Sex-specific regulation of body temperature in Id2−/− mice. (a) Body temperature measurements of WT and Id2−/− male (left) and female (right) mice under normal temperature (upper panel) or thermoneutral temperature (lower panel) over 24 hrs. Values are mean ± SEM. Two-factor ANOVA was performed. ANOVAs revealed significantly lower body temperatures for both male and female Id2−/− mice compared to WT mice. The genotypic effect was independent of the prevailing ambient temperature. (b) Upper: regression analysis of body weight to body temperature of WT and Id2−/− mice under normal ambient temperature (left: male at ZT5.5 and ZT20.5; right: female at ZT5.5 and ZT17.5). Lower: regression analysis of body weight to body temperature of WT and Id2−/− mice under thermoneutral temperature conditions (left: male at ZT5.5 and ZT17.5; right: female at ZT5.5 and ZT17.5). Values are individual animal body temperatures and their respective measures of body mass. Note that no linear regression was found to be significant (n.s.), indicating that body mass does not predict body temperature for any group analyzed, examined under either 21°C or 30°C environmental temperatures.