Scratches signal to youngsters that they need to get a move on.

When it’s time to get moving, mothers can be very no-nonsense. The same is true of orangutan mothers: when ready to relocate, they vigorously scratch themselves to tell their infants to follow.

Scientists already knew that some great apes scratch their shoulders to invite their offspring to climb aboard or travel with them. To explore whether Sumatran orangutans (Pongo abelii) do the same, Marlen Fröhlich at the University of Zurich in Switzerland and her colleagues analysed more than 40 hours of video footage of 17 wild orangutans.

For each of almost 1,500 scratching bouts, the researchers assessed features including the duration of the scratch and surrounding orangutans’ responses. Scratching bouts were so loud that a person could hear them at a distance of 15 metres in the noisy rainforest.

Loud scratches that preceded movement to a new place tended to be longer and more common among mothers than among males or adolescent females. And when mothers scratched themselves, their infants were more likely than unrelated orangutans to come closer or follow.