To challenge the sheep even further, scientists showed them the same celebrities in photos captured from a different, tilted angle. Those celebrities were journalist Fiona Bruce, actor Jake Gyllenhaal, former US President Barack Obama and actress Emma Watson.
The researchers published their findings today in the Royal Society Open Science journal with the title "Sheep recognize familiar and unfamiliar human faces from two-dimensional images".
Picking the celebrity earned a sheep a food-pellet reward.
In subsequent tests, the sheep chose the learnt celebrity face eight times out of every ten, said the research team. The sheep could even recognize images of faces shown at an angle, though their ability to do so declined by about 15 percent - the same rate at which a human's ability to perform the same task declines. "Our study gives us another way to monitor how these abilities change", Morton said.
"Sheep are capable of sophisticated decision making", said study author Jenny Morton, a neurobiologist at the University of Cambridge.
"Sheep are long-lived and have brains that are similar in size and complexity to those of some monkeys".
Sheep, on average, chose the celebrity faces correctly in 8 out of 10 trials.
Morton is using sheep as models for studying human brain disorders. Training involved the sheep making decisions as they moved around a specially-designed pen. "I guess they have extended our work to show that sheep generalize viewpoints of the faces, which does require a rich representation of the identity".
Face recognition is a critical social skill in humans, and we are able to identify a known person within milliseconds of seeing them. The sheep were still able to choose the correct person, showing that they weren't simply memorizing what a 2D photo looks like, but instead were understanding the 3D idea of a human head. Maybe they just didn't like that the non-familiar lacked a reward, for example.