Orangutans adapt their movements to swamp forest

Orangutans living in thin forest growing in peat swamps in Borneo have different ways of getting about to their cousins in drier rainforest on the neighbouring Indonesian island of Sumatra - but not as different as scientists expected.

New research shows that the Borneo apes have to move through sparser forest canopies with bigger gaps between thinner and more pliant trees. So they travel lower down in the canopy and use special techniques like swaying trees to make them bend towards others nearby to form improvised bridges over gaps, turning thinner trunks and boughs to their advantage.

But after following orangutans during several gruelling field trips, Kirsten Manduell, a PhD student in the Locomotor Ecology and Biomechanics lab at the University of Birmingham and lead author of the paper, was surprised to find that how they get about doesn't seem to depend much on their size and age. She'd expected that sparser vegetation in peat forests would make life harder for big males, forcing them to use different tactics from their slighter peers.

'I'd expected that in this environment, the big flanged males might not be able to do things that smaller orangutans could, because they weigh so much more - a flanged male can weigh up to twice as much as a female,' says Manduell. 'So it was quite surprising to find out that age and sex don't seem to make much difference to how they move in the canopy.'

Male orangutans come in two varieties - flanged and unflanged. The latter are smaller, usually younger individuals who travel around the forest looking for females. In later life they may develop the characteristic facial ridges of the flanged male, becoming much bigger and heavier. From then on they don't need to go looking for females; they stay put and attract mates with their sonorous calls and impressive appearance.

Orangutans live in a wide range of habitats across Indonesia and Malaysia, but previous studies have focused on their behaviour in drier rainforests, many of them not seriously affected by deforestation. This is the first study of how they travel in swamp forest, based on data gathered on three trips to Borneo's Sabangau National Park. It shows that orangutans can adapt to relatively sparse forest, even when it has been severely harmed by logging in recent decades.

Peat swamp is an important habitat for orangutans. Logging has hit the swamp forest of Sabangau hard, creating gaps in the canopy that cause additional problems for travelling orangutans. The canals that loggers dig to float timber out of the forest make the situation still worse; they drain the swamp, making its peat dry and highly flammable. Forest fires have become a serious problem in recent years.

The previous logging concession in Sabangau expired in 1996, but unlawful tree-felling continued for some time, as well as people illegally hunting fruit bats by creating clearings in which to set up large nets.

The local conservation patrol group that Manduell worked with is now trying to address these problems; it has successfully stopped illegal hunting and logging, and is working on firefighting and on blocking canals in the hope of stopping the forest drying out. The patrol is run by the Center for International Cooperation in Sustainable Management of Tropical Peatland at the University of Palangka Raya, which also acted as the local sponsor for Manduell's research.

On Manduell's trips to Borneo, she travelled with local conservationists and researchers around the Sabangau areas until she spotted an orangutan in the trees.

She'd then follow it about, noting where it nested each night and returning the next morning, and gathering data on the type of forest it was moving through and what movement strategies it was adopting to cope. When eventually that orangutan moved out of the research area, she had to return to searching until she found another one. On returning to the UK, she analysed the data she'd gathered to arrive at a mathematical representation of how and where the apes move.

Manduell notes that her work suggests orangutans can move about comfortably even in badly disturbed peat-swamp forest, but that they are still likely to be in danger for other reasons. 'From a locomotive perspective they seem to be fine, but you have to look at the big picture,' she says. 'The Sabangau forest is huge but not very productive, and they may have adapted to logging disturbance by having larger ranges, or other aspects of their ecology may have been altered. They have very low population densities compared to other areas, so they are still at risk.'

The study is published in the American Journal of Physical Anthropology.