No one expects a human infant to slide into the world with a good grasp of grammar. Marmosets, another kind of chatty primate, are also poor conversationalists when they're young. But their parents seem to teach them how it's done. Young marmosets learn the cardinal rule of having a conversation: don't interrupt. And if they mess up, their parents give them the silent treatment. Common marmosets (Callithrix jacchus) live in large family groups in the forests of Brazil. "Because marmosets live in dense forests and are very small, it is difficult for them to maintain visual contact," says Cory Miller, a psychologist and neuroscientist at the University of California, San Diego. So the little monkeys call to each other often, using a variety of yelps, trills, and whistles. "These vocal exchanges are essentially social interactions," Miller says. So there are rules about how they should go, just as there are unwritten rules in human social interaction—say please and thank you, don't stand too close, use your indoor voice. When adult marmosets "converse," they take turns and don't talk over each other. To find out how marmosets learn the rules, Miller and his coauthors studied two captive marmoset families, including four parents and ten juveniles. (These were really five pairs of siblings, since most marmosets are born as twins.) Twice a month while the young monkeys were between four and twelve months old, the researchers brought them to a room with recording equipment. A juvenile sat on one side of the room while a sibling or one of its parents sat on the other side. In between the monkeys was a cloth divider so they couldn't see each other, as in a dense forest. The the researchers recorded the calls between the two marmosets for the next 30 minutes. In this setting, Miller says, the most common vocalization marmosets make is a simple contact call. Scientists call it a "phee," which is pretty much what it sounds like. The call tells other monkeys "I'm over here!" while also conveying information about the caller's identity, sex, and so on. https://soundcloud.com/epreston8/common-marmoset-phee-call Young marmosets were prone to two kinds of errors: making a non-phee call, and interrupting. It wasn't until they were eight or nine months old that they learned to take turns when calling, and to always make the right kind of vocalization. The monkeys learned from their parents' feedback. If a juvenile started to make the wrong call type, its parent might interrupt it. And if a juvenile interrupted, its parent was likely to stop responding altogether. "Parents effectively ignored offspring following interruptions," the authors write. Miller thinks these "corrective measures" from marmoset parents help their offspring learn. And that might teach humans, in turn, a little more about the evolution of language. "If marmoset parents do guide the development of turn-taking, it would suggest that this ability may have evolved early in our primate ancestry," Miller says. Having conversations without interrupting could be a rule we learned way back in our evolution. When people forget that rule, maybe we can correct them using a marmoset-style silent treatment.
Image: by Francesco Veronesi (via Flickr). Audio courtesy of Cory Miller.
Chow, C., Mitchell, J., & Miller, C. (2015). Vocal turn-taking in a non-human primate is learned during ontogeny Proceedings of the Royal Society B: Biological Sciences, 282 (1807), 20150069-20150069 DOI: 10.1098/rspb.2015.0069