and clutch-size. In a follow-up experiment, we accompanied egg removal with sequential blood sampling in an attempt to further investigate any hormone-trait relationships. Egg removal significantly increased clutch-size (from ~6 to 15 eggs, on average), exposing latent phenotypic plasticity, and was associated with changes in circulating prolactin levels. Still, the nature of this relationship was not consistent with a role for either absolute threshold or rate of prolactin increase accompanying the cessation of laying. These findings continue to challenge the applicability of the only mechanistic model available to explain a key fitness trait in birds.