Individual differences in the development of thermoregulation among littermates of the domestic rabbit.

The tight regulation of body temperature is essential for the maintenance of homeostasis in homeothermic endotherms. However, newborn altricial mammals are limited in their ability to maintain body temperature independently because they usually lack an insulating layer of fur, they have a relatively large body surface to volume ratio, and they have immature vasomotor control. Thus, altricial young are generally considered ectothermic poikilotherms at birth, but rapidly develop homeothermic endothermy. However, in polytocous species it is not known whether there are differences among littermates in the temporal development of endothermy and if so, what might account for this. Our aim was to determine the contribution of interactions among littermates to possible individual differences in the onset of homeothermic endothermy in altricial pups of the domestic rabbit. In 10 litters we evaluated individual differences in pups' position within the litter huddle on differences in their ability to maintain a stable body temperature when exposed individually to an acute daily cold challenge across the first 15 postnatal days. Greater relative body mass at birth was associated with greater body mass gain, occupancy of central positions in the litter huddle, and more rapid growth of fur, which together were associated with the earlier development of homeothermic endothermy. We conclude that early interactions among littermates associated with relative differences in initial body mass contribute to individual differences in the development of endothermy, with possible long-term consequences for individuals' fitness and metabolic phenotype.