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Orangutan mothers use loud scratches to tell their infants that it is time to leave one area and move to another, possibly to avoid attracting predators or other orangutans.

Primate experts are increasingly interested in behaviours such as scratching, self-grooming and face touching, debating whether these activities are intentional or simply due to psychological or physiological arousal.

Marlen Fröhlich at the University of Zurich in Switzerland and her colleagues noticed that wild Sumatran orangutans living in the trees of the  in Sumatra would sometimes scratch themselves in a loud and exaggerated way.

“We have found individuals in the forest just from hearing these loud scratches above our heads,” says Fröhlich.

The scratches are a “rhythmic, harsh sound” due to the leathery skin and long hair of the orangutans, she says, and are loud enough to be heard by humans at least 15 metres away in a noisy rainforest.

In contrast, the normal self-maintenance scratches involve smaller movements, happen less regularly and are less noisy.

Fröhlich and her colleagues analysed 1457 bouts of scratching produced by 17 different orangutans, including four mothers and their dependent offspring, and the behaviour that occurred before and after each.

“We found that orangutan mothers use their loud scratches to tell their infants that it is time to leave,” says Fröhlich.

These exaggerated scratches were overwhelmingly produced by mothers, shortly before moving. They were usually directed towards a dependent offspring who was paying attention to them and who responded by moving towards the mother, she says.

As a result, these loud scratches could be reliably distinguished from regular self-maintenance scratches.

Living bridges

Orangutans travel almost entirely in the trees, swinging from branch to branch and using their body weight to bend branches to bridge the gap between neighbouring trees. However, infants are too light to bend branches this way and so rely on their mothers to create “living bridges” to cross, says Fröhlich.

Orangutans aren’t very vocal towards their infants, and this probably protects them from the attentions of predators and potentially dangerous male orangutans, according to the researchers. Instead, the loud scratches may be used to communicate low urgency messages.

Adults don’t rely on each other to travel, which is likely to explain why this signalling behaviour wasn’t seen outside of mother-infant pairs, says Fröhlich.

Fröhlich says that until this behaviour is studied in other orangutan populations, it is unclear whether it is evolved or culturally learnt.

“However, it would not surprise me if they did invent such ‘signals’ from scratch, so to speak, because they are smart, have intentional communication, and can save time and energy by telling their children it is time to go,” she says.