LAUSR

The decline of resistance in old age Infection, immunity, and demography are rarely measured simultaneously, despite being intertwined. Froy et al. measured an immune marker of resistance to infection by worm parasites (helminths) in Soay sheep off the remote Atlantic island of St. Kilda (see the Perspective by Gaillard and Lemaître). They used a library of 2000 blood samples from 800 known individuals that have been left to run wild and unmanaged. Resistance declines as the sheep age, which reduces a sheep's chances of surviving the winter. Helminths are an important component of many natural systems, including humans, and may thus become an increasing burden on health with age. Science, this issue p. 1296; see also p. 1244 Aging brings declining immune resistance to helminth parasites and affects host survival in wild Soay sheep. Our understanding of the deterioration in immune function in old age—immunosenescence—derives principally from studies of modern human populations and laboratory animals. The generality and significance of this process for systems experiencing complex, natural infections and environmental challenges are unknown. Here, we show that late-life declines in an important immune marker of resistance to helminth parasites in wild Soay sheep predict overwinter mortality. We found senescence in circulating antibody levels against a highly prevalent nematode worm, which was associated with reduced adult survival probability, independent of changes in body weight. These findings establish a role for immunosenescence in the ecology and evolution of natural populations.