Getting a head start in life is no sure path to success, a study of wild sheep on St Kilda suggests.

Researchers studying Soay sheep have found that male lambs which grow big horns are more likely to die if their first winter is a particularly harsh one.

Young rams which develop the longest horns would be expected to grow into the strongest, fittest adults and win more mates.

But they face a genetic trade-off if they are to survive, according to a 17-year study of sheep on the island of Hirta.

By diverting too much energy into growing big horns, the lambs leave themselves vulnerable to extreme cold and hunger when environmental conditions are poor.

The discovery will help zoologists to conserve wild populations of rare and endangered species.

Matthew Robinson of Edinburgh University's School of Biological Sciences, who led the study, said: "Young rams are taking a gamble by putting lots of energy into growing horns, which may pay off if their first winter is mild.

"Those rams with big horns which make it through their first winter go on to become the fittest of the flock, being better able to compete with other males to win mates.

"However, if the winter is a difficult one, lambs with big horns are most at risk. They have invested resources which could have been put to better use - helping them survive the winter."

St Kilda's wild sheep have been monitored over 17 years, by scientists from the Universities of Edinburgh, Cambridge and the Macaulay Institute in Aberdeen.

The island flock are an ideal study population because they are isolated from human interference.

Their latest study, published today in the journal Current Biology, was funded by the Natural Environment Research Council, the Wellcome Trust, the BBSRC and the Royal Society.