Soay Sheep Study Shows Parasite Load Impacts Prolificacy
AUgust 15, 2014

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 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.

Reported in the journal PLoS Biology, the finding could provide the groundwork for boosting the resilience of humans and livestock to infection. The researchers used 25 years of data on a population of Soay sheep living on an island in northwest Scotland to assess the evolutionary importance of infection tolerance. They first examined the relationship between each sheep's body weight and its level of infection with nematodes, tiny parasitic worms that thrive in the gastrointestinal tract of sheep. The level of infection was determined by the number of nematode eggs per gram of the animal's feces.

While all of the animals lost weight as a result of nematode infection, the degree of weight loss varied widely: an adult female sheep with the maximum egg count of 2,000 eggs per gram of feces might lose as little as 2 percent or as much as 20 percent of her body weight. The researchers then tracked the number of offspring produced by each of nearly 2,500 sheep and found that sheep with the highest tolerance to nematode infection produced the most offspring, while sheep with lower parasite tolerance left fewer descendants.

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