The purpose of the research was to determine the basis for variation in tolerance to Gastrointestinal Nematode parasites in  sheep. The goal was to determine if the tolerance level was a heritable trait and if not what explained the variation. Animals can react in two ways to parasites. This article focuses on tolerance, which is the ability for an animal to live with the parasite, instead of trying to kill it. The authors’ hypothesized a negative relationship between the body weight of the sheep and the growing number of parasites within it. The data was collected on the Soay sheep from 1988 to 2012. Annual survival, reproductive rates, and fecal egg counts  of the nematodes were data collected. Results showed how some sheep did lose more weight with an increasing nematode egg count, exhibiting lower tolerance. Also, the cause for the difference in how sheep tolerated the parasites was not genetic. The sheep which had the greater body weight (how their fitness is measured) had higher levels of tolerance than those which had less weight. These sheep survived and had offspring which inherited their parent's body weight. The ability to have better tolerance is not genetic, but weight is inheritable. They proved that there is a negative relationship between body weight and the growing number of parasites in a sheep and they also proved that natural selection does play a hand in tolerance, though indirectly. 

Hayward, Adam D, Nussey, Daniel H, Berenos, Camillo, Pilkington, Jill G, Watt, Kathryn A, Pemberton, Josephine M, Graham, Andrea L, Wilson, Alistair J. "Natural Selection on Individual Variation in Tolerance of Gastrointestinal Nematode Infection"  PLOS Biology. (2014). Web.