Social environment and weather during early life influence gastro-intestinal parasite loads in a group-living mammal.

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.