Early development of thermoregulatory competence in chickens: Responses of heart rate and oxygen uptake to altered ambient temperatures

Avian embryos need a heat supply from the external environment to develop, because heat loss from an egg overwhelms heat production of an embryo and accordingly they cannot maintain body temperature constant during incubation. In the domestic fowl, a minute homeothermic competence appears during the end of incubation with subsequent large augmentation soon after hatching. However, hatchling's metabolic rate still produces less heat than that required to offset heat loss. This stage prior to reaching "full-blown" homeothermy is designated as "power-limited". In this power-limited stage of thermoregulation, development of thermoregulatory competence in response to altered ambient temperature (T_a) involves the rapid maturation of heart rate (HR) regulation. The first part of this report reviews briefly: (1) early development of homeothermic thermal and metabolic responses to altered T_a in chick embryos and hatchlings, and (2) HR responses to altered T_a in newly hatched chicks during the power-limited stage of thermoregulation, which depend on the state of development of thermoregulatory competence. The last part concerns our recent preliminary experiment and hypothesis with regard to the early development of thermoregulatory competence in hatchlings developing in the power-limited stage. The preliminary experiment shows possible influence of brooding temperatures (i.e. preferred brooding T_a of 35°C and low brooding T_a of 24-27°C) to the development of thermoregulatory competence in terms of HR responses to altered T_a. We hypothesise that homeothermic-metabolic response to altered T_a develops in parallel with the homeothermic HR response and additionally the brooding temperature will influence the maturation of homeothermic-metabolic response so that hatchlings maintained at low T _a will develop the homeothermic-metabolic response earlier than those at the preferred brooding T_a. The result shows that homeothemic-metabolic response obtained in the white leghorn chickens does not develop in parallel with the homeothermic HR response obtained previously in the broiler chickens. In addition, maturation of homeothermic-metabolic response is observed on just hatched chicks maintained at both the preferred brooding T_a and low T_a. The failure of proving the hypothesis suggests other possibilities of which investigation will improve our knowledge of thermoregulation in avian embryos and hatchlings.