Gas exchange in the incubation mounds of megapode birds

Summary

The brush turkey (Alectura lathami) and mallee fowl (Leipoa ocellata) are megapode birds that incubate their eggs by burying them in mounds. Respiratory gas exchange between the buried eggs and the atmosphere occurs mainly by diffusion through about 60 cm of decomposing forest litter (brush turkey) or sand (mallee fowl).

Gas fluxes in the brush turkey mound are greatly influenced by the respiration of thermophilic microorganisms which consume O₂ at rates over eight times that of all of the eggs. The respiratory exchange ratio ($\dot V_{{\text{CO}}_{\text{2}} } / \dot V_{{\text{O}}_{\text{2}} }$) of the microorganisms is 0.75 and theQ ₁₀ for metabolism is 2.56. Fermentation and nitrogen fixation do not occur in the mounds.

If the mound becomes too wet, gas tensions near the eggs can become critical because water increases rates of microbial respiration and impedes gas diffusion. However, field mounds are relatively dry, possibly because the adult bird modifies the shape of the mound and affects the entry of rain water. At egg level in field mounds,$P_{O_2 }$ and$P_{{\text{CO}}_2 }$ are about 132 and 21 Torr, respectively, in both species. Embryonic respiration decreases$P_{O_2 }$ and increases$P_{{\text{CO}}_2 }$ about 5 Torr in the immediate environment of individual eggs in late development. Due to a high eggshell gas conductance, which increases during incubation, the gas tensions within the shell of late embryos ($P_{O_2 }$ ca. 108 Torr,$P_{{\text{CO}}_2 }$ ca. 47 Torr) are not far from the mean values found in species that nest above ground.