Meeting Abstract
12.5 Monday, Jan. 4 Trade-off between migration and reproduction, and the physiological basis of egg size dimorphism in Macaroni penguins. CROSSIN, G.T.*; TRATHAN, P.N.; PHILLIPS, R.A.; WILLIAMS, T.D.; Simon Fraser University (Canada) and Centre for Ecology and Hydrology (UK); British Antarctic Survey; British Antarctic Survey; Simon Fraser University crossin@interchange.ubc.ca
Crested penguins (Eudyptes spp.) are unique as they lay 2-egg clutches in which the first egg (A-egg) is much smaller (55-84%) than the second egg (B-egg). Macaroni penguins (E. chrysolophus) have one of the greatest degrees of egg-size dimorphism within the genus, and a previous study presented some evidence that inter-individual variation in dimorphism is a function of the amount of time spent in the breeding colony prior to laying. This suggests that the female reproductive system only fully develops after arrival at the colony, and that eggs initiated and developed while still at sea are subject to a constraint imposed by the demands of migration. To address this, we studied the reproductive physiology of Macaroni penguins breeding at Bird Island, South Georgia (54°S), with the aim of describing the physiological mechanisms underlying egg dimorphism. Specifically, we tested the hypothesis that “reproductive readiness”, as measured by plasma yolk precursor levels, increases with time after arrival, and that the smaller A-eggs, which begin developing at sea, do so when yolk precursor levels are below maximal relative to the B-egg. We collected serial blood samples from 48 female penguins from the time of arrival at nest sites to just after the laying of their A- and B-eggs. We also took egg measurements. As yolk deposition is driven by the 17β-estradiol-induced production of vitellogenin (VTG, the principal yolk protein), we predicted that concentrations of circulating VTG would increase through time and show strong correlations with egg-size dimorphism. Results are discussed with emphasis on the timing and costs of reproductive preparedness in a migratory species.
