Kim Mills: If you open your high school yearbook, you’ll probably recognize many of the faces of your classmates, even if you haven’t seen them in years. That might seem like a uniquely human ability, but recent research with chimpanzees and bonobos has found that they too can recognize individuals—even after spending many years apart. Today we’re going to talk with a researcher who studies these great apes about how humans’ closest relatives navigate their social world.
So how closely are we related to these non-human primates? What else can they do that might surprise us? What catches their attention in the social world and what can their behavior teach us about the evolution of human social intelligence?
Welcome to Speaking of Psychology, the flagship podcast of the American Psychological Association that examines the links between psychological science and everyday life. I’m Kim Mills.
My guest today is Dr. Laura Simone Lewis, an assistant professor in the departments of psychological and brain sciences and anthropology at the University of California, Santa Barbara. She studies social cognition in bonobos and chimpanzees, including how these great apes form, build, maintain, and repair their long-term social relationships. Her research findings have been published in peer-reviewed journals and covered in media outlets including Scientific American, NPR, The Washington Post, and The New York Times.
Dr. Lewis, thanks for joining me today.
Laura Simone Lewis, PhD: Thank you so much for having me. It’s a joy to be here.
Mills: I’d like to start with some context. As I said, you study chimpanzees and bonobos. Where do they fall on the evolutionary tree?
Lewis: That’s right. So I study chimpanzees and bonobos, who are both our closest living phylogenetic relatives. So we share, humans share a last common ancestor with them that lived somewhere between 5 to 9 million years ago. The numbers change all the time, but somewhere between 5 to 9 million years ago. And when I say we share a last common ancestor with them, the way I like to think about it is that last common ancestor that we share that lived 5 million years ago is not alive today. So I like to think of this last common ancestor as our great, great, great, great, great, great, great grandma.
And 5 million years ago sounds like a ton of time and it is, but in terms of our evolutionary history and the age of the earth, it’s actually just a blip in evolutionary time, which means that we are extremely closely related to them. We share almost 99% of our DNA with them. And it means also we have a lot of similarities in terms of our psychology and especially our social psychology, which is what we study. Of course, there’s many differences as well, and it’s these similarities and differences in our social psychology that I explore in order to think about our evolutionary history and our shared evolutionary history.
Mills: Now, one of your best known findings, which I mentioned in my introduction a moment ago, is that chimps and bonobos can recognize former groupmates even after many years apart. So can you walk us through how you tested that and what you found?
Lewis: Absolutely. Let me start by talking about what we know in ourselves as humans. We have remarkable long-term memory. And as you mentioned in your intro, we could pick up a yearbook from 30 years ago and look through that yearbook and recognize the faces of our classmates, even if we haven’t seen them or seen a picture of them in 30 years. And the other thing we know about our own human memory is that our memory, our long-term memory for others is shaped by our social relationships and social roles. So we remember others in clusters based on our previous relationships with them. So first, that’s what we know already about human long-term memory. And what we didn’t know is whether and to what extent chimpanzees and bonobos could remember the faces of their previous groupmates and for how long, and whether their social relationships also shaped their long-term social memory.
So this is how we came to think about designing a study around long-term social memory. And I just want to give you a few anecdotes before we jump into the details of the study, which is that as researchers who work with great apes, these are very precious and endangered populations, and we often have to travel around the world to get to work with these populations. And what researchers noticed when they would travel back and forth to go study these great ape populations is that it seems like the great apes we studied would remember us year after year, even if we hadn’t been there in a year or two to that site or that population. They expressed what seemed like behaviors that indicated they remembered us, even if they hadn’t seen us in years. And so we wanted to go test this idea. So that’s how we kind of came up with this study idea, and it was based on anecdotally what great ape researchers had been noticing, and we really wanted to get some data to find this out.
So how we designed the study is this is a non-invasive eye tracking study with chimpanzees and bonobos. Non-invasive, meaning it’s not harmful at all to the animals. And eye tracking, meaning we use an eye tracker in order to determine where on a screen chimpanzees and bonobos are looking at any given time. So in this way, we can show two different pictures on a screen, and the eye tracker sits directly below the screen, and it records how long bonobos and chimpanzees are looking at one image over the other. So in this way, we designed a pretty simple study. The study design itself was pretty simple. We showed one picture of a previous groupmate who they hadn’t seen in years, and one picture of a complete stranger. And what we found is that both chimpanzees and bonobos looked significantly longer at these images of their previous groupmates as compared to the images of the strangers.
