Some furry faces are unforgettable — at least to other sheep.
It turns out the animals use similar neural networks as humans to both recognize and remember each other's faces.
According to a new study, sheep and people use one set of neural networks for remembering very familiar faces, and another to recognize faces that just ring a bell for some forgotten reason. And both can hold memories of the very familiar for long periods of time.
"We know that sheep can not only recognize other sheep, they can remember some faces of sheep for up to two years," said Keith Kendrick, a neuroscientist at the Babraham Institute in Cambridge, England who authored the study in this week's issue of the journal Nature.
"That begs the question of whether they can think about, perhaps even miss individuals they haven't seen in a long time."
Why care about the inner thoughts of sheep?
Clues to Human Disease
By understanding how sheep recognize and store memories of each other, Kendrick hopes to learn more about human recognition, and possibly find clues to curing conditions that mar people's ability to recognize others and the expression of others.
For example, patients with autism, schizophrenia and other conditions often respond inappropriately to visual images, including facial expressions. And those who incur damage to the right side of the brain can have trouble recognizing new faces.
Since the neural and visual networks in sheep are very similar to ours, learning more about how the animals recognize each other could shed light on these complex human conditions.
"There are a lot of common principles that cut across the boundaries," explains Vilayanur Ramachandran, a neuroscientist at the University of California, San Diego. "It can be extremely valuable to study other animals like sheep for this field."
Following a Face to Food
To test how well sheep recognize each other, Kendrick trained a group of 20 sheep to associate the faces of other sheep with food.
He set up a barn-like structure with two separate entrances. At the front of one entrance, he posted a life-sized digital image of one sheep's face, and another at the other entrance. The sheep eventually learned which face to follow to find a food reward inside the entrance. The animals learned to choose the right face among 25 different pairs of sheep faces. Even when the facial images were shown in profile, the sheep recognized which face to follow.
Kendrick then tested the same sheep a month later, another month later and eventually more than two years later. He found that, for up to 600-800 days, the sheep retained the memory of which faces led to food and continued to correctly choose the entrance with that face at the front.
Next, Kendrick's team monitored specific nerve cell clusters in the sheeps' brains. From past work, he knew that a small number of cells in the right side of the sheep brain respond to somewhat familiar faces. Another set respond to the faces of highly familiar individuals so the faces are recognized in an emotional context. This is also true in humans.
To see how a sheep retains the memory of a fellow flock member, he removed individuals from the group and then monitored the neural networks in the right region of the sheeps' brains using electrodes. When showing the sheep images of the lost flock members and monitoring brain activity, Kendrick learned the sheep had registered memories of 50 faces of lost individuals.
The more time that elapsed since the individual was removed, the less the brain responds to seeing an image of the long lost sheep. He also learned that eventually the memory of a face is transferred from the neural network used for recognizing highly familiar faces to the area of the brain associated with recognizing somewhat familiar faces.
"The difference is analagous to knowing a person, being able to recognize them and place them in context and recognizing a person, but not being able to place them in much context," he said.
Kendrick says this could also mean that sheep, like people, use their neural system not only to recognize other faces, but also to bring the faces of other sheep to mind.
"It suggests the sheep could be conscience and could actually think about each other," he said.
Further tests showed that sheep can also retain memories of very familiar people, although his tests showed the animals had a harder time doing this unless the person was a very regular presence in the sheep's life.
Horses, Cows, Monkeys Also Good at Recognition
Some pet owners might wonder if dogs and cats might have the same ability since these animals' behavior often indicate they recognize their owners. Unlikely, says Kendrick. These animals likely use other senses, namely smell, to recognize others since their vision is not as highly developed as sheep.
But other animals with sophisticated vision systems, including cows, horses and monkeys, likely have the same ability. Among this group, Kendrick says, the sheep is often dismissed as simple and dim-witted, but he argues his studies prove the sheep's intellect should not be overlooked.
"If I give you a monkey brain a human brain and a sheep brain, you'd be hard pressed to see major differences," he said. "Their brains are sophisticated-looking organs."