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No monkey business: For Zanzibar’s red colobus, speed bumps are lifesavers

  • Half of the 6,000 remaining Zanzibar red colobus monkeys, a species endemic to the island, are found in Jozani Chwaka Bay National Park, where getting hit by a vehicle is a leading cause of their death.
  • Speed bumps significantly reduce road mortality for this endangered African monkey, an analysis by primatologists at Bangor University in the U.K. and researchers at the Zanzibar forest department found.
  • About half of the roadkill on a stretch of a paved road that cuts across the southern edge of the national park were red colobus.
  • Understanding how animals respond to human-made features that have cropped up in their environment is a key question in behavioral ecology today, experts say.

With fewer than 6,000 of Zanzibar’s iconic red colobus monkey (Piliocolobus kirkii) left in the world, there’s much about their well-being that worries scientists. Near the top of that list: their road sense.

Or the lack of it, as evidenced by videos captured by primatologists showing them sauntering across a road amid heavy traffic, scurrying up to cars, or just parking themselves in the middle of all the action.

“We know from studies in chimpanzees in Uganda and Guinea that they show some kind of awareness of the road, but the colobus they look kind of oblivious,” said Alexander V. Georgiev, an ecologist at Bangor University in the U.K. and co-author of a recent study in the conservation journal Oryx. “They don’t differ in the way that they cross the big road where they are likely to get hit versus the small dirt road where there is hardly any traffic.”

With no larger animals to prey on red colobus monkeys, getting hit by vehicles has become a leading cause of death for the endangered primates, research from Zanzibar’s Jozani Chwaka Bay National Park shows. Cars, motorcycles, trucks and shared taxis called dala dala all whiz past on the paved road that intersects the national park.

One of the solutions the new research confirmed is simple: speed bumps.

These monkeys are endemic to Unguja, the main island of Zanzibar, an autonomous region of Tanzania. Their habitat has shrunk over the years, from covering the entire island of 2,460 square kilometers (950 square miles) to just about half of it. Jozani Chwaka Bay National Park, occupying 50 km2 (19 mi2) in the island’s center, hosts half of the wild population. The main road that connects the east of Zanzibar to the west grazes the park’s southern border.

As for the answer to the question: why did the red colobus cross the road? In some cases, it could be a particularly enticing roadside tree. Several accidents have happened in the vicinity of a favored Indian almond tree, Georgiev said. They also enjoy munching on mango leaves and have a bizarre taste for charcoal, much of it supplied by humans.

“Road accidents are a big problem here, especially in the summertime because of a scarcity of food. Since they prefer young leaves, so during that time, they expand their home range and cross the main road several times,” said study co-author Mzee Khamis Mohamed, from Zanzibar’s forestry department.

The research, led by one of Georgiev’s students, Harry Olgun, relied on roadkill data and population census data of 18 groups of red colobus living sufficiently close to the road to attempt crossing it. They used information collected by staff at the national park and reports from the public over a period of four years to estimate roadkill numbers. More than half of the carcasses reported were of red colobus.

Their analysis builds on an earlier study that found that after the installation of four speed bumps and wildlife crossing signs near the park entrance, mortality rates fell by 80% for red colobus living in the vicinity. But these speed bumps occur only on a 600-meter (2,000-foot) stretch of road. The study authors suggest that installing more of them would help save the clueless red colobus.

Zanzibar red colobus. Image courtesy of Alexander Georgiev.

Understanding how animals respond to new features that have cropped up in the environment thanks to humans is a key question in behavioral ecology today. Human disturbances in habitats are nothing new. For the red colobus themselves, the main road traversing the park has been around for generations. But a lot of work on primate behavior has focused on the study of primates in wild, undisturbed environments rather than their interaction with human-made structures like roads.

This also means that historical data is scarce in road ecology studies.

“This dataset is incredibly important as it lays the groundwork for site-specific mitigations, but it also provides information to primate conservation managers globally to assess risks in their own areas even if they do not have the long-term data themselves,” said Pamela Cunneyworth, who heads Colobus Conservation, a nonprofit that works to save colobus species in neighboring Kenya.

Unlike other national parks, Zanzibar’s only national park, created in 2004, has seen some success in tackling conventional challenges to conservation.

The road has been bumpy. Friction with the communities hampered efforts initially as the monkeys would help themselves to crops in adjacent fields. This led villagers to view them as pests, sometimes leading to the monkeys being killed. Today, the forest department, working with NGOs, compensates villagers for crop losses. There are still those who say this is compensation not adequate, but the monkeys are much less likely to become victims of mob justice.

Financing the scheme is possible because half of the tourism revenue from the park is set aside for the communities.

Its popularity with tourists, however, has downsides. “The study highlights the inherent danger of improving access to protected areas to support tourism and the risks associated with increased tourist traffic with the very attraction that people are often coming to see,” said Rodney van der Ree, director of Ecology and Infrastructure International, an Australia-based consultancy.

According to Mohamed from the forestry department, even a quick fix measure like installing speed bumps will not be easy, in particular finding funds for it. He also says they are not the only solution. A crossing bridge would help, Mohamed said. There is only one crossing bridge at the park currently.

Studies have shown that they save wildlife from human-made risks and also keep humans safe.

“As most primate populations are declining, mitigations to reduce the anthropogenic risks are important,” Cunneyworth said. “Mitigations such as canopy bridges and speed bumps need further investigation, and we look forward to analyses which determine how well these work to reduce the rate of collisions.”

It remains to be seen, according to Georgiev, if the monkeys grasp that speed-bump-ed zones are their safest bets as zebra crossings.


Olgun, H., Mohammed, M. K., Mzee, A. J., Green, M. E., Davenport, T. R., & Georgiev, A. V. (2021). The implications of vehicle collisions for the endangered endemic Zanzibar red colobus Piliocolobus kirkii. Oryx, 1-9. doi:10.1017/s0030605320000605

Struhsaker, T. T., Cooney, D. O., & Siex, K. S. (1997). Charcoal consumption by Zanzibar red colobus monkeys: Its function and its ecological and demographic consequences. International Journal of Primatology18, 61-72. doi:10.1023/A:1026341207045

(Banner Image: Harry Olgun collecting data at Jozani-Chawaka Bay National Park as one of the Zanzibar red colobus runs across the main road. Image courtesy of Alexander Georgiev.)

Malavika Vyawahare is a staff writer for Mongabay. Find her on Twitter: @MalavikaVy

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