Marmosets eavesdrop on conversations and form judgements

In a new study from the University of Zurich, researchers report that marmoset monkeys eavesdrop on conversations among other individuals. Not only do the marmosets seem to understand these conversations, but they also appear to form judgements based on what they hear. 

It is very common for humans to observe and evaluate interactions between other people as they decide who they would interact with in the future. With animals, it is difficult to measure what they are picking up on as they eavesdrop. Even if they do understand a chat between other individuals, animals do not express their reactions in ways that can be easily observed.

A team of anthropologists at Zurich used behavioral analyses and thermal imaging to investigate the emotional states of the marmosets.

Thermal imaging provided the researchers with a noninvasive tool to measure temperature changes in the faces of marmoset monkeys to identify subtle emotional responses. 

“We were able to use this technique to show that the marmosets did not perceive the vocal interactions between conspecifics as the mere sum of the single call elements but rather perceived them holistically, as a conversation,” said study first author Rahel Brügger.

When an animal experiences an increase in emotional arousal, this corresponds with a drop in facial surface temperature – especially in the most exposed regions like the nose. Thermal imaging makes it possible to record these changes.

The researchers obtained recordings of vocal exchanges between marmosets, as well as calls of nearby animals that were not involved in the interaction. As the recordings were played from a hidden loudspeaker, the team used thermography to measure the monkeys’ reactions. 

“This showed that the response to call interactions was significantly different than the response to individual calls,” said Brügger. “Marmoset monkeys are thus able to distinguish a dialogue among conspecifics from a pure monologue.”

The researchers distinguished between the recorded interactions that were cooperative and those that were competitive.

After the marmosets heard the different interactions, they had the opportunity to approach the sources of the sounds. This experiment revealed that the monkeys preferred to approach individuals who had been involved in a cooperative interaction with a third party.

“This study adds to the growing evidence that many animals are not only passive observers of third-party interactions, but that they also interpret them,” said study co-author Professor Burkart. “In addition, our study shows that thermography can help unveil how these social interactions are perceived by nonverbal subjects.”

The research is published in the journal Science Advances.

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By Chrissy Sexton, Earth.com Staff Writer