VEZINA, F.*; WILLIAMS, T.D.; Simon Fraser Univ., Burnaby, BC, Canada: Metabolic adjustments at the organ level: Does avian egg production result in energy conflicts between organs?

Basal metabolic rate (BMR) can be defined as the sum of the energy expenditure of all physiological systems and organs at rest; i.e., the maintenance cost of an animal. Studies investigating the basis of variation in BMR often compare it to the size or mass of internal organs, assuming that elevated daily energy expenditure needs to be supported by larger machinery resulting in an increased maintenance cost. However, i) the mechanistic link between BMR and organ mass across species is not obvious, and ii) organs significantly related to BMR are inconsistent between seasons and species. This may not be surprising since an organism under high demand may readjust the mass-specific energy use between its various physiological systems to compensate for the increased effort. Egg production is known to induce significant increases in resting or basal metabolism in at least three species of birds. However, in egg-producing European starlings (Sturnus vulgaris), the elevated metabolic rate is only significantly related to oviduct mass and this organ only accounts for 18% of the variation. Therefore, other organs may undergo an increase in mass-specific energy consumption without a change in mass. In this study, using citrate synthase as an indicator of energy use, we investigated metabolic adjustments in four key organs in reproductive females starlings throughout the breeding season to determine if the cost of egg production results in readjustments in mass-specific energy use between organs. Despite an active liver involved in yolk precursor production and significant changes in organ mass between breeding stages and years, metabolic adjustments do not appear to be related to the demand of egg production but rather respond to a transition between seasons.