Sam Trull was never prouder of Kermie than when she watched him fight another sloth for the first time.

The two-toed sloth was hanging upside down from a tree branch when the scuffle began. Kermie—strawberry blonde, orphaned, hand-raised by Trull, still learning how to be wild—scrapped with the bigger, older, and wilder Diablo, who clearly wasn’t thrilled with this newcomer to his small patch of Costa Rican rainforest.

Unexpectedly, a sloth fight isn’t some sort of slow-motion martial art that's not actually violent, like capoiera or tai chi. Instead, it involves quite dangerous combatants armed with sharp, bacteria-riddled teeth and dagger-like curved claws on their hands moving at a not-unusual speed. Yes, sloths are slow, but they can move faster than you think.

As Trull watched, Kermie and Diablo hissed and snapped and scratched at each other. The fight didn’t last long. After a few minutes, Diablo backed down.

Kermie had won.

“He was the first baby I ever raised from a newborn,” she says. “To know that he can interact with other sloths he’s never met—and then fight them and win—is amazing.”

Trull runs the Sloth Institute Costa Rica, a small nonprofit organization—co-founded by Trull, a zoologist, and her business partner, Seda Sejud, in August 2014—that rescues, rehabilitates, and releases orphaned sloths into the wild. She is one of the handful of researchers studying the six sloth species that live in Central and South America, and her work with the creatures is documented in her new book Slothlove (inkshares). Filled with gorgeous photos and fascinating facts, Slothlove touches on Trull’s journey from primatologist to sloth-ologist. But its focus is the many sloths she’s tried to help.

Wildlife rescue centers aren’t uncommon in Costa Rica, and several handle sloths. But few currently attempt to release the creatures back into the wild. In the past, that’s often been a death sentence for the animals.

Trull is trying nevertheless. She’s faced with an extraordinarily difficult task: rescue and heal a biologically unique mammal whose physiology is only somewhat understood; nurture it without taming it; train it in the sloth skills it will need to survive in the wild, even though we know little about how sloth mothers teach their babies; and release it into the rainforest, where it may not know which plants to eat (because we only know some of them) or how it will interact with wild sloths (because we know even less about their social interactions).

That’s why Kermie’s fight was such a huge milestone for Trull. When she met him, he was a tiny week-old orphan, the only survivor of a set of twins. She fed, nurtured, played with, nuzzled, snuggled, trained, and then eventually let go of Kermie, who after several months in a large training compound has been largely living independently since October 2015. Somehow, through a mix of instinct and, Trull hopes, her training, the human-raised Kermie is managing not only to live wild, but—if his scuffle with Diablo is any indication—even to kick some ass.

Trull arrived in Costa Rica in 2013 after spending nearly two decades working with primates both in the U.S. and abroad. With a master’s degree from the UK's Oxford Brookes in primate conservation, she spent most of that time at the Duke Lemur Center, the largest prosimian sanctuary in the world.

In 2007, Trull lost both her fiancé in a car crash and her father to bone marrow cancer; the two died just six months apart. She spent the next few years wandering between the U.S. and Africa, grief stricken, directionless.

Then she moved to Costa Rica. She found a job at a small wildlife rehabilitation clinic on the Pacific coast called Kids Saving the Rainforest, which opened in 1999 in Manuel Antonio thanks to the efforts of two 9-year-old girls (with a lot of help from their mothers). The lush region draws many tourists, both Tico (Costa Rican) and foreign, and many expats have settled there. Between the hotels, homes, roads, and infrastructure, habitat encroachment has taken a serious toll on the region’s wildlife. The clinic needed someone trained in wild animal care to look after the many animals people brought to the clinic—squirrel monkeys zapped by electrical wires, sloths mauled by dogs, a wide variety of animals hit by cars.

Trull had experience in the field, and she needed a new purpose. In healing wild animals, she might also heal herself.

Less than a month after she arrived at KSRT, Kermie was brought in.

As tree dwellers, sloths cling to each other for both safety and comfort; and so Kermie clung to Trull. As she would with the others, she fed him milk through a nipple-capped syringe, wrapped him in blankets, cradled him against her all the time. She had no idea how to care for a newborn sloth, but she had to do something.

