Sheep living on a remote island off the coast of Scotland have been shrinking for 20 years. Now it seems shorter winters caused by climate change are responsible.
Soay sheep are a primitive breed of domestic sheep, which live on the island of Hirta, in the St Kilda archipelago, without human interference. From 1955 onwards, the population has been closely studied.Over the last 20 years, the average size of the sheep has been getting smaller, but it has been unclear why – particularly as natural selection would tend to drive the development of bigger bodies.
Sheep stats
To explore the effects of environmental change and natural selection, Timothy Coulson of Imperial College London and colleagues modified the Price equation, which is used to describe how natural selection changes a population from one generation to the next.
Coulson's team extended the equation so that it could reproduce the effects of a variable environment: how weather and seasons have changed from one year to the next, for example. They also modified it so that they could split the population up into different age groups, and describe changes in them separately.
This modification allowed them to pin down the factors that have affected the size of the sheep.
Natural selection pushes the sheep to get bigger, as the smallest individuals tend not to survive through hard winters to reproduce, they found.
However, this size increase is largely offset by the so-called "young mum effect" – the tendency for female sheep in their first breeding seasons to have offspring that are smaller than they themselves were at birth. The study is the first to take this effect into account.
Dearth of deaths
Over and above these factors, the modelling revealed that one of the most powerful influences on size was the gradual warming of the climate, driven by changes to the North Atlantic Oscillation ocean current, which has led to shorter winters on the island. As a consequence, the vulnerable smaller sheep were more likely to survive the winter, pushing average size down over successive generations.
"Because fewer sheep are dying, I think that means the environment is getting better for them," says Coulson. "The winters are less harsh than they used to be."
Kaustuv Roy, an evolutionary biologist at the University of California San Diego, who was not involved in the study, is impressed. "Their results are really useful, because they tease apart the different processes. It's a really nice study," he says.
Roy adds the team's modification of the Price equation could be used widely. "They've come up with a new approach, which people will definitely apply to other systems," he says.
Journal reference: Science, DOI: 10.1126/science.1173668 (in press)
Climate change is shrinking sheep
Victoria Gill, BBC News 2 Jul 09;
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.
"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.
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."
Climate change: Bye-bye, black sheep?
Yahoo News 21 Jul 09;
PARIS (AFP) – Another clue has been found in the Case of the Shrinking Sheep, an animal mystery in which climate change features as the principal culprit.
The tale of scientific sleuthing is unfolding on two Scottish islands, Soay and Hirta, in the remote Outer Hebrides.
Their sole inhabitants are wild sheep which probably arrived there with the first human settlers some 4,000 years ago.
The sheep's isolation and lack of predators make them terrific candidates for studying the impact of weather, food and genetics on a wild animal population. The flock, suffering occasional surges and crashes in numbers, has been closely scrutinised since the 1950s.
Two years ago, researchers came across a strange thing: The average size of the Soay sheep was progressively falling.
That finding ran counter to Darwinian intuition. Evolutionary theory said that, given the cold, rough winter on the islands, bigger sheep had the better chance of survival, so their genes should progressively dominate the flock.
The solution to this enigma, suggested Imperial College London scientists earlier this month, lies in global warming.
Milder winters in recent decades have enabled smaller lambs, which otherwise would have died after birth, to survive into adulthood and then reproduce, they said.
The climate whodunnit has now been backed by a trio of Australian experts, who have matched weather and population records with the colour of the sheep's coats.
The smaller sheep that now dominate the flock are also lighter-haired ones, a link that has been proven by gene analysis. Bigger sheep tend to be darker.
Why would coat colour make a difference?
The answer, suggests the team led by University of Western Australia's Shane Maloney, is that, in colder times, sheep with darker coats have an advantage.
Mammals with darker coats absorb more solar radiation and thus need to expend less food energy to keep warm than do their lighter counterparts.
But, as the climate has warmed in the North Atlantic, this advantage has diminished, which gives more of a chance for lighter-haired (and smaller) rivals in the struggle to survive.
"If environmental effects are the cause of the decline, then we can expect the proportion of dark-coloured Soay sheep to decrease further," the fleece police add soberly.
The study appears on Wednesday in Biology Letters, published by the Royal Society, Britain's de-facto academy of sciences.