Can manmade rope bridges offer relief for the world's rarest primate - the Hainan gibbon? A new study shows encouraging results.

by Larry Powell

Hainan gibbons started to use the canopy bridge 176 days after installation. Females and small juveniles were the faithful users.

The Hainan gibbon (Nomascus hainanus) is described as "The world's most critically endangered primate." In fact, its numbers are said to be much lower than any other primate on Earth, with only about 30 individuals remaining for the entire species. It's found in just a single block of forest on Hainan Island, China, and nowhere else. 

The adult male is jet black with a hairy crest.

The female is golden yellow with a black crown patch.
The immature gibbon is black, regardless of gender.

Like so many similar creatures, it travels through the forest canopy, tree-to-tree. But major disturbances, like roads or landslides, can produce major gaps or gorges which seriously restrict its movements. This, in turn, can make it harder for it to feed or breed, but easier to be killed by predators.

Professional tree climber in action constructing the canopy bridge, December 2015. Mountaineering-grade ropes were tied to sturdy trees to provide connectivity at the forest gap created by a landslide.
All photos by Kadoorie Farm and Botanic.Garden.

So researchers decided to conduct an experiment. In 2015, they installed motion sensor cameras and enlisted the help of professional tree-climbers (above). They used strong ropes tied to sturdy trees to bridge a 15-metre-wide gully formed by a natural landslide. It had become a barrier separating two areas of gibbon habitat.

After a wait of almost six months, their efforts paid off. The gibbons - mostly mothers with younger members of the family - were first captured on the camera traps - actually using the bridge. (See top photo, above.) Adult males and larger juveniles seemed to prefer leaping across the gap together, instead.

Well over a year after the experiment began, more than two hundred photos and over fifty videos showed many gibbons belonging to the family group involved in the study, using the bridge. (Gibbons are territorial and live in family groups.)

As the authors conclude, "The study highlights the use and value of rope bridges to connect gaps in forest canopies. Although restoring natural forest should be a priority conservation intervention, artificial canopy bridges may be useful short-term solutions."  

In an e-mail to PinP, the head of the study, Dr. Bosco Chan of the Kadoorie Conservation China Dept. in Hong Kong, explains further. 

"Landslides created by heavy rains occur throughout Hainan Island, and I believe it is very common in areas affected by tropical cyclones throughout the world. So, yes I believe rope bridges can provide short-term solution to restore forest canopy connectivity in these places.

"In fact, artificial canopy bridges are quite widely applied in South America, Australia, and also see some attempts in Africa and more recently Asia, for natural disasters like the one we described, but also in forest fragmented by roads, pipelines and other artificial structures or disturbances.

"I do not believe the survivorship of this critically-endangered gibbon relies on building canopy bridges. But it surely helps alleviate impacts of forest fragmentation locally."

The findings were published earlier today in Nature.

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