The ontogeny of nest-building behaviour in Sumatran orang-utans, Pongo abelii

Nest building is an important subsistence behaviour that young great apes must learn to become competent adults. Orang-utans show a remarkable degree of variability and selectivity for a broad range of features in their nest building. However, the details of when different aspects of nest-building skills emerge remain unclear. We used data on 27 immature Sumatran orang-utans and 20 mothers collected over a decade at Suaq Balimbing, Sumatra to investigate when immatures develop their nest-building skills and examine when nest tree species preferences emerge. We found that young orang-utans showed interest in nest building from 6 months of age and begin to construct day nests at around 1year of age, whereas night nests were not practised until close to the third year of life. Nest-building practice peaked around age 3–4years and then steadily decreased as immatures approached the age of nutritional independence, around age 8years. By then, immature orang-utans were competent nest-builders, but their nests differed from adult nests in several aspects, such as fewer multitree nests and additional comfort elements, which seemed to be mastered later in development. All age classes demonstrated stronger selectivity towards tree species used for night nests and immatures eventually had similar preferences to mothers. We conclude that the ontogeny of nest-building behaviour and the selection of nest tree species in Sumatran orang-utans is a multiyear learning process that requires intense practice.