Swinging to safety: How canopy bridges may save Costa Rica’s howlers

by Noah Tobias

  • New research shows that building rope bridges over roads and buildings protects howler monkeys from needless deaths in Costa Rica.
  • Breaks in the tree canopy from roads, buildings and other developments pose a threat to howlers, which are often struck by moving vehicles or electrocuted on power lines while trying to cross these gaps.
  • Researchers built simple rope bridges over interrupted canopy and monitored them over the course of six years, finding that the bridges have led to a decrease in howler deaths and a rebound in their population.
  • Howlers monkeys are vital ecosystem engineers due to their seed dispersal and their ability to live in fragmented, disturbed habitats, so protecting them goes a long way toward protecting the ecosystem.

One of the first times Inés Rojas saw a dead howler monkey, it was lying on a road with its body curled up in a ball. The monkey’s palms were charred, its muscles seized and contorted. While traveling between two patches of forest, the monkey was forced to traverse the tarmac that separated them — using an overhead electrical cable. Somewhere between the two safe ends, the monkey touched a live wire.

“Seeing them burned, all the injuries from the electrocutions — that’s what sensitized me to continue researching,” Rojas said. Driven by anger at a needless death, Rojas, a primatologist at the University of Costa Rica, gathered a team and began looking for strategies to save lives. According to her new study, published in the journal Folia Primatologica and based on six years of fieldwork in Costa Rica’s Guanacaste province, Rojas may have found a way to keep golden-mantled howler monkeys (Alouatta palliata palliata) safe by building rope bridges between the treetops.

“If we want a healthy planet,” she says, “we need to maintain connectivity for howlers, who are in charge of keeping forests biodiverse.”

A howler monkey and its young cling to a tree. Howlers have active social lives, and often move between groups in search of mates – a journey that sometimes leads across roads to new forest patches. Image by Inés Rojas.

Bridges for monkeys

Between 2018 and 2019, electrocution killed nearly 1,000 monkeys across Costa Rica, according to research from the Ministry of Environment and Energy. Vehicle collisions and dog attacks also take a toll on monkeys.

Golden-mantled howlers, the only howler species in Costa Rica, are listed as endangered on the IUCN Red List. They spend most of their time in the trees, and without development and deforestation would never have been forced to the ground.

To tackle the problem, Rojas and her team decided to go to the source.

Every morning, in the humid darkness before sunrise, Rojas and her team tracked down troops of howlers by listening for their calls. They followed the monkeys until dusk, dodging the urine and feces that rained down from the treetops.

After two months of close calls, the team identified 20 hotspots where the monkeys crossed roads or other gaps between the forests, including over building or deforested areas. They began installing bridges at these hotspots, working closely with local communities to make sure they weren’t disturbing anyone.

“Clearly, we are outsiders,” Rojas says. “We had to earn [the locals’] trust and respect so that they would allow us to know the [monkeys’] routes.” The researchers returned year after year, maintaining the bridges and rebuilding them as they collapsed.

Rather than a traditional thick, flat-bottomed passage, Rojas’s canopy bridge consists of two lines. The top line is a simple rope, nothing more, and the bottom is composed of an agricultural mesh rolled into a cylinder. Howler monkeys usually walk on all fours across tree limbs, holding their tails above them to support them on unsteady surfaces. For howlers, the top and bottom rope design turn dangerous traverses into a cakewalk.

Plus, the team reasoned, the mesh looks ugly, so nobody will steal it.

Airplane view of rain forest in Costa Rica. Image by Rhett A. Butler/Mongabay.

From 2015, when Rojas and her team began installing bridges, to 2021, the howler monkey population nearly doubled in this forest, which stretches around the beach town of Playa Hermosa. Rojas set up cameras at the first 20 bridges, capturing photos of 76 separate howler crossings within a few months.

Howlers quickly adapted to their new options. “In the field,” the team wrote in the paper, “we frequently see them lounging and playing” on the lines.

Forest heroes

The rope bridges not only help howler monkeys, but also the forest they live in.

Mantled howler monkeys are highly intelligent, adaptable creatures. Their diet includes fruits and leaves from hundreds of plants, and their open-minded eating habits play a crucial role in forest ecology.

“[Howlers] are the gardeners of the forest,” Rojas says. When howlers chow down, seeds pass undamaged through their digestive system and drop to the forest floor. Some research argues that howlers’ life-giving dung is the most important factor in plant reproduction, especially in fragmented landscapes that don’t have other large fruit eaters.

“Many of these plants are also dispersed by the pollen monkeys carry from flower to flower in their mouths,” Rojas says.

A healthy howler population, according to a 2018 paper in Conservation Biology, can help keep an ecosystem stable, even as human development carves away chunks of wilderness. Their flexibility and accommodating palate mean that when their habitat shrinks, as roads split forests into fragments, for example, howler monkeys still survive.

When roads separate stands of trees, the forest’s microclimate changes due to a process called the “edge effect.” The new “edges” are exposed to more sun and wind. Plants that are used to a darker, moister climate begin to disappear. The new plant community is smaller, less diverse, and offers a poorer choice of food for most monkeys.

Research suggests howlers don’t seem to mind these changes too much, as long as the forest patches are large enough to support them. In smaller fragments, though, monkeys in search of food or mates must cross roads, either on foot or by swinging on power lines.

Building bridges

The team’s work has produced encouraging results.

“We managed to connect most of Playa Hermosa, end to end,” Rojas says. In addition to howlers, researchers have also caught glimpses of squirrels, kinkajous and opossums using the bridges, and counted fewer wildlife deaths in this area from electrification, vehicle strikes or dog attacks compared to past years.

A howler carries its young while crossing a bridge. Howlers have an outsized impact on their environment, carrying pollen between fragments of forest and helping plants to reproduce. Image by Inés Rojas.

Yet it’s not an unqualified success. Howler monkeys seem at ease crossing canopy bridges, but some other species may not be so brave. Spider monkeys, for example, haven’t been observed using the bridges.

“It’s not about placing a structure and that’s it,” Rojas says. “Data is needed, information that allows us to know places of passage for the animals, and this must be known prior to the installation of roads, power lines and buildings.”

Costa Rica, which has more canopy bridges for wildlife than any other country, is doubling down on its commitment to wildlife. The government is considering, and set to pass, a new law that mandates building canopy bridges over all new roads and buildings. The legislation includes current infrastructure, too, requiring cities to place bridges over every road where wildlife crossings have been observed.

Rojas says she hopes the new law will give howlers the dignity and peace they need. “It’s a way of being respectful,” she says, “of giving rights to animals that are as important as us.”


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