Lactation at hot temperature: a test of heat dissipation limitation in mice divergently selected for basal metabolic rate.

The HDL (heat dissipation limitation) hypothesis posits that mammalian energy budgets (SuSMR, sustained metabolic rate) are limited by the ability to dissipate metabolic heat. The HDL hypothesis has been tested in lactating mice but rarely systematically differs in SuSMR. Here, we used lines of laboratory mice divergently selected for basal metabolic rate (BMR) and effectively co-selected for SuSMR. We exposed lactating females to 23 and 30°C and manipulated their heat dissipation abilities by fur shaving. Exposure to 30°C did not affect the high BMR mice's litter mass but increased litter mass in the low BMR mice. Fur shaving did not affect litter mass. However, it decreased body temperature (T_b) by 0.2°C in the shaved mice, independent of line affiliation and ambient temperature. In both lines exposed to 30°C, the T_b increased by 0.2°C, while daily energy expenditure (a proxy of SuSMR) decreased by 20% and still was higher in the high BMR mice. These results do not support the HDL hypothesis. Low SuSMR individuals may benefit from higher ambient temperatures because of reduced costs of thermoregulation. It may change the course of natural selection towards reducing SuSMR and BMR.