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To be honest, I struggled to believe the headline news: “Climate change is shrinking sheep” – surely April fools day was over 3 months ago? Reading on, the story becomes more intriguing… Apparently, researchers have conducted detailed measurements on the body weights of a population of Soay sheep on the island of Herta off of the Scottish coastline since 1986. Soay sheep are an intriguing bunch, first brought to the island in 1936 and remaining isolated since, making a perfect study subject for investigating the effects of environmental change on physical characteristics. Analysis of these measurements revealed that not only is the population of sheep putting on less body mass (an average decline of 5% over the past 24 years), but are also affected by a decrease in the length of their hind legs, suggesting that the Soay population really is declining in size, rather than a decline in body condition.

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Mean annual August weights of Soay sheep showing a pattern of decline across all age catagories

So what factors are driving this apparent phenotypic change over such short time scales? Apparently the answer isn’t evolution: selective pressures explained little of the observed pattern, instead environmental change (in this case the warming related to the North Atlantic oscillation index) is a more likely explanation:

In the past, Hirta’s sheep gorged on grass during their first summer, the team notes, piling on the weight in order to make it through the island’s typically harsh winters. But over the past quarter-century, Hirta has had unusually short and mild winters. As a result, Ozgul and colleagues propose, grass has become available for more months of the year, meaning the Soay sheep do not have to bulk up as much. In addition, Hirta’s harsh winters used to kill small ewes born to young mothers. But now these small ewes survive–and because of their low birth weight, they never get as big as normal sheep. That drives down the average size of the entire population, the team reports.  (Read More)

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