Baby common marmosets, small primates found in the forests of northeastern Brazil, must learn to take turns when calling, just as human infants learn not to interrupt. Even though the marmosets (Callithrix jacchus) don’t have language, they do exchange calls. And the discovery that a young marmoset (as in the photo above) learns to wait for another marmoset to finish its call before uttering its own sound may help us better understand the origins of human language, say scientists online today in the Proceedings of the Royal Society B. No primate, other than humans, is a vocal learner, with the ability to hear a sound and imitate it—a talent considered essential to speech. But the marmoset researchers say that primates still exchange calls in a manner reminiscent of having a conversation because they wait for another to finish calling before vocalizing—and that this ability is often overlooked in discussions about the evolution of language. If this skill is learned, it would be even more similar to that of humans, because human babies learn to do this while babbling with their mothers. In a lab, the researchers recorded the calls of a marmoset youngster from age 4 months to 12 months and those of its mother or father while they were separated by a dark curtain. In adult exchanges, a marmoset makes a high-pitched contact call (listen to a recording here), and its fellow responds within 10 seconds. The study showed that the youngster’s responses varied depending on who was calling to them. They were less likely to interrupt their mothers, but not their dads—and both mothers and fathers would give the kids the “silent treatment” if they were interrupted. Thus, the youngster learns the first rule of polite conversation: Don’t interrupt!