BioEd Online

Gibbons swing through the air with the greatest of ease, but Evie Vereecke, a PhD student at the University of Antwerp in Belgium, wondered how they fared on the ground. Turns out, they do just fine on two legs, but their gait is much different from ours.

"It looks like a playful run," Vereecke says, "a silly walk".

To get more specific, Vereecke and two colleagues introduced an 'instrumented walkway' to the enclosure of four white-handed gibbons (Hylobates lar) in a Belgian zoo. As the apes ambled over it, their every move was filmed and every footfall recorded. An analysis is published this week in the Journal of Experimental Biology¹.

Always on the run

Whereas humans and horses have two very distinct gaits, walking and running, gibbons only rarely engage in a way of locomotion that resembles our walk, where the legs are swung like pendulums. Instead, at all speeds, they propel themselves in a springy, bouncy fashion closer to our run.

But unlike humans, they do not use their Achilles tendons as the main spring. Vereecke hypothesizes that they might use their quadriceps muscles instead.

Also unlike a human, they never have both feet off the ground at the same time. Vereecke suggests that this 'aerial phase' (think of this lovely expression on your next jog) should not be a requirement to call something a run.

Swingers

Vereecke says that the apes are good at bipedalism, even though the animals are much more often found swinging from limb to limb. She wonders whether there is some fundamental aspect of physiology that might make swinging animals adaptable to walking. "Probably there is a biomechanical overlap between bipedal and arboreal locomotion," she says, "which might have something to say about the evolution of human bipedalism."

Liza Shapiro, a primate locomotion specialist at the University of Texas, Austin, is a bit less impressed with the gibbon bouncy gait. She points out that the study also shows that the gibbons' centre of mass tends to roil about during this funny walk. "I get the impression that maybe their bipedalism isn't very energy-efficient at all," she says.

Still, she says, she is intigued by Vereecke’s hypothesis: perhaps there is some common energy-efficiency pattern in all primates that are fundamentally adapted to arboreal environments, but that can also stand up and walk.

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