By Griet Scheldeman
Science reporter, BBC News
Orangutans have a unique way of moving through the trees
The deft way that Sumatran orangutans are able to swing from branches seemingly too weak to take their weight has been described by scientists.
The animals use their hands and feet to move through the canopy in a completely different way from other primates, a team from Birmingham, UK, reports.
Sumatran orangutans have been in rapid decline and risk becoming extinct.
The scientific team said the animals' conservation could be helped by better understanding the habitat they need.
The research is published in the Proceedings of the National Academy of Sciences.
Bioscientist Susannah Thorpe and colleagues studied wild Sumatran orangutans as they moved through the treetops.
Dr Thorpe told BBC News that she had followed the primates round the forest on the ground, from dawn (when they get up if they are feeling energetic) to dusk (when they make their nests for the night).
"Every minute I recorded what they did, which was great fun. I did not only get an insight into how they move, but also how they get on with one another and how the infants grow up."
Dr Thorpe and her colleagues showed that the tree vibrations, which could unsettle the animals' balance, were countered by their ability to move with an irregular rhythm.
They observed how the apes managed to reach the ripe fruits at the end of narrow unstable branches without falling down, by swinging flexibly.
Unlike other primates, which move through trees at a regular pace with fixed body positions and stretched limbs, the orangutans were seen to travel both upright and horizontally, above and under branches, holding on with both their hands and feet.
And more than 29% of the time, the orangutans held on to more than four branches at once.
"The apes can support their weight in almost any way possible. They can hold many bits of foliage so tightly that they don't slip off," Dr Thorpe explained.
A key finding, said the researchers, was the realisation that Sumatran orangutans rely on woody vines, or lianas, which are often depleted in logged forests, for their exceptional style of locomotion.
Dr Thorpe clarified: "The risk is that in disturbed forest where there are more gaps in the canopy and fewer chunky supports, the orangutans might be forced to use the extremes of their locomotor ability. This will probably be energetically more costly and place them at a more frequent risk of falling or injury".
"Unless habitat degradation in Sumatra slows down, Sumatran orangutans are likely to be the first great ape to go extinct. This could occur in less than a decade," Dr Thorpe warned.
The scientists believe their work is likely to aid conservation.
"If the destruction of forest land does not slow down, the Sumatran orangutan could be extinct within the next decade," the University of Birmingham researcher said.
"Now that we know more about how they move through the trees and the unique way that they adapt to challenges in their environment, we can better understand their needs. This could help with reintroducing rescued animals to the forests and efforts to conserve their environment."