Humans see faces everywhere. We see a face in the craters of the moon, in wall sockets, sideways in punctuation :-) and just about anywhere else two dots and a line are arranged in even approximately the same positions as two eyes and a mouth.
Once we recognize something as a face, we process it differently from other visual stimuli. Certain parts of the brains are triggered preferentially by faces. We are especially good at perceiving faces: we can pick out matching faces faster than matching abstract patterns, and distinguish non-matching faces more easily than other images. This only works, however, when our brains recognize the faces as faces: if you flip faces upside-down, they no longer trigger the “face” switch for us, and we become much worse at distinguishing them. The same thing happens if you digitally scramble facial features, so that there’s an ear in the middle and an eye on the chin and a mouth slanting across the forehead, or any other mix-up that makes the face no longer be arranged like a face. Our brains are specialized to perceive face-shaped patterns much better than other patterns.
Why? The answer is what you would probably guess: we need to be able to tell each other apart. We use faces to identify individual people. If one person regularly steals your food, and another regularly gives you fresh-baked cookies, you’ll want to be able to identify each of them so that you can avoid the thief and hang out with the generous baker.
But are humans the only animals who need to be able to identify individuals? Clearly not—social animals need to be able to distinguish among their fellows for exactly the same reasons that humans do: to know who is friendly and who is not, to keep track of who owes you a favor and to whom you owe something. We might expect that some other social animals would also have specialized facial recognition; and, indeed, they do.
Sheep can tell each other apart by their faces. They are better at discriminating among faces of sheep of their own breed than of sheep of an unfamiliar breed, and they are considerably worse at discriminating among upside-down sheep faces than right-side-up sheep faces. They even have trouble with faces in profile, doing much better when the faces are seen from the front (Kendrick et al. 1996).
That sheep even possess facial differences may surprise anyone who has not spent a lot of time with sheep: they can look all the same, from afar.
But I worked with sheep a bit in college, and I can attest that you can quickly learn to pick out a sheep by its face.
And as a human, I am undoubtedly much worse at distinguishing sheep faces than sheep are.
Sheep aren’t alone. Many animals have been found to use faces as markers of identity, including various fish, birds, and—not surprisingly—our relatives the primates.
One animal in particular has received a lot of attention recently regarding its ability to recognize faces. This animal is… well, if any animal could be the opposite of sheep, in size, fluffiness, and overall impression, this is the animal.
It’s a wasp.
Not all wasps, though: Polistes fuscatus has individual facial recognition, while many other wasps, including its close relative Polistes metricus, don’t. Polistes fuscatus wasps can rapidly learn to distinguish the faces of other wasps of their species—they are faster at learning faces than at learning non-face-like geometrical patterns. But they have to process the images as faces in order to discriminate them: if you show the wasps images of wasp faces with the antennae digitally removed from the image, the wasps stop being good at distinguishing them. Presumably, taking the antennae away stops the image from triggering the “face” switch in the wasp’s brain. The same thing happens if you scramble the features in an image of a wasp face: like us, they don’t recognized mixed-up faces as faces. For something to be a face, it has to have the right features and those features have to be in the right places.
Of course, for the wasps to identify each other by their faces, they have to have facial differences. Do they? Look at the following images and see if you can see differences among the wasps’ facial markings.
Did you see differences? If not, look again, focusing on the color of that shield-like plate in the part of the face we might think of as the “nose.” More subtle, but also different, are the shapes of the yellow “eyebrows” above the antennae. Undoubtedly a wasp would see still more differences; those are just the ones that I see, with my non-attuned human brain.
It is thought that Polistes fuscatus is good at recognizing faces, and has facial differences that can be recognized, because it often lives in small cooperative groups. The ability to distinguish individuals is thought to be a critical condition of cooperation in animals: if you can’t tell your cooperating buddies from non-cooperating strangers, you’ll be taken advantage of by those strangers, doing them favors and receiving none in return. Polistes fuscatus can know that Wide-Eyebrows Wasp and Red-Nose Wasp are friends, while Yellow-Nose Wasp is a creepy interloper. Non-cooperative wasp species, on the other hand, don’t have specialized facial recognition because it isn’t important for them to tell individual wasps apart (Sheehan & Tibbetts 2011).
Of course, even animals that lack facial recognition may have other ways of identifying individuals. Many mammals and some birds identify each other by smell; many birds identify each other by the sound of their calls. Facial recognition is far from the only game in town. But it’s interesting to know, as we go about our daily lives picking out the faces of our friends, learning the faces of new acquaintances, and seeing phantom faces in every random squiggle, that we aren’t the only animals doing so.
Kendrick KM, et al. 1996. Are faces special for sheep? Evidence from facial and object discrimination learning tests showing effects of inversion and social familiarity. Behavioural Processes 38:19-35.
Leopold DA, Rhodes G. 2010. A comparative view of face perception. Journal of Comparative Psychology 124(3):233-251.
Sheehan MJ, Tibbetts EA. 2011. Specialized face learning is associated with individual recognition in Paper Wasps. Science 334:1272-1275.
*Photos obtained from Flickr and used via Creative Commons. Many thanks to these photographers for using Creative Commons!