Development of the hypothermia in obese mice (genotype ob/ob).

Heavier-obese mice (genotype, ob/ob) displayed their characteristic hypothermia at ambient temperatures and obese mice at all body weights did so after 60 min in the cold (3.0 - 5.5 degrees C). The fall in core temperature was least in slightly obese (3.5 degrees C) and greatest in extremely-obese animals (12.5 degrees C). The fall could be prevented by previous pair feeding with wild-type animals, which limited the body weight by 52 per cent. Limiting the body weight of wild-type mice (23 per cent) was not followed by a change in core temperature. Neither grouping nor floor insulation altered the fall in core temperature of obese or wild-type mice. These results suggest the hypothermia of the mutant to be as much under a physical as a metabolic influence, such as the abnormal body form and consequent inactivity. Heat production in muscle would decrease with the inactivity, contributing to the hypothermia. The normothermia in obese mice of limited body weight could result from their increased (normal) activity, previously demonstrated.