Lessons From Squirrel Monkeys

17:32 minutes

very cute squirrel monkey holding baby at her waist while sitting in tree
An adult female squirrel monkey with her infant. Credit: Anita Stone

Squirrel monkeys have big brains for their size, they’re chatterboxes, and they’ve even been to space. There may even be parallels between squirrel monkey communication and the evolution of human language, says primatologist Anita Stone, professor at California Lutheran University. In this segment, she joins Ira to translate the culture of our primate cousins, and talks about what they can teach us about ourselves. (Plus, we’ll hear about a couple of the monkeys’ incredibly strange habits.) Thanks to Dr. Michelle Mulholland, who recorded the squirrel monkey sounds heard in this episode at the captive colony of the MD Anderson Cancer Center, Texas.

[How do bumblebee colonies crown the next queen?]

squirrel monkey sitting alne surrounded by leaves
An adult female squirrel monkey. Credit: Anita Stone

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Anita Stone

Anita Stone is an assistant professor of biology at California Lutheran University in Thousand Oaks, California.

Segment Transcript

IRA FLATOW: This is Science Friday. I’m Ira Flatow. In May of 1959, a small squirrel monkey, from the jungles of Peru, sat atop an American Jupiter rocket awaiting her blastoff into space. Miss Baker was her name. And after nine minutes of weightlessness, she fell back to Earth where she became an instant celebrity inundated with letters from schoolchildren and even mobbed at press conferences.

One of the first two American animals successfully launched into and returned from space, Miss Baker’s flight was a study in how we humans might fare in orbit, in space. But there’s a lot to learn by studying these small cousins of ours right here on Earth– analyzing their communications, the structures of their societies, and the brainy ways that they navigate their world as my next guest does. Anita Stone, assistant professor of biology at California Lutheran University here in Thousand Oaks. Welcome to Science Friday.

ANITA STONE: Thank you. It’s great to be here.

IRA FLATOW: I remember that squirrel monkey from my childhood as one of the first animals that went into space.


IRA FLATOW: Well, what got you fascinated with studying squirrel monkeys?

ANITA STONE: So I’m from Brazil. And I went to grad school here in the States. And I knew that I wanted to work in the Brazilian Amazon for my research. And I started reading about squirrel monkeys and became fascinated with them. And then I saw them in the wild and it was just a perfect combination to look at the questions that I was interested in. So off I went to Amazon in Brazil for 12 months.

And I thought it would just be one project, one season. But the more time I spent there, the more I became fascinated with all kinds of questions about them. And I couldn’t find a lot of answers in the literature because there really aren’t that many people studying them in the wild. I kind of know why now after so many years. They’re a little bit of a pain to study. They’re very challenging. But yeah, that’s how it all started.

IRA FLATOW: Wow. So how many do you study? Is there a group, a number you study?

ANITA STONE: Yeah. So the groups are very large. My groups have 30 to 50 animals. So I monitor three groups regularly.

IRA FLATOW: And do they each have personalities of their own? Do you give them names? You keep track of them?

ANITA STONE: Some. So one of the challenges of working with squirrel monkeys is that because they’re so small– and by small, we’re talking a kilogram, max, so that’s about two pounds. There’s 50 of them. And they move quite fast. They’re pretty active– so it’s not that easy to individually identify them, particularly the juveniles, because they look very similar. But there are some adults that stand out, if they have a certain more dominant or forceful personality, yeah, they do stand out.

IRA FLATOW: OK. Give me a typical day in the life of a squirrel monkey.

ANITA STONE: OK. So squirrel monkeys wake up really early. Usually, by 5:00 AM, they’re already moving out of their sleeping tree. And they spend a lot of time active all day. They’re nonstop. They eat fruit and they eat mostly insects. And so they spend a lot of time foraging in the morning for insects.

But, really, that goes on all day. When it gets a little hotter in the day, they tend to rest. But rest, for a group of squirrel monkeys, is about 10 minutes. And then they keep on foraging and they don’t go to sleep. We follow them pretty consistently, and they don’t usually go to their sleeping tree until about 7:00.

IRA FLATOW: So they’re pretty active all day, 12 hours. I hear that they have a strange way of eating grasshoppers.

ANITA STONE: Yes. So grasshoppers are one of their favorite items, so to speak. And they catch these pretty large grasshoppers sometimes. And the way they do it, the way they handle it once they grab it, is they hold it by the bottom with the head up. And then they bite off the head kind of like an ice cream cone. And then they slurp the insides. And then finally, they’ll eat the legs and the wings sometimes. And they seem very happy while they’re doing it.

IRA FLATOW: Wow. And I understand they’re not afraid to attack a wasp nest.


IRA FLATOW: Wow. How do they do that?

