Oxygen Consumption, Gas Exchange, and Growth of Embryonic Wedge-Tailed Shearwaters (Puffinus pacificus chlororhynchus)

The mass of the wedge-tailed shearwater (Puffinus pacificus chlororhychus) egg is approximately the same as that of the chicken (≃60 g), but the incubation period is more than twice as long (52 days vs. 21 days) and the water vapor conductance, $$G_{H_{2}O}$$, less than one-half that for the chicken's egg. These differences appear related to constraints on water loss by the egg, since eggs of both species lose about 15% of their initial mass as water during prepipping incubation. Air-cell gas tensions ($$P_{A_{O_{2}}}$$, $$P_{A_{CO_{2}}}$$) of shearwater and chicken eggs are similar throughout incubation-close to 100 torr, $$P_{A_{O_{2}}}$$, and 42 torr, $$P_{A_{CO_{2}}}$$, just prior to shell pipping. The low gas conductance of the shearwater eggshell is matched by a low egg O₂ uptake (Ṁo2), relative to the chicken egg, throughout incubation. Just prior to pipping, the Ṁo2 of the shearwater egg (300 ml O₂ STPD·day⁻¹) is less than one-half that of the chicken egg. At pipping, the Ṁo2 rises rapidly, tripling by the time hatching occurs, so that at hatching the Ṁo2 of shearwater and chicken hatchlings are similar. The postpipping, prehatching period is considerably longer in the shearwater than in the chicken. The low Ṁo2 of the developing wedge-tailed shearwater embryo may be related to its reduced rate of growth relative to the chicken embryo. The total oxygen cost of incubation for the shearwater egg (8 liters O₂) is greater than that for the chicken egg (5.5 liters O₂), but the oxygen consumed during prepipping incubation is similar. However, the shearwater embryo appears to pip at a smaller embryonic mass than the chicken embryo, so that the oxygen cost of producing a unit of embryonic tissue is greater for the shearwater for both prepipping and total incubation. The higher energetic requirements of the shearwater embryo appear to be met by increasing the amount of yolk in the egg.