VEZINA, F.*; WILLIAMS, T.D.: What drives the metabolic cost of egg production?

Preliminary data from a study on egg production in European starlings showed a positive correlation between resting metabolic rate (RMR) and plasma levels of the two main yolk precursors, vitellogenin (VTG) and very low density lipoprotein (VLDL). At that time we suggested that the energy cost of producing the precursors in the liver was enough to have an impact on the overall energy expenditure of resting females during egg production. We measured RMR of females from the pre-breeding stage (before the development of the reproductive organs) through to clutch completion. RMR at the one egg stage was increased by 30% over the pre-breeding values. Although plasma levels of VTG and VLDL were still correlated with RMR, lean dry liver mass did not explain the variation in metabolic rate. Yolk precursor levels, as well as the reproductive organ masses, closely followed a pattern associated with the number of developing ovarian follicles. Our analysis showed that the mass of the reproductive organs, mainly the oviduct, explained the largest part (32%) of the variation in egg producing RMR. Therefore the metabolic cost of egg production was not entirely influenced by precursor production, but was mainly explained by the physiological processes involved in oviduct activity and maintenance.