An opposable thumb is supposed to be a sign of a sophisticated species. But apes called bonobos make little use of their thumbs when they hang from tree branches – even though we use ours to keep a tight grip.
are our closest living relatives, along with chimpanzees. We are all descended from the same common ancestor, so studying bonobos can give us clues to what that common ancestor was like.
at the University of Kent, UK and her colleagues studied bonobos living at , a zoo in Antwerp, Belgium. The team wrapped a pressure-sensing mat around a wooden beam in the bonobos’ enclosure, and recorded the apes’ behaviour with a high-speed camera. At different times, the mat was on horizontal or vertical beams, which the bonobos used in different ways such as knuckle-walking or hanging by their fingers.
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The parts of the mat touched by the bonobos’ thumbs recorded extremely low pressures, or no pressure whatsoever.
“We were a bit surprised that, even though it’s clear from the video that the thumb is fully contacting the pole, there’s very little pressure there,” says Kivell. The bonobos may simply use their thumbs to guide the rest of the hand into place, she suggests.
In an unpublished study, one of Kivell’s students tried a similar experiment with human subjects. As expected, the thumb pressure readings were much higher.
A history of the hand
The data are “very rare and precious,” says at George Washington University in Washington, DC.
Almécija has studied the evolution of human and ape hands. In he found a wide range of different shapes and finger-to-thumb ratios. Notably, bonobos have quite small thumbs and long fingers. Almécija thinks their hands likely evolved like this for safer tree-climbing. Unlike us, they rely almost entirely on their fingers for gripping branches.
His current working theory is that the common ancestor of humans and apes had a more human-like hand. Since then, the hands of tree-climbing species like bonobos have actually changed more than ours have, to suit their arboreal lifestyle. Meanwhile human thumbs became indispensable for manipulating objects. The new study fits this story.
Almécija emphasises that bonobos do benefit from their thumbs. “It’s still really important,” he says. “They use it for feeding or grooming each other.”
Journal of Experimental Biology
