Golden lion tamarins use artificial canopy overpass to get around: a new road for their conservation?

In early 2020, fences and wildlife safe passages were established as wildlife mitigation measures on a recently twinned stretch of the BR-101/RJ Norte highway, which crosses part of the State of Rio de Janeiro, along the Atlantic Forest biome, in Brazil. Historically, the highway has caused collisions between vehicles passing through it and regional fauna (Grilo et al. 2018), such as crab-eating fox (Cerdocyon thous), southern tamandua (Tamandua tetradactyla), nine-banded armadillo (Dasypus novemcinctus), and Brazilian common opossum (Didelphis aurita), along with many other species. It also acted as a geographic barrier to the natural movement of some species such as the golden lion tamarin (Leontopithecus rosalia). This situation geographically isolated this endemic, endangered species, which is approaching ever closer to the threat of extinction. They have a population estimated at less than 4000 individuals in the wild (only 1400 mature) (Ruiz-Miranda et al. 2019). The BR-101/RJ Norte Highway cuts the species’ distribution area by dividing protected areas (União Biological Reserve and Poço das Antas Biological Reserve) and large forest patches maintained by rural landowners where the largest population densities of golden lion tamarins occur. Therefore, the connection between these areas needs to be restored, to enable natural mating patterns to reestablish, which in turn diversifies the gene pool, and ensures the species’ long-term viability (Ascensão et al. 2019). The persistence of the species is directly associated with its capacity to cross the highway lanes and other elements that are part of the anthropized landscape matrix to establish contact with other groups. In this context, 10 canopy overpasses (six composed of concrete corridors and four metal structures) were set up focussed on arboreal fauna, with golden lion tamarins distributed along an approximately 50 km stretch of the BR-101/RJ Norte (see Figure 1). Each structure received ropes connecting the surrounding trees to the entrance to the passage, as well as crossing from the inside, allowing different means of movement for the arboreal fauna inside the structure (Figure 2). The concrete canopy overpasses are 2.97 m high, 4.15 m wide, and 40 m long, while the metal canopy overpasses are 2.73 m high, 2.20 m wide and 40 m long. Given the scarcity of similar case studies, the two types of structure were designed by a group of specialists focussed on the golden lion tamarin, for later evaluation of their respective effectiveness during the monitoring of the crossings, with the agreement of the environmental agency licensor. In September 2021 (approximately 14 months after these structures were installed), systematic monitoring of the use of eight of these structures for the safe crossing of arboreal fauna began (km 195 + 500; 202 + 760; 212 + 600; 215; 223; 234 + 600; 235 + 200; 240 + 100). Monitoring was, and continues to be, carried out by installing camera traps at both entrances of the canopy overpasses, to capture images of the approach and crossing of any animal that interacts with the safe passages. Fortunately, with only 4 months of monitoring carried out between September and December/2021, the analysis of the images of these overpasses has already justified the installation of these structures, given that numerous arboreal species have used them, including golden lion tamarins (km 212 + 600; 240 + 100), as well as marmosets (Callithrix sp.) (km 212 + 600; 240 + 100), opossums (Didelphis aurita) (km 202 + 760; 215; 234 + 600; 235 + 200) and porcupines (Coendou spinosus) (km 212 + 600; 240 + 100). The overpass located at km 240 + 100 m of the highway (passage no. 10 in Figure 1), composed of a metal structure and installed in May 2020, was the passage with the highest number of species and crossings recorded. The passage is located in an area where patches of Atlantic Forest vegetation reach both sides of the highway, forming a movement corridor for