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.

Over-winter survival in subadult European rabbits: weather effects, density dependence, and the impact of individual characteristics.

The survival probability of an individual may be limited by density-dependent mechanisms and by environmental stochasticity, but can also be modified by individual characteristics. In our study, we investigated over-winter survival of subadults of an enclosed European rabbit Oryctolagus cuniculus population in a temperate zone habitat over the period 1992-2002. We: (1) selected for appropriate models to explain individual variation in over-winter survival and the animals' autumn body mass, the latter was used as a measure of the individual pre-winter body condition; and (2) aimed to compare the sensitivity of the target variables on the realised variation of the factors considered. Model selection based on information theory revealed that individual over-winter survival was best explained by the combination of autumn body mass, winter temperature, population density and sex, where the probability of survival was higher in females than in males. According to this model, the probability of survival reacted most sensitively to variation in the autumn body mass and in winter temperature. Individual autumn body mass was best explained by the combination of the date of birth, population density, and weather conditions by means of the percentage of rainy days during the first 2 months after the animals had emerged above ground, where the autumn body mass was negatively related to the percentage of rainy days. The chosen model suggested that the autumn body mass reacted most sensitively to variation in the date of birth. Combining these models, we found that weather conditions during two different periods of time as well as population density, sex and the date of birth operated together to determine the probability of over-winter survival. In particular, the study points out the high impact of environmental stochasticity on over-winter survival: (1) by direct effects of winter temperature conditions, and (2) by the indirect action of weather conditions to which the animals were exposed during the early period of juvenile development.