Senescence has been hypothesised to arise in part from age-related declines in immune performance, but the patterns and drivers of within-individual age-related changes in immunity remain virtually unexplored in natural populations. Here, using a long-term epidemiological study of wild European badgers (Meles meles), we (i) present evidence of a within-individual age-related decline in the response of a key immune-signalling cytokine, Interferon-gamma (IFNγ), to ex vivo lymphocyte stimulation, and (ii) investigate three putative drivers of individual...