ONTOGENY OF BRAIN TEMPERATURE REGULATION IN QUAIL CHICKS (COTURNIX-COTURNIX JAPONICA)

Development of brain temperature regulation was studied in the precocial chick of the Japanese quail from hatching to 21 d of age during exposure to ambient temperatures of 35-degrees, 30-degrees, 25-degrees, and 20-degrees-C. The newly hatched quail maintained a stable body-to-brain temperature difference from the day of hatching, although at relatively hypothermic levels compared to adults. Brain temperature and body temperature increased as a power function of age, approaching adult levels within 6 d and 9 d, at 30-degrees-35-degrees-C and 25-degrees-C, respectively. The chicks could not survive exposures to 20-degrees-C during the first 5 d of age and approached homeothermy at an extrapolated age of 24 d. The significantly different exponents of the equations relating brain temperature and body temperature to age resulted in a significant linear increase in body-to-brain temperature difference with age, except at 20-degrees-C. The slope of the relation between brain temperature and body temperature (0.932) was significantly lower than 1.0 (P < 0.001), indicating an increased brain cooling capacity at higher body temperatures. The developmental pattern of brain temperature regulation revealed in the present study resembles in general those of precocial fowl and altricial pigeon chicks. The present findings that heat-defense responses develop earlier than cold-defense responses are consistent with the existence of a hierarchical order in the maturation of the central thermoregulatory system.