Body temperature and oxygen consumption of newborn rats at various ambient temperatures

The thermogenic response and rectal temperature (Tre) of newborn rats (0 to 32 days old) on exposure to various ambient temperatures (Ta) were investigated. The O2 consumption (VO2 in ml . min-1 per 100 cm2 surface area) was measured for one rat at a time by means of a closed circuit system. The results were that (1) within the thermal neutral ranges, Tre stood at 36--37 degrees C regardless of the age and the value of VO2 was lowest at the age of 6--7 days, after this period increasing and reaching a level of 1.4 ml . min-1 . cm-2 X 10(-2) after 18--21 days old (the weaning period); (2) the thermogenic response to cold (delta VO2/--delta Ta) was higher at ages prior to the weaning period than after the weaning period; (3) in spite of the higher thermogenic responses, Tre decreased markedly on exposure to cold prior to the weaning period, because of a poor development of thermal insulation; and (4) the achievement of homeothermy of the rat at subneutral temperatures was observed after the weaning period.