One sheep can tell when another is unhappy by its expression, according to scientists.

The discovery could have implications for the way animals are treated in captivity, they say.

The study was carried out by Scotland's Rural College (SRUC) and the French National Institute for Agricultural Research.

Dr Lucille Bellegarde, of the SRUC, said: "This study showed for the first time that sheep are not only able to discriminate between facial expressions but they are also able to perceive the valence - in other words, negativity or neutrality - of the expressions displayed.

"In terms of animal welfare, it is essential to be able to understand how emotions are perceived between animals reared in groups.

"Because this emotional perception is likely to affect their own emotional state, it might take just one happy or unhappy sheep to make an entire flock happy or unhappy."

During the study, researchers photographed sheep in three different situations.

To obtain neutral expressions they were photographed in their pens and for negative expressions they were either photographed alone or during aggressive social interactions.

Sheep were then tested on whether or not they could discriminate between the two expressions using a reward system.

The scientists believe goats and deer may also be able to recognise negative expressions.