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The tale of the sleeping tree

Pohon tidur – ‘sleeping tree’.
Even the English name has a certain poetry to it.
Rather than a fairy-tale tree that sleeps, this refers to the tree that gibbons and many other species of primate choose to use each night as their bed.
Can you imagine sleeping in a new bed everynight? When I stay over at a hotel, I know that I usually find it difficult to get a good night’s sleep in an unfamiliar room. But for orangutans and gibbons, this is just a part of daily life.
Orangutans build a new nest to sleep in each night, rarely ever re-using a nest. 
Southern Bornean gibbons move between sleeping trees, resting by lying or sitting on branches that are large enough to take their weight. Sometimes they may re-use a sleeping tree, but very often they will bed down on the branches of a completely different tree from the one they used the night before. This is probably because both their day’s travels in search of food bring them to a different end point each night and to eliminate the risk of parasites setting up home in permanent sleeping locations.
Two new papers (one on gibbons and one on orangutans) based on data collected at our Sabangau site reveal some of the preferences that gibbons and orangutans have for different kinds of trees for sleeping in.
It has previously been suggested that gibbons’ tendency to sleep much higher up than orangutans is because of their smaller size, which means they are more concerned with keeping themselves safe from predation.
For gibbons, OuTrop’s new data suggest that choosing a tree that minimises the risk of predation is a factor. Trees they selected were larger and more exposed – with fewer trees of similar heights surrounding them – and had fewer lianas, possibly to make sure that predators could not access them.
Gibbons also seem to be actively avoiding trees with food in. This could be a strategy to make sure their sleeping is not disturbed by animals which are awake for longer and still feeding, such as orangutans and civets.
This could also explain why they avoid resting during the day and, here at Sabangau, enter the sleeping tree earlier than gibbons at other sites.
In a related study on orangutans, 120 nest trees were measured between July and September 2008. Understanding nest selection criteria is important for assessing the overall impact that human disturbance to the forests has on these apes.
No evidence was unearthed suggesting selection of nest trees based on avoiding predation. Instead, orangutans favoured stable trees that afforded good protection from the wind and rain, and they generally make nests lower down in trees for the same reasons. Six particular tree species also accounted for just over half of all nests, suggesting that these tree species may be particularly important for orangutans.
A safe place to sleep at night is as important as food sources and other factors for apes’ daily lives. The knowledge gained through this research, with the help of research students at OuTrop, is therefore an important resource for informing conservation strategies for orangutans, gibbons and their habitats.

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