Contagious yawning may act as a kind of social glue that strengthens emotional bonds, research suggests.
Scientists studying bonobos, a chimpanzee subspecies, found yawning was more catching in animals that were relatives or friends.
A similar effect is seen in humans, with yawns less likely to be triggered by strangers than people we are close to. Although this aspect of yawning is well known, its origins and significance are still unclear.
The research indicates that, even before the appearance of humans, yawn-catching evolved as a form of emotional communication. The findings are published in the online journal Public Library of Science ONE.