Humans can recognize hundreds, if not thousands of individual faces. Sheep, it appears, may not be far behind.
A team of British scientists has shown that sheep are able to recognize the individual faces of at least 50 sheep and remember them for more than two years.
"If sheep have such sophisticated facial recognition skills, they must have much greater social requirements than we thought," said Keith Kendrick, of the Babraham Institute in Cambridge, England, and an author of the recent report published in the November 8 issue of the journal Nature.
Kendrick and his colleagues presented sheep with 25 pairs of sheep faces and associated one face within each pair with a food reward. After about 30 trials with each face pair the sheep were able to correctly recognize all 50 faces about 80 percent of the time.
In the following six weeks the sheep were shown each face pair up to 500 times. These additional trials revealed that the sheep were also able to recognize profiles of the 50 sheep without having seen those particular perspectives before.
The ability to recognize these faces only began to decline between 600 and 800 days later.
To understand how these visual memories form and gradually fade, Kendrick measured the responses from cells in a part of the sheep's brain known to control facial recognition. Sheep were shown mug shots of unfamiliar and familiar sheep while an electrode measured cell activity in their brains.
"Sheep, like humans, have specialized areas in the brain for face recognition," said Kendrick, and they have a separate system, far less specific, for dealing with the recognition of other objects, such as rocks and trees.
"Whereas you can measure a cellular response to a face, you would have a hard time finding a cellular response to a banana," he added.
Kendrick's team discovered that a large network of cells responded to faces in general. A smaller number of cells respond to familiar sheep faces. An even tinier subset of cells responds to specific, very familiar individuals, such as pen mates of the sheep.
"There may even be cells that respond only to a particular individual," said Kendrick.
Kendrick suggested that memories may fade when circuits dedicated to recognition of a specific individual somehow become more general and are downgraded to "code" for just a familiar face.
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