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The Big Picture
Reproduction comes at a cost


Studies of a wild population of sheep in the St Kilda archipelago in Scotland have found that ewes can end up paying a heavy price. Kathryn Clark explains

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

Reproduction is a biological imperative but brings with it some less-than-positive consequences.

Data from the long-running St Kilda Soay sheep project suggest that ewes that have a lamb, and those that successfully rear their lamb to weaning, are less likely to survive the winter to reproduce again.

The Soay sheep project has been running in its current form since 1985 and is studying population dynamics, evolution and genetics, ageing and parasite infection in a natural setting. Its database contains details for more than 8000 individuals that have lived in the Village Bay area of the island of Hirta. The sheep are not managed in any way – the only human input comes when they are caught for the purposes of tagging after birth and for data collection.

In a new study, which has been published in the journal Ecology Letters, researchers from the Moredun Research Institute and the universities of Stirling, Lancaster and Edinburgh used data collected from 741 individuals between 1989 and 2008 to examine how reproduction influenced faecal egg counts of strongyle nematodes.

They found that ewes that produced a lamb in spring had bigger gut worm infections than ewes that did not reproduce. Ewes that successfully raised their lambs to weaning had higher parasite counts than those whose lamb died soon after birth.

Mathematical modelling revealed that the parasite burdens came with additional costs – ewes with heavier burdens in spring had lower bodyweights in summer and were less likely to survive over the following winter and breed again.

‘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.

‘Our results also suggest that lactation is particularly costly, because females that weaned their lambs had even more parasites than those whose lambs died and therefore didn’t need to lactate.’

Vets know that lambing time is associated with high levels of infection in ewes, added Adam Hayward, a research fellow at the Moredun Research Institute and senior author of the study. The Soay study showed that this was also the case in the wild and that, in the absence of supplementary food and veterinary treatment, parasitic infections could have long-term consequences.

‘We’ve known for a long time that reproduction can affect survival,’ he said.

‘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!’

Leivesley JA, Bussière LF, Pemberton JM, et al. Survival costs of reproduction are mediated by parasite infection in wild Soay sheep. Ecology Letters

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