How to design a city for sloths


It’s hard to think of an animal less suited for jaywalking than a sloth.

As pedestrians, sloths do not walk as much as ooze, inching forward commando-style with their bellies to the ground, as if trying to dodge a museum’s laser-security system. From a distance, the 10 long minutes it takes an average sloth to walk across the street might look more like the end of a yoga class.

It’s not their fault that sloths are so bad at crossing the street. Far from a symptom of the deadly sin from which they get their name, the mammals’ molasses-slow movement is adaptive: Sloths are leisurely digesters, and by conserving energy they can survive on an extremely low-calorie diet of plants. Their ideal habitat is a dense forest canopy, which well-camouflaged animals can navigate without drawing attention to themselves or running into ground-dwelling predators.

But humans have been encroaching on these habitats. And with their urbanizing ways have come fast cars, humming electrical wires, a thinning canopy — and a dangerous new world for sloths.

The Sloth Conservation Foundation, based in southern Costa Rica, is trying to help sloths adapt to this changing environment, by stringing rope above roads that sloths can traverse by their signature crawl in just three minutes flat. So far, the foundation has built more than 130 such sloth crossings, which cost $200 each to install. They’ve also worked with the local electric company and with nearby property owners to sloth-proof their power lines, which sloths sometimes cling to when trees are scarce.

“If you’re putting up safe ways for them to travel from tree to tree, then they’re not getting attacked by dogs because they’re up in the canopy; they’re not getting electrocuted, because they’re using the ropes instead of the cables; they’re not being hit by cars,” says Rebecca Cliffe, the founder of the foundation. “Nobody bothers them up there.”

Six species of sloths live in Central and South America, but they face increasing pressure from development, deforestation and illegal pet-trade trafficking in counties like Venezuela, Panama and Brazil. Costa Rica, too, has experienced rapid urbanization, growing in population by almost five times between 1950 and 2000; by 2016, more than 70% of Costa Ricans lived in cities. Though widespread deforestation has slowed, Costa Rican cities, especially San José, have continued to sprawl.

“The growth of the metropolitan built-up area encroached on the rural spaces that formerly separated cities and towns in the region, with a resulting loss of environmental quality,” wrote researchers Rosendo Pujol-Mesalles and Eduardo Pérez Molina in a 2013 report for the Lincoln Institute of Land Policy.

The South Caribbean coast of Costa Rica was slower to modernize. When Cliffe arrived there a dozen years ago, the roads were still unpaved, cars traveled carefully, and there was at least a sloth in every tree. But in the years since, tourism has grown substantially. “I’ve seen it explode into this massively developed region,” she says. “Now there’s a highway, and there are traffic jams for miles down the road every night.”

In other parts of Costa Rica, the sloth population has already been all but eliminated, Cliffe says. But she’s hopeful that it’s not too late to protect the creatures. “We’ve got such a good opportunity in this region to try and achieve this coexistence and balance, because it’s an area that has undergone development so recently,” she says. “There’s still a chance to reverse the damage and do things the right way.”

When it comes to adapting to the pace and disruption of urbanization, sloths are just about the most vulnerable imaginable form of wildlife — which means that interventions designed to help them can end up benefiting other members of the animal kingdom. More than 20 other rainforest-dwelling species have also been seen traversing the sloth crossings, including red-eyed tree frogs who jump across the ropes, and primates who swing on them like monkey bars.

Wildlife crossings like these have sprung up across the world, in an effort to help wildlife negotiate the human-altered landscape, facilitate mating and migration, and connect surviving fragments of animal habitat. There are red crab bridges on Australia’s Christmas Island, a salamander tunnel in Massachusetts, and turtle underpasses on some train tracks in Japan.

Car collisions with wildlife cost the U.S. an estimated $8 billion a year in repairs and injuries. A study published in Biological Conservation found that between mid-March 2020 and mid-April 2020 — when car traffic thinned during pandemic lockdowns — highway mortality of large wild animals like deer and coyotes fell 34% in four U.S. states. Vehicular movement has swiftly returned to normal, and though helping animals cross the street is only one part of a larger conservation puzzle, the effort has gained traction and political support. The U.S.’s recently passed infrastructure bill includes $350 million for such interventions, like wildlife crossings and underpasses.

“It’s not a lost cause that humans and wildlife can coexist,” says Cliffe.

Navigating the rope bridges is not exactly intuitive for sloths, she admits. They’re creatures of habit, reluctant to change their ways when faced with new elements in their environment. When they approach the structures, they don’t always know exactly what to do. But “once they use it the first time, they’ll use it very, very frequently after that.”

Stringing up several kilometers of rope can’t entirely make up for widespread habitat destruction. To that end, the foundation is also trying to plant more trees. But it can put more space between sloths and their new, unnatural automobile predators.

“People look at them and think that they’re so poorly equipped to survive because you see them crossing roads and trying to to move around and they look so awkward and useless,” says Cliffe. “But if you put them in a well-connected rainforest, then they are masters of survival.”, 30 November 2021