CO₂ and avian eggshell formation at high altitude

We tested the hypothesis that altitude-induced hypocapnia in hens reduces eggshell conductance to water vapor (G_H2O). Seven laying hens (Gallus domesticus) native to 1200 m were chronically exposed to high altitude (3800 m), and then to high altitude with sufficient inspired CO₂ to relieve hypocapnia (3800 m + CO₂). Egg G_H2O was measured gravimetrically, shell thickness was measured with a micrometer, and aggregate pore area was calculated from measured values using Fick's law. Comparing results at 1200 m (n = 118) and 3800 m (n = 102), G_H2O was reduced from 13.9 ± 0.2 to 12.6 ± 0.2 mg/(d·Torr) (mean ± SE), shell thickness was reduced from 0.297 ± 0.003 mm to 0.287 ± 0.03 mm, and calculated aggregate pore area per egg was reduced from 1.97 ± 0.03 mm²to 1.72 ± 0.03 mm². When hypocapnia was relieved at 3800 m + CO₂ (n = 82), G_H2O was reduced even further to 11.1 ± 0.2 mg/(d·Torr), shell thickness increased to 0.305 ± 0.003 mm, and aggregate pore area was reduced 1.61 ± 0.03 mm². Based on these results we reject our hypothesis. We conclude that hypocapnia is responsible for thin eggshells at altitude. Other physiological stimuli must cause the reductions in eggshell G_H2O and pore area.