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R ebecca Cliffe Photography


Suzi E szterhas


Both two- and three-fingered sloths have a whole array of fascinating anatomical features that allow them to hang inverted for up to 90 percent of the day. These include circulatory adaptations that prevent the blood from rushing to the head and a highly specialized esophagus that allows them to swallow while upside down.

C an introduction to the world ’s slowest mammal

onventional wisdom has it that sloths are simple, lazy creatures that do very little other than sleep all day. The very name “sloth” in most languages translates to some version of “lazy.” It seems astonishing that such an animal survives in the wild at all. But the truth is, sloths are profoundly misunderstood, ranging from benign speculations – that they sleep all day – to creative anecdotes such as, “Sloths are so stupid that they mistake their own arm for a tree branch.” Sloths are incredibly slow movers, but for a very simple reason: survival. The fact that sloths have been on this planet for almost 64 million years shows they have a winning strategy in taking it slow. But to understand exactly what makes them such slow movers and why this strategy works so well, let’s take a look at these unusual animals in a bit more detail. First, sloths don’t actually sleep all day – research has shown that, on average, wild sloths only sleep for 10 to 14 hours a day. In comparison, koalas regularly clock up 20 hours. Instead, sloths do move, but very slowly and always at the same, almost measured, pace. Moving slowly requires less energy than moving fast, and it is this principle that underlies the sloths’ unusual ecology. Everything about the sloth is geared toward saving energy and avoiding detection by predators. Due to their specialized, hook-like fingers, hanging inverted from a branch requires almost no energy. Sloths can eat, sleep, and even give birth in this position. They have very little muscle mass and survive with an incredibly low and variable core body temperature compared to other mammals. Sloths burn very little energy on movement, and they don’t need to spend much time or energy looking for food or metabolizing it. Their predominantly leaf-based diet provides few calories, and their incredibly slow digestive rate can take up to 30 days to extract the energy from just one leaf. Since the sloth’s large four-chambered stomach is constantly full, they can’t eat



Sloths’ “claws” are actually their finger bones – the “nail” simply forms a sheath over the bone, which is continually worn down by climbing.

not winning any medals for speed or grace, all six sloth species are surprisingly fantastic swimmers. Indeed, a submerged sloth can move up to three times faster in water than it can move on land! Sloths are perfectly adapted for a quiet life upside-down in the rainforest canopy, but their slow habits do not help them survive our quickly changing modern world. Rapidly expanding human population is driving deforestation to an unprecedented level and the rainforest habitat that the sloths depend on is becoming more and more fragmented. Roads, farms, towns, and cities now dominate the landscape, cutting the once continuous forest into smaller and more isolated segments. For animals that have limited dispersal abilities, habitat fragmentation can be detrimental to the species, with modern-day sloths now facing the perils of power lines, cars, and dogs, as well as genetic isolation. To ensure sloths will continue to hang from the treetops for generations to come, we must establish a way for humans and sloths to coexist. This will involve careful research and a better understanding of the sloths’ unusual ecology. While these animals are undeniably slow, they certainly aren’t lazy. Taking life at a slow pace helps them save energy, avoid the rush and tumble for food, and keep a low profile to help avoid being identified as prey. There must be a lesson somewhere in that for all of us.


very much on a daily basis. In fact, they can function with the lowest metabolic rate of any non-hibernating mammal. With little energy at their disposal, sloths do not have the capacity to defend themselves or run away from predators, as a monkey might. Instead, their survival is entirely dependent upon camouflage. It is likely that sloths simply move at a pace that doesn’t get them noticed by predators that detect their prey visually – big cats such as jaguars and ocelots and birds such as harpy eagles. To help them remain hidden, sloths have hairs with a specialized structure allowing over 80 different species of algae and fungi to grow, making sloths appear green and thus blend in with the rainforest canopy. Some species of fungi living in sloth fur have even been found to be active against certain strains of bacteria, cancer, and parasites! Also buried within the sloth’s fur are several species of moth and beetle, which are found nowhere else on earth. Each sloth provides a home to an entire unique ecosystem. Their hair grows in the opposite direction to every other mammal, parting on the stomach and making it easier for water to run off the body in heavy rain. The only time sloths will descend to the ground is to defecate, which they only do once a week, or to travel between unconnected trees. While on the ground they move awkwardly and are incredibly vulnerable. While certainly


