Overhead road crossings avoid deaths of primate threatened with extinction, study confirms

A project supported by environmental agencies, local government and private enterprise, as well as FAPESP, compared the use of a wooden pole bridge and a rope ladder bridge between tree canopies over a local road for three years in a municipality with an Atlantic Rainforest conservation unit. Monitoring focused on the Black lion tamarin, an environmental heritage species symbolizing wildlife conservation in the state of São Paulo.

By Karina Ninni  |  Agência FAPESP – Roads adversely affect wildlife that inhabits adjacent ecosystems, but their impact can be mitigated by animal crossings such as canopy bridges for tree dwellers and underpasses for terrestrial species. 

On a single-lane local road in the municipality of Guareí, state of São Paulo, Brazil, Francini de Oliveira Garcia, a biologist affiliated with the Federal University of São Carlos (UFSCar), installed canopy bridges with the aim of finding out how effective they would be to avoid roadkill of Black lion tamarins (Leontopithecus chrysopygus). The study also compared the use of wooden pole and PVC/nylon rope ladder canopy bridges by the primates.

The road (GRI 253) bisects an area of riparian forest fringing the Guareí River and connecting Atlantic Rainforest fragments. Groups of L. chrysopygus live in the area. Part of the nearby Angatuba Ecological Station is also in the municipality of Guareí, and is home to a group of these primates believed to have 46 members. The species is endemic to the Atlantic Rainforest biome in the state of São Paulo and is classified as endangered by the International Union for Conservation of Nature (IUCN).

“The advisory board of the conservation unit and the local community realized they were losing this population to roadkill. Five tamarins were killed by road traffic in 2013 alone. I was doing my master’s research and raised funding for the canopy bridges. I also established partnerships, mainly with the municipal government and the Forestry Institute, an arm of the São Paulo State Department of the Environment that manages the conservation unit. I monitored the area and the canopy bridges for 13 months for my master’s research,” Garcia said.

After defending her dissertation, she continued to monitor use of the crossings for almost two years on her own account. “From the standpoint of road ecology, a relatively new and interdisciplinary scientific field, 13 months wasn’t long enough. I wanted more robust results and kept up the partnerships, which were very important,” she recalled.

No roadkills were recorded after the canopy bridges were installed, except for one tamarin that was run over two days afterward. “Two days is too short a period for comparison with what happened before the crossings were put up,” Garcia said. “What matters is that we’ve had no more roadkills apart from that one incident. Moreover, the number of individuals in the group has risen.”

The results were written up in an article by Garcia and colleagues, and published in the European Journal of Wildlife Research. The study was supported by FAPESP via the project “The effect of fragmentation on primate ecological functions”, led by Laurence Marianne Vincianne Culot, a professor at São Paulo State University (UNESP) in Rio Claro and second author of the article.

Bridges and cameras

The two canopy bridges were installed in 2017 at a site where tamarins frequently crossed and where the five roadkills occurred in 2013. Both are 13 m long and 6 m from the ground. They are supported by timber poles on either side of the road. One is an untreated eucalyptus tree trunk (Eucalyptus umbra) with a diameter of about 20 cm. The other resembles a horizontal rope ladder and is 50 cm wide, with 40 cm nylon “steps” secured by flame-resistant PVC piping.

“This is the first study of the kind ever conducted in South America, in that we placed two different crossings at the same site in order to find out if the animals preferred one over the other,” Garcia said.

The bridges were monitored continuously using camera traps placed on the supporting uprights. Garcia collected the images monthly for two years. Rafaela de Carvalho, then a project intern at UNESP and a co-author of the article, collected images for an additional year.

Garcia watched the recordings, separating the images in which animals could be seen crossing the road via the canopy bridges by species and bridge type. The rope bridge and camera traps were put up by local government workers via the partnership with Guareí.

“We recorded nine species of mammals and one lizard species crossing the canopy bridges, as well as 13 bird species that used them as perches,” Garcia said. The pole bridge was used by the Black lion tamarin, the Brazilian squirrel (Guerlinguetus brasiliensis), an unidentified rodent, another rodent belonging to the family Cricetidae, and a lizard in the family Scincidae. Two species of mammal – another rodent in the family Cricetidae, and the Bare-tailed woolly opossum (Caluromys philander) – used only the rope bridge. Three used both: the Paraguayan or Orange-spined hairy dwarf porcupine (Coendou spinosus), the Black-eared opossum (Didelphis aurita), and another rodent in the family Cricetidae.

Most of the 702 crossings recorded by the camera traps occurred on the pole bridge (527), while only 175 used the rope bridge, with a total of 500 occurring in the dry season and the rest (202) in the rainy season.

“We suggest that the movement of the tamarins from one side of the road to the other is driven by biological requirements within their home range, which is bisected by the road,” Garcia said. “The animals crossed in both directions, and most crossings occurred in dry months, when less fruit is available and more prey has to be hunted, so that foraging efforts must increase compared with the wet season.”

According to her, during the rainy season, these primates feed on sweet fleshy fruit, while in the dry season their diet includes invertebrates and tree sap, leaves and shoots.

“One question we had was whether animals would use the crossings while vehicles were traveling past on the road. The answer we arrived at was that the traffic didn’t interfere with or limit use of the crossings. Nor did the noise of car engines or the glare from vehicle headlights. Even when a vehicle was right under the bridge, animals felt safe enough to cross,” she said.

In the case of the Black lion tamarin, use of the pole bridge increased gradually. “The pole wasn’t used much in the first year. Its use increased in the second, in terms of frequency and the number of individuals crossing it. In the third year, the nine members of the group crossed at the same time more frequently,” she added.

According to Garcia, the individual primates studied had distinct personalities. “They each took their own time to trust the structure,” she said. “Hence the importance of taking time to analyze the use of these crossings. There are always those who go first. The others see that and follow, and the young learn from their parents.”

Public policy

The results of the study show that both canopy bridge designs worked well for arboreal species, but the wooden pole bridge was used by a larger number of species, including the endangered primates. It is worth noting that in 2014 the Black lion tamarin was declared part of São Paulo state’s environmental heritage and a symbol of its wildlife conservation policy.

For Garcia, the study can help foster measures to mitigate threats to wildlife from roads and road traffic. “It produced preliminary conclusions on the efficacy of canopy crossings, especially for these tamarins,” she said. “In the specific case of the GRI 253 [road], we also suggested the installation of speed bumps to help avoid wildlife-vehicle collisions.”

In her view, the research could not have been carried out without the partnerships involving researchers affiliated with public universities, the municipal authorities, government environmental agencies, private enterprise, and civil society. “Partnerships such as these are essential to road ecology actions that can benefit wildlife conservation and management,” she said.

The article “Functionality of two canopy bridge designs: successful trials for the endangered black lion tamarin and other arboreal species” is at: link.springer.com/article/10.1007/s10344-022-01569-8.