Thermal niches within the nest contribute to the long-term development of individual differences in metabolic rate in an altricial mammal.

Early life circumstances can affect an individual's phenotype into later life. In some polytocous altricial mammals, individuals' occupancy of micro niches within the litter huddle during early postnatal life contributes to the emergence of phenotypic variation among them across development. Rabbit pups with a greater birth mass typically occupy thermally more favorable positions towards the center of the huddle, whilst their lighter littermates are pushed to the less well insulated, cooler periphery. We therefore asked whether occupying different thermal niches within the huddle during early development has short- and long-term consequences for the resting metabolic rate (RMR) as measured by oxygen consumption of individual rabbits during preweaning, juvenile, subadult and adult life. For each of the 10 litters studied, we chose the individual in body contact with the largest number of littermates within the huddle, and the individual in body contact with the fewest, thus reflecting their more central (well insulated) or more peripheral (poorly insulated) positions, respectively. Within litters, we found differences in RMR between littermates that persisted across development, and even into adulthood. Notably, RMR from preweaning until subadult life and tendentially into adulthood, was consistently and significantly higher in individuals which had occupied poorly insulated peripheral positions in the litter huddle. In conclusion, our results provide an example of the priming effect of thermal conditions during early postnatal life on RMR, a major variable related to the body's energy homeostasis.