Metabolism of Avian Embryos: Ontogeny of Oxygen Consumption in the Rhea and Emu

Abstract

Oxygen consumption of Rhea (Rhea americana) and Emu (Dromiceius novaehollandiae) eggs increases exponentially during the first 70% of incubation and reaches a maximum about three-quarters of the way through incubation. Rate of O₂ consumption then declines to about 75% of the peak value, increasing again just prior to pipping. We suggest the decline in rate of O₂ consumption is due to a decline in growth rate, and that growth of the embryo of ratites is essentially complete at the time of the peak in O₂ consumption. Completion of growth prior to the normal end of incubation may permit ratite eggs of different ages to synchronize hatching within a clutch. Rates of O₂ consumption just prior to the initiation of pulmonary respiration are 104 ± 7 cm³ h⁻¹ in Rhea eggs and 75 ± 7 cm³ h⁻¹ in Emu eggs. Calculated and measured air-cell gas tensions at this stage of incubation vary systematically with egg size between species of birds. Large eggs have higher air-cell O₂ tensions and lower air-cell CO₂ tensions than do small eggs. Water vapor conductance of Emu eggs is 46.2 ± 9.6 mg·day⁻¹ torr⁻¹, much lower than predicted on the basis of egg size and incubation period.