SOTHERLAND, P.R.*; KILLPACK, T.L.; SELBO, B.G.; DZIALOWSKI, E.M.; Kalamazoo College; Kalamazoo College; Kalamazoo College; Univ. of North Texas: A change of heart in developing birds at the onset of endothermy

In pythons, postprandial upregulation of physiological systems is large, rapid, and correlated with a noticeable rise in standard metabolic rate. Cardiac hypertrophy of pythons in response to this increased metabolism led us to investigate whether a similar change in hearts of developing birds occurs with the abrupt increase in metabolism associated with onset of endothermy at hatching. We measured rate of oxygen consumption at 35 ^oC and 25 ^oC as well as body size and masses of heart and small intestine of paranate and neonate chickens. We obtained the same measurements on neonates maintained at 35 ^oC (normothermic) or 25 ^oC (cold-stressed) for two days post-hatching. Neonates manifested an endothermic response to low ambient temperature (paranates remained ectothermic) and had significantly larger hearts than paranates even though body masses of the two groups did not differ. During two days post-hatch, normothermic neonates maintained a progressively increasing and higher body temperature than cold-stressed neonates, whose body temperature decreased after one day post-hatch. Both groups of neonates decreased in dry body mass and residual yolk mass, while intestine dry mass increased in both groups. However, dry heart mass increased only in cold-stressed neonates. Normothermic neonates retained an endothermic response to acute cold-stress whereas cold-stressed neonates lost this response after two days of exposure to 25 ^oC. These results support the notion that ontogenetic changes coinciding with the metabolic metamorphosis from ectothermy to endothermy in birds might be similar to changes associated with feeding in large snakes and to differences between adult ectotherms and endotherms.