VEZINA, F.*; SALVANTE, K.G.; WILLIAMS, T.D.; Simon Fraser University, Burnaby, B.C., Canada: The Metabolic Cost of Avian Egg Formation: Possible Impact of Yolk Precursors Production?

The metabolic cost of egg production in birds is poorly understood. Previous work done by our lab with European Starlings (Sturnus vulgaris) showed a 22% increase in resting metabolic rate (RMR) in laying females. However only 21% of the variation in laying females RMR was explainable (oviduct and heart mass explaining 15% and 6% respectively). It is possible that other processes like the production of the yolk precursors, vitellogenin (VTG) and yolk-targeted very-low density lipoprotein (VLDLy) by the liver, are responsible for part of the remaining variation in laying RMR. To test whether yolk precursor production affects RMR in birds we administered 17-beta estradiol (E2) to male zebra finches (Taeniopygia guttata) to stimulate yolk precursor production in non-reproductive birds and measure its effect on RMR. The metabolic rates of sham and E2-treated birds were measured before and during the E2 administration period (after two days of injections). After four days of injections, E2 administration resulted in an increase in plasma VTG to levels comparable to breeding females at the one egg stage. Neither RMR nor circulating VLDL was affected by E2 treatment.