(STOCKHOLM-AFP) - Human yawns are contagious for chimpanzees but, like children, this only happens among apes that have grown beyond infancy, scientists said Thursday.

Playing with a researcher, orphaned young chimps aged between five and eight years began to yawn after their human chum did so, investigators at Sweden's Lund University found.

Infant apes, though, seemed immune to yawn contagion.

Humans behave in a simAping ilar way, with contagious yawning starting when they are around four -- about the same age when behaviours emerge that suggest a capacity for empathy.

Humans are also likelier to yawn when it is a friend or relative who does it, rather than someone they do not know.

Previous studies have found that dogs, too, are susceptible to yawn contagion from their owners, but less so from strangers.

But this may not hold true for chimpanzees, the new experiments suggest.

"It is possible that younger chimpanzees switch from a generalised empathy to all individuals to a more targeted empathy as they mature into adults," said researcher Elainie Madsen.

The study took place at the Tacugama Chimpanzee Sanctuary in the west African state of Sierra Leone, involving 33 orphaned apes aged between 13 months and eight years.