Infanticide is an important source of mortality of dependent offspring in several mammal species, whereas female conspecifics are often the perpetrators. This has led to maternal counter‐strategies, such as the defence of the nests. However, cases of infanticide are hard to detect in the field, and studies on maternal offspring defence behaviour under natural breeding conditions are scarce. We conducted such a study on the European rabbit (Oryctolagus cuniculus), which is usually considered to show low maternal care. The study was carried out over 5 yr on a field enclosure population. We (1) studied infanticide rates and the impact of potential determinants: the group density and age structure of the females’ rank hierarchy within the groups; we used the latter as an estimator of social group stability. We (2) studied if mothers defend their breeding burrow against approaching, potentially infanticidal females. Overall, we recorded infanticide in 5% of all litters; infanticide was the cause in 12% of cases of litter mortality. The proportion of infanticide was 7% higher in groups where same‐age females occupied successive rank positions than in groups where the females’ rank hierarchy had a more heterogeneous and linear age structure. We hypothesize that social instability in the former groups was the reason for the increased infanticide risk. Infanticide rates were not correlated with group density and did not differ among mothers with different social ranks. Infanticide occurred exclusively during the first 10 d after parturition. During this time, mothers stayed closer to their breeding burrows than shortly before parturition or during later lactation. Moreover, mothers were more aggressive against other females in proximity to their breeding burrow than in more distant areas. We suggest that the pattern of spacing behaviour and intrasexual aggression of rabbit mothers are an adaptive response to the occurrence of female infanticidal behaviour.

We conducted a study on nest mortality of an individually marked population of European rabbits (Oryctolagus cuniculus L., 1758) living in a field enclosure. Over 4 years, we determined maternities and quantified pup mortality during the nest period of 703 pups born in subterranean breeding burrows. Overall, pup mortality occurred in 42.7% of the litters, whereas 32.4% of all born pups were affected. Mortality was highest during the first few postnatal days. In about 50% of the cases, we managed to quantify different causes of mortality such as malnutrition, flooding, cooling of the pups, infanticide, or predation. The pups' body mass on postnatal day 1, the thermal environment and the number of litter mates were the most important predictors of nest mortality. Litter mortality risk decreased with increasing soil temperature around the subterranean nests. A comparatively higher average pup body mass lowered the nest mortality risk of a litter, whereas this effect was more pronounced when soil temperatures were low. Furthermore, mortality was lowest in medium-sized litters, most probably due to the balance between the thermal benefits of huddling with litter siblings and the costs of having them due to the lower share of milk obtained by the individual pups in larger litters. In addition, nest mortality depended on characteristics of the mother; mortality was increased in litters of low-ranking females and of mothers with lower body mass. In conclusion, our study highlights multiple causes and the effects of different environmental and social factors on nest mortality of this small mammal.

Genes of the major histocompatibility complex (MHC) play a fundamental role in the vertebrate immune response and are amongst the most polymorphic genes in vertebrate genomes. It is generally agreed that the highly polymorphic nature of the MHC is maintained through host-parasite co-evolution. Two nonexclusive mechanisms of selection are supposed to act on MHC genes: superiority of MHC heterozygous individuals (overdominance) and an advantage for rare MHC alleles. However, the precise mechanisms and their relative importance are still unknown. Here, we examined MHC dependent parasite load in European rabbits (Oryctolagus cuniculus) from a distinct population with low MHC diversity (three alleles, six genotypes). Using a multivariate approach, we tested for associations of individual MHC class II DRB constitution and the rabbits' intestinal burden with nematodes and coccidia. Rabbits having a particular allele showed lower infestations with hepatic coccidia (E. stiedai). However, a comparison of all six genotypes in the population revealed that carriers of this allele only benefit when they are heterozygous, and furthermore, MHC heterozygosity in general did not affect individual parasite load. In conclusion, this study suggests an immunogenetic basis of European rabbit resistance to hepatic coccidiosis, which can strongly limit survival to maturity in this species. Our study gives a complex picture of MHC-parasite correlations, unveiling the limits of the classical hypotheses of how MHC polymorphism is maintained in natural systems.

The authors investigated in an observational study the consequences of the presence of litter sisters on the social interactions and on reproductive performance of young female European rabbits (Oryctolagus cuniculus) from a population living under seminatural conditions. In early winter, when all young females were settled in a social group, they were characterized by the presence or absence of litter sisters in their groups. Females with litter sisters in their groups displayed significantly more positive social interaction with females of their social group compared to females without sisters, whereas this difference was mainly due to the high frequency of positive social interactions among the respective sisters. Such differences between the females of both categories were already apparent during the animals' juvenile phase, before the females integrated into a particular group: females which later had litter sisters in their group showed more positive social behavior than females which later no longer had litter sisters. During their first breeding season, females with litter sisters present generally started to breed earlier by an average of 14 days. A more favorable social environment might have possibly facilitated the earlier onset of breeding in females with present litter sisters by attenuating the negative consequences of stress.

Conditions experienced during early life have been frequently shown to exert long-term consequences on an animal's fitness. In mammals and birds, the time around and shortly after weaning is one of the crucial periods early in life. However, little is known about how social and abiotic environmental conditions experienced around this time affect fitness-related traits such as endoparasite loads. We studied consequences of social interactions and rainy weather conditions around and after weaning on gastro-intestinal nematode loads in juvenile European rabbits Oryctolagus cuniculus. Infestations with the gastric nematode Graphidium strigosum and with the intestinal nematode Passalurus ambiguus were higher in animals experiencing more rain during early life. This might have been due to the higher persistence of nematodes' infective stages outside the host body together with the animals' lower energy allocation for immune defence under more humid and thus energetically challenging conditions. In contrast, infestations with P. ambiguus were lower in animals with more positive social interactions with mother and litter siblings. We propose that social support provided by familiar group members buffered negative stress effects on immune function, lowering endoparasite infestations. This is supported by the negative correlation between positive social behaviour and serum corticosterone concentrations, indicating lower stress in juveniles which integrated more successfully into the social network of their group. In conclusion, the findings offer a pathway showing how differences in the abiotic environment and social life conditions experienced early in life could translate into long-term fitness consequences via the effects on endoparasite loads.