( Indonesia. Orangutans spend almost the...)Lessons from the orangutans: Upright walking may have begun in the trees

To collect the data, Thorpe spent a year living in the Sumatran rainforest and recording virtually every move the orangutans made. Then, she and her colleagues used these observations to test the hypothesis that bipedalism would have benefited tree-dwelling ape ancestors.

Because these ancestors were probably fruit-eaters, as orangutans are, they would have needed a way to navigate the thin, flexible branches at the tree's periphery, where the fruit typically is. Moving on two legs and using their arms primarily for balance, or "hand-assisted bipedalism," may have helped them travel on these branches.

The researchers analyzed nearly 3,000 examples of observed orangutan movement, and found that the orangutans were more likely to use hand-assisted bipedalism when they were on the thinnest branches. When bipedal, the animals also tended to grip multiple branches with their long toes.

On medium-sized branches, the orangutans used their arms more to support their weight, changing their moving style to incorporate hanging. They only tended to walk on all fours when navigating the largest branches, the researchers found.

Hand-assisted bipedalism may have offered several advantages that allowed our arboreal ancestors to venture onto thin branches. They could have gripped multiple branches with their toes and distributed their center of gravity more effectively, while keeping one or both of their long arms free to reach for fruits and other supports.

Orangutans also keep their legs straight while standing on bending branches, the authors report. The exact benefit of the straight legs is still unclear, but when humans run on springy surfaces, we also keep our weight-bearing legs relatively straight, so this may have an energy-related advantage.

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Source:American Association for the Advancement of Science