Monkeying Around

Primate study finds a scandalous similarity to humans

Do you judge strangers? This animal does, too.

Common Marmoset

As our closest living relatives, primates share a lot in common with humans. From their ability to forge social bonds to understanding video games, primates and humans are perhaps more alike than we realize. New research shows we can add one more slightly scandalous similarity to the list.

Primates eavesdrop! And they make snap judgments! Yes, really, it's not just you, Karen.

In research published in the journal Science Advances, scientists reveal marmosets are able to eavesdrop on and evaluate the conversations of other monkeys, expressing an affinity for cooperative individuals over non-cooperative monkeys.

Rahel Brügger is lead author on the study and a researcher in the anthropology department at the University of Zurich. She explains the study adds to the mounting evidence humans are not alone in how much attention we pay to others' actions.

"The study adds to the growing evidence that common marmosets and many animals are not only passive observers of third-party interactions; they are able to understand and evaluate these interactions," she tells Inverse.

A marmoset interacting playfully.

Khatawut Chaemchamras / EyeEm/EyeEm/Getty Images

Here's the background — Previous studies examined the outward behavioral responses of primates to the third-party conversations of other monkeys, but that approach has significant limitations, according to this new study.

Relying on behavioral reports doesn't provide the kind of "insider perspective" that gives insight into the mind of the monkey — in other words, how the animals are processing the conversations in their heads.

But thermography — also known as heat mapping — can help scientists understand how animal subjects are feeling about certain events, based on their bodily responses.

In this study, researchers utilized a combination of thermography and behavioral responses to understand how primates interpreted strangers' conversations.

"We used thermography to get a glimpse into the minds of the marmosets," Brügger explains.

How they did it — Brügger's team focused on 21 monkeys, including both breeding monkeys and non-breeding (helper) monkeys of both sexes.

In phase A, the researchers played audio recordings of six different monkey "conversations" — both cooperative and non-cooperative — which could provoke an emotional response in the listening monkeys. These calls, made by unrelated monkeys to those used in the study, included everything from infants crying for food to adults chattering.

A figure from the study illustrating the setup of the experiment.

"We simulated interactions between [marmosets] with playbacks that used calls from the natural repertoire of the common marmosets to simulate cooperative or non-cooperative interactions between strangers and strangers being alone," Brügger says.

At the same time, the researchers observed the monkeys on an infrared heat camera.

Importantly, the researchers in this study did not intervene in the monkey's socialization, reducing the possibility of bias or interference from human third parties in their results. This breaks from past research, which has typically used humans as an integral part of the experiment.

"The crucial difference to previous studies on social evaluation in different animals is that in our experiment, we did not use human actors," Brügger says.

In phase B, the researchers opened the doors to a test area where they broadcast the audio recordings to the monkey listeners. Using a carefully-placed mirror and set design to rival a low-budget Hollywood movie, the researchers made it appear as if there were another marmoset monkey in the test area.

The researchers gave the monkey subjects the option to venture into the test area, or return to their home enclosure, following the playback of the different social interactions.

What's new — In Phase A, the researchers found some intriguing results in how the marmosets' body temperatures fluctuated in response to the calls.

According to the study, marmosets, like humans, experience heightened sensitivity in the nasal region, correlated with sexual arousal. So the researchers paid careful attention to temperature changes in this region.

The monkeys' nasal temperature significantly decreased or increased in response to the audio recordings collectively, suggesting the monkeys perceived the calls as part of a conversation, rather than as a set of individual calls.

With that understanding, the researchers were better able to validate the results gathered in Phase B.

In this phase, the researchers found male and female breeding monkeys reacted to three out of five of the types of audio playbacks by displaying significant temperature changes.

Specifically, the monkeys entered the test compartment after hearing playbacks of positive, cooperative conversations, indicating that the monkeys wanted to interact with these cooperative monkeys, even though they were complete strangers.

But the converse wasn't necessarily true. The monkeys didn't seem to want to punish non-cooperative marmosets, according to the study, suggesting marmosets may hold back their social judgment depending on the situation.

"In the context of cooperation, where socially evaluating other individuals is much less costly compared to the context of fighting or mating, the social evaluation does not necessarily lead to punishment of non-cooperators and appears to be used flexibly," Brügger explains.

A baby marmoset with an open mouth. The researchers played calls of juvenile marmosets crying in their experiment.


Digging into the details — Interestingly, there were some sex-specific differences in how the monkeys' responded, as well as differences between breeding and helper monkeys.

  • Female helper monkeys had the strongest temperature drops in response to calls, indicating a desire to mate.
  • Male helpers, meanwhile, "showed strong increases in nasal temperature, indicating a decrease of arousal, and thus relaxation," according to the study.

Breeder monkeys did not show very significant temperature drops, on average. But there were some curious differences:

  • Female breeder monkeys — which are "very food motivated," per the study — did experience changes in arousal levels after food calls.
  • Male breeder monkeys showed greater arousal when hearing negative interactions between a juvenile monkey and an adult monkey.

This last result speaks to the fact male breeder monkeys are typically the caregiving members in the marmoset family. The negative audio calls may have triggered a male breeder monkey's more protective, fatherly, "fight or flight" response.

Why it matters — Overall, marmosets have keen skills of social perception, the study suggests. They may use these skills to identify certain positive characteristics about future mates — including cooperation.

"[Marmosets] are able to understand and evaluate these interactions, and this very likely helps them to decide which future interaction partners to choose," Brügger says.

Importantly, marmosets are remarkably similar to humans in their social behaviors. Marmosets, like humans, rely extensively on social cooperation, so being able to identify cooperative characteristics in their fellows is crucial to the survival of their species.

Human infants as young as three months old can distinguish between anti-social and positive behavior, and express a preference for strangers who cooperate in social interactions, according to the study.

This research suggests a similar process of socialization occurs in the primates, establishing close evolutionary ties between humans and our primate kin.

What's next — The study's unique blend of thermography and behavioral research paves a way for future research tracking primate socialization.

"The study also highlights that non-invasive methods able to track arousal levels such as thermography can help unveil how monkeys perceive their conspecifics," Brügger adds.

Future studies would benefit from a larger sample size of breeding males and females to "systematically address sex differences in the breeders," this study's authors say.

But for now, we can take heart in knowing primates and humans share a similar sense of gossipy social judgment when it comes to eavesdropping on strangers' conversations.

Abstract: What information animals derive from eavesdropping on interactions between conspecifics, and whether they assign value to it, is difficult to assess because overt behavioral reactions are often lacking. An inside perspective of how observers perceive and process such interactions is thus paramount. Here, we investigate what happens in the mind of marmoset monkeys when they hear playbacks of positive or negative third-party vocal interactions, by combining thermography to assess physiological reactions and behavioral preference measures. The physiological reactions show that playbacks were perceived and processed holistically as interactions rather than as the sum of the separate elements. Subsequently, the animals preferred those individuals who had been simulated to engage in positive, cooperative vocal interactions during the playbacks. By using thermography to disentangle the mechanics of marmoset sociality, we thus find that marmosets eavesdrop on and socially evaluate vocal exchanges and use this information to distinguish between cooperative and non-cooperative conspecifics.
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