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Deterioration of the immune system with age increases winter mortality in Soay sheep Understanding how aging occurs is a substantial challenge that requires integrative studies of aging performed through the lens of evolutionary biology (1). Demographic senescence is the decline in fitness components—survival, fertility, fecundity—with advancing age and has been described in the majority of vertebrates (2). However, little is known about physiological senescence, which is the decline in organismal functions—body mass, biochemical constants, immunocompetence—with age in nonmodel organisms. On page 1296 of this issue, Froy et al. (3) investigate a specific aging phenotype, immunosenescence (the decline in immune performance with age), in a wild population of Soay sheep from St. Kilda, Scotland. They find that a decline in one component of the immune system is associated with increased risk of adult mortality the following winter. This suggests a link between physiological and demographic senescence shaping a unified aging phenotype in a wild animal population.