Thermoregulatory responses of two mouse Mus musculus strains selectively bred for high and low food intake

We examined the thermoregulatory responses of mate and female mice Mus musculus that had been divergently selected on voluntary food intake, corrected for body mass, to produce a high-intake and a low-intake strain. Resting metabolic rate was determined by indirect calorimetry (at 30 degreesC, 25 degreesC, 15 degreesC and 5 degreesC). Body temperature responses were measured in a separate group of mice in a parallel protocol. High-intake mice had significantly elevated body masses compared to low-intake mice in both sexes. Lower critical temperature in both strains appeared to be around 28 degreesC. At 30 degreesC there was a significant strain effect on resting metabolic rate, with high strain mice having greater metabolism than low strain mice. Sex and body mass were not significant main effects on resting metabolic rate and there were no significant interactions. Body temperature measured at 30 degreesC, 25 degreesC, 15 degreesC and 5 degreesC differed significantly between sexes (females higher) and there was a significant sex x body mass interaction effect, but there was no difference between strains. Thermal conductance was significantly related to strain and sex, mice from the high strain and males having greater thermal conductances than mice from the low strain and females. Artificial selection has resulted in high-intake mice having greater body masses and greater thermal conductances, which together account for up to 45% of the elevated daily energy demands that underpin the increase in food intake. The greater levels of food intake were also associated with higher resting metabolic rates at 30 degreesC.