Monster, Elvis, Ellie, Newbie, and Chuck (named after her father) all followed. Soon Trull had a half-dozen orphaned sloths living in her apartment. They were a mix of two-toed and three-toed sloths belonging to one of the two sloth species that live in Costa Rica: the brown-throated three-toed sloth (Bradypus variegatus) and Hoffmann’s two-toed sloth (Choloepus hoffmanni).

They all needed basically the same thing: a mother. (Fathers have no role in rearing.) In the wild, up in the trees, a baby sloth spends about six months in gestation (Hoffmann's two-toed sloths for 11 months) and about the same amount of time after birth clinging to its mom. It’s clear this is a safety issue; a fall from a tree can be deadly. Even if a baby sloth manages to survive, it’s then largely helpless on the forest floor, unable to escape predators or survive on its own.

It’s also a temperature issue. As heterothermic creatures, sloths regulate their body temperature through their environment. Holding onto mom keeps a baby sloth warm. It may also keep its gut bacteria at the right temperature to digest its leaf-heavy diet.

But there’s more to it than that. Sloths are intense cuddlers. They must touch, at least early on in their lives. Whatever physical contact they don’t get from Trull, they get from each other, forming ongoing alliances and relationships. Chuck rides around on Monster. Elvis play-fights with Bruno. If no companions are around, a stuffed animal will do for comfort.

Two-toed sloths also—for lack of a better term—make out. Tongues are involved. “They ‘French kiss,’” says Trull. While kissing appears to be a sign of affection, it also likely fulfills a biological function—perhaps the exchange of bacteria and enzymes.

Mothers also teach survival skills. Diet must be part of it. But does a mom sloth teach her child how to hide, sleep in a tree, or move from branch to branch? Or is this all instinctual? We don’t know. Sloths are notoriously difficult to observe because they are masters of stealth.

Trull did what she could to keep them fed, play with them, and let them explore within a protected environment.

Sam Trull

Chuck snuggles.

Her devotion to the sloths soon caught the attention of the BBC, who profiled her work in a series called Nature’s Miracle Orphans. (It aired in the U.S. on Nature.) The clip below highlights her attempts to care for four-month-old Newbie, a three-toed sloth whose mother had been killed by a dog.

As she did with the other orphan sloths, Trull tried to replace the mom sloth Newbie had lost as best she could. She fetched Newbie the perfect guarumo leaf to snack. She positioned Newbie’s cuddle pillow in the perfect patch of doze-worthy afternoon sun.

Then Newbie got pneumonia. Four months of twice-daily oxygen treatments and injections couldn’t save her. After she died, Trull cradled her body for three hours. Her experience with Newbie cemented her desire to focus on sloths.

About a year later, in October 2014, someone brought to KSTR a pregnant sloth who had a severe head injury after falling out of a tree. She was having seizures—slow seizures, in sloth fashion. And then she went into labor. It wasn’t productive. Trull started to worry both mother and baby would die.

Detecting health problems in sloths is difficult because they are also masters of deception. When hiding is your best defense, cloaking your vulnerabilities is an important survival mechanism. “That makes it really hard to care for them because you don’t know something’s wrong oftentimes until it’s too late,” Trull says. “So you have to guess. Sometimes I go on my gut reaction. I can’t explain why something’s wrong, but something’s wrong. If you try to tell that to a vet, they look at you like you’re crazy.”

In this case, the vet Trull consulted about the pregnant sloth didn’t look at her like she was crazy. Instead, she ordered an x-ray. It revealed that the baby was breech. The two decided on an unprecedented course: a sloth C-section. Fully documented by Trull’s ever-present camera, the surgery made headlines worldwide.

Unfortunately, mother and baby both died a week later. The mother’s necropsy was inconclusive. The baby’s cause of death was no clearer. They classified it as a failure to thrive.

It was around that time that Trull shifted her attention to the Sloth Institute, which she and Sejud had launched just a few months before. The focus is the three Rs of wild animal care—rescue, rehab, release—along with one additional R: research.

