The last few dozen of the world’s most endangered primates, Hainan gibbons, live in small patches of rainforest on Hainan Island off southern China. In 2014, a landslide fractured their habitat, forcing the canopy-dwelling primates to make dangerous leaps to reach their food. Conservation scientists came up with an alternative path: a simple rope bridge that spans the gap between the sections of trees.
Now a study published on Thursday in Scientific Reports shows that Hainan gibbons can and do make use of the high ropes course. The gibbons took about six months to warm up to the idea, but 176 days after the rope bridge was installed, a few females and juveniles began to use it, study author Bosco Pui Lok Chan of the Hainan Gibbon Conservation Project tells Mary Bates at National Geographic. The gibbons invented unexpected bridge-crossing strategies, but the researchers emphasize that the bridge is a temporary solution.
Hainan gibbons spend their lives in the rainforest canopy, swinging from branch to branch with their long arms. (Gibbons are apes, which do not have tails, unlike monkeys, which generally do.) According to the New England Primate Conservancy, Hainan gibbons have never been seen on the ground.
So when faced with a 50-foot-wide gap, the gibbons didn’t climb down, walk across the rubble of the landslide, and climb up the trees again. Instead, the gibbons launched themselves across the gap from about 100 feet in the air.
“It was pretty scary to watch – my heart just popped out of my throat,” Chan tells Clare Wilson at New Scientist. He adds that mother gibbons made the jump with their babies holding on, and “if the infant-carrying mother falls, that would have been two down out of 25.”
To help the gibbons safely cross the divide, the researchers set up a group of mountaineering-grade ropes fastened to sturdy trees by professional tree climbers, per the paper. The gibbons didn’t swing underneath the ropes like they do from tree branches. Instead, they began to walk along one rope while holding on to another for support, which the researchers called “handrailing.” On occasion, the gibbons held on to the ropes with all four limbs like a sloth and crossed upside-down, Lucy Hicks writes for Science magazine.
The research team documented eight of the gibbons—all but the males—crossing with the rope bridge a total of 52 times.
“There are many different designs of canopy bridges used all over the world, but this one is particularly cool because it is simple, low cost, and well adapted to this species,” says conservation biologist Tremaine Gregory, of the Smithsonian Conservation Biology Institute, to National Geographic. “As we chop up the world into smaller and smaller piece with roads and other infrastructure, it’s important to think about solutions for maintaining connectivity between fragments of habitat.”
Gregory, who wasn’t involved in the new research, adds to National Geographic that conservationists working with other tree-dwelling animals might take note of the gibbons’ success. The Smithsonian National Zoo has its own rope line infrastructure for the orangutans to travel between two enclosures.
Most of the Hainan gibbons’ population loss happened between the 1950s and the 1970s, when the population fell from about 2,000 to less than ten. Poaching and habitat loss had the greatest impact, and 50 years of conservation efforts have slowly brought the population up again.
A pair of Hainan gibbons established a family unit in a new patch of rainforest in 2019. Local rangers identified the pair in their new home because they sing a duet, Elizabeth Claire Alberts reported for Mongabay this May. In April, the Kadoorie Farm and Botanic Garden recognized five Hainan gibbon family groups outside of the central fragment of forest where they live.
“Finding ways to restore natural forest corridors should be a priority,” Chan says to National Geographic, and adds to New Scientist that the trees in the area affected by the landslide have now regrown.