And then we also added one more detail where we surveyed the keepers who were working with these chimpanzees and bonobos who were living in zoos and sanctuaries. We surveyed them about the participants in our study and their previous relationships with the previous groupmates who they were seeing in the images. We asked them what was the social relationship like? What were the rates of negative social interactions between the participants and the previous groupmates? What were the rates of positive social interactions and what was the relative dominance between the participants in our study and the previous groupmates who they were seeing on the screen? We found something fascinating here, which is that chimpanzees and bonobos looked even longer, significantly longer at images of previous groupmates with whom they had higher rates of positive social interactions or whom I might call their previous friends. So we think what we’re seeing here is there’s some distinction in their attention where they’re paying even more attention to their previous friends.
And the last thing I’ll note about this study is that we had a bonobo, Louise, who hadn’t seen her sister and her nephew for almost 27 years. Now, we couldn’t collect a ton of data from Louise. We had a few trials from Louise, so we couldn’t analyze them on their own. But what we noticed, and in our plots, when we plotted this data, we found that Louise was looking a lot longer at the images of her sister and her nephew who she hadn’t seen in 27 years. And we think that this indicates that she may have some recognition of her sister and her nephew for almost three decades, which is very similar to our own human memory and really remarkable. And I would say the longest long-term social memory ever found in an animal species besides humans.
Mills: Wow. Well, what do you think this means for how we treat animals in captivity? I mean, I don’t want to anthropomorphize this too much, but did Louise maybe miss her sister and her nephew the same way we humans would?
Lewis: It’s a great question. And I think I have to say, we don’t know. And I think this is an area that is rich for further exploration. And I hope graduate students in my lab will be able to explore this question further is what is the nature of those memories look like? Do they come with emotions that we humans feel when we have, think about previous social partners? What is the nature of those memories in terms of the content? Do they remember, for example, grooming with their previous groupmates or getting into conflicts with their previous groupmates? We don’t know that yet. It’s something that we still have to test. I would assume and hypothesize that they do remember they have a fuller, richer content in their memories of these individuals rather than lower level, just simple recognition, but it’s something that we have to explore and we’re designing studies now to try and get at that question.
Mills: It’s interesting that they could recognize former groupmates or family members from photos. I mean, a lot of animals don’t understand photos at all. You can show a picture of a dog to your dog. My dog is down here. If I showed her a picture of a dog, it’d be like, What the hell? But how is it that these great apes actually can understand what they’re seeing is another bonobo or chimp?
Lewis: Absolutely. Well, with the first eye tracking studies with great apes, a mentor and colleague and collaborator of mine, Dr. Fumihiro Kano, in his first studies with eye tracking with great apes, he found that chimpanzees and bonobos look at images of others in a similar way to humans. So they focus a lot on the eye regions and the mouth regions of the face rather than other parts of the face. And as humans, these are regions that we also focus on. So we could tell just from these, even from these first instances of eye tracking, that they’re looking at images of others in similar ways to humans. And I’ll say one more point about this that I love in Kano’s work is he found that chimpanzees pay much more attention to the mouth region while bonobos pay more attention to the eye region, like us, humans.
As humans, we pay a lot of attention to the eye region. We know that eye contact is very important. We communicate a lot with our eyes. It turns out bonobos are also really interested in looking at the eyes of other individuals. And so we think that this means that there’s something about the way they communicate. There’s something important about the eyes for them as well. So we can tell just based on them looking at images of others and tracking their eye movements as they look at these images, that they’re at least looking at these images in the same ways as humans or in very similar ways as humans. And then as we note that there are differences in their attentional patterns between different individuals based on their group membership and based on their social relationships, we can tell that they are processing these faces as similar or as a representation of an individual in real life.
Mills: So when you’re showing them pictures of other apes they would presumably recognize, what’s the expression on the face of the photograph? I mean, do you show them like a neutral face, a happy face, an angry face, a frightened face?