upside down energy savers


rom their long, gangly limbs with elongated hook-like fingers to their dense, shaggy coat of hair, modern sloths are well adapted for an energy-efficient life hanging upside down in the rainforest canopy. For most animals, hanging upside down all day would cause some gravityinduced stress on their bodies. But to combat this, sloths have evolved a suite of adaptations that allow them to spend up to 90 percent of their time completely inverted. For example, the weight of internal organs can press down on the lungs and make breathing difficult. A sloth’s oversized stomach can account for up to a third of its body weight, so this is particularly problematic for an animal that doesn’t have much spare energy to play with. Sloths’ bodies, however, have evolved an ingenious solution. All of their organs are effectively bound to their rib cage via special internal adhesions, which are not seen in any other animal. These “organ anchors” not only prevent the lungs from being squashed but are estimated to reduce a sloth’s energy requirements by up to 13 percent. Two-fingered sloths further support their stomach when hanging upside down with an



impressive 46 ribs, which is more than any other mammal (in comparison, humans have 24 and whales have just 18). Their ribs are remarkably flexible, allowing sloths to frequently survive huge falls of up to 90 feet to the forest floor. While three-fingered sloths have fewer ribs, they have developed nine cervical neck vertebrae (almost all other mammals have seven). This extra-long neck gives three-fingered sloths the unusual ability to turn their head 270 degrees and helps them view the world right side up, despite being upside down. This looks quite strange, but it allows sloths to scan for danger effectively without having to change position. And while sloths are capable of seeing where they are going, their vision is certainly not their best asset. All sloths have a very rare condition called rod monochromacy - meaning that they completely lack cone cells in their eyes. As a result, they are color-blind, see poorly in dim light, and are completely blind in bright daylight. Thankfully, they compensate for poor vision by having a phenomenal sense of smell! In fact, a sloth will smell every leaf before eating it to determine how old it is and thus the toxin content. They will even smell a tree branch to check if it is rotten before trusting it with their body weight. By carefully moving through the rainforest canopy in this way, sloths burn very little energy while avoiding the attention of predators. It is an incredibly successful lifestyle strategy that pushes “taking it slow� to the extreme.

Secret Swimmers


A sloth in water may seem about as likely as a fish in a tree. However, sloths are surprisingly at home in the water, where they can move up to three times faster than on land! Swimming may seem like a pointless skill for an animal that spends most of its time high up in the canopy, but living in the rainforests of Central and South America means that swimming is essential for survival. In these regions, vast rivers fragment the forest, so sloths, unable to leap from branch to branch to cross breaks in the canopy like a monkey, can swim as a strategy to avoid geographic isolation. A closer look reveals some unusual features of the sloth’s body that help it to swim. The sloth’s enormous stomach creates a lot of gas and thus acts as a giant flotation device. Its long neck allows the sloth to keep its nose high above the water with ease, much like a swimming elephant uses its trunk as a snorkel. And much like its energy-efficient ability to hang upside down, once free from the need to invest effort in staying afloat, the sloth can simply bob along, using its long arms to control the direction of travel.


Blending in with their background, sloths spend virtually their whole life in the rainforest canopy, descending only occasionally to cross open ground between trees or to answer the call of nature. They make the long journey down to the forest floor just once a week to urinate and defecate at the base of their chosen tree before climbing back up to safety.


Sloths have an unusual method of camouflage. Cracks in their hair allow several species of algae and fungi to grow which makes them appear green and helps them to remain hidden among the rainforest leaves. Sloth hair also provides home to an entire ecosystem of invertebrates, some species of which are found nowhere else on earth!


As well as drinking milk, baby sloths will begin to sample leaves from around their mother’s mouth from as early as 1 week old.

life cycle Birth A two-fingered female sloth will give birth to a single baby after a gestation of 11 months; a three-fingered sloth after approximately six months (although this is still unconfirmed by science). When it’s time to give birth, the mother will usually descend from the canopy to a lower branch and give birth while hanging upside down. In this position, if the baby falls to the ground, it won’t fall far, and the mother can climb down to retrieve her young. Male sloths don’t care for their young, so the female sloth will spend the next 12 months of her life raising her baby alone. A baby sloth is born ready to face the world with eyes open, sharp claws, and fully formed teeth. It will instinctively cling to the mother’s fur immediately after birth and crawl up toward the safety of her chest. Once there, the baby will cling on for the next six months, regularly suckling small amounts of milk throughout the day.

