Dynamics of behavioral thermoregulation in the rat

Past studies have found that the laboratory rat placed in a temperature gradient prefers temperatures that are markedly below its lower critical ambient temperature (LCT), whereas other rodents (e.g., mouse, hamster, and guinea pig) generally select thermal environments associated with minimal metabolic expenditure. To further study the rat's thermoregulatory behavior, a temperature gradient was designed to monitor the selected ambient temperature (STa) and motor activity (MA) of food-deprived rats of the Long-Evans (LE), Fischer 344 (F344), and Sprague-Dawley (SD) strains over a 22-h period. All three rat strains selected relatively cool STas of 21-26 degrees C during the first 1-3 h in the temperature gradient. This was followed by a gradual increase in the STa that peaked at 4 (F344) to 6 h (SD and LE) after being placed in the gradient. The LE strain had a significantly lower STa during the initial period in the gradient. There were slight decreases in the STa during the nocturnal phase in the F344 and SD strains concomitant with marked increases in MA. These results indicate that the rat requires a relatively long adjustment period in a temperature gradient before it exhibits STas that are associated with minimal metabolic expenditure. Given adequate time for accommodation, behavioral thermoregulatory responses of the rat appear to be similar to those of other rodents.