Marmosets are fluffy, 8-inch-long monkeys native to South America. They are also very polite. New research shows that these little mammals carry on lengthy, back-and-forth discussions without interrupting one another. This is a conversation style adopted by only one other kind of primate: humans.
The Cooperative Monkey
Marmosets are a group of monkeys that include some 22 species, and they're considered to be "primitive" for a number of reasons — for example, they have claws rather than nails, and their thumbs are not opposable. But these primitive primates do seem to be quite advanced when it comes to socializing. Not only are they good listeners, but they also engage in cooperative breeding.
"Marmosets are one of the few primates where both parents will take care of the offspring," said Asif Ghazanfar, a primate neurobiologist at Princeton University. What's more, older siblings help to care for younger siblings, and sometimes other adults lend a hand, too.
This behavior may have evolved in marmosets due to various selective pressures, Ghazanfar told io9. Certain pressures, whatever they may be, caused the primates to become smaller, which may have made them more susceptible to predators. To counteract this effect, marmosets almost always give birth to twins or triplets. "But this is very different from other primate species and required them to adopt a different care strategy," Ghazanfar said. Mothers couldn't take care of the multiple babies by themselves, so the fathers and other monkeys in the group stepped in.
Marmosets are very friendly with one another and very talkative, including when they're out of sight of each other. Given these cooperative traits, which marmosets share with humans, Ghazanfar and his colleagues wondered if the monkeys also engage in vocal turn-taking (as humans also do) to enhance their cooperation.
To find out, the researchers placed dyads (pairs) of adult common marmoset monkeys (Callithrix jacchus) at opposite corners of a room, which had a visually opaque but acoustically transparent curtain splitting the room in half, diagonally. Over the course of the experiment, they paired the 10 non-related monkeys in various combinations — five cagemate pairs and 22 non-cagemate pairs.
The team recorded 1,415 "phee" calls in 54 sessions. These calls, Ghazanfar explained, are very loud, high-pitched, long-distance vocalizations that marmosets use when separated from other group members.
Ghazanfar and his colleagues discovered that the dyads wouldn't call out at the same time. Instead, the monkeys would take turns — about five seconds after one monkey called out, the other would respond. Not once did the dyads ever interrupt one another.
This vocal turn-taking is especially remarkable considering that some sessions lasted 30 to 40 minutes. Ghazanfar also noted that the phee calls between the dyads didn't always alternate one-to-one. "If I am talking to someone, I could make a couple of statements before I get a response," Ghazanfar said. "It was similar with the marmosets: They do take turns, but not every single call that one marmoset produces gets a response."
Importantly, phee calls, though simple to the human ear, contain important information about identity, gender, social group and context. The researchers suggest that the vocal turn-taking may help the animals extract this information from each other's calls. In the wild, it would be difficult to glean information from the phee calls if the marmosets were constantly interrupting each other, especially when you consider the noisy forest environment they live in.
Talking Like Humans
"It bears emphasis that marmosets are not the only example of animals that engage in vocal turn-taking," Ghazanfar said. Frogs and crickets are known to do similar things. "But the marmosets' behavior is much more human-like because they will carry out the calls with any other marmoset — it's not only for pair-bonded individuals and it's not territorial."
And when Ghazanfar and his team took a closer look at the audio spectra of the phee exchanges, they discovered that the marmoset conversations are similar to human conversations in a couple of other ways. For one, the dyads' conversations were periodically coupled, meaning that there was a predictable timing between the vocalizations. Also, the marmosets' calls were "entrained" so that when one monkey sped up or slowed down the timing of its call, the other monkey would follow.
"This suggests they are acting like coupled oscillators and making a true connection via this signal coming through the air," Ghazanfar said. Scientists have previously described human conversational turn-taking as a kind of coupled oscillation. What this means, essentially, is that cooperative conversation doesn't require the higher-order cognitive capacities seen in humans and other apes.
And since apes don't appear to use vocal turn-taking, the results also suggest that this kind of "vocal cooperative behavior" evolved as a byproduct of prosocial behaviors (specifically, cooperatively breeding), Ghazanfar said.
Using the marmosets as a model, the researchers are now interested in studying how human communication develops and potentially breaks down. Babies are not born with the ability to communicate — they adopt conversational rules through interactions with their parents. The same is true for marmosets.
"That's huge because we can now do experiments and longitudinal tracking, " Ghazanfar said. "We can see how different factors — prenatal, neuronal, parental — influence ultimate vocal behaviors."
Ghazanfar and his colleagues detailed their work in the journal Current Biology.