Lewis: So in these studies, I show them images of faces with neutral expressions so that there weren’t any sort of emotional expressions that may be biasing their attention to one image over the other. Some other researchers like Dr. Mariska Kret and others do explore how emotional expressions bias attention in great apes. Often they are interested in negative emotional expressions, but here in this study, we didn’t want any emotional expressions to bias their attention to one image over the other. And so we used all neutral expressions in these studies.
Now I’ll say that we were one thing that we’re always worried about when working with great apes, we’re very conscious of when we’re working with great apes in studies like these is we don’t want to create any adverse effects for them. We don’t want it to be harmful for them in any way. And so in this study we were showing them images of their previous groupmates. We didn’t want there to be any adverse effects or negative harmful effects. And so we watched very carefully at the beginning of this study to ensure that the participants themselves weren’t expressing any negative emotions and they weren’t. But what I found and was shocked by is that sometimes the chimpanzees and bonobos in this study, they would be drinking juice as they looked at these images. They would be drinking a bit of diluted juice to help keep their heads still in order to ensure the accuracy of our eye tracking recordings. And sometimes when they saw the images of their previous groupmates, they would stop drinking the juice, pull away from the straw and just stare with their jaws hanging open. And it was fascinating. I had never seen these behaviors before when doing eye tracking studies. And I was amazed and delighted and just praying that their eyes movements were still being recorded even though they had moved slightly—and they were.
But I was really fascinated and delighted to see, at least anecdotally when watching these participants in my study, that they were showing even some behavioral signs of recognition. Now I couldn’t, until I saw the data, I couldn’t say one way or the other whether we really could capture that recognition. But then once I saw the data, I was thrilled that we were actually capturing a difference in attention and what we think is recognition of these previous groupmates.
Mills: Now you purposely did not show them pictures of another groupmate who was maybe an enemy or somebody they just didn’t like.
Lewis: We did. We did. Exactly. Yes. So we showed them images of previous groupmates with whom they had positive social interactions and images of previous groupmates with whom they had more negative interactions. And especially with chimpanzees, but also bonobos, we know that social conflicts are very important in their societies, actually just like in ours. They resolve conflicts with aggression. They use aggression to help form and maintain their dominance hierarchy. And so I thought when designing this study that they actually may pay more attention to individuals with whom they had more negative interactions, but we found exactly the opposite. We found that both chimpanzees and bonobos were paying more attention to previous groupmates with whom they had these positive social interactions. We think that that is because positive social interactions also play a key and fundamental role in chimpanzee and Bonobo’s social lives, that their closest friends, the individuals with whom they interact with the most, these individuals they’re grooming with, they’re playing with, they’re spending the most time in proximity with these individuals, they’re spending a lot of time and a lot of quality time with their friends.
And we think that for these reasons that they’re spending a lot of time in close proximity with these individuals with whom they had positive social interactions, we think that’s actually shaping their long-term memory for these individuals and that they have some especially different recognition of these previous groupmates, of these previous friends.
Mills: Recognition between bonobos and chimpanzees, did they differ in any way when they were reacting to someone, an animal they had known?
Lewis: We didn’t find any species differences, significant species differences in this study. We found that the bonobos in our study looked slightly longer at previous groupmates as compared to chimpanzees. We think that maybe because of the institutions where these animals were tested, we tested the—bonobos that we tested had more familiarity with these eye tracking setups, with seeing images on the screen. So it may be because of their experience, they may be recognizing these individuals even more, but we didn’t find any significant species differences.
Mills: Do you get to do some of this work in the field?
Lewis: That’s a great question. So I also work with chimpanzees living at a sanctuary in Uganda. It’s called the Ngamba Island Chimpanzee Sanctuary, and that is a sanctuary that’s on an island in the middle of Lake Victoria off the coast of Entebbe. And it’s a wonderful sanctuary that rescued a lot of chimpanzees about 25 years ago who were orphaned because of deforestation and poaching. Now here on the sanctuary, what’s so special about it is that the majority of the island is lush tropical forests that chimpanzees can roam and range in throughout the day. At night, they have the option of returning to sleeping rooms where in the morning then we wake up early and do a few of these behavioral tests with them and cognitive tests with them, and then they’re let out back into the forest for the rest of the day.