Occasionally, sloths have been known to give birth to twins, but there is only enough room on the female’s chest for one baby, so the weakest twin will be rejected.




As a baby sloth grows, it learns what to eat by copying what its mother is eating. At first, the baby will eat the food from around the mother’s mouth. As it gets older, it will begin to reach off her body to grab leaves for itself while she is feeding. Through this process, the baby sloth will learn essential lessons about which tree species are safe to eat. Although sloths are known to feed from more than 90 different tree species, an individual will rotate among approximately seven to 12 favorite feeding trees – a strategy that prevents them from overeating specific toxins present in the leaves. Due to their incredibly slow metabolism and rate of digestion, if sloths were to feed from the same tree species for too long, it is thought that they would intoxicate themselves. Sloths learn this behavior and inherit their feeding tree preferences from their mothers, which may be why releasing hand-reared sloths into the wild can be particularly difficult. At about six months old, the baby sloth will begin to venture off the mother’s body and spend more time hanging out in the tree next to her. As independence draws closer, the distance between mother and baby slowly grows. Eventually, the mother sloth will completely leave her original home range and move to a neighboring, sometimes overlapping, patch of forest. This is an unusual method of separation for a mammal, as typically the baby is expected to disperse and establish a territory elsewhere. After separating from its mother, a baby sloth becomes completely solitary and will only interact with others when finding a mate.


Present in urine and faeces, pheromones are an incredibly important method of communication for all types of sloths and are likely the reason behind their unusual defecation habits. While sloths will typically only descend from the canopy to relieve themselves once a week, a female sloth will make the long journey down to the forest floor every single day when she is in estrus.

Mating Sloths reach sexual maturity when they’re between one and two years old. However, when you are a slow mover without much energy to spare, finding a mate can be difficult. Female three-fingered sloths have found an unexpected solution to this problem - they

scream! When she enters estrus, a female will emit high-pitched vocalizations to attract the attention of males. She will do this for eight to 10 days every single month, with the vocalizations increasing in frequency until she is screaming every 10 to 15 minutes.


S sloth science: dispelling the lazy myth

loths are some of the world’s most interesting yet misunderstood mammals. Due to their enigmatic nature and ability to blend seamlessly into the rainforest canopy, they are notoriously difficult to study. Scientists simply cannot see what the sloths are doing high up in the trees, and therefore we know very little about how they live in the wild. Further complicating matters is the tendency of these secretive animals to live entangled in dense, impenetrable jungles, which are not only hard for us to reach, but are also fraught with danger. From venomous snakes and tropical diseases to scorching heat and 100 percent humidity, it is not a comfortable study site for any biologist. As a result, research is limited and much of the published work covers horrifying topics such as “if you chop the head off a sloth it’s heart will continue to beat for 15 minutes”, or “if you hold a sloth under water it can survive 40 minutes before drowning”. These historical studies are extremely questionable and thankfully modern research ethics prohibit the replication of this type of work. With so little knowledge of how wild sloths live, developing conservation strategies is incredibly difficult. If we don’t fully understand the sloth’s basic habitat requirements and what the species needs to survive, then it becomes almost impossible to protect those necessities when faced with a rapidly changing world. Thankfully, research practices have progressed in recent years, and new work is beginning to shed light on what these mysterious mammals are really getting up to.



When sloths are either abandoned or orphaned as babies, they require careful hand-rearing and round-the-clock care. Because we know so little about how these animals live in the wild, it is incredibly difficult to successfully reintroduce a sloth that has been reared by humans. For example, the baby must be taught which combinations of leaves are safe to eat by feeding it from the exact trees it will find after being released.

Right: Every day hundreds of animals are falling victim to the land development and urbanization occurring throughout South and Central America. As a result, local authorities are delivering injured and orphaned sloths to rescue centers in unprecedented numbers. We do not know how many sloths remain in the wild today, but when we consider that these animals only raise one baby every three years, it becomes clear that conservation action is essential if we want to see sloths in the future.