The big idea was to keep these animals as wild as possible during their time in full-time human care so that they could hopefully thrive in the jungle on their own. The tricky part was that the very human Trull was going to have to train them to be wild. Trull reached out to other sloth researchers in Costa Rica and Colombia for advice and insights and pored over the relatively scant scientific literature on sloths.

Then, inspired by the “boot camp” at the Duke Lemur Center—a forested enclosure where lemurs practiced being lemurs before being released in Madagascar—she and Sejud built a 19-foot-by-19-foot-by-19-foot cage near the field site where they hoped to eventually release sloths. (Trull found housing nearby; her home is currently sloth free.) The sloths spend several months in the enclosure, which provides a protected slice of rainforest where they can work on survival skills like climbing, finding food, not falling, moving slowly, being very still, and sleeping.

When Trull thinks they’re ready, she gives them a “soft release”: “The doors open, and then they can come and go as they please until they feel comfortable enough to be on their own and they’re eating enough wild food on their own,” Trull says.

So far, just two sloths have had a soft release: Kermie and Ellie, another two-toed sloth. Both are doing well so far.

Their movements are tracked thanks to the VHF (“very high frequency”) collars they wear. Trull’s four research assistants observe and track Kermie and Ellie every night from 4 p.m. to 6 a.m. (Two-toed sloths are nocturnal.) The first time Kermie slept all day in the rainforest—the nocturnal sloth equivalent of a first sleepover party—was, like the fight with Diablo, a milestone.

They also spy on wild sloths to find out what they eat and how they behave. If the researchers note a wild sloth eating a particular kind of leaf, for instance, they’ll gather some of those leaves and introduce them to the orphan sloths’ diet. Just last week they got permission to collar their first wild sloth, whom the research assistants, all British, decided to call Percy. Because Percy is of similar age and size to Kermie, “he will give us a better idea of what Kermie should be doing,” Trull says.

Understandably, there has been a lot of trial and error during this process. Not every sloth has survived. They generally don’t do well in captivity. Some have succumbed to the injuries that landed them in Trull’s care in the first place. Others withered away for mysterious reasons; they may have had genetic conditions that led to their abandonment. The first iteration of the boot camp cage didn’t work so well; built far from the release site, it wasn’t snakeproof, and Kermie and Ellie’s companion there, Pelota, was fatally bitten by a venomous fer-de-lance pit viper. Her death was a heartbreaking loss—and bitter lesson—for Trull, who is as openly in love with these animals as she is fascinated with them as a scientist.

“It was so devastating,” she says. “But it was also obviously a big learning experience.”

Not being snakeproof wasn’t the only problem with the cage. It also enclosed a patch of jungle that was less than ideal. If the trees had been taller, for example, Pelota may have been able to climb higher than a fer-de-lance can go.

They built the second cage near the release site, where the trees are “better,” Trull says, and the rainforest the sloths are meant to call home after release is right outside the cage door.

One of the main challenges facing sloth rescue, rehab, and release is that there is still so much we don’t know about sloth biology, ecology, reproduction, social structure, or intelligence. Much of their lives remains mysterious.

Here is what we do know. There are two families and six species. Two are two-toed sloths, and four are three-toed sloths. Calling them two- or three-toed is a misnomer; it’s their hands that are different, so it’s more accurate to call them two- or three-fingered. (For simplicity’s sake, we’re calling them “-toed” throughout this article.) All species have three toes. They can have more bones in their neck than a giraffe—and the number of vertebrae varies by individual.

Two-toed sloths are substantially bigger than three-toed sloths (13 pounds versus 9 pounds on average) and they have a broader diet than the strictly herbivorous three-toed sloth, eating eggs, insects, small vertebrates, and even dirt. In Costa Rica, hibiscus flowers and cinnamon tree leaves are favorites.

Their teeth are unique among mammals, lacking both incisors and enamel, which leads to discoloration from the leaves they eat. Their mouths are riddled with bacteria, and the two-toed sloths have a particularly nasty bite. One sloth clamped down on Trull’s ring finger while she was feeding it, and it was only thanks to people nearby prying the sloth’s mouth open that she was able to free her finger. She had to take systemic antibiotics for weeks, and in the end she still lost the nail.

