How Speed Bumps Are Saving Endangered Monkeys in Africa

"Cars are not selective in the animals they kill."

A closeup of a Zanzibar red colobus

Manoj Shah / Getty Images

Speed bumps are saving the lives of the endangered Zanzibar red colobus, one of Africa’s rarest primates. After four speed bumps were installed along a road crossing the Jozani-Chwaka Bay National Park on the Zanzibar archipelago, the number of colobuses killed by vehicles dropped dramatically, according to a new study.

Roads affect wildlife in many ways. When first constructed, they can remove habitat, and later, they can be responsible for vehicle collisions as animals try to cross them.

Cars can become more dangerous than predators.

“Cars are not selective in the animals they kill,” study senior author and director of the Zanzibar Red Colobus Project, primatologist Alexander Georgiev, said in a statement. “This means that while natural predators may target the very young and old more often, cars are equally likely to kill reproductively active young adults, who would contribute the most to population growth. And this may be a problem.”

Zanzibar red colobus (Piliocolobus kirkii) are classified as endangered by the International Union for the Conservation of Nature (IUCN) Red List. They're found only on the Zanzibar archipelago and about half of the species population is found in the Jozani-Chwaka Bay National Park.

“A main road passes through Jozani National Park where a number of Zanzibar red colobus groups are habituated for tourism,” study co-author Tim Davenport, director of species conservation and science in Africa at the Wildlife Conservation Society (WCS), tells Treehugger.

“These animals have also become accustomed to foraging outside the park for food, in part because forest quality has diminished. As a result, they cross the road, many die and so we wanted to quantify this and look for solutions.”

When the road was resurfaced in 1996, vehicles started traveling faster and roadkill became more common. National Park staff members estimated at the time that on average one Zanzibar red colobus was killed every two to three weeks by traffic on the road.

One study at the time suggested that of the estimated 150 colobuses exposed to the road, as many as 12% to 17% were lost to vehicle accidents each year.

After four speed bumps were installed, colobus road fatalities have dropped to about one every six weeks.

“Vehicles, especially tourist vehicles and taxis were forced to slow down and so the death rate dropped,” Davenport says.

The Impact of Speed Bumps

Zanzibar red colobus
Zanzibar red colobus.

Tim Davenport

For the study, researchers relied on staff members working at park headquarters who commuted from nearby villages via the main road. They reported seven species of roadkill including elephant shrews, rats, squirrels, and bushy-tailed mongooses, although they were more likely to notice colobuses versus smaller animals.

“Other species also cross, such as elephant shrews, white collared guenons, etc but not to the same extent and they do not seem to get hit quite as much,” says Davenport.

Staff members also monitored the part of the road near headquarters for animals while they led groups of tourists throughout the day. Members of the public also reported dead animals to park staff. Again, researchers assumed that people were more likely to report dead colobus over smaller species.

Based on those reports, descriptions, and locations, researchers were able to estimate a lower mortality rate in the study period between 2016–2019. They found one colobus road fatality occurred nearly every six weeks with an estimated annual mortality loss of 1.77% to 3.24%.

The results were published in Oryx – The International Journal of Conservation.

While the speed bumps certainly had an impact, due to insufficient road maintenance, they now need upgrading, Davenport says. New ones need to be installed so they can continue to be effective.

The conservation takeaways from the findings are fairly straightforward, he says.

“Generally, that science is so important in defining, quantifying and understanding conservation challenges and finding solutions to them,” Davenport says.

“Specifically, that measures that slow vehicles down in this area have a positive conservation impact on a very rare primate species and we can and now will try and build on that as well as monitor it.”

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