The Amazon Basin is the world’s largest tropical forest, hosting approximately 5,000 tree species and an extraordinary wealth of animal life. Three-quarters of vertebrate species in the Amazon are arboreal or semi-arboreal, meaning they live in or around the trees rather than on the forest floor. These canopy-dwelling vertebrates, particularly mammals, play critical roles in maintaining ecosystem structure and promoting plant diversity through seed dispersal. Understanding their ecology is therefore essential not only for their own conservation but for the health of Amazonian ecosystems as a whole. Yet the vertical dimension of tropical forests—the complex layering of habitats from the ground to the treetops—remains poorly studied compared to terrestrial strata, largely because observing animals dozens of meters overhead is logistically difficult.

This knowledge gap has taken on new urgency as forest fragmentation continues to alter the Amazon Basin. When roads, agriculture, and development break continuous canopy into isolated fragments, arboreal mammals can become stranded on forest islands, unable to move between patches to forage, mate, and disperse. Conservationists worldwide have responded by installing canopy bridges—ropes, poles, and other structures spanning gaps—to reconnect fragmented habitat. These bridges have successfully promoted connectivity for critically endangered species and reduced vehicle collisions and mortality. However, the extent to which arboreal species use artificial bridges across different canopy layers, and when they use them, remains poorly understood across diverse Neotropical habitats. How mammals use artificial structures in an unfragmented forest can inform best practices for designing conservation bridges in fragmented landscapes.

The research team took advantage of the ACTS Canopy Walkway, a series of platforms and bridges spanning multiple forest levels and built for researcher and visitor use—one of the longest artificial canopy walkway bridge systems in the Americas. Through July and August 2022, the researchers deployed four camera traps continuously for 21 days over a canopy height gradient, fastening each camera to the nearest tree or beam overlooking a potential wildlife pathway. The cameras were placed at 12.1 meters, 20.8 meters, 26.5 meters, and 33.5 meters above the ground, chosen for their similar connectivity to nearby vegetation, canopy spacing, and elevation. All infrared sensors were set to high sensitivity, allowing the cameras to capture smaller animals and those at a distance. During the first half of the study, each camera recorded 15-second videos throughout the 24-hour operation period; during the second half, cameras captured three photographs per trigger event to save memory storage and battery usage.

The camera traps operated for a total of 1,680 camera trap hours and recorded 35 independent mammal observations, with independent events separated by 30 minutes as suggested by similar camera trap studies. No daytime observations were recorded, so the analysis quantified mammalian walkway use at night, extending from approximately sunset at 1800 hours to sunrise at 0600 hours. Linnaeus’s two-toed sloth (Choloepus didactylus) was the most frequently observed species overall, with 14 independent sightings exclusively on the ropes of the walkway bridge. The long-tailed porcupine (Coendou l. longicaudatus, now considered a morphotype of Coendou prehensilis) followed with 10 independent sightings on both the beams and ropes. Several observations of the more reclusive and data-deficient streaked dwarf porcupine (Coendou ichillus) were also recorded, along with the bicolor-spined porcupine (Coendou bicolor), the brown-eared woolly opossum (Caluromys lanatus), a saki monkey (Pithecia sp.), and Neotropical spiny rats (Echimyidae). Bats triggered cameras in flight but were classified as incidental species, since mammals accessing the canopy walkway were the focus of the study.

The spatial pattern of walkway use was strikingly stratified. Most mammal observations occurred mid-canopy at 20.8 meters, where all seven species were recorded, and mammals were next most frequently observed in the upper canopy at 33.5 meters. No mammals were recorded by the camera trap at 12.1 meters, the lowest camera position. Of the two walkway components—the walkway bridges and their supporting beams—the walkway bridge received the greatest overall use, though a greater diversity of mammals was observed on the beams, which many species used as an access point to the rest of the walkway platform. Notably, most mammals used the rope bridges of the walkway for movement; although bridge floor planks were available, no usage was observed. There was little evidence of foraging, though porcupines chewed on walkway beams. This suggests the canopy walkway is used primarily by mammals as movement corridors rather than foraging hotspots.

