Hatching and the Establishment of Thermoregulation in the Wedge-Tailed Shearwater (Puffinus pacificus)

Abstract

Late-incubation unpipped eggs, pipped eggs, and hatchlings of the wedge-tailed shearwater (Puffinus pacificus) were exposed to constant ambient temperatures ( $T_{a}'s$ ) between 28° and 38°C (eggs) or 15° and 43°C (hatchlings). Below 36° C (incubation temperature for shearwater eggs), oxygen consumption ( $\dot{V}O_{2}$ ) and body temperature ( $T_{b}$ ) of embryos varied directly with $T_{a}$ , even with individuals in eggs with well-developed pip holes. Thus, shearwaters appear unable to initiate and sustain any effective cold-induced thermogenesis before hatching, even though access to oxygen appears to improve substantially during pipping. In contrast, hatchlings increased their $\dot{V}O_{2}$ by as much as 74% during cooling, with the maximum rate occurring at a $T_{a}$ of 25°C. The consequent increase in heat production served to maintain $T_{b}$ near 35°C at $T_{a}'s$ between 25° and 35° C, but hypothermia tended to develop below 25°C. Hatchlings increased pulmocutaneous evaporation at $T_{a}'s$ above 36°C, but this only produced a limited capacity for heat defense. Establishment of endothermy in young of some altricial birds has been linked with increased activity of Ca-, Mg-activated myofibrillar ATPase. However, no significant difference in protein-specific activity of this enzyme was found between embryos in pip-holed eggs and hatchlings. This observation provides tentative support for the view that the appearance of endothermy in wedgetailed shearwaters at hatching is linked more with the elimination of the physiological consequences of physical confinement than with abrupt biochemical maturation.