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Costly kids have survival consequences for sheep mothers, shows new study

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

Soay ewe
Soay sheep ewe on St Kilda.

Any parent will be able to tell you that, along with the many joys that raising children brings, it comes at a cost: less free time, insufficient sleep and more sickness are just some of the downsides of having kids.

High parasite burden associated with lactation

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.

Further, those ewes which successfully suckled their lamb through to weaning had higher parasite counts than those whose lamb died soon after birth.

Jessica Leivesley, who led the research while an undergraduate student at the University of Stirling, and is now a PhD researcher at the University of Toronto, said:

“The resources which a female must channel into producing her lamb means that less energy remains to fight infections. 

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.

Dr Adam Hayward, a Research Fellow at the Moredun Research Institute and senior author of the study, said:

We’ve known for a long time that reproduction can affect survival.  What our new study does is to provide an explanation for why this might be the case: we’ve discovered a complex but clear pathway linking reproduction to increased infections and reduced survival. 

While all organisms strive to reproduce, it has its costs, and as the father of an eight-month-old this research has recently taken on a new relevance to me!

The paper is published in Ecology Letters.

Leivesley, J.A., Bussière, L.F., Pemberton, J.M., Pilkington, J.G., Wilson, K. & Hayward, A.D. (2019) Survival costs of reproduction are mediated by parasite infection in wild Soay sheep

Related links

St Kilda Soay sheep project

Moredun Research Institute