So these are not wild populations of great apes. What we refer to them as semi-wild populations of great apes, that they’re ranging in a wild environment for most of the day, but they also then return to sleeping rooms built by humans, and it’s where we can also do some cognitive behavioral tests with them, of course, completely non-invasive and not harmful to the animals at all. So this is a really special place because we can actually do some eye tracking with them and other behavioral experiments with them. Whereas when working with wild populations, we now have to be incredibly thoughtful and considerate about what type of instruments, tools, objects we may be introducing into environments like these because we don’t want to disturb these wild populations too much. And so instead, we work with what we call semi-wild populations where we can get as close to wild as possible, but they are familiar with humans and human objects and different tools.
Mills: Now, are there other non-human animals that have the same recognition skill? I mean, like some pet owners think that their dogs and cats can remember people. And then you see all these videos on social media where somebody’s coming back from a long deployment in the military and they get reunited with the dog after 2 years and the dog goes crazy. Is that the same thing that you’re seeing in chimpanzees and bonobos or is that a different reaction that we’re seeing?
Lewis: Yeah. I love those videos. They make me tear up every time. So I will say yes, we know that other animals—So we know that pet dogs, some monkeys like Japanese macaques and elephants, sheep, these animals can remember faces for a few years. So maybe up to 3 years is what’s typically found in the literature. We know that orangutans also can remember faces for about 3-ish years. We know from previous research by Kenan and colleagues that bonobos can recognize the vocalizations of previous groupmates for up to 5.5 years. So they can recognize the voices of their previous groupmates for up to 5.5 years. And then I’ll say another amazing study found that dolphins can remember the vocalizations of their groupmates for up to 20 years, so a really long time. So we do know that there are some animal species and there are many animal species that can remember the faces or the vocalizations of previous groupmates for a few years.
And then we’ve only found in a few species like dolphins that they can remember vocalizations for two decades. And the last thing I’ll say is that the only other study that we know of that really has found that social relationships shape long-term memory in non-human animals is with ravens, and that was found with corvids that their social relationships do affect their long-term social memory as well. That’s the only other study besides this one where we find that social relationships are impacting social memory as they do in humans.
Mills: What made you glom onto bonobos and chimpanzees? Why not some other great apes?
Lewis: Yeah, that’s a great question. I have been studying bonobos and chimpanzees for 13 years, and I find them fascinating precisely because they are so similar to us. They are our closest living cousins, and they are so similar to us in many ways, and I think especially in their social environments. These individuals, these species can live in groups of up to 150, sometimes even 200 individuals in a single community. So they live in these large communities like we do as humans, but these communities aren’t together at all times of the day. They have what we call fission-fusion social systems. So smaller groups will break off from the community and they will go and forage on their own, or a group of male chimps may go and patrol the borders of their territory. And so they will break off, this is the fission part of their social dynamics.
And then at other times of the day, they’ll come back together, let’s say at a big fruiting tree, and especially at night, they’ll come back together for safety at night into bigger groups. And this is called the fusion part of their social system. So they have these fascinating fission fusion social dynamics, again, just like we do as humans, right? This is the same type of social dynamics that chimpanzees and bonobos experience as well, and this is why I’m so drawn to study their social psychology.
Mills: I want to talk about another one of your recent studies. You found that both human children and chimps like to watch social interactions.
Lewis: That’s right.
Mills: What else did you find in that study? Tell us about that one.
Lewis: Yeah. So this is a fun study. Now, instead of using eye tracking techniques here, I designed a new kind of paradigm or method that I call the curiosity boxes. So here what I’m studying is social curiosity, curiosity being the drive or the motivation to gain information. And so what I wanted to study here is often curiosity is studied in physical context. So curiosity about an object and what it may do, curiosity about the location of a hiding reward. But here, I wanted to study social curiosity. So the drive to gain social information. And what I was really interested in is whether children and chimpanzees demonstrate similar social curiosity or not. So I designed these curiosity boxes, which are two wooden boxes placed on tables spaced about three feet a meter apart. And inside these boxes are tablets with videos playing on them. And I designed them both for children and chimpanzees.
