Climate change is causing a breed of wild sheep in Scotland to shrink, according
to research.

Scientists say milder winters help smaller sheep to survive, resulting in this
"paradoxical decrease in size".

Classic evolutionary theory would predict that wild sheep gradually get bigger,
as the stronger, larger animals survive into adulthood and reproduce.

Reporting in Science journal, the team says this shows the "subtle interplay"
between evolution and the environment.

Scientists first began studying Soay sheep, on the island of Hirta in the St
Kilda archipelago, in 1985.

Since then, the sheep have decreased in size by 5% - their legs getting steadily
shorter and their body weight decreasing.

This strange phenomenon was first reported in 2007, but the reason for it
remained under debate.

'A natural laboratory'

The lead researcher in the study, Tim Coulson from Imperial College London, said
the island provided an ideal opportunity to tease apart the factors driving the
sheep's physical change.


In the past, only big, healthy sheep... could survive the harsh winters
on Hirta

Tim Coulson
Imperial College London
"The island is almost like a natural laboratory - there are only the sheep and
the vegetation there," he said.

He and his team had access to detailed information about the sheep that had been
collected over more than two decades.

"We have so much great data," said Professor Coulson, "that we were able to
write a ledger of how much of an effect each of the different factors had on the
sheep."

They used a formula called the "Price equation", which was designed by
evolutionary theorist George Price to predict how a physical trait, such as body
size, will change from one generation to the next.

With all of this data, the team was able to "rearrange the equation" and use it
to work out how much of a contribution each driver made to the sheep's body
size.

They found that the local environment had a stronger effect on the animals than
the evolutionary pressure to grow larger.

"In the past, only the big, healthy sheep and large lambs that had piled on
weight in their first summer could survive the harsh winters on Hirta," said
Professor Coulson.

Because of climate change, he explained, grass for food is now available for
more months of the year on the island.

"Survival conditions are not so challenging - even the slower growing sheep have
a chance of making it, and this means smaller individuals are becoming
increasingly prevalent in the population," he said.


"Young mums" tend to give birth to smaller lambs
The team also found that younger sheep tended to give birth to smaller lambs - a
phenomenon they termed "the young mum effect".

This effect, said Professor Coulson, combined with environmental changes had
"overriden what we would expect through natural selection".

As for the future of the sheep, the team believes that they are still shrinking.

"The next step is to extend our description of past change into a predictive
model," said Professor Coulson.

"But it's too early to say if, in 100 years, we will have chihuahuas herding
pocket-sized sheep."


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