Gas fluxes in avian eggs: Driving forces and the pathway for exchange

Abstract

1. 1. In this study, we review reported values for fluxes of water vapor and oxygen across the pores in avian eggshells, pore numbers per egg, the changes in O 2 and CO 2 tensions in the air space during development, and the absolute humidity of bird nests in various climates. 2. 2. With egg mass from 1 to 1500 g as the independent variable we use regression analysis of daily water vapor loss, O 2 uptake at the preinternal pipping (PIP) stage of development, and pore numbers to show that O 2, CO 2 and water vapor fluxes per pore are 68, 50 and 49 μl/day, respectively, independent of egg mass. When these fluxes are divided by the invariant pore conductance previously established by Ar and Rahn ( Respir. Physiol. 61, 1–20, 1985), predicted air cell O 2 and CO 2 tensions of ca 100 and 40 Torr just prior to the initiation of lung function are obtained, values which agree well with mean measured gas tensions in 25 species. 3. 3. Our analysis complements the model proposed by Ar and Rahn, in which pores serve as basic respiratory units for most bird eggs. In this model, pore number per egg is matched to O 2 demand at the PIP stage of development to produce the commonly observed air cell gas tensions. Average, total diffusive water loss in 117 species is 15%, SD 2.6, of initial egg mass. To achieve this value requires the proper combination of pore number, egg temperature and nest water vapor tension; the latter is also a function of nest construction and incubation behavior. Examples of nest absolute humidity are cited for 20 species which incubate in diverse climates with ambient absolute humidities from 4 to 22 Torr. Exceptions to the model are seen in eggs incubated under environmental conditions which are unusual in temperature, humidity, or altitude.