Sloths have the slowest metabolisms and least muscle mass of any mammal of their size (and yet “abs of steel,” Trull writes, allow them to spend much of their lives upside down). It takes a long time for them to digest food; leaves can take a month to process. And although they can be affectionate in captivity, the famous sloth “smile” is actually caused by a lack of expressive facial muscles. In fact, they stress easily. A good indication of a freaked-out sloth is large pupils.

A sloth may only poop weekly or even monthly, and three-toed sloths take a long and dangerous descent to the forest floor to defecate and urinate. When they do go, they can lose about one-third of their body weight. Three-toed sloths bury their waste using their stubby tail to dig a hole. In captivity, they may relieve themselves almost daily.

They are indeed slow. Their sluggish metabolism, combined with a leaf-rich diet—which, from a food energy perspective, isn’t very rich at all—keeps sloths in first gear almost all of the time. That can be good news when it comes to avoiding attention. Sloth movements are so slow, they fall beneath the detection threshold of most predators. But it can be bad news for a sloth that mistakes an electrical wire for a branch, as happens often enough in Costa Rica. If it grabs on, it’s likely to get severely sizzled before it can let go. Its muscles simply can’t respond fast enough before it gets injured.

Despite their lazy reputation, sloths may not sleep as much as we once thought. A recent study [PDF] of wild sloths in Panama that found they sleep on average 9.5 hours a day in the wild. (They can sleep as long as 16 hours a day in captivity.) Being on the alert for predators may keep them awake longer.

Their hair is uniquely structured. They grow algae both on and inside their hair. The benefit of this is unclear: It may help them to blend into the trees better. It may also have a nutritional benefit. While some research suggests sloths may eat the algae, Trull is doubtful. (“I’ve never seen a sloth lick its arm,” she notes.) She is more inclined to support another theory: that the hair acts like a straw sucking the algae close to the skin, where its nutrients are absorbed. (One study found that a type of algae that only lives on sloths is passed from mother to child.) Their hair can be home to a range of other creatures, including moths, beetles, fungi, and roaches.

Their hair sucks up scents, too, and will hold onto them for weeks. For this reason, anyone who works with sloths slated for return to the wild can’t wear perfume, lotions, or—hard to imagine in the rain forest—bug spray.

They can live a relatively long time for mammals of their size: anywhere from 10 to 50 years.

Despite their similarities, two-toed sloths and three-toed sloths are quite different. Two-toed sloths are more active, aggressive, and nocturnal. Three-toed sloths are less energetic and confrontational, and they’re mostly diurnal.

A clue to their differences lies in their evolutionary history. Remarkably, two-toed sloths and three-toed sloths aren’t closely related. They split from each other at least 40 million years ago, and perhaps as far back as 64 million years ago. While two-toed sloths are descended from giant ground sloths, three-toed sloths owe their genetic line to some arboreal creature.

Sloths are one of the most extreme examples of convergent evolution, when the same environmental pressures cause similar adaptations in different creatures, resulting in uncanny similarities despite a lack of common ancestry (at least recently). While convergent evolution is a long-known, fascinating, and yet quite common phenomenon in its own right—another example is the long, sticky tongue that arose separately among anteaters, armadillos, aardvarks, and pangolin—it's notable that the adaptations these different sloth families made over millions of years rendered them so similarly unique. Or uniquely similar. Among mammals, sloths are, well, weirdos. Tens of millions of years of natural selection has led the different families to be weird in similar—though not identical—ways.

“They’re just completely different animals. They probably shouldn’t have the same name,” says zoologist Becky Cliffe. A Ph.D. student at Swansea University who has been studying sloths in affiliation with the Sloth Sanctuary, another wildlife rehabilitation clinic in Costa Rica, Cliffe is looking into the ecology and genetics of sloths. Her main tool? A backpack. Specifically, a GPS-loaded animal backpack originally designed for penguins by biologist Rory Wilson, Cliffe’s Ph.D. supervisor. In the past six years, Cliffe has strapped these backpacks onto 15 three-toed sloths and 9 two-toed sloths. Some she followed for three years continuously. She has noted multiple differences between the two species in Costa Rica, which represent the two sloth lineages.

