Sumatran orang-utans are more sociable than their Bornean relatives, due in part to the mast fruiting of the fig trees, where large groups come together to feed (7). Orang-utans are long-lived and females tend to only give birth after they reach 15 years of age (2). The infant spends its first two to three years being carried constantly and will still remain close to the mother for at least another three years (7). The interval between births is the longest for any mammal and may be as long as eight years (4). Orang-utans move slowly through the trees, and will sway trees in order to cross larger gaps (7). Nights are spent in nests built high up in the canopy, constructed from branches and leaves (6). Because of increased availability, the diet of Sumatran orang-utans has a higher percentage of pulpy fruit and figs compared to that of Bornean orang-utans (7). Orang-utans are highly intelligent and some populations in Sumatra have learnt to use tools, passing this knowledge on through generations. Sticks are used to probe for termites in termite mounds or to extricate seeds from the large Neesia fruit, which has stinging hairs (2).