Role of huddling on the energetic of growth in a newborn altricial mammal.

Huddling is considered as a social strategy to reduce thermal stress and promote growth in newborn altricial mammals. So far, the role of huddling on the allocation of saved energy has not been quantified nor have the related impacts on body temperature rhythms. To determine the energy partitioning of rabbit pups either raised alone or in groups of eight, four, or two individuals, when thermoregulatory inefficient (TI) and efficient (TE), we first investigated their total energy expenditure and body composition. We then monitored body temperature and activity rhythms to test whether huddling may impact these rhythms, centered on the suckling event. Pups in a group of eight utilized 40% less energy for thermogenesis when TI than did pups alone and 32% less energy when TE. Pups in groups of eight and four had significantly lower thermoregulatory costs in the TI period, whereas pups in groups of two, four, and eight had lower costs during the TE period. Huddling pups could therefore channel the energy saved into processes of growth and accrued more fat mass (on average 4.5 +/- 1.4 g) than isolated pups, which lost 0.7 g of fat. Pups in groups of four and eight had a body temperature significantly higher by 0.8 degrees C than pups in groups of two and one when TI, whereas no more differences were noted when the TE period was reached. Moreover, pups alone showed an endogenous circadian body temperature rhythm that differed when compared with that of huddling pups, with no rise before suckling. Thus huddling enables pups to invest the saved energy into growth and to regulate their body temperature to be more competitive during nursing, particularly at the early time when they are TI.