Sheep can remember up to 50 different sheep faces for more than two years, say researchers in Britain, raising the possibility of sheep emotions and even sheep consciousness.

Keith Kendrink and his colleagues at the Babraham Institute in Cambridge report in Thursday's issue of Nature that sheep have a mechanism for storing memories that is similar to that of humans.

"This suggests that sheep may be capable of using the same system to remember and respond emotionally to individuals in their absence," wrote Kendrink in his report.

Can you tell these two sheep apart? Other sheep can

The researchers discovered the sheep's remarkable memory by showing 20 of them pictures of 25 pairs of sheep. One picture in each pair was associated with a food reward.

The sheep were sent into a maze with branches marked by pictures of the pairs of sheep. By selecting the correct face at each branch, the sheep could finish the maze and get their reward.

After about 30 tries the sheep could complete the course correctly 80 per cent of the time.

The researchers then tested the sheep in the maze at different times over the next several months. The sheep were able to negotiate the maze just as well as before. It was only after almost two years that their performance diminished significantly.

The animals were even able to follow the maze when the pictures were shown in profile, rather than from the front, without having to learn the course over again.

Sheep brains similar to ours

The researcher say sheep, like humans and monkeys, have a special part of the brain that remembers faces, as opposed to other shapes.

By monitoring the activity in this part of the brain, the researchers found the sheep could not only remember the faces of other sheep, but the face of their shepherd as well.

The sheep would also "baa" at the old pictures just as they would at pictures of current acquaintances.

The scientists say their work could have implications on how we treat farm animals and on branches of medicine that deal with the brain.

Because the visual system of sheep is so similar to ours, it could serve as a model for discovering how the human brain works, say the researchers.