Analyze This: Ropes restore a gibbon highway through a rainforest
Simple bridges may reconnect forest habitats separated due to human activities
Scampering or climbing along ropes, Hainan gibbons (Nomascus hainanus) can now cross a great gully carved by a landslide.
These endangered primates live in the forest on China’s Hainan Island. A 2014 landslide had damaged the preferred route that these tree-dwelling apes took through the forest. They could cross by vaulting across the gap, catching onto a palm frond. But when the frond started to sag, researchers rushed to provide a safer way.
The gibbons were slow to adopt the new route. But they increasingly traveled a bridge made of two ropes across the 15-meter (49-foot) gap. The scientists shared their success October 15 in Scientific Reports.
When people build roads or other structures in a forest, it can carve animal habitats into fragments. This can split populations into smaller groups that may struggle to survive, says Bosco Chan. A conservation biologist, he works at the Kadoorie Farm and Botanic Garden in Hong Kong.
Only about 30 Hainan gibbons remain. Chan and his team were worried about a group of nine that had been affected by the landslide. They wanted to prevent the animals from getting hurt while jumping. So the researchers built a rope bridge.
About 176 days after the bridge went up, cameras caught the gibbons taking to the ropes. “I was very excited when [the gibbons] first started using it,” Chan says. Eventually, the team observed the gibbons crossing the bridge about as often as the gibbons had traveled that stretch of forest before the landslide.
Learning the ropes
To cross a gully created by a landslide, some Hainan gibbons climbed across a canopy bridge, either using the top rope as a handrail (a) or with arms and legs below the ropes (b). Others walked as if on a tightrope (c) while some dangled and swung (d).
The Hainan gibbons’ adoption of the bridge suggests that other primates may also use rope bridges in fragmented forests, says Susan Cheyne. Based in Oxford, England, Cheyne studies primates. She also serves as vice chair of the IUCN Primates Section on Small Apes. This community of experts works to save gibbons and related species. These gibbons are a “relatively fickle species,” says Cheyne, who advised the Hainan project. “They are not overly keen on using new things,” she says.
The gibbon group’s two females and two small juveniles favored crossing on the bridge. But an adult male never used it and three nearly grown juveniles rarely did. An infant always made the crossing carried by a female. The bridge provides a temporary solution while trees, including native transplants, grow in to fill the forest gap.
Scientists used cameras near a rope bridge to photograph female and juvenile Hainan gibbons in action as they crossed. These pie charts tally the methods seven apes used on their crossings. Some gibbons climbed across the bridge while holding the top rope (handrailing) or with arms and legs below the ropes (underneath). Others walked as if on a tightrope. Still others used a dangle-and-swing technique, called brachiation, similar to how you might cross monkey bars.
- Looking at the pie charts, which is the most common way that gibbons use the bridge?
- Which is the least common way for gibbons to cross the bridge? Why do you think this is?
- How does the way that small juveniles cross the bridge differ from how females cross?
- Why do you think females and small juveniles used the bridge most?
- This bridge was made of two ropes, one above the other, connected to trees. What other designs can you imagine for a rope bridge? What other materials might work as a “bridge” for these gibbons?