Contagious yawning may act as a kind of social glue that strengthens emotional bonds, research suggests.
Scientists studying bonobos, a chimpanzee subspecies with human-like behaviour traits, 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 social aspect of yawning is well known, its origins and significance are still unclear.
The new 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.
Authors Dr Elisabetta Palagi, from the University of Pisa in Italy, and colleagues wrote: "The importance of social bonds in shaping this phenomenon in bonobos suggests that a basic form of empathy may play a role in modulating yawning behaviour."