Diani Beach, on the coast of Kenya, south of Mombasa, is a popular tourist resort. The busy main road bisects the remaining patches of the ancient Diani forest, where there are 285 primates per square kilometre.
This is one of the densest populations of primates in East Africa, with six species of monkey, including about 400 Angolan Colobus. They are often hit by cars, bitten by dogs or electrocuted on power lines.
The danger from cars came to the fore in 1997 when 17 Colobus were killed in only three months. This high death rate prompted local residents to set up the Colobus Trust.
"It was incredibly important for the survival of these monkeys that we invented a way for them to cross between forest patches without the risk of being hit by fast vehicles," said Shelley Petch, head of research and development at the trust.
They came up with the Colobridge, a chain-link ladder connecting the tree canopies eight metres above the road. The ladder runs between two platforms on telegraph poles either side of the road. "We carried out research to identify the most effective locations for the Colobridges because primates generally use the same path to cross the road," she said.
Mike Waterland, manager of the Colobus Trust, highlights the importance of the bridges. "During the recent Equator Rally, I watched a monkey cross the bridge just as a rally car flashed by underneath at exactly the point below where the monkey was sitting and metres from what would have previously been a fatal accident," he said.
Six new bridges have been built in the past year, bringing the total to 25 along the five-kilometre Diani Beach road. Research shows that the bridges are used by 80 per cent of all primates that cross it. They make almost 150,000 crossings a year - about 3,000 a week. "The monkeys seem very pleased with this lifeline," said Waterland.
"They are a big hit with local and worldwide companies and holiday-makers who want to be associated with conservation," said Petch. "This is because each bridge can carry the name of its sponsor on a large plaque attached to a support pole."
Replicas of the Diani Colobridges have been installed elsewhere in East Africa. The trust recently set them up in Zanzibar to help save a population of Red Colobus.