The scientists returned to the remote Scottish island again and again and counted enough sheep to send even the most dedicated researchers into a sound slumber.

Yet by the end of the exercise, they had confirmed a remarkable phenomenon. The sheep were shrinking. Quite rapidly.

Over the 25-year period of their study the Soay sheep on the island of Hirta in the Outer Hebrides had shrunk by about 2cm (0.8in) or 81g (2.9oz) per year, amounting to about 5 per cent of their body mass.

The scientists attributed the change to short-term changes in climate rather than to the long-term pressures of natural selection, which would favour a larger — not a smaller — body size.

Previously the smallest lambs would have perished in the cold conditions of early spring. But shorter, milder winters and warmer springtimes mean that more small lambs survive and pass their “small” genes on to their offspring, according to the study, which is published today in the journal Science.

The sheep studied were completely untended and the authors say that the effect is unlikely to be seen in farmed populations. Farmers tend to select strongly for larger sheep, to maximise profits. This would mask any trends caused by environmental change.

“We’re certainly not predicting that we’ll be seeing pygmy sheep being herded by chihuahuas,” said Tim Coulson, a specialist in population biology at Imperial College London and a co-author of the paper.

Likewise, in the human population factors other than climate are playing a bigger role in changing our size, shape and behaviour.

The average human height in Britain has increased by more than 5cm in the past century, mainly because of changes in nutrition, for instance. And progress in medicine means that the Briton’s average lifespan is increasing by about five hours every day.

However, isolated human populations that depend strongly on their natural environment and are relatively immune to cultural and technological changes could conceivably undergo physical changes because of climate change, according to Professor Coulson.

Similar effects have been observed in fish, including cod, salmon and sticklebacks, marine iguanas, large-horned Canadian sheep, North American squirrels and blue tits. “A growing number of cases like this are being reported,” said Professor Coulson.

Ecologists have also reported changes in behaviour. The migratory patterns and breeding times of some bird species have altered. And flowers such as daffodils are blooming earlier.

In the Hirta study about 30 surveys of the sheep were carried out each year from 1986.

Ewes and their female offspring in the flock were weighed and measured as lambs, yearlings and at various ages as adults. Only ewes were included because it was difficult to determine fatherhood at the start of the study, before genetic testing was readily available.

During the course of the study, measurements of the lambs’ legs showed that they were getting smaller, not just skinnier, as the sheep’s average weight fell.

Because the researchers had accumulated data on both the climate, and the parenthood of sheep, they were able to work out the extent to which size was being driven by evolutionary factors and how much it was driven by environmental factors.

“The Soay sheep provides another example of how far-reaching and unpredictable the effects of climate change can be,” said Professor Coulson.

Peter Morris, head of the National Sheep Association, agreed, despite admitting that it was counter-intuitive to believe that shorter winters would cause sheep to shrink.

“The smaller lambs surviving makes sense, but you’d expect that to be counter-balanced by the longer summers when more food would be available,” he said.

In reality, because more animals are surviving there is more competition for food, the study showed.

So while the size of the flock grew significantly, the size of individual animals did not.

“More food doesn’t necessarily equate to bigger animals; it often means more animals,” Professor Coulson said.

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