Physiological Response of Chicken Hatchlings to Cold-stress
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Postprandial upregulation of physiological systems in pythons is large, rapid and associated with a substantial increase in metabolic rate. Significant growth in heart mass in these snakes occurs in response to increasing metabolic demands. Intrigued by these phenomena, we developed an experiment to look for a similar change in hearts of young birds associated with onset of endothermy and subsequent exposure to a thermally stressful environment. We measured metabolic rate at 35 DC and 25 DC as well as body size and masses of yolk free body, yolk sac, heart and small intestine of recently hatched (day 0) neonates as well as those maintained at 35 DC (normothermic) or 25 DC (cold-stressed) for two days post-hatching. During two days post-hatch, normothermic neonates maintained a progressively increasing body temperature that was higher than that of cold-stressed neonates, whose body temperature began to decrease one day after hatching. Yolk-free body mass and residual yolk mass decreased during the first two days post-hatch in both groups of neonates, while intestine dry mass increased in both groups. Dry heart mass, however, increased significantly in only cold-stressed neonates. Normothermic neonates retained a healthy endothermic response to short periods of cold-stress whereas cold-stressed neonates lost this response after two days of exposure to 25 DC. Thus, growth of the heart in these neonates responds to metabolic demands associated with thermoregulation.