Scientists have found a link to reduced fertility due to strong immunity that may play a role in determining long life.

The scientists from Princeton University found this in an 11-year study on wild sheep in Scotland. They found the sheep had strong immunity but at the cost of fertility levels. They found the sheep that contained the highest number of anti-bodies lived the longest, but failed to produce offspring regularly. Sheep with fewer anti-bodies in the blood gave birth to more lambs every year, though they tended to die earlier.

This could also give new insights on the immunity of human beings too. Some people get sicker than others when exposed to the same illness.

The research into the immunity of wild sheep to infection may yield new insights in why some people get sicker than others when exposed to the same illness.

"This is a groundbreaking study that to my mind will change our whole understanding of the immunoheterogenity in animal populations," said Peter Hudson, the Willaman Professor of Biology and director of life sciences at Penn State University.

"Graham and colleagues show beautifully the tradeoffs in the immune system as a balance ... that maximizes reproductive output."

The study, which was led by Princeton ecologist Andrea Graham has been published in the Oct. 29 issue of Science.

"We have long suspected that strong immune responses should prolong life in the face of infections, but might also be costly to reproduction," Graham said. "To find evidence for such tradeoff may clarify why animals vary so much in the strength of their immune responses, and even in their predisposition to infection or autoimmunity.