Secret of Scotland's Shrinking Sheep Solved

ScienceNOW Daily News ^ | 2 July 2009 | Nayanah Siva

[neverdem]

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Slimming down. Sheep on the remote Scottish isle of Hirta have been getting smaller.

Credit: A. Ozgul/Science

Call it the case of the shrinking sheep. On the remote Scottish island of Hirta, sheep have been getting smaller, shrinking an average of 5% over the last 24 years. Don't blame evolution, though. Researchers say climate change is the real culprit.

The Hirta sheep belong to a breed known as Soay, after the remote Scottish island where they arose. One of the most primitive forms of domestic sheep, Soays first came to Hirta in 1932. Because Hirta is a remote island, its sheep have remained genetically isolated, and no other sheep have been brought in for breeding. That's made Hirta's Soays ideal subjects for scientific study.

In 2007, scientists first reported that the sheep were smaller than they had been in the past. This prompted biologist Arpat Ozgul of Imperial College London and colleagues to analyze body weight data going back 24 years. The researchers confirmed that the Soays had indeed been getting smaller. And, as they report online today in Science, the reason appears to be climate change.

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. Further mathematical modeling allowed the researchers to propose that natural selection has played little--if any--role in the shrinkage of the Hirta sheep.

Malcolm Gordon, an ecologist and evolutionary biologist at the University of California, Los Angeles, praises the study. But he says that other mechanisms may be at work. "Changing [environmental] conditions on the island ... [may] have led to changes in the chemical composition and nutritional value of the plant foods the sheep eat," he says, and that may have shrunk the sheep. Though at the end of the day, he says, climate change could still be the root cause.

Related Site

The Dynamics of Phenotypic Change and the Shrinking Sheep of St. Kilda

[Battle Axe]

Sheep or anything with low birth weight are not prone to being small. The genes control that as well as amount of feed. If the winters are less harsh, then small born sheep would grow more not less.

I agree with the shrinking breeding population, called sympatrick breeding populations...no outside genes coming in.

Bulking up would be a yearly thing, and growth is altogether a different thing. They would grow to their potential if nutrients are available.

This is some crappy dim trying to push global warming etc, which does NOT exist. If anything it is global cooling. We have had two hot days all summer, now the leaves are turning.

BA

[Mr. K]

"Researchers say climate change is the real culprit."

DING! DING! DING! I KNEW THERE WAS A *GOOD* REASON!