Milk energy output in Swiss mice throughout the first, second, third and fourth lactation events

Most studies on the factors limiting sustained energy intake (SusEI) during peak lactation period have been performed in females at the 1st lactation event. However, an inconsistent change in SusEI is observed between the 1st and 2nd lactation event. Thus, the limits to SusEI may be associated with reproductive experiences, but the effects of reproductive experiences on SusEI or reproductive output remain unclear. Here, food intake, reproductive output, suckling behaviour and serum prolactin levels were measured in female Swiss mice throughout the 1st, 2nd, 3rd and 4th lactation periods. Asymptotic food intake was significantly elevated during the 2nd lactation period relative to that observed during the 1st lactation period. Females in the 2nd lactation period exported significantly more energy in milk than those in the 1st lactation event and consequently raised larger litters with heavier litters at weaning. This was inconsistent with the prediction of the peripheral limitation hypothesis, but also did not provide support for the heat dissipation limitation hypothesis. Neither food intake nor reproductive output, indicative of litter size, litter mass and milk energy output (MEO), was different between the 1st, 3rd and 4th lactation event. Differences in suckling behaviour and serum prolactin levels were not significant between the four lactation events. Correlations of prolactin levels with asymptotic food intake, MEO and mammary gland mass were only observed in females during the 1st lactation period. This may suggest that prolactin is not a key factor in stimulating milk production when the mammary glands work at their maximum during the peak lactation period.