Orangutans show unique moves
29 July 2009
Orangutans can move in unique ways specially adapted to getting about in the rainforest canopy.
Researchers believe their tactics are unlike those of any other kind of ape, and let orangutans cross thin branches, sway trees together to move between them, and find the tastiest morsels of food. The findings come from a new study, published in Proceedings of the National Academy of Sciences.
Understanding the unique ways the apes move may give us insights into the kinds of habitat they need, and so help protect them from extinction.
Some of orangutans' styles of movement are probably retained from the common ancestor of all the great apes, which is thought to have lived almost entirely in the treetops, like orangutans but unlike other apes like chimpanzees and gorillas.
These species have become adapted to spending part of their time on the ground; because of this, they've lost some of their special adaptations for treetop living.
Orangutans still inhabit what ecologists call the 'terminal branch niche', venturing out onto thin boughs to find fruit and to move from tree to tree while rarely setting foot on the ground. They therefore retain the 'orthograde' or upright body posture that's believed to have characterised the ancestral ape.
Other distinctive orangutan climbing behaviours have probably evolved since the species diverged from the other great apes.
Dr Susannah Thorpe, a lecturer in Locomotor Ecology and Biomechanics at the University of Birmingham, spent a year studying ten Sumatran orangutans in the wild in Indonesia.
She followed individuals at ground level as they travelled around the canopy far above, noting how they moved and the details of the environments they moved through, such as how thick and flexible the branches they were using to support themselves were. As well as keeping the apes under near-constant observation, she took video footage of any particularly interesting behaviour.
They identified several ways of moving between branches that seem to be unique to them, such as using all four limbs to scramble across gaps between trees, with each limb grasping any available support - a technique the researchers call 'pronograde bridging'. Orangutans also apply a similar technique while hanging from all four limbs beneath branches, a technique called 'pronograde suspension' that is unknown in their evolutionary cousins. This technique offers even more stability, as the animal has effectively already fallen off the branch and is now hanging underneath it.
Other apes like chimpanzees move differently in trees, relying on bent limbs to absorb unexpected shocks and keep them stable.
Many of Thorpe's results surprised her. For example, she had expected that adult males would be most conservative in their style of movement; they are the heaviest members of the species, so they are at greater risk of injury or death if they fall.
But it turns out this is not the case. 'We had expected to find adult males were the most conservative in how they move, but we found that they're as adventurous as most other individuals,' says Thorpe. 'It seems that adult females have the most conservative method of locomotion, although this only happens once they have had their first infant. This behaviour continues even after their infant has grown up,' she adds. Why this should be the case isn't yet understood.
Thorpe believes that understanding orangutans' habitat needs can help us conserve them.
'Understanding the kinds of support orangutans need to move on will help us better understand the kinds of habitat and movement pathways they need,' Thorpe argues.
For example, many efforts to curb deforestation have focused primarily on preserving trees. But lianas, the woody vines stretching between branches in a rainforest canopy, turn out to be crucially important for orangutans. Selective logging techniques are intended to preserve trees, but lianas are often cleared away by falling trees. More effort to preserve liana-rich habitats could benefit endangered orangutans. 'We hope this will let us conserve the most appropriate areas of forest to protect orangutans,' Thorpe adds.
Orangutans are critically endangered, and conservationists fear they could become extinct within a decade. This is partly because their rainforest habitat in Indonesia and Malaysia is being devastated as trees are cut down for timber and to clear room for palm oil plantations. Thorpe suggests that consumers can help protect the apes by refusing to buy palm oil or wooden products unless they come from certified sustainable sources.
Keywords:
Australasia,
Biodiversity,
Environmental change,
Hazards,
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