Development of Endothermy : Getting to the Heart of the Problem
Faber, Alan R. III
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Precocial birds develop from ectothermy to endothermy during the final days of incubation and the first 24 hours after hatching. Onset of endothermy and the ability to thermoregulate is concurrent with ductus arteriosus closure, two physiologically stressful events that have extensive and still fully-unknown developmental impacts. The present study examined changes in heart mass and whole animal oxygen consumption as an indicator of endothermic capacity in precocial Pekin ducklings (Anas pekin). Parameters of heart mass, hemoglobin, and hematocrit were measured to elucidate changes in blood oxygen carrying capacity. Mitochondrial respiration in permeabilized cardiac fibers from left and right ventricles was measured to examine cellular level metabolic capacity. Measurements were made on day 24 of incubation, at internal and external pipping stages, and at 12 to 24 hours after hatching. There was a significant increase in body mass, coupled with a decrease in yolk sac mass, as ducks aged. No significant differences were documented in hemoglobin or hematocrit between age groups. At the onset of endothermy, hatchling relative ventricle size was significantly larger than in earlier stages of development. Increased heart mass was associated with an increase in cardiac oxidative phosphorylation (OXPHOS) through Complex I in the left, but not right ventricle. In hatchlings, there was a significant increase in OXPHOS through Complex I + II within the left ventricle compared with the right. These results suggest that increased ventricle size and mitochondrial respiration within the left ventricle of hatchlings occurs in response to increased energy demand brought about by acquisition of endothermy and the closing of the ductus arteriosus and could be invaluable in uncovering the physiological and developmental mechanisms behind the ontogeny of endothermy.