New research published by scientists at the Moredun Research Institute and the Universities of Stirling, Lancaster and Edinburgh shows that, for at least one population, the link between offspring production and infections can have deadly consequences.

Scientists studying a population of wild Soay sheep, living in the remote St Kilda archipelago off Scotland’s west coast, found that during the lambing season, females that had a lamb had bigger gut worm infections than females which didn’t reproduce. And, those ewes which successfully suckled their lamb through to weaning had higher parasite counts than those whose lamb died soon after birth.

Kenneth Wilson, Professor of Evolutionary Ecology at Lancaster University and co-author of the paper, said: “We have known for a while that there is a ‘cost of reproduction’, but in this study we demonstrate that this cost is mediated, in part at least, by parasites. Producing and weaning lambs in spring increases a ewe’s susceptibility to worms and the more worms a sheep has, the lower her subsequent weight gain over the summer and the lower her over-winter survival.”

“The resources which a female must channel into producing her lamb means that less energy remains to fight infections”, said Jessica Leivesley, who led the research while an undergraduate student at the University of Stirling, now a PhD researcher at the University of Toronto. “Our results also suggest that lactation is particularly costly, because females that weaned their lamb had even more parasites than those whose lambs died and therefore didn’t need to lactate.”

The researchers went on to show that the higher worm counts in reproducing females came at a very high price: ewes with bigger worm infections in spring had lower body weight in summer and were less likely to survive over the following winter to breed again in the future.

The St Kilda Soay sheep project is one of the foremost studies of a wild mammal population in the world. Since 1985, animals living in the Village Bay area of the island of Hirta have been monitored from birth until death, with data collected on body size, survival, reproduction, genetics, infections, immunity and population size. Over 200 scientific articles have been published on the sheep, transforming our knowledge of population demography, evolution and host-parasite interactions in the wild. More information can be found at http://soaysheep.biology.ed.ac.uk/.