A Change of Heart in Birds: Cardiac Response to the Onset of Endothermy

Abstract

Precocial birds, like chickens, experience a large, rapid increase in metabolism at hatching, with the 'physiological stressor" of the establishment of endothermy. In post-prandial pythons, a comparable upregulation of metabolism is accompanied by a rapid increase in heart mass, though in the absence of other body growth. Therefore, we hypothesized that heart mass would increase between 21-day paranatal and 21-day neonatal chickens with the establishment of endothermy. We expected other body parts to grow little or not at all in this 12-hour period. We tested this hypothesis on chickens (Gallus domesticus, White Mountain Broiler) by measuring metabolic rate of 18 (18) and 19-day (19) prenates, 21-day paranates (21p), and 21-day neonates (21 n) at 25°C and 35 °C. We dissected and massed the heart, yolk, intestine, and yolk-free carcass, and measured culmen and tibiotarsus, of the chickens at these stages and also of 16-day incubated embryos. Results of our experiment confirmed the onset of endothermy between 21p and 21n; neonates had higher metabolic rates at 25°C than 35 °C, while paranates did not. The 53% increase between the ectothermic paranates and the endothermic neonates was the most striking change in heart mass that we observed. The most significant increases in intestine and body mass occurred between 19 and 21 p, prior to the metabolic increase at the onset of endothermy. These results could lead to insights into the ontogeny of endothermy in mammals, including humans, and into the responses of these animals to the physiological stressors that accompany these developmental changes.