Optimal litter size for individual growth of European rabbit pups depends on their thermal environment

In altricial mammals and birds, the presence of a large number of litter or brood mates often affects the development of individual offspring by reducing the share of resources provided by the parents. However, sibling presence can also be favourable, conferring thermoregulatory benefits when ambient temperatures are low. Consequently, shifts in the relation between costs and benefits of sibling presence can be expected as a function of the thermal environment. In a study of a European rabbit population (Oryctolagus cuniculus) living in a field enclosure, we investigated the effects of litter size and soil temperature on pup growth over 7 years. Temperatures inside the subterranean nests were positively correlated with soil temperature and with litter size. Soil temperature varied strongly across the breeding season, ranging from 3 to 21 degrees C. Under warmer soil temperature conditions (10-15 degrees C and > 15 degrees C), pup growth decreased with increasing litter size, where litters of two pups (smallest litter size considered) showed the highest growth rates. In contrast, under colder soil temperature conditions (< 10 degrees C), the highest growth rates were found in litters of three pups. We also asked if such temperature-dependent differences in the optimal pup growth rates might be explained by differences in maternal characteristics, which might affect lactational performance. We assessed maternal performance using females' postpartum body mass and social rank. However, we did not find consistent differences in maternal characteristics between females giving birth to different-sized litters during different soil temperature conditions, which would have provided an alternative explanation for the observed differences in litter size-dependent pup growth. We conclude that under colder soil temperature conditions, the thermal benefits of a greater number of littermates outweigh the negative consequences of competition for milk, leading to an environment-dependent shift in the optimal litter size for individual growth in this species.