There is a handle on the door of the box that they could slide up and a vertically sliding door would slide up so that children and chimpanzees could look inside and watch these videos. And importantly, they had to keep holding the door up in order to keep watching the videos. As soon as they let go of the door, the door would drop down and they could no longer access these videos. And so what we think we’re measuring here is a measure of curiosity, that there’s this drive or this motivation to continue holding the door up and gaining the information inside. Now, importantly, the videos inside are videos of social interactions in one box versus non-social interactions in another box or just a single individual in the other box. So we had videos of social interactions. Children would see videos of other children interacting, chimpanzees would see videos of chimpanzees interacting, and we had them in different contexts like playing and grooming, et cetera.
We found that both children and chimpanzees looked significantly longer at the videos of the social interactions as compared to the videos of a single agent acting alone. And we think that they’re actually attentive to how much social information is in each of these videos, that a video of a single agent is just all the information you can get is just about that individual alone. Whereas with the videos of the social interactions, not only can you gain information about two different individuals, but you can also gain some information about their relationship as well. And so we think that chimpanzees and children are attentive to these differences in amount of social information and they’re drawn or they’re motivated to go and gain more social information from these social interactions.
Mills: And were these social interactions, for the most part, positive? In other words, you weren’t showing them videos of people or animals fighting with each other, were you?
Lewis: We actually were. Yes. So yes, in another study around the same time, we had positive social interactions in one box versus negative social interactions in another box. So the positive interactions were things like playing or grooming. For chimpanzees, that’s really where they’re grooming one another. For children, that’s where they’re doing each other’s hair, for example. And in the other box, we had videos of conflicts. Now, for chimpanzees, that was actual videos of conflicts, so physical, aggressive fights between two chimpanzees. Whereas for the kids, we didn’t want to show them videos of children beating each other up. And so instead we took videos from YouTube where children were, for example, playing tug of war or fighting over a toy. So they weren’t aggressive and violent videos, but there was some conflict involved. Now, interestingly, we found that chimpanzees did not show any significant differences in their curiosity for positive versus negative social interactions, but we found something fascinating with the children.
We found that at 4 years old, both girls and boys, at 4 years old, were somewhat more curious about positive social interactions. As they aged, boys and girls split in terms of their curiosity. Girls became more and more curious about positive social interactions, whereas boys became more and more curious about negative social interactions, such that by age six, we see a significant difference in their social curiosity, where boys are watching these videos of negative conflicts significantly more than our girls, and girls are watching the videos of the positive social interactions significantly more than our boys. I want to talk about one interesting piece of this finding though, which is the time at which girls and boys split. So we see the time at which they split is right around 5 years old. And this is a very important time for kids in the US at least.
This is right around the time that they’re entering school. So we think that they are now at age five, they are starting to be socialized as girls and socialized as boys more and more by their teachers, by the other adults in the schools, by their peers as well. So I think that what may be happening here, what may be driving these patterns of social curiosity is a combination of factors. I think it’s a combination of evolutionary selective pressures. It is certainly the case that boys and men had to be more attentive to conflicts and were more involved in conflicts throughout our evolutionary history, in physical conflicts throughout our evolutionary history. But also, I think that it’s also a huge factor here maybe the socialization patterns and the differences in how we socialize girls and boys. So I think it’s a combination of socialization patterns and evolutionary patterns as well that may be shaping social curiosity differently in girls versus boys.
Mills: What are the biggest challenges in working with non-human primates?
Lewis: That’s a wonderful question. And since this is a podcast all about psychology, I’ll talk a little bit about the psychology of these apes and trying to understand what they may be thinking. I think that’s the biggest challenge. Of course, they’re dangerous and so we have to be very conscious of working with them just physically because they’re- They’re very strong. They’re very strong and they can be aggressive, not always, but they can be. So that’s one challenge. But I would say the biggest challenge when trying to study the minds of these great apes is how do you get at their inner worlds, their inner workings? Now, this is a challenge that I share with developmental psychologists who work with infants. We can’t just ask infants questions and then have them answer, right? Instead, we have to get creative and come up with designs where we can ask them questions in these creative ways.
