Sil H. J. van Lieshout, Elisa Perez Badas, Michael W.T. Mason, Chris Newman, Christina D. Buesching, David W. Macdonald, Hannah L Dugdale 

Evidence for age-related changes in innate and adaptive immune responses is increasing in wild populations. Such changes have been linked to fitness, and understanding the factors driving variation in immune responses is important for the evolution of immunity and senescence. Age-related changes in immune profiles may be due to sex-specific behaviour, physiology and responses to environmental conditions. Social conditions may also contribute to variation in immunological responses, for example, through transmission of pathogens and stress from resource and mate competition. Yet, the impact of the social environment on age-related changes in immune cell profile requires further investigation in the wild. Here, we tested the relationship between leukocyte cell composition (agranulocyte proportion, i.e. adaptive and innate immunity) and age, sex, and group size in a wild population of European badgers (Meles meles). We found that the proportion of agranulocytes decreased with age only in males living in small groups. In contrast, females in larger groups exhibited a greater age-related decline in the proportion of agranulocytes compared to females in smaller groups. Our results provide evidence for age-related changes in immune cell profiles in a wild mammal, which are influenced by both the sex of the individual and their social environment.