Like humans, marmosets – tiny monkeys with Einstein-like ear tufts native to Brazil – eavesdrop on conversations between others, and prefer to approach individuals they view positively, a study in the journal Science Advances showed Wednesday.
While behavioral research has built up knowledge around the social lives of primates, it has tended to lack reliable ways to determine an individual's "inside perspective," or the inner workings of her or his mind.
Marmosets are an ideal species to study because of their close-knit social structure: They live in highly cooperative groups of around 15 family members, with the entire extended clan responsible for rearing children.
How do they decide who is reliable and who is not?
A team led by Rahel Brugger at the University of Zurich (UZH) presented 21 captive-born adult marmosets with recordings from a hidden speaker of an opposite sex adult making either food-offering calls or aggressive chatter calls in response to begging infants.
As a control, they also played the marmosets calls made by a single individual.
The scientists then pointed infrared cameras at the marmosets' faces to record the nasal temperatures – looking for drops that indicate the monkeys were alert and engaged.
The tests found the marmosets only responded to combined and not individual calls, indicating they understood when real conversations were occurring.
After playing them the recordings, the team let the marmosets enter a room filled with toys and a mirror.
Marmosets don't recognize their own reflection, and so believed that it represented the monkey who made the recorded call.
The researchers found that overall, the marmosets preferred to approach when the recordings indicated the individual was helpful.
"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 the paper's senior author and professor of anthropology at UZH, Judith Burkart.
The team plans to use this temperature-mapping approach for future investigations, such as into the origin of morality.