Limits to sustained energy intake. X. Effects of fur removal on reproductive performance in laboratory mice

The maximum rate of sustained energy intake (SusEI) may limit reproductive effort and other aspects of animal performance. We have previously suggested that lactating mice are not limited centrally by the alimentary tract or peripherally by the mammary glands, but that the limits to SusEI are imposed by the capacity of the animal to dissipate body heat generated as a by-product of processing food and producing milk. To explore the nature of the limits to SusEI, we bred MF1 laboratory mice at 21 C and then dorsally shaved lactating females to reduce their external insulation and thereby elevate their capacity to dissipate body heat. These mice increased their food intake by 12.0% and assimilated on average 30.9 kJ day(-1) more energy than unshaved animals. With nearly identical mean litter sizes (11.4 pups for shaved and 11.3 pups for unshaved mice), shaved mothers exported 15.2% (22.0 kJ day(-1)) more energy as milk than control individuals. The elevated milk production of shaved mice enabled them to wean litters that were 15.4% (12.2 g) heavier than offspring produced by unshaved mice. Our results argue against central, peripheral or extrinsic limits to SusEI at peak lactation and provide strong support for the heat dissipation limit hypothesis. More generally, we see many situations where heat dissipation may be a previously unrecognised factor constraining the evolution of endothermic animals - for example, the latitudinal and altitudinal trends in clutch and litter sizes and the migration patterns of birds.