Tree-loving orang-utans hang out on the forest floor
Colin Barras
Getting on down (Image: Brent Loken)
Orang-utans may be perfectly adapted for swinging through trees, but new observations suggest they also spend a surprising amount of time hanging out on the ground.
The finding could have implications for conserving the endangered species – and perhaps help efforts to understand why our ancient ancestors left the trees.
With their long, strong arms and short bowed legs, it's easy to see why orang-utans are considered the most arboreal of all great apes. But male Bornean orangs have been known to sometimes climb down to the ground.
To find out just how often the apes leave the canopy, Brent Loken at Simon Fraser University in Burnaby, Canada, and colleagues placed a network of ground-based cameras across a 38-square-kilometre region of forest in north-eastern Indonesian Borneo. Remarkably, they found that orang-utans were almost as likely to be caught on camera as a ground-dwelling primate called the pig-tailed macaque, which is roughly as abundant as orang-utans in the forests. The macaques were photographed 113 times over an eight month period, the orang-utans 110 times.
Down time
"We didn't expect to get so many photos," says Stephanie Spehar of the University of Wisconsin–Oshkosh. "Individuals of all ages and sexes use the ground regularly."
The results suggest orang-utans may already have coping strategies in place as deforestation fragments their home. "If forced to, it may be that orang-utans could travel [on the ground] between forested areas," says Loken. "However, we are unsure about the long-term prospects for the apes living within degraded landscapes."
Travelling on foot may also offer a more direct and therefore energy efficient route to a destination than zig-zagging through the canopy, he adds.
Ben Buckley at the University of Cambridge says that travelling on the ground is tough in deforested areas. "Males are usually searching for and eating termites from rotten logs whilst on the ground," he says. "These [logs] are much less likely to be found in deforested areas."
Upright living
The idea that orang-utans spend a significant proportion of their time on the ground is intriguing in light of emerging evidence about our own evolutionary past. Recent studies have suggested that our ancestors began to stand upright on two legs while they lived in the trees – something that orang-utans occasionally do today.
Those ancestors, like orang-utans, lived in forest habitats, says Tracy Kivell at the University of Kent in Canterbury, UK. A more thorough investigation of the energy costs and benefits orang-utans face both in the canopy and on the forest floor could throw light on the factors that encouraged our tree-dwelling forebears to shift permanently to ground level.
"This study does raise some interesting questions," says Kivell. "It could be a catalyst for looking to new reasons as to how and why terrestrial bipedalism evolved in our lineage."
Journal reference: American Journal of Primatology, doi.org/m2p
