Thermal preference behavior of genetically obese (ob/ob) and genetically lean (+/?) mice.

Chronically lower colonic temperatures (TcS) of genetically-obese (ob/ob) mice at ambient temperatures below thermoneutrality have led to speculation that these mutants regulate a lowered thermal setpoint relative to lean mice. Previous experiments, however, have not provided an opportunity for obese mice to exhibit compensatory thermoregulatory behaviors which might reinstate normal body temperature. In the present experiment, adult obese and lean (+/?) mice were tested at room temperature (25 degrees C) on a copper bar, thermal gradient to determine what temperatures they would select and what effect their selection would have on their TcS. The data revealed that ob/obs were more frequently observed within 25-35 degrees C locales than were lean controls, which spent more time in the below-25 degrees C zone (p less than 0.005). Ob/obs also raised their TcS to pretest values of leans' TcS, although ob/obs' TcS remained significantly lower than those of leans at the conclusion of testing. These data suggest that the hypothermia exhibited by the ob/ob may reflect both the absence of the opportunity to behaviorally thermoregulate and a genetic defect in thermogenesis.