Oxygen and avian eggshell formation at high altitude

Many birds at high altitude lay eggs with reduced eggshell diffusive conductance to water vapor. Disagreement exists about the cause, but hypotheses include physiological acclimatization and genetic adaptation. To investigate this, we tested for the occurrence of physiological acclimatization and quantified the effect of relieving hypoxia at high altitude. Ten laying hens (Gallus domesticus) were exposed to elevations of 3800 m (Pi_O2 90 Torr), 3800 m with supplementary oxygen (Pi_O2 140 Torr), and 1200 m (Pi_O2 125 Torr). 573 eggs were collected and analyzed during the 17-week experiment. Shell conductance, aggregate pore area, and shell thickness were reduced at 3800 m compared to 1200 m or 3800 m with supplementary oxygen. There was a lag in the response to changes in altitude or Pi_O2 that corresponded to a time constant of approximately 2.5 weeks. We conclude that physiological acclimatization of eggshell conductance occurs in some chickens, and that it is probably stimulated by hypoxia.