REED, W.L.*: The plateau phase in egg metabolic rates: evidence for oxygen limitation.
In bird eggs, rates of gas exchange across the shell are limited by diffusion and, thus, a function of the physical properties of the shell and the surrounding environment (i.e. conductance, surface area:volume ratio). As metabolic rates increase over the incubation period oxygen demand may eventually exceed oxygen supply and ultimately limit metabolism. In many birds a plateau in metabolic rate is reached prior to hatching and this plateau may be a function of oxygen limitation. I tested this hypothesis by incubating American coot eggs in hyperoxic (60% O2) and normoxic (21% O2) conditions. Metabolic rates of eggs incubated in hyperoxic conditions were greater than metabolic rates of eggs in normoxic conditions during the last two days prior to convective breathing. This suggests that metabolism is compromised by diffusion of oxygen across the shell late in incubation and the plateau phase of metabolic rates is due to oxygen limitation. Hatchlings from the hyperoxic treatment were heavier than hatchlings from the normoxic treatment, however most of this mass difference was due chicks from the hyperoxic incubation environment having more water in their tissues than chicks from the normoxic incubation environment.