Why are infectious diseases still the major causes of animal mortality, despite the strong and ever-present evolutionary pressure to eliminate traits that make individuals susceptible to infection? Why hasn’t natural selection weeded out weak immune systems? An emerging and rapidly growing field, known as ecological immunology or ecoimmunology, is trying to answer these questions by investigating how variation in immune response in free-living animal populations affects fitness. On page 662 of this issue, Graham et al. (1) provide an illuminating answer for feral sheep living on the Scottish island of Hirta in the St. Kilda archipelago. The sheep have varying blood levels of antibodies, and individuals with higher levels appear to confront a fitness trade-off: Although a strong immune response helps adult females survive harsh winters, it also reduces reproduction.
One journal (2) has dedicated a special issue to ecoimmunology, another special issue (3) is forthcoming, and the U.S. National Science Foundation recently funded a Research Collaborative Network (www.ecoimmunology.org) to help shape the field. Some of the first papers in the area (published just ∼20 years ago) have been cited more than 1000 times (4, 5). Recently, investigators have also begun to develop the study of human ecoimmunology (6–8), which may augment another nascent discipline known as Darwinian medicine (9, 10).
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