SUMMARYSeveral studies have suggested that lactation performance may be programmed by the number of fetuses during pregnancy, whereas other studies indicate that processes during lactation are more important. As gestation litter size and litter size in lactation are usually strongly correlated, separating the roles of pregnancy and lactation in lactation performance is difficult. To break this link, we experimentally manipulated litter size of MF1 mice to five or 16 pups per litter by cross-fostering. Litter size and mass at birth were recorded on day 1 of lactation prior to litter size manipulation. Maternal body mass and food intake, litter size and litter mass were measured daily throughout. After weaning, the potential differential utilisation of body tissues of the mothers was investigated. Relationships between maternal mass and food intake, including asymptotic daily food intake at peak lactation, offspring traits and other maternal parameters suggested that the number of fetuses the females had carried during pregnancy had no effect on lactation performance. Litter mass increases depended only on maternal food intake, which was highly variable between individuals, but was independent of fetal litter size. The sizes of key organs and tissues like the liver and alimentary tract were not related to maximal food intake at peak lactation or to fetal litter size, but the masses of the pelage, mammary glands and retroperitoneal fat pad were. These data suggest that while growth of the mammary glands and associated structures may be initiated in gestation, and vary in relation to the number of placentas, the ultimate sizes and activities of the tissues depends primarily on factors during lactation.

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 taken. Offspring from the first lactation were also bred to investigate their reproductive success. In their first lactation, mothers with enlarged litters (=9, 16 pups) weaned significantly smaller pups, culled more pups, and increased MEO and food intake compared with mothers with reduced litters (=9, 5 pups). In the second lactation, no significant differences in pup mass or litter size were observed between groups, but mothers that had previously reared enlarged litters significantly decreased pup mass, MEO and food intake compared with those that had reared reduced litters. Female offspring from enlarged litters weaned slightly smaller pups than those from reduced litters, but displayed no significant differences in any of the other variables measured. 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. Hence, the 'limit' on performance may not be a hard physiological limit. These data support the ecological context hypothesis.

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