Far Right: Sloths are creatures of habit. They do not have the ability to change their behavior in response to humans suddenly disturbing their habitat. When roads, power lines, and fruit plantations break up the continuous rainforest canopy that they rely on, sloths simply have nowhere left to go. This particular sloth was found bewildered, crawling across a busy road surrounded by fruit plantations. He was taken by bus to a rescue center where he was released in a protected area of forest.



loths are perfectly adapted for life upsidedown in the rainforest canopy. Unfortunately, the rainforest habitat that the sloths depend on is becoming more and more disturbed as a rapidly expanding human population drives deforestation to new levels. Roads, farms, towns, and cities now dominate the landscape, cutting the once continuous forest into smaller and more isolated segments. There is no way to escape the fact that one or two acres of rainforest land are cleared every single second. Due to their highly specialized lifestyle, sloths do not adapt well to such rapid environmental change. The ultimate creatures of habit, sloths inherit specific home ranges from their mothers, along with everything else they need to know. If a road is built that divides the sloth’s patch of forest in half, which may have been passed down through generations, the sloth cannot adapt and will descend from the safety of the canopy to crawl across the road. The magnitude of this problem is only just becoming apparent, with sloths now being admitted to rescue centers in unprecedented numbers. Sloths are facing a long and sad list of threats, not just deforestation but risks of power line electrocutions, dog attacks, road collisions, and human cruelty. As a result, the Pygmy threefingered sloths of Panama are now listed as critically endangered on the International Union for Conservation of Nature (IUCN) Red List, while the Maned sloths of Brazil have been listed as vulnerable.


auth o r


Rebecca Cliffe, is a British zoologist, one of the world’s leading authorities in sloths, and the founder and executive director of the Sloth Conservation Foundation (www.slothconservation. com). While living in the Costa Rican rainforest for many years, she spearheaded the longest recorded study into wild sloth ecology for her PhD and has since expanded her work to encompass many different aspects of sloth biology, physiology, and conservation genetics. Although her areas of research are broad, Rebecca’s work has always been focused on one central goal: helping sloths – whether through improving rehabilitation programs or tackling the problems sloths are facing in the wild. Her scientific work has been featured in a range of distinguished journals, as well as media outlets including the BBC, Discovery Channel, National Geographic, Animal Planet, Ranger Rick, The Independent, and Scientific American. With her wealth of experience from living with and observing sloths in their natural habitat, Rebecca has gained an unparalleled knowledge of the little-known habits of these mysterious mammals.

ph oto gr apher Suzi Eszterhas, is an award-winning wildlife photographer and has spent months photographing sloths in the jungles of South America. Her photographs have been published in over 100 magazine cover and feature stories in publications such as TIME, Smithsonian, BBC Wildlife, GEO, Popular Photography, Ranger Rick, and National Geographic Kids. As an author, Suzi has fourteen books in print with another four in progress. Her books have been featured on and Having lived in a bush camp in Africa for three years and photographed wild animals on all seven continents, Suzi has accumulated scores of adventure stories during her prolific career. She has hugged a baby whale, swam with sloths, fended off curious grizzly bears, had hyenas chew on her engine, had insects lay eggs in her feet, was chased by a green mamba, and hand-raised and released an orphaned serval. Suzi is passionate about wildlife conservation and helps raise funds and awareness for several organizations, including the Sloth Conservation Foundation. For more info, please visit


Sloth expert Rebecca Cliffe has teamed up with world renowned wildlife photographer Suzi Eszterhas to take readers on an immersive journey through the jungles of South America and discover the secret lives of sloths. The story is illustrated through spectacular imagery capturing some of the most intimate and rarely seen moments, while the narrative reveals some of the latest scientific discoveries and provides fascinating insights into the previously unknown habits of these unusual animals. A portion of all proceeds from the sale of this book go towards supporting the work of the Sloth Conservation Foundation. 22

Available for purchase online A significant portion of the proceeds going directly to The Sloth Conservation Foundation.

SLOTHS Life in the Slow Lane by Rebecca Cliffe and Photography by Suzi Eszterhas ISBN 978-0-692-86436-4 $25.00 us 144 pp; 10� x 8�, full color throughout. Publication date: September 2017

Sloths Life in the Slow Lane  

Sneak preview of book available for pre-order from The Sloth Conservation Foundation (SloCo) is a registered n...

Sloths Life in the Slow Lane  

Sneak preview of book available for pre-order from The Sloth Conservation Foundation (SloCo) is a registered n...