Environmental conditions significantly varied by canopy layer, and these microclimatic gradients likely shape mammal habitat selection. Temperature and wind speed were highest in the upper canopy and decreased toward the forest floor, whereas humidity was greatest at the forest floor and decreased toward the canopy. All pairwise differences in temperature and wind speed between layers were statistically significant, while humidity differences were significant except between mid-canopy and the forest floor. The researchers used a linear mixed effect model to test whether environmental conditions varied with canopy strata, with time as a random effect, followed by Tukey post hoc comparisons to determine differences between specific layers. The mid-canopy preference observed in the mammals—roughly 21 to 27 meters above ground—may reflect a choice for lower-risk, more concealed pathways through the canopy, consistent with trends for small mammals observed in other studies.

The temporal pattern was equally revealing. Most mammal activity was first recorded between 1900 and 2100 hours and concluded by 0200 hours, though Linnaeus’s two-toed sloths continued to be active until 0400 hours. Mammal diversity peaked between 2000 and 2200 hours; during this period, mean temperature was 24.3 degrees Celsius and mean humidity was 84.6 percent, both significantly higher than the nighttime means of 23.7 degrees Celsius and 82.0 percent. Wind speeds during the peak observation period did not differ from mean nighttime wind speed. The activity pattern corresponds closely to findings from other studies of arboreal mammal behavior on natural canopy bridges, including a larger study that recorded 5,000 observed individual events across 27 arboreal mammal species, with 87.4 percent of events occurring between 1800 and 0600 hours. The researchers suggest that peak activity while temperature and humidity are relatively high may indicate favorable conditions for arboreal mammal foraging and social activities, or may be correlated with increased foraging motivation during the first few hours of nightly activity.

Of particular note is the streaked dwarf porcupine (Coendou ichillus), a data-deficient species that was only relatively recently recognized by Western science when researchers noted distinctive morphological traits on a specimen collected from a 1936 expedition in Ecuador. The species is smaller, darker, and has a smaller tail and bristle quills than its relatives, allowing researchers to discriminate it from Coendou bicolor in camera trap images. Erethizontids, the porcupine family to which C. ichillus belongs, are characterized by reclusive cryptic behavior that contributes to the limited available data. Previous work identified seasonal variation in the species’ activity and similar activity times to those identified in this study, with activity peaking around 2000 hours. Observing their activity periods could provide additional insights into their behavior, leading to appropriate conservation measures.

The findings provide baseline data on arboreal mammal use of canopy walkways in a tropical forest with continuous canopy cover, which may help inform the design of human-created arboreal corridors to connect forest fragments. As fragmentation of tropical forests increases, understanding how mammals use artificial canopy structures in an unbroken forest may enable future planning of more naturally structured or accessible artificial bridges. The researchers also suggest that efforts could be taken to minimize disturbance to artificial canopy bridges in fragmented forests during peak movement hours and while temperature and humidity remain relatively high. Much remains to be explored regarding the long-term ecological impacts of these structures on populations and their role in supporting biodiversity, and further research is needed to assess their effectiveness more broadly in the Amazon and across various levels of forest fragmentation. But this study demonstrates that many arboreal mammal species, even cryptic and reclusive ones, actively use human-made canopy structures—even within a continuous, non-fragmented forest—offering a promising foundation for conservation engineering in an increasingly fragmented world.

Subject of Research: Biology

Article Title: Arboreal mammal use of canopy walkway bridges in an Amazonian forest with continuous canopy cover

Article References: Santiago, J., & Swierk, L. (2025). Arboreal mammal use of canopy walkway bridges in an Amazonian forest with continuous canopy cover. Neotropical Biology and Conservation, 20(3), 281-294. https://doi.org/10.3897/neotropical.20.e154791

Image Credits: AI Generated

DOI: 10.3897/neotropical.20.e154791

Keywords: Arboreal, mammal, canopy, walkway, bridges, Amazonian, forest, continuous, cover, scientific research