How did the monkey cross the road?

Courtney Maiden, Endangered Wildlife Trust MSc Student

Over 750,000 km of roads crisscross South Africa, and the country’s natural habitats and wildlife are gravely threatened by further road development. Furthermore, with the anticipated increase of vehicles on the roads over the coming years, the likelihood of more wildlife-vehicle collisions is worrying. The Vervet Monkey (Chlorocebus pygerythrus) has adapted to thrive in human-altered landscapes. However, this also means they face many risks from humans and their activities. Vervet monkeys face daily challenges living in an urban environment due to increasing habitat loss and fragmentation. The clearing of treed areas for roads and other purposes forces monkeys to the ground, increasing their risk of being hit by vehicles. An important first step in reducing this outcome is the design of safe and cost-effective structures by which animals can safely cross the road.

Over the past two decades,  wildlife crossing structures have been installed to facilitate wildlife movement over or under roads and railways to connect habitats and reduce roadkill. These structures are often custom-designed for each site and according to the needs of the targeted species. Yet, less than a handful of studies look at the effectiveness of different measures in reducing wildlife‐vehicle collisions in South Africa. Moreover, systematic assessments on designing safe and cost‐effective crossing structures for wildlife have not been carried out to date in our country, despite their importance in preventing Vervet Monkeys and other animals from becoming roadkill.

In March 2022, Courtney Maiden from the Endangered Wildlife Trust tested three different canopy bridge designs for Vervet Monkeys to identify one standardised design for the benefit of free‐ranging Vervet Monkey troops. The observational experiments took place at the Centre for the Rehabilitation of Wildlife (CROW) in Durban, where the three bridge designs were installed in two Vervet Monkey enclosures. Testing different canopy bridge designs in an ex-situ environment such as CROW allows us to establish design guidelines that can be used for free-ranging Vervet Monkeys in-situ (in their natural habitat). These ex-situ experiments are vital for understanding how Vervet Monkeys behave on different canopy bridge designs and identifying the most suitable bridge design to increase the likelihood of the bridges being used in the wild.

The bridges were made using polypropylene rope and recycled plastic. The design is adaptable to varying installation lengths, heights, and crossing environments. By observing the monkeys directly and using camera trap footage, we found that the ladder bridge was used most often and showed great potential for being the most suitable design.

The positioning of any wildlife crossing structure is equally as important as its design. As the Vervet Monkey is a territorial species with daily foraging paths, installing canopy bridges along preferred movement pathways is vital for maximum benefit. Examining troop territories, crossing areas, and frequency of use can inform the best bridge location. Through the EWT-WTP student mentorship and with help from Wendy Collinson-Jonker (EWT) and Sandra Jacobson (US Forest Service Wildlife Biologist), Courtney has visited potential bridge installation sites in KwaZulu Natal and is currently studying road crossing hotspots to determine where bridges would be most likely to be used as intended.

Once suitable sites have been identified, we can begin the exciting part – testing the design identified as most suitable (the ladder bridge) on free-ranging monkeys. Watch this space for updates! All information and research updates can also be found on Instagram (@wildways_sa), Facebook (Wild Ways South Africa), and EWT platforms.