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Two wild marmosets in the Caatinga forest in northeastern Brazil
Judith M. Burkart
By Tess Joosse
Like a curious neighbor, marmosets listen to other people’s conversations – and judge them based on what they “say”, new research shows. Pint-sized primates might use the behavior to screen out strangers, preferring to mingle with those they think will make the best nannies for their offspring.
“This study is really cool because it identifies what’s going on inside the animal” when they listen, says Sonja Koski, an evolutionary anthropologist at the University of Helsinki who was not involved in the work.
Common marmoset (Callithrix jacchus) are native to the forests of northeastern Brazil, where they scurry between the branches like squirrels, thanks to their clawed nails. They’re tiny, weigh around 250 grams, and have white ear tufts reminiscent of Albert Einstein’s untamed hair. But it’s their social structure that really sets them apart.
Extended families of up to 15 marmosets live, eat and spend time together, but only one or two pairs in each group breed. When the babies are born, the whole clan intervenes: brothers and sisters, cousins, aunts and uncles all take turns to take care of the young. It takes a village to breed a marmoset.
Because marmosets rely on others for help, they need to assess who is and is not good at cooperation, says Judith Burkart, evolutionary anthropologist at the University of Zurich (UZH). This is where eavesdropping comes in. But what exactly goes on the minds of the kids when they eavesdrop on conversations?
To find out, Burkart teamed up with evolutionary anthropologist from UZH Rahel Brügger. The duo and their colleagues placed a single marmoset in a room and played recordings of marmoset vocalizations from a hidden speaker. The chatter originated either from a positive interaction, such as an infant marmoset calling for food and an adult responding softly, or from a negative interaction, such as the adult reacting to the hungry baby with aggressive speech. As a control, the scientists played the calls of a single animal.
The researchers then pointed an infrared camera at the marmosets’ faces to record the temperature of their noses – “one of the only places on the face that isn’t covered in fur,” Brügger explains. They tested 21 marmosets out of a total of 90 sessions, looking for drops in nasal temperature, which indicates that the marmoset is alert and engaged. The animals caught fire during the combined calls but not during the individual vocalizations, indicating that they perceived them as conversations and not just noise.
After the playbacks, the scientists left the marmosets in a nearby room filled with toys and a mirror. Because primates do not recognize their own reflections, they are likely to approach a mirror and socialize with the image as if it were an unfamiliar monkey. The researchers set up the interaction so that the animals assume that the calls they just heard were from the mirrored room and the individual in the mirror reflection.
After hearing the reading of a positive interaction, the marmosets easily entered the room and ran to the mirror ready to socialize with the supposed vocalizer, the researchers say. But after the uncooperative calls, the marmosets were reluctant to approach the monkey in the mirror. They were more interested in interacting with a “stranger” who was cooperative, report researchers today in Scientific advances.
The results indicate that marmosets are not just passive observers, but make decisions about others based on what they hear – just like humans, say the researchers. The team plans to use this temperature mapping approach to investigate even more important questions about the origin of human traits like morality.
Koski is on board. Using monkeys to understand the evolution of human behavior “is based on the idea that animals understand what is going on in the interactions of others,” she says. “They really brought it out here.”
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