Adaptive variation in avian eggshell structure and gas conductance across elevational gradients?

Many tropical species have restricted elevational distributions, which are potentially bounded by constraints imposed by physical conditions on physiological processes. Although some studies have examined variation in the physiology of adult birds with respect to elevation, little attention has been paid to the structure and function of eggshells, which mediate gas exchange between the embryo and the environment. At high elevations, dry air is expected to increase water loss from the egg; selection to avoid desiccation might therefore be expected to favor reduced gas conductance by means of increased eggshell thickness or reduced pore size. We used gas diffusion experiments and scanning electron microscopy to examine water vapor conductance rates and eggshell structures in 197 bird species distributed along an elevational gradient in the Andes. As predicted, water vapor conductance across the eggshell declined in a narrow range with elevation among all species and among species within families, but not among individuals within species. Variation among species in eggshell conductance was lower at high-elevation sites, potentially indicating greater constraints at such sites. Structural changes in eggshells with respect to elevation varied among taxonomic families of birds, suggesting potentially different adaptive responses to common selective pressures in terms of eggshell thickness and pore density, and size. We suggest that considering functional and structural traits of eggshells, which influence embryo development, may help one to better understand the elevational distributions of species and to forecast their responses to global climate change.