So we’re not verbally asking them questions and asking them to verbally respond, but instead we’re designing different games and different techniques and different methods in order to measure their behavior on these tasks that then where we can actually, based on their behavior, we can test some hypotheses about what may be happening in their minds. So just as with infants, we can’t just ask chimpanzees and bonobos, Hey, what do you think about this individual? Do you find this more interesting? Do you remember this guy from 10 years ago? Instead, we have to design these creative games and methodologies in order to understand and try and answer some of these questions. So it’s both the biggest challenge for me when working with great apes and also the biggest joy. I love trying to be creative when trying to understand something about the minds of our closest cousins, where we can’t just ask them a question or ask them to fill out a survey.
Mills: Right. So big picture, what does studying chimps’ and bonobos’ social cognition teach us about human abilities and how they evolved?
Lewis: Absolutely. Well, I was trained in departments that asked this question. What makes humans unique? Often the question comes from what is so special about humans? What makes us so special? I instead am much more focused on another question, a similar but different question, which is, what makes humans so similar to our great ape cousins? And I think this is the question that I continuously try to answer in my work is how similar are we? Are we curious about the same things? Does our memory last the same amount of time? Do our friendships matter in the same way to us? Do we think about individuals in the same way? And of course then from there, also what may be different, but I’m much more fascinated by the question of what makes humans so similar to our great ape cousins. And so by studying chimpanzees and bonobos and humans in comparison, I can think about the and test hypotheses about the selective pressures that have shaped our social cognition in similar or differing ways.
So we can think about the evolutionary history and the time, the 5 million years since we split, since we split from these species. And we can think about the different selective and evolutionary pressures that we’ve faced. For example, we have continued to evolve in large social groups. That means we need to be able to identify, discriminate between, recognize and remember other individuals in our groups for years. However, for example, bonobos evolved to have females be the most dominant members of their groups. So this is an evolutionary pressure that is very specific for them in terms of their social dominance and the individuals who hold the highest roles in their dominance hierarchy. For humans, that may be a little bit different, right? For chimpanzees, that’s also different. Chimpanzees are male dominant. So I like to think about what are the similarities between humans and our closest cousins, and then also what are the differences and what are the evolutionary pressures that have shaped these similarities and differences in our social psychology.
Mills: So what’s next for you? What are you working on now? What are the big questions you want to answer?
Lewis: Yes. Well, this is an extremely exciting time and chapter in my life. I’ve just started as an assistant professor at UC Santa Barbara, and I’ve just started my very own lab. It’s called The BOOM Lab, the Biological Origins of Minds Lab. And I have my first PhD student, Amanda Goguen, along with six undergraduate research assistants who are helping to start new research with chimpanzees at the Los Angeles Zoo. So we’re just getting started. We’re just getting started with some of these projects, but I’m very excited to bring some of this research to the Southern California area to start this own research with my lab and to start exploring some new aspects of social psychology, including around friendships with new mothers and their infants, and how different social relationships may affect interactions with new infants in the community.
Mills: Well, that all sounds really exciting. Well, Dr. Lewis, I want to thank you for joining me and wish you luck in your future research.
Lewis: Thank you so much for having me. It’s been such a joy to talk to you. Thank you, Kim.
Mills: You can find previous episodes of Speaking of Psychology on our website at speakingofpsychology.org or on Apple, Spotify, YouTube, or wherever you get your podcasts. If you like what you’ve heard, please follow us and leave a review. If you have comments or ideas for future episodes, you can email us at speakingofpsychology@apa.org. Speaking of Psychology is produced by Lea Winerman.
Thank you for listening. For the American Psychological Association, I’m Kim Mills.
Most of us could look at a decades-old yearbook and recognize many of the faces of our former classmates—but humans aren't the only ones with this remarkable ability. Laura Simone Lewis, PhD, of the University of California, Santa Barbara, discusses her research on the long-term social memories of chimpanzees and bonobos, including a bonobo who remembered family members after 26 years apart. She explains how these great apes navigate their social worlds and what they can teach us about the evolution of our own social cognition.
