Ontogeny of homeothermy in the immature rat: metabolic and thermal responses

Steady-state thermoregulatory responses were measured in the immature rat at 5, 7, 9, 11, 13, 15, 17, and 19 days of age. Tests were conducted at controlled ambient temperatures (Ta) ranging from 22.5 to 37.0 degrees C. Colonic (Tco) and skin (tail, interscapular, abdominal) temperatures were measured, as was O2 consumption from which metabolic rate (M) was calculated. Significant improvements in homeothermic ability occurred from 5 to 19 days of age. Although the resting level of M (RMR) increased by 6.9 W/m2 and the lower Ta limit for RMR (LCT) decreased by 2.5 degrees C as age advanced from 5 to 19 days, Tco at LCT was 36.8-37.1 degrees C at all ages studied. Below LCT the elevation of M to a given decrease in Tco was greater the older the animal. A comparable response to a change in skin temperature was not age dependent. Improvement in thermal insulation was the primary factor responsible for increases in homeothermic ability between 5 and 19 days of age.