As sanctuary-kept chimpanzees grow from infant to juvenile¹, they develop increased susceptibility to human yawn（哈欠） contagion², possibility due to their increasing ability to empathize（移情，神会）, says a study published October 16, 2013, in the open-access journal PLOS ONE by Elainie Madsen and colleagues from Lund University. Scientists examined the extent to which two factors affected³ chimpanzees' susceptibility to yawn contagion: their age, and their emotional closeness to the person yawning. Thirty-three orphaned⁴ chimpanzees, 12 infants 1 to 4 years old, and 21 juveniles⁵ 5 to 8 years old, were included in the trials. A trial sequence consisted of 7 five-minute sessions: a baseline session, followed by three experimental sessions, where the human repeatedly either yawned, gaped⁶ or nose-wiped, and three post-experimental sessions, where social interactions continued without the inclusion of the key behaviors. Each chimpanzee separately observed an unfamiliar⁷ human and a familiar human preforming the sequence.

 

Researchers found that yawning, but not nose-wiping, was contagious⁸ for juvenile chimpanzees, while infants found neither yawning nor nose-wiping contagious. Specifically, human yawning elicited⁹ 24 yawns from the juvenile chimpanzees and zero yawns from the infants. Chimpanzees appear to develop susceptibility to interspecies contagious yawning as they grow from infant to juvenile, possibly due to their developing ability to empathize with the person yawning. Emotional closeness with the yawning human did not affect contagion. Madsen added, "The results of the study reflect a general developmental pattern, shared by humans and other animals. Given that contagious yawning may be an empathetic response, the results can also be taken to mean that empathy develops slowly over the first years of a chimpanzee's life."

 

Aside from humans, cross-species yawn contagion and a gradual development yawn contagion, has previously¹⁰ only been demonstrated in dogs.

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