Female sheep have been found to use their horns to fight over food, researchers say.
In many species males use weapons, such as horns, in competing for mates. But it was thought some females developed horns as an evolutionary by-product.
However, Edinburgh University scientists have found females will use their horns to fight against other females over food.
Researchers studied wild Soay sheep on the island of Hirta, St Kilda.
They found many fights took place during the lambing season, when food was scarce.
The study, published in Biology Letters, backs previous theories which suggest male sheep's horns are curved to withstand head-on clashes, while female sheep's horns are spiked in order to push competitors away.
It was also found that female sheep with fully-developed horns were found to be more aggressive than those with partial horns or no horns.
Matthew Robinson, from the university's School of Biological Sciences, said: "Males use weapons to win mates and females use weapons to win food.
"This shows that females will behave aggressively towards each other using weapons when resources are in short supply.
"It was previously thought that for females, horn growth might be a drawback and a waste of nutrition, but in fact it might give a competitive advantage meaning that they may be able to better provide for their offspring."
Soay sheep are primitive domestic sheep and originate from the island of Soay.