It's the news that the introverted, less-well-endowed male of the species has been waiting to hear: researchers studying a herd of sheep on a remote Scottish island have found that rams with the biggest horns are the most likely to die off when the going gets tough. The findings are the latest from a study of Soay sheep on Hirta, St Kilda, a long-running project that has produced several surprises. Last October scientists demonstrated that ewes from the herd used their horns in clashes over food - previously it was thought that only rams would use their headgear for fighting.

The same Edinburgh University team has now uncovered the unexpected among the rams' horns.

“You would expect big horns to be a benefit for the males, but that is not necessarily the case. It all depends on the conditions they are born into,” said Matt Robinson, a PhD student whose thesis addresses the selection and genetic variation of weaponry in a large mammal. In other words, beasts with big horns must be lucky with the weather if they are to survive. Anything that saps their energy or lowers their food intake could lead to an early death.

The sheep on Soay and Hirta, tiny islands that form part of the St Kilda archipelago, have survived in isolation since the Bronze Age, amid a treeless landscape exposed to North Atlantic storms. Like any isolated group the herd is a subject of endless fascination to biologists. The Soay's origins are unknown, but they have outlasted the island's human population, which was evacuated in 1930, and stoically endure the attentions of the researchers who have arrived three times a year since 1985.

Counts of the animals have found that the herd can vary from 500 to 2,000 animals. Genetic variations, population density and climatic conditions have all been the subject of study in an attempt to understand the huge fluctuation in numbers.

“The fittest always survive. It's just that different things can affect how you survive, depending on the environment you are born into. Individuals that invest the most resources in horns which are really useful when they are breeding are the ones who are most vulnerable when they need more energy to survive in tough conditions,” Mr Robinson said. “These same young males with large horns might also get involved in a rut at an earlier age - and that is a very high energy activity too,” he added.

Evolutionary biologists are fascinated by variations within individual species. Peter and Rosemary Grant have spent 30 years studying Darwin's finches on the Galapagos islands, a species of bird whose beak comes in different shapes and sizes. However, Mr Robinson said that the St Kildan findings broke new ground.

“This is the first example of a trait which is only used in reproduction,” said Mr Robinson. “In one good year, when conditions are favourable a cohort of big-horned rams will survive, because reproductory success is skewed towards the dominant males.

“But the survival of their offspring will depend on the environment they are born into. The findings suggest we need to look at the costs and benefits of things over the course of a life - there might not be a single optimal strategy for growth.”

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