Synchronous Hatching and Oxygen Consumption of Darwin's Rhea Eggs (Pterocnemia pennata)

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Abstract
Eggs of Darwin's rhea may hatch synchronously, even though incubation periods may differ for individual eggs. We attempted to measure the capacity for synchrony and to determine whether differing developmental rates were responsible for unequal hatching times. Our measurements demonstrated no differences in the rates of oxygen consumption, diameters of blastoderms on incubation day 7, eggshell temperatures, or physical parameters of groups of eggs that hatched after longer or shorter incubation periods. Therefore, the rates of development appear to be similar in all eggs, regardless of age at hatching. The most likely explanation for synchronous hatching is that, once eggs reach a certain developmental level, they are susceptible to a hatching stimulus from other eggs. Embryos that reach this level may hatch less developed, with incomplete yolk sac resorption. Eggs that have not yet attained this development at the time that other eggs begin to hatch will never hatch, owing to nest abandonment by the male.