Pores in avian eggshells: gas conductance, gas exchange and embryonic growth rate

The number of pores (N) in eggshells of birds were counted in 161 species ranging in egg mass (W) from 0.9 to 500 g. In addition the water vapor conductance of the shell (G) mg X (d X Torr)-1, the water loss in the nest (M) mg X d-1, the incubation duration (I) d, and the shell thickness or pore length (L) micron are listed for each species when available. Allometric equations for all variables are given when regressed on egg mass. When log G is regressed on log N the slope is essentially 1.0 indicating that the G X N-1 for average pores in bird eggshells is 1.5 micrograms H2O X (d X Torr)-1 regardless of egg mass or incubation duration. According to Fick's law of diffusion such pores have a cross-sectional area to pore length ratio of 0.67 micron 2 X micron-1. Further analysis show that N, G, M, and the rate of oxygen consumption at the pre-internal pipping stage, are all directly proportional to the absolute mean growth rate of embryos (defined as 0.67 [W/I], g X d-1). Thus, single pores of typical eggshells not only have a similar conductance G X N-1, but also the metabolic rate and rate of water loss are matched to the pore conductance so that O2, CO2 fluxes per pore at the pre-internal pipping stage and water vapor flux per pore are similar among species, namely 68, 49 and 50 microliters X d-1, respectively. The partial pressure differences across the shell at the same stage are 42, 40 and 27 Torr, respectively. Ecological as well as taxonomical variation may alter some of the relationships predicted for the 'typical' egg in order to conserve a typical overall diffusive water loss.