Wild Scottish Sheep Could Help Explain Differences in Immunity

http://www.sciencedaily.com/releases/2010/10/101028162156.htm

ScienceDaily (Oct. 28, 2010) — Strong immunity may play a key role in determining long life, but may do so at the expense of reduced fertility, a Princeton University study has concluded. An 11-year study of a population of wild sheep located on a remote island off the coast of Scotland that gauged the animals' susceptibility to infection may give new insight into why some people get sicker than others when exposed to the same illness.

The answer to this medical puzzle may lie in deep-rooted differences in how animals survive and reproduce in the wild, according to the study, which was led by Princeton ecologist Andrea Graham and published in the Oct. 29 issue of Science. The research revealed that the sheep population over time has maintained a balance of those with weaker and stronger levels of immunity and fertility.

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Graham, an assistant professor of ecology and evolutionary biology at Princeton who also is on the faculty of the University of Edinburgh, led the study of wild Soay sheep on the remote island of Hirta in the St. Kilda archipelago, about 100 miles west of the Scottish mainland. The scientists tested the animals for levels of antibodies, natural molecules produced by the sheep's immune systems to fend off infections such as influenza or those caused by parasitic worms.

The sheep whose blood contained the most antibodies lived the longest, the researchers found. These animals also were most likely to survive harsh winters. However, they failed to produce as many offspring each spring as other sheep. Sheep with lower levels of antibodies tended to die earlier, they found, but also gave birth to more lambs each year.

Viewed in terms of breeding and, ultimately, evolutionary success, the differing groups of sheep were equally successful in that the longer-lived but less fertile sheep and the shorter-lived but more fertile sheep produced about the same number of progeny over the course of their lives. The overall balance, the researchers said, could help explain why immunity varies so much among individuals.

The tendency to form either strong or weak responses to infection ran in families in the sheep, the researchers also found. "This genetic basis means that natural selection has the chance to shape the trait," Graham said. If differing responses to infection still result in equal long-term reproductive success, she said, this means "selection seems actually to be maintaining this genetic variation in immunity."

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