“They’re 64 million years apart,” Cliffe says. “It’s like calling an anteater a sloth. They’re not the same. We can’t really group them as the same animal when we’re talking about them scientifically, and much less when we’re talking about rehabilitation programs. I think they both have very different requirements. But I think it’s something we’re only just really beginning to understand fully.”

Here’s one thing all sloth species have in common: in the forest, they're really good at being sloths. Overall, they’re quite successful and common throughout Central and South America. Their uniqueness works in the trees. The problem is, it doesn’t work well outside the forest, which is constantly getting smaller thanks to human encroachment.

Beyond their physiology, much of the mystery surrounding sloths has to do with their interactions: namely, how they interact with their environment and with each other.

Thanks to the training and now the tracking, Trull is gathering a lot of new data about how they navigate their environment. One key insight: in order to survive, a sloth must be a careful cartographer of its local forest.

“For sloths, one of the most important things for them to learn is to basically map out the forest in their minds and to learn where they’re going,” she says. “That’s their biggest obstacle.”

She compares them to monkeys. “They’re similarly sized, and they’re both mammals that are arboreal,” Trull says. “But monkeys can just bounce around from tree to tree—bing bing bing! It’s no problem. And they can also see their food from far away. They can be like, ‘Oh, I want to bounce over to that tree that’s 15 meters away because they have some really big yummy berries.’”

Sloths are incapable of such improvised exploration, Trull says. “They need to know that tree’s there, they need to know they can get there, they need to know they can cross every single route through the forest and that berries will be waiting for them, because they can’t waste energy climbing 15 meters through a puzzle maze of trees just to get there and have no food.”

Cliffe says sloths are “masters of energy savings.” Every movement happens at the same speed, from blinking to grasping. She recently studied the metabolic rate of wild sloths by injecting them with doubly labeled water (in which certain isotopes of hydrogen or oxygen are replaced to allow for easier tracking) to measure how much energy they used during a two-week period. Sloths don’t eat much, because it can take them 30 days to digest the leaves in their diet.

“The energy supply is so restricted, that they have to save energy or they’re not going to be able to get from this tree to the next tree where they know they can eat the leaves safely,” she says. “I like to say they’re on the edge of the ‘energy budget.’ If they spend too much energy doing one thing, they’re not just going to have anything left to compensate. I don’t know if there are many other mammals that live quite on the edge like that. But they’re doing it quite well. They’ve been around for about 64 million years, so they’ve got the balance just about right. And it’s happening in both kinds of sloths, they’re just doing it in different ways.”

That likely explains their strategy from getting from place to place, Trull says. Once they find a route between points, they take it again and again. This also explains why roads are so dangerous to sloths. They don’t improvise well.

Observing their locomotion has been revealing. Sloths use their body weight and careful timing to move from branch to branch. They also follow a basic rule that all rock climbers (and ladder users) know: always maintain three points of contact. Sloths have three limbs on the next branch before they let go of the previous one, Trull says.

Two-toed sloths also appear to be nearsighted, so visual acuity isn’t much of a factor in their forest navigation. Unlike a monkey, which can spot a sweet treat in your hand from far away, a sloth can’t see very far. That means they’re likely unable to spot food from a distance. They can’t rely on their eyes to plot a course.

All of this has both Trull and Cliffe curious about the nature of sloth intelligence. “The fact that have to have these mental maps of the forest just to get around must show some level of memory ability,” Trull says. “Also, I think—and I don’t really have any evidence yet to back this up except my gut, but—they definitely get a bad rap for being stupid and lazy. Yes, they’re slow, but their slowness is a part of their genius.”

While Trull is still mulling over a study design to test out sloth intelligence, Cliffe has attempted to measure it. “It didn’t go well,” she admits. They placed a three-toed sloth in an outdoor tree maze. It didn’t move. At all. “We gave up in the end. When sloths aren’t sure what’s going on or where they are, they sit still. That’s their defense mechanism.”