ANITA STONE: Yeah. So I’ve only seen adults do it, but I’ve actually seen it quite a bit. So what they do is they will approach a wasp’s nest, a particular species, and they will knock it down very quickly to the ground. So these are nests that hang pretty low. And once the adult wasps desert the nest, the adult squirrel monkey will basically move in and eat all the larvae that’s left abandoned in the nest.


ANITA STONE: So they’re pretty brave.

IRA FLATOW: They’re pretty brave and patient.

ANITA STONE: And patient, yes.

IRA FLATOW: Those wasps are not leaving so fast.

ANITA STONE: Right. Yeah.

IRA FLATOW: I’ve done this myself. We have a clip I’d like to play– and then maybe you can translate for us what’s going on here– of the spider monkeys. Let’s hear that clip.


Squirrel monkeys. I said spider monkeys.

ANITA STONE: Everybody does that.

IRA FLATOW: Everybody does that, right? Yeah. OK. Tell us what we’re listening to.

ANITA STONE: OK. So this was recorded in captivity. And what it sounds like, to me, is that these are contract calls. But these are made by animals that are not very close by. These are longer contact calls. So they’re basically talking to each other from not very close spatial proximity.

IRA FLATOW: Can you actually tell the individual monkeys in there? Can you recognize their own?

ANITA STONE: I can recognize, yes, the sender and the receiver. But when there’s 10 of them in a tree, it’s hard to figure out exactly who’s making the vocalization.

IRA FLATOW: We have another clip that sounds a little bit more, how shall I put it, contentious.


ANITA STONE: OK. So that’s definitely a little squabble. It doesn’t sound like a serious fight, to me, like something we’d hear in the wild. My guess is that there’s an adult male, at least, in there making one of the noises. And my guess is that it’s a quick fight over some food item.

IRA FLATOW: You know, what’s interesting, as I was listening to that, it just struck me that when we see movies of jungles, that’s the sound that we hear. Is that right? Is those squirrel monkeys that we’re listening to?

ANITA STONE: It might be birds, actually, because squirrel monkeys sound a lot like birds when you’re in the forest. That’s what my students usually notice is that, isn’t that a bird? And you have to listen really closely to be able to identify that it’s a group of squirrel monkeys instead of birds.

IRA FLATOW: How much time has it taken you to know the difference and to study the sounds that they make?

ANITA STONE: So it’s not something that I personally study. But just from studying there for 18 years, from being with the monkeys, you just get used to it. It probably took me the whole 12 months to figure out what the different vocalizations were. But my field assistant, who’s a local field assistant, he’s the monkey whisperer. And he knows everything about squirrel monkeys. And so he can tell right away if it’s a bird or a troop of monkeys.

IRA FLATOW: How long has he been doing that for? A little longer?

ANITA STONE: He started a couple of years before I got there. So he’s been doing it for about 20 years.

IRA FLATOW: The monkey whisperer.


IRA FLATOW: Wow. Do you call him that?

ANITA STONE: Yeah, sometimes.

IRA FLATOW: Does he like it?

ANITA STONE: He loves it.

IRA FLATOW: Is there anything that we can learn about human language, or evolution of our language, from studying the squirrel monkeys?

ANITA STONE: Yeah. So it’s interesting that you bring that up, because there’s a hypothesis that was proposed about 20 years ago called the vocal grooming hypothesis. And the idea here is that you can try to understand the evolution of human language. So the idea is that humans started living in social groups that were increasingly large. And so manual grooming, the kind we’ve seen baboons and macaques, became less feasible really.

And so the idea’s that humans would have invented an alternative form of grooming, vocal grooming, so it would be a way to maintain contact in social bonds with their friends. And that would leave the hands also free for other tasks. And the idea is that that eventually would have evolved into human language.

Now squirrel monkeys, for living in groups so large, they’re actually pretty asocial. They don’t groom. We expect primates all to do that. But they don’t actually groom each other. They, too, live in social groups that are large. They don’t groom. And they talk all the time. They’re constantly chattering.

It’s nonstop sound when they’re foraging, when they’re traveling, when they’re just resting. And so I wouldn’t be surprised if one of the ways that they maintain their social connections is actually through vocalizations. In some ways, you could say that it’s not that different from keeping in touch with your friends by texting when we’re far away from our friends, that’s what we do all the time is talk.

IRA FLATOW: So instead of the grooming, they’re bonding, or talking, by actually talking. They’re communicating by talking instead of bonding that way.

ANITA STONE: That is my guess,

IRA FLATOW: And we might have done that ourselves–


IRA FLATOW: –evolutionary.

ANITA STONE: Yeah. It hasn’t been studied in squirrel monkeys, but I think that’s a pretty good guess.

IRA FLATOW: Oh, well, there’s something for you to do next.

ANITA STONE: Yeah, really.

IRA FLATOW: Study those squirrel monkeys. Are they sort of the pacifist compared to some of the more litigious primates, like chimps, who are actually out there being aggressive all the time?

