Limits to sustained energy intake XXVII: trade-offs between first and second litters in lactating mice support the ecological context hypothesis

Increased reproductive effort may lead to trade-offs with future performance and impact offspring, thereby influencing optimal current effort level. We experimentally enlarged or reduced litter size in mice during their first lactation, and then followed them through a successive unmanipulated lactation. Measurements of food intake, body mass, milk energy output (MEO), litter size and litter mass were made. Offspring from the first lactation were also bred to investigate their reproductive success. In their first lactation, mothers with enlarged litters (n=9, 16 pups) weaned significantly smaller pups, culled more pups, and increased MEO and food intake compared to mothers with reduced litters (n=9, 5 pups). In the second lactation no significant differences in pup mass or litter size were observed between groups, but mothers that previously reared enlarged litters significantly decreased pup mass, MEO and food intake compared to mothers with reduced litters. Female offspring from enlarged litters displayed no significant differences in any of the other variables measured compared to female offspring from reduced litters, but did wean slightly smaller pups. These results suggest that females with enlarged litters suffered from a greater energetic burden during their first lactation, and this was associated with lowered performance in a successive reproductive event and impacted on their offspring's reproductive performance. Female ‘choice’ about how much to invest in the first lactation may thus be driven by trade-offs with future reproductive success, and hence the ‘limit’ on performance may not be a hard physiological limit. These data support the ecological-context-hypothesis.