VEZINA, F.*; SALVANTE, K.G.; WILLIAMS, T.D.: The Metabolic Cost of Yolk Precursor Production

The metabolic cost of egg production in birds is poorly understood. Previous results with European Starlings ( Sturnus vulgaris) suggested that physiological adjustments, namely production of the yolk precursors, vitellogenin (VTG) and yolk-targeted very-low density lipoprotein (VLDLy), may be responsible for an increase in resting metabolic rate (RMR) during follicular development and egg production. To test this hypothesis we administered 17-beta estradiol (E2) to male and female 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 after the E2 administration period (four daily injections). E2 administration resulted in an increase in plasma VTG levels, as well as an increase in oviduct mass, but to levels lower than natural reproductive state. RMR was not affected by E2 treatment, nor were plasma levels of VLDL or ovary mass. Resting metabolic rate was not related to oviduct mass and was repeatable when comparing before and after treatment.