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

Abstract

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, 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 (18g/d at 21°C vs 10g/d at 30°C), thermal conductance (lower by 20-27% at 30°C than 21°C), litter mass and MEO decreased significantly in the females raising large litter size at 30°C compared to those at 21°C. Furthermore, an interaction between ambient temperature and litter size affected females' energy budget, imposing strong constraints on SusEI. Together, out data suggest that the limitation may be caused by both mammary glands and heat dissipation, i.e. the limits to mammary gland is dominant at the room temperature, but heat limitation is more significant at warm temperatures. Further, the level of heat dissipation limits may be temperature dependent, shifting down with increasing temperature.