Sheep Photo (image) Princeton University Caption In the first evidence that natural selection favors an individual's infection tolerance, researchers from Princeton University and the University of Edinburgh have found that an animal's ability to endure an internal parasite strongly influences its reproductive success. The researchers used 25 years of data on a population of wild Soay sheep (above) living on the island of Hirta in northwest Scotland to assess the evolutionary importance of infection tolerance. The researchers tracked the number of offspring produced by each of nearly 2,500 sheep and found that those with the highest tolerance to infection produced the most offspring. Credit Photo by Arpat Ozgul, University of Edinburgh Usage Restrictions None Disclaimer: AAAS and EurekAlert! are not responsible for the accuracy of news releases posted to EurekAlert! by contributing institutions or for the use of any information through the EurekAlert system.