The avian embryo undergoes a thermoregulatory transition from being a poikilotherm in its egg to being a homeotherm at or shortly after hatching. Thermoregulation in hatchling chicks relies on non-shivering endogenous heat production to control

The avian embryo undergoes a thermoregulatory transition from being a poikilotherm in its egg to being a homeotherm at or shortly after hatching. Thermoregulation in hatchling chicks relies on non-shivering endogenous heat production to control body temperature (Tb) (Whittow and Tazawa, 1991; Roberts, 1996; Tazawa et al., 2001), but the physiological adjustments and the timing of their onset as they occur during the transition to homeothermy have not been well described. In a companion study (Black and Burggren, 2004), we showed that late-stage chicken embryos (HH 41–44; Hamburger and Hamilton, 1951) maintained their rate of O2 consumption (and presumably the attendant heat production) during acute reductions in body temperature, suggesting the onset of at least some of the physiological control mechanisms necessary for thermoregulation. Black and Burggren (2004) also investigated the influence of chronic hypothermic incubation in chicken embryos. Not surprisingly, fundamental developmental parameters such as growth and the time of hatching events (internal and external pipping) were slowed by chronic hypothermic incubation (35°C). These changes, however, were all relative – i.e. when expressed as a percentage of total incubation period, embryonic growth and hatching events occurred at predictable points in development in hypothermic chicks. These data suggested that temperature alone did not induce heterokairic changes (heterokairy is the change in the relative timing of onset of physiological processes and their control; Spicer and Burggren, 2003). However, late-stage embryos incubated under hypothermic conditions were not as proficient at maintaining metabolism during a gradual decline in ambient temperature (Ta) as were populations incubated at the normal 38°C, suggesting that chronically low incubation temperature interfered with the normal onset of maturity of their thermoregulatory system, and in particular the ability to elevate oxygen consumption and heat production. Maturity of both the respiratory and cardiovascular systems is crucial to ensure the efficient delivery of O2 to support metabolic heat production and regulate body temperature. In this study we examine the effects of hypothermic incubation on the metabolic and thermoregulatory physiology of chicken embryos. We examine hematology, blood O2 transport The Journal of Experimental Biology 207, 1553-1561 Published by The Company of Biologists 2004 doi:10.1242/jeb.00910