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Among orangutans, scratching is contagious 鈥� just as yawning is among humans. When an orangutan sees another scratch, they often start scratching themselves.

However, the behaviour differs from contagious yawning in one crucial respect. Humans catch yawns more readily from close family and friends, but the orangutans were more likely to catch scratching if they did not know the other orangutan well.

鈥淚 was doubting the results initially, but after checking everything it seemed to really be there,鈥� says Daan Lam茅ris at the University of Antwerp in Belgium.

While contagious yawning has been studied extensively as a form of empathy, contagious scratching has received little attention. A 2004 study showed Japanese macaques are susceptible, and in 2013 it emerged that rhesus macaques also do it.

Lam茅ris and his colleagues studied nine adult Bornean orangutans living in captivity in the Netherlands. They recorded instances of yawning and scratching, as well as whether the apes seemed to be relaxed or aroused 鈥� for instance, if males were aggressively displaying or charging each other. They also monitored the quality of the orangutans鈥� relationships by noting friendly behaviours like grooming.

The orangutans rarely yawned, so the team could not find evidence of contagious yawning. However, contagious scratching was evident. An orangutan would typically scratch itself within 90 seconds of seeing another scratch. They became three times more likely to scratch if another orangutan scratched first.

Scratching was more likely to be contagious if the initial scratch took place in a tense context rather than a relaxed one, and if the two orangutans had a relatively poor relationship.

This makes sense if you consider when orangutans and other primates tend to scratch, says Lam茅ris. 鈥淪cratch rates increase during arousing events,鈥� he says, such as when one animal is aggressive, or if a predator has attacked. 鈥淪cratching is often seen as an indicator of arousal within an individual, often in negative contexts.鈥�

Lam茅ris points out that aroused or stressed individuals often behave unpredictably, stressing other members of the group 鈥� especially if they are not close friends or family. 鈥淚f I would see my family and friends being really stressed, I would probably approach them and ask what鈥檚 wrong,鈥� he says. 鈥淚f it鈥檚 a complete stranger you鈥檙e maybe a bit suspicious. Why is this individual so stressed? It becomes a bit scary.鈥�

However, it鈥檚 not clear whether emotions like stress are being transferred between the orangutans with the scratching. 鈥淲e couldn鈥檛 objectively measure what the underlying emotion was in the first place, and whether this emotion was conveyed to the observer,鈥� says Lam茅ris.