Adaptive reduction in permeability of avian eggshells to water vapour at high altitudes

WE report here the existence of an altitudinal cline in permeability of eggshells to water vapour for eggs of a North American species of bird. This pattern of geographic variation in permeability of eggshells seems to have arisen as an adaptive response to altitudinal variation in the diffusivity of water vapour through air. Diffusivity of atmospheric gases has not previously been widely recognised as an important component of the physical environment to which organisms are exposed, and our findings provide further evidence for the pervasiveness of natural selection.