Despite a greater abundance of food, milder winters and longer summers means that the wild Soay sheep of the St Kilda archipelago are shrinking by 3.5 ounces (100g) a year.
Over nearly a quarter of a century the sheep, one of the oldest breeds in the world and already half the size of a normal domestic sheep, have dropped in weight and height by five per cent.
Researchers believe that the hotter weather means that the weaker, smaller lambs that are usually wiped out by harsh winters are surviving – bringing down the average of the 2,000-strong wild flock.
The milder weather is also allowing younger, smaller mothers to have children early, meaning they give birth to smaller offspring.
Professor Coulson suggests that this is because shorter, milder winters, caused by global climate change, mean that lambs do not need to put on as much as weight in the first months of life to survive to their first birthday as they did when winters were colder.
Although Soay sheep, which can live up to 16 years-old and weigh 100lbs (45kgs), get their name from the small outcrop of rock of the same name, they now inhabit other islands in the St Kilda archipelago.
The scientists led by Professor Tim Coulson, from Imperial College London, chose to study the population on the 1,500 acre island of Hirta which has been uninhabited since 1930.
Over 24 years they have studied the relative size of the flock, capturing, marking and measuring them every summer. Their research, published in the journal Science, shows that over the period the average size of the flock has dropped from 66lbs (30kgs) to 61lbs (28kgs).