Cliffe says, “I don’t think they’re intelligent in the way you think a monkey or a dog is intelligent, but they’re smart in their own way in their mental maps and their memory.” In her six years of tracking, she could predict which branch of which tree they’d be on during a given day. “But if you cut that tree down, they’d be stumped. I think they’re smart in the ways they need to be, but beyond that, there’s not much there.”

Whatever their baseline intelligence, to survive and reproduce, sloths need to have some sort of social intelligence. While sloths are classified as solitary creatures, it’s clear there’s a strong bond between mother and offspring, and the orphan sloths in Trull’s care rely on each other for companionship, play, and comfort. As we mentioned, two-toed sloths kiss, sometimes upon encountering each other.

In the wild, there’s also the issue of sharing the forest. Kermie and Diablo have now scuffled three times (Kermie won twice), but another wild male in the vicinity shows little tendency to mix it up. What this indicates about sloth social structure is unclear, Trull says—“We’re still so new in our research”—but she notes that one previous study found what may be an “alpha” sloth with a group of females.

While they haven’t observed that behavior, it’s something to keep in mind when planning to release sloths. “The ratio of sexes in the wild is probably pretty important,” she says. “We probably shouldn’t release too many sloths in one area, because they’re just going to end up fighting, eventually.”

Diablo has also shown interest in Ellie. If they decide to mate, we know the basics of how that might unfold. Like humans, a female sloth goes into estrus once a month for about a week. She’ll make a high-pitched scream to attract a male. After mating, he’ll guard her for three or four days, and then the two will go their separate ways.

“In terms of everything else, we don’t really know much about it,” Cliffe says. “There’s very limited observation as to what’s going on.”

After mothers raise their babies, which takes about six months, they leave the territory to their child, and move on themselves. But their ranges aren’t big, males and females live near each other, and sloths live quite a long time; one 25-year-old sloth Cliffe has observed is still going into estrus. That means generations of sloths can overlap in specific regions. Which makes Cliffe wonder: “How are they not inbred? Or maybe they are really inbred. Maybe that’s why they’re so weird.”

Cliffe has hair samples of all the sloths she’s been tracking, and a genetic analysis is underway that will reveal their genetic diversity—or lack thereof. “It’ll show me who is related to whom, and who’s the father of which baby,” she says. “So we’re going to get a lot of information from it.”

Kermie and Ellie are soon to be joined in the wild by Monster and Piper, both three-toed sloths. They’ve been in the boot camp cage for months. They’re slated for soft release on May 1, and they too will be collared and tracked.

Monster, who came in at two weeks old and is now 2.5 years old, is “my slothy soulmate,” Trull says. “Releasing her will be very emotional. I’m so excited for her. She’s done amazingly. Obviously, through the whole process, you know that they’re wild animals. But it’s also very amazing and reassuring to see their instincts kick in with certain things. At least they’re coming at this with some knowledge, and I don’t have to teach them everything. But to see them learn everything I’ve taught them is very rewarding as well.”

Hopefully, Monster and Piper will fare as well as Kermie and Ellie have. Trull and her team will continue to monitor their lives, all the while preparing the next batch of sloths for release.

In Slothlove, the animals look downy, soft, and clean. That’s not because Trull ever gave them a bath. Being hand-raised by a human, they simply spent less time in the wild. But now Kermie and Ellie are out and about, and they’re changing. They have little interest in Trull, which is exactly what she was hoping for. Their fur is growing algae. And their scent is different.

“As they become more wild, their smell has changed. They smell really good now,” Trull says. “Not that they smelled bad, but Ellie and Kermie smell distinctly different hanging out in the trees all the time than they did before that—which is interesting.”

Their new scent is kind of a subtle testament to the fact that they seem to be moving—slowly, of course—towards a fully wild life in the forest. “It’s probably just coming from being in the trees and whatever saps that are getting on them from sleeping and moving in the trees versus living in my living room or sleeping in bags,” Trull says. “But they smell like trees now.”

All photos © Sam Trull in Slothlove

You can keep up with the Sloth Institute’s work at their website, as well as through Trull’s Tumblr and Twitter feeds. And if you’d like to support their work, you can donate here.