In the 1970s, psychologist Paul Ekman created a system to catalogue human facial expressions based on individual muscle movements. Since then, his Facial Action Coding System, or FACS, has proven surprisingly powerful at uncovering hidden emotions. FACS has attracted a diverse following, including CIA interrogators trying to detect tiny facial twitches that belie deception and animated filmmakers hoping to design lifelike expressions for their cartoons.
Comparative biologist Lisa Parr took Ekman’s research and added a hairy twist. Instead of cataloguing human expressions, she studies chimpanzee faces. Many evolutionary biologists think the ability to communicate with facial expressions evolved alongside big brains and upright posture. If so, one would expect to find similar expressions in primate species close together on the evolutionary tree—like chimps and humans, who shared a common ancestor only six million years ago.
But before comparing chimp expressions to our own, Parr and her colleagues at the Yerkes National Primate Research Center in Atlanta needed an objective way to study primate faces. In 2005, they created the first version of “Chimp FACS,” a facial expression coding system analogous to Ekman’s. Next, they designed a computer program with a three-dimensional, anatomically accurate chimp character who can replicate any of the natural primate facial expressions. Parr has taught five chimps to do facial expression matching exercises on the computer. The patterns of their responses offer insight into how chimps recognize emotions and the faces of their kin.
At the 2009 meeting of the American Association for the Advancement of Science in Chicago, Parr joined other comparative and evolutionary biologists to discuss some fundamental questions about primate expressions: How do chimps communicate with their faces, and how closely do their facial expressions resemble ours?
How did you first become interested in studying the emotional lives of primates?
That’s not an easy question! When I was an undergraduate, I was a biology major, and I knew I wanted to work with animals because I didn’t want to do the kind of wet lab biology where you’re stuck in a lab working with chemicals. Right before I graduated, a new faculty member arrived who did field work with monkeys in Brazil, and that seemed really exotic. Through her, I met my graduate adviser, and he got me working with chimpanzees. Once you start working with chimps, it’s pretty hard to avoid thinking about them in emotional terms.
When did people first decide to replicate what Paul Ekman did for humans, in chimps?
Well, we invented Chimp FACS, so it was us. The idea started to brew around 2002 or 2003. Our big question was, how similar are facial expressions between humans and chimpanzees? But without a way to compare the expressions, everything was just subjective—like saying, “Well they kind of look the same, but…” The first goal was to find out how similar facial expressions are in terms of muscle movements.
"Chimps can do certain facial expressions that we can't, mostly because they've got these stretchy, rubbery lips, and they can curl them in all sorts of positions we can't replicate. Our lips just aren't long enough."
To develop Chimp FACS, you said your group watched hundreds of hours of chimpanzee videos.
After we validated how different muscles change the appearance of the face, we needed to find examples of chimp expressions from naturally occurring behavior. So we collected as many videos as we could find of chimps hanging out in their natural habitat. Basically, we watched each video until we saw a movement that we recognized. Then we’d go back and compare it to our anatomy diagrams. If we said, “Yeah, that definitely looks like movement number one,” then we’d catalogue the video clip—that’s how we put together the FACS manual.
Did you find similar facial expressions between chimps and humans?
If you ignore upper facial movements, which chimpanzees don’t have a lot of, then the facial expressions actually match up pretty well. For example, “screaming” is their excited face, and it looks just like when your mouth is wide open and you’re sort of screaming or making loud noises because you’re excited. The “pout” matches up almost identically in terms of movements and how it’s used in both chimps and humans. And I think some components of the “bulging lip face” are similar to what kids do when they’re pouting or showing a mad face.
Why don’t chimps have as many upper facial movements as we do?
Chimpanzees have this heavy, thick brow ridge that constrains the way their upper face can move. They don’t have any visible cues there, like a hairy eyebrow or a different coloration on their heads. So if they were doing something subtle with their brows, no one would know. And if you can’t see a movement, it’s not going to persist [through evolution]—there’s no point in making a movement that doesn’t contribute to the overall expression.
Many people have compared the chimps’ “bared-teeth face” to a human smile. Do they use the bared-teeth face the same way we use a smile?
