Like a nosy neighbor, marmosets eavesdrop on the conversations of others—and judge them based on what they “say,” new research finds. The pint-size primates might be using the behavior to screen strangers, preferring to mingle with those they feel will make the best nannies for their offspring.
“This study is really cool because it pinpoints what’s happening inside the animal” when they eavesdrop, says Sonja Koski, an evolutionary anthropologist at the University of Helsinki who was not involved with the work.
Common marmosets (Callithrix jacchus) are native to the forests of northeastern Brazil, where they scurry between branches like squirrels, thanks to their clawed fingernails. They’re tiny, weighing about 250 grams, and have white ear tufts that evoke the untamed hair of Albert Einstein. But it’s their social structure that really sets them apart.
Extended families of up to 15 marmosets live, eat, and hang out with each other, but only one or two pairs within each group breed. When babies are born, the whole clan pitches in: Siblings, cousins, aunts, and uncles all take turns caring for the young. It takes a village to raise a marmoset.
Because marmosets rely on others for help, they must evaluate who is or isn’t good at cooperation, says Judith Burkart, an evolutionary anthropologist at the University of Zurich (UZH). That’s where the eavesdropping comes in. But what exactly is going on in the marmoset mind when they spy on the conversations?
To find out, Burkart teamed up with UZH evolutionary anthropologist 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 was either from a positive interaction, like an infant marmoset calling for food and an adult responding gently, or a negative one, like the adult reacting to the hungry baby with aggressive talkback. As a control, the scientists played calls from a single animal.
The researchers then pointed an infrared camera at the faces of the marmosets to record the temperature of their noses—“one of the only places on the face that is not covered by fur,” Brügger says. They tested 21 marmosets over 90 total sessions, looking for drops in nasal temperature, which indicate the marmoset is alert and engaged. The animals got fired up during the combined calls but not during the individual vocalizations, indicating they perceived them as conversations and not just noise.
After the playbacks, the scientists let the marmosets into an adjoining room stocked with toys and a mirror. Because the primates don’t recognize their own reflections, they’re likely to approach a mirror and socialize with the image like it’s an unknown monkey. The researchers set up the interaction so the animals would assume the calls they just heard were coming from the mirrored room—and from the individual in the mirror’s reflection.
After hearing the playback of a positive interaction, the marmosets readily entered the room and ran up to the mirror ready to socialize with the supposed vocalizer, the researchers say. But after the uncooperative calls, the marmosets were hesitant to approach the monkey in the mirror. They were more interested in interacting with a “stranger” who was cooperative, the researchers report today in Science Advances.
The findings indicate marmosets aren’t just passive observers, but make decisions about others based on what they hear—just like humans, the researchers say. The team plans to use this temperature-mapping approach to investigate even bigger questions about the origin of human traits like morality.
Koski is on board. Using monkeys to understand the evolution of human behaviors “relies on the idea that animals understand what’s happening in others’ interactions,” she says. “They have really pinpointed that here.”
sciencemag.org, 3 February 2021