ANITA STONE: They’re definitely not like chimps. They’re like chimps only in the sense that they have really large brains. But in terms of aggression, they don’t defend territory. So the groups are large. They don’t defend territories. So they can be overlap in areas, home ranges, for two or three groups.

They don’t like to meet very much. But when they meet, they pretty much just go their separate ways. There’s not a ton of aggression in the group, except during the mating season between adult males. Then things can get pretty ugly.

IRA FLATOW: Well, speaking of that, you bring up a good point because squirrel monkeys, their society’s female dominated.

ANITA STONE: Correct. For this species, yes.

IRA FLATOW: For this species.


IRA FLATOW: Does that have anything to do with the fact that they’re not that aggressive?

ANITA STONE: Possibly. So for female-dominated species, we mostly see that in lemurs really. But for anthropoid primates, that’s pretty rare really. The one thing about squirrel monkeys is that they have very interspecific differences in their life histories.

And all species have a very high degree of maternal investment. But it’s interesting, to me, that the two species where we see female dominance, the females nurse the infants for a very long time. So possibly they getting priority of access to resources, that might be something that’s going on.

IRA FLATOW: So with the domination of females in these species, are the men pulled back? Do they know their place a little bit more?

ANITA STONE: Yeah. That’s exactly how I would describe it. There’s not that much aggression from females to males. But the males know their place. The center of the groups is formed by adult females and their juveniles. And the males, except for the mating season, they pretty much stay on the periphery of the group minding their own business. And if they try to get too close to the infants and juveniles, the females are not happy.

IRA FLATOW: I’m Ira Flatow. This is Science Friday from WNYC Studios talking with Anita Stone about the squirrel monkeys. So any idea why society would evolve one way, or the other, to have one gender dominate versus the other?

ANITA STONE: Well, we know that for most species, females invest a lot more in reproduction than males do. In primates, you have gestation, lactation. You have the difference between sperm and eggs. So from studying primates where there is female dominance, we know that sometimes they live in very harsh environments.

And so females having that– this is true in lemurs, where the females have priority of access to resource, may have something to do about where they live and how the resources are distributed. In squirrel monkeys, we’re not exactly sure, but of the seven species, there’s only two that we know of that are female dominant.

IRA FLATOW: Let me ask you about something I learned is that the squirrel monkeys have something called– we might consider it a strange behavior, but for them– it’s called urine washing.

ANITA STONE: Yes. So this became a side project in my research group because a couple of students got really interested in it. So there’s this odd behavior that’s done. It’s not just squirrel monkeys, but capuchins do it too, where an individual will urinate a few drops onto the palms of their hands and rub it on the sole of the foot and, sometimes, repeat it on the other side. It doesn’t take very long. It’s pretty quick.

And we don’t really have a good idea about why this happens. If you look at the literature, most studies were done with capuchins in captivity. And different studies points in all different directions. Some things that have been proposed are that it could be a form of sexual communication between adult males and females, even thermal regulation as a way to cool down, because squirrel monkeys don’t sweat.

But what we found, in our population, is that juveniles do it much more than adults. And so that really almost rules out the sexual communication idea because juveniles are doing it more. But it also seems to be linked to stressful situations, so fights in the group, or some sort of turmoil. And so one thing that had been suggested, which I think our data support, is that it’s some form of anxiety displacement, like a stress reliever, much like wringing your hands when you’re nervous or something like that. But we definitely need to dig deeper into that.

IRA FLATOW: So it’s not like a territory marking–


IRA FLATOW: –thing that just the other animals do, right?

ANITA STONE: So that actually has been looked at, a long time ago, in a semi-captive population that it’s not territory marking. It’s not a way for one individual to know where they’re supposed to go to follow the group. That doesn’t seem to be the function of it. But it could be that it has multiple functions. It could mean something for adults and something else for juveniles.

IRA FLATOW: You know, we’ve seen issues where animal cultures, we’re learning more, are passing their culture down from one to another. Do you see that in squirrel monkeys at all?

ANITA STONE: So I’ve never actually seen a squirrel monkey do what we would consider tool use in the wild. I did hear about something that sounded kind of like it in captivity. So in terms of passing things down, I would think more that the constant proximity of the females with the juveniles, and their constant communication, I don’t know what they’re talking about, but the wasp’s nest foraging could very well be something the juveniles learn by observing the adults.

IRA FLATOW: That’s terrific. I want to thank you both for taking time– thank you for taking time to be with us today. Anita Stone, assistant professor of biology, California Lutheran University here in Thousand Oaks. Fascinating stuff. Well, hope you get back in the jungle soon.

ANITA STONE: Thank you. It was great to be here.

IRA FLATOW: We’re going to take a break. When we come back, we’re going to talk about salmon swimming hundreds of miles to spawn and the magnetic sense of direction they have. Stay with us, we’ll be right back after this break.

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