Yes and no. A lot of times the bared-teeth face is used after an individual has been in a fight, and that led to it being called a “fear grin.” But chimps use bared teeth in so many other contexts—I’ve seen it during play, when individuals greet one another, when they seek reassurance of some kind. People who study the cause and consequence of the bared-teeth face refer to it to a signal of “benign intent.” That’s a really boring term for it, but it’s basically a pacifying signal. Like, “Hey, I don’t mean any harm—I come in peace, and I’m not going to try anything.”
That sounds a lot like how humans use a smile.
Yeah, absolutely. The bared-teeth expression seems to put everyone at ease, just as our smile puts other people at ease when we meet. If you greet someone and they’re frowning at you, grimacing and making an angry mouth, that doesn’t usually lead to a good interaction. But if you meet someone for the first time and they smile, usually there’s a good interaction that follows.
Does learning about primate facial expressions tell us something about ourselves?
I study chimps because I think they’re amazing animals and I want to know about them—so I don’t think it necessarily has to tell us anything about ourselves. But until we understand what the chimps’ facial expressions mean and how they’re used, we won’t know what kinds of comparative questions to ask. Because chimpanzees are our closest living relatives, the best way to understand something about our ancestral condition is to look at chimp behavior. By using chimp FACS, we can compare what kinds of facial signals the animals use to communicate, and we can look at similarities and differences in humans. Then we can go back and come up with some sort of evolutionary explanation of why that might be.
Can you give an example?
The bared teeth is one example. Given the social structure of chimps and humans, it was probably very important that they develop a signal to pacify individuals before any conflict would happen. So the chimps started using a version of the bared teeth face to signal this pacifying intent—and obviously it was something that was important in our ancestors as well.
Are there facial movements that are unique to chimpanzees, that humans don’t have?
Not movements, but combinations of movements for sure. The chimps can do certain facial expressions that we can’t, mostly because they’ve got these big stretchy, rubbery lips, and they can move them and curl them in all sorts of positions we just can’t replicate. For example, the “whimper” is one where the lips are puckered but also retracted. It’s like smiling and making a kissing face at the same time, and it’s very difficult for us to do—our lips just aren’t long enough.
What about expressions that we use, but chimps don’t?
We’ve looked at so much video, but we haven’t seen anything that looks like frowning. It’s probably one of the most common movements in humans, but I honestly don’t think they have it.
In humans, FACS has been used to decipher very complex emotions, like deception and guilt. Do chimps have “self-conscious” expressions like ours?
That’s a really tricky area. Just in terms of facial expressions, I would say no. But chimps definitely do things that we want to interpret as very high-level conscious behavior, like deception. For example, you often see two individuals walk very quietly away from the group and then mate behind a rock. It implies that they know they’re out of view, and that’s kind of a deceptive move. But it’s really hard to do experiments that elicit deceptive behavior, so you’re waiting for these anecdotes of behavior to happen. That’s a dangerous game to play. We want to interpret it a certain way because that’s what we would do, but it’s not necessarily what a chimp would do.
How hard was it to train the chimps to use the computer? Was it something they figured out pretty quickly?
Both chimps and monkeys are pretty interested in the computer. They’re clever, curious creatures, and they like to interact with things that give them rewards and food. So it’s not very difficult. A couple of months working with them and they become pretty proficient at it.
Has your work with primate expressions changed the way you view chimpanzees and their social interactions?
Absolutely. When I was trying to learn all the different basic categories, I wasn’t aware of all the subtleties and the differences among expressions. It just seemed like a blur. But having a tool like the chimp FACS to standardize and characterize the expressions has been really helpful. Now we know we’re calling the same expression the same thing across different individuals. Things make a lot more sense using these tools, and now I can appreciate that chimpanzees use an extremely complicated and highly variable system.
Do you find yourself being able to read their expressions more easily now?
Yeah, I think so. You can see their faces start to move sometimes and think, “Oh they’re going to do a bared-teeth face.” And then maybe the individual will just kind of get up and walk away. I definitely look for those subtle little changes, and now I can read a little more into them. It helps me read human behavior better as well. I think most FACS people will tell you that: After you learn it, it fundamentally changes the way you look at people.
Hadley Leggett, a graduate student in the Science Communication Program at UC Santa Cruz, earned a B.A. in biochemistry and Spanish from Rice University and an M.D. from UC San Francisco. She has worked as a reporting intern at the Salinas Californian, the San Jose Mercury News, and the news office of Stanford Medical Center. She will work as a summer reporting intern for the science unit of Wired.com in San Francisco.
© 2009 Hadley Leggett
