Development of endothermy and concomitant increases in cardiac and skeletal muscle mitochondrial respiration in the precocial Pekin duck (Anas platyrhynchos domestica)

ABSTRACT Attaining endothermic homeothermy occurs at different times post-hatching in birds and is associated with maturation of metabolic and aerobic capacity. Simultaneous measurements at the organism, organ and cellular levels during the transition to endothermy reveal means by which this change in phenotype occurs. We examined development of endothermy in precocial Pekin ducks ( Anas platyrhynchos domestica ) by measuring whole-animal O 2 consumption ( V O 2 ) as animals cooled from 35 to 15°C. We measured heart ventricle mass, an indicator of O 2 delivery capacity, and mitochondrial respiration in permeabilized skeletal and cardiac muscle to elucidate associated changes in mitochondrial capacities at the cellular level. We examined animals on day 24 of incubation through 7 days post-hatching. V O 2  of embryos decreased when cooling from 35 to 15°C; V O 2  of hatchlings, beginning on day 0 post-hatching, increased during cooling with a lower critical temperature of 32°C. Yolk-free body mass did not change between internal pipping and hatching, but the heart and thigh skeletal muscle grew at faster rates than the rest of the body as the animals transitioned from an externally pipped paranate to a hatchling. Large changes in oxidative phosphorylation capacity occurred during ontogeny in both thigh muscles, the primary site of shivering, and cardiac ventricles. Thus, increased metabolic capacity necessary to attain endothermy was associated with augmented metabolic capacity of the tissue and augmented increasing O 2 delivery capacity, both of which were attained rapidly at hatching.