Effect of cold exposure on energy budget and thermogenesis during lactation in Swiss mice raising large litters.

In Swiss mice sustained energy intake (SusEI) during peak lactation has been previously suggested to be constrained by the capacity of the mammary glands to produce milk, supporting the "peripheral limitation" hypothesis. Here we experimentally examined if SusEI in these mice was not only limited peripherally but also constrained by the ability to dissipate heat. Female Swiss mice were provided with additional offspring above their natural litter sizes and were maintained during lactation either in warm (23°C) or cold (5°C) conditions. Food intake, thermogenesis, litter size and mass, and the weight of the mammary glands were measured. No differences were observed in asymptotic food intake at peak lactation, litter mass and thermogenesis between females raising litters of different size. Cold-exposed females increased food intake and thermogenic capacity, but weaned significantly smaller and lighter litters with smaller pup sizes compared with females in warm conditions. The weight of the mammary glands did not differ between warm and cold-exposed females, but within temperatures was positively related to litter mass. These data suggested that cold exposure increased food intake, but had no effect on the capacity of the mammary glands to secret milk because they were already working maximally in the females raising larger litters. The factors causing this limit in the mammary capacity remain elusive.