Effects of body mass and huddling on resting metabolic rates of post‐weaned European rabbits under different simulated weather conditions

Summary 1.  During the early juvenile period, animals are especially prone to heat loss due to their high surface area to volume ratio. Consequently, adverse environmental conditions such as low ambient temperature and wetness affect the energy balance in juveniles more strongly than in adults. 2. Our main study goal was to experimentally measure the influence of low ambient temperatures and wet fur on resting metabolic rates (RMR) and on thermal conductance in young, pre‐weaned (wild‐type) European rabbits ( Oryctolagus cuniculus L.), i.e. shortly after young rabbits in the wild usually leave the breeding burrow. 3.  We predicted that a higher body mass and therefore a more favourable surface area to volume ratio might decrease the responses of the juveniles to adverse weather conditions. Furthermore, huddling with siblings might buffer the negative influences of low temperature and wet fur. 4.  Using studies in field enclosures, we first evaluated the occurrence of huddling in juvenile European rabbits, and recorded the variation in ambient temperature experienced by the young. In laboratory experiments, we then measured RMR in single and pair‐housed (huddling) juveniles with dry and wet fur under different ambient temperatures reflecting the conditions experienced by same‐aged rabbits in the wild. 5.  We found interacting effects of temperature and wetness on RMR and on thermal conductance, which increased more strongly with decreasing temperature when the animals were wet. In addition, RMR decreased significantly with increasing body mass, but only under wet conditions when the temperature was low. 6.  Pre‐weaned juveniles in the field enclosures almost exclusively huddled in pairs but not in larger groups, probably because of the high juvenile mortality, which was strongly limiting the number of litter mates. In laboratory experiments, we did not find significantly lower RMR or thermal conductance in huddling pairs than in single juveniles. The occurrence of huddling, in the field and in the laboratory, was also not related to ambient temperature conditions. 7.  Our study highlights the importance of a high juvenile body mass which helps to reduce maintenance costs under adverse environmental conditions. Such effects might have important implications for the individual’s pre‐weaning growth and development and may exert long‐term effects on fitness‐related traits.