Sustained energy intake in lactating Swiss mice: a dual modulation process.

Limits to sustained energy intake (SusEI) during lactation are important because they provide an upper boundary below which females must trade off competing physiological activities. To date, SusEI is thought to be limited either by the capacity of the mammary glands to produce milk (the peripheral limitation hypothesis) or by a female's ability to dissipate body heat (the heat dissipation hypothesis). In the present study, we examined the effects of litter size and ambient temperature on a set of physiological, behavioral and morphological indicators of SusEI and reproductive performance in lactating Swiss mice. Our results indicate that energy input, energy output and mammary gland mass increased with litter size, whereas pup body mass and survival rate decreased. The body temperature increased significantly, while food intake (18 g day-1 at 21°C versus 10 g day-1 at 30°C), thermal conductance (lower by 20-27% at 30°C than 21°C), litter mass and milk energy output decreased significantly in the females raising a large litter size at 30°C compared with those at 21°C. Furthermore, an interaction between ambient temperature and litter size affected females' energy budget, imposing strong constraints on SusEI. Together, our data suggest that the limitation may be caused by both mammary glands and heat dissipation, i.e. peripheral limitation is dominant at room temperature, but heat dissipation is more significant at warm temperatures. Further, the level of the heat dissipation limits may be temperature dependent, shifting down with increasing temperature.