NEWLY PUBLISHED – the sloths extraordinary metabolic response to temperature!

We are delighted to announce that our latest sloth research paper was published today in the peer-reviewed scientific journal PeerJ! It is freely available for everyone to access and an online version is available here.
The results are fascinating and underpin much of the unusual behavioural ecology that we see in sloths, however we understand that not everyone will have the time or sloth-like patience to trawl through the actual publication (and unless you are a scientist, you might find the paper a little tedious). For those of you who don’t fancy reading the full manuscript, we have written a short, simple summary of the project below.


Typically, mammals have something called a thermo-neutral zone. This is basically a comfortable range of temperatures in which they use very little energy because they don’t need to control their body temperature (or thermo-regulate). This thermo-neutral zone differs between species, but it is usually always there at some point. On either side of this zone, either when it is too cold or too hot, mammals use lots of energy trying to make sure that their core temperature stays at a comfortable level. Reptiles and birds, however, don’t have this zone. They use very little energy when it’s cold and a lot of energy when its hot (because they can’t control their body temperatures and all metabolic processes work faster in hotter temperatures).

To complicate matters, when faced with extreme conditions (such as hot/cold temperatures, or a lack of resources) many mammals can enter a ‘hypometabolic’ state (i.e. lowering metabolic rate) to conserve energy and aid survival until conditions become favourable again. Such states, including hibernation, daily torpor and aestivation, all vary in length but are characterised by a drop in both body temperature and metabolic activity. The molecular mechanisms underlying the active control and suppression of metabolic rate in mammals remain poorly understood and are the subject of a lot of recent research due to the potential far-reaching medical implications.
But where do sloths fit into all of this? Given the strong link between sloth body temperature and that of the environment (which has led to them being likened to reptiles), an increase in temperature should, theoretically, result in an increase in metabolic rate. However, nobody really knows. Considering the sloths limited energy supply and the potential knock-on effects of a warming climate, we designed an experiment to find out.



We determined the resting metabolic rate (RMR) of 8 adult three-fingered sloths (B. variegatus) using indirect calorimetry (i.e. by monitoring the oxygen consumption and carbon dioxide production of sloths that were sleeping inside of a metabolic chamber in Costa Rica). We changed the temperature of the chamber (within the natural range that sloths would typically experience in the jungle) while simultaneously recording the sloths body temperature, activity and postural adjustments.



It appears that sloths do not behave like mammals, reptiles OR birds when it comes to their metabolic response to temperature. They use very little energy when it is cold (just like a reptile), lots of energy in the middle (between 26-30 degrees), but then as they get too hot, they begin to use less energy again. This reduction in metabolic rate at high temperatures is the exact opposite of what typically happens in all other animals.

So why would they do this? Well at cold temperatures, the sloths are in a similar situation to reptiles (unable to raise their body temperature and so the metabolic rate is very low). In the middle zone, the temperature is ideal and so they can burn energy at the optimal rate (this looks like using lots of energy, but it might actually just be normal energy expenditure for a sloth). We nominally refer to this as the sloths “thermally-active zone”. This middle temperature range coincides closely with average daytime temperatures in tropical forests, when three-fingered sloths happen to be the most active and feed the most. This links in well with one of our previous publications that shows that sloths eat more at hotter temperatures due to their increase in metabolic activity increasing their rate of digestion (see: ‘Sloths like it hot: ambient temperature modulates food intake in the brown-throated sloth, Bradypus variegatus’).

Finally, and here is the odd part, when it gets too hot the sloths can temporarily (and perhaps strategically) actively depress their metabolism in a manner which seems unique in the animal kingdom. While we don’t know exactly how they are doing this, to our knowledge this is the first physiological evidence of a mammal quickly invoking reversible metabolic depression without entering a state of torpor, aestivation or hibernation. But what could be the benefit? Well, when it gets too hot in the wild, all a sloth can really do is to move into the shade to cool down. It would therefore make sense to sit still, sleep through it, and wait for the conditions to become more favourable for activity. Whilst sitting still, by depressing their metabolism they are simultaneously saving energy and also reducing the amount of metabolic heat being produced by their body (and therefore staying a little bit cooler).

Due to slow rates of digestion limiting the rates of energy acquisition, all sloths exist under severe energetic constraints. It appears that delicate adjustments of metabolic rate – in part as a response to temperature – are one way in which sloths conserve energy. Reductions in metabolism therefore minimise energy expenditure at cold temperatures and reduce the risk of hyperthermia at hot temperatures.

Ultimately this broadens our knowledge of how animals deal with variation in temperatures, and further work to determine the underlying molecular mechanisms controlling the metabolic depression in sloths could provide important insights into the active control and suppression of metabolic rate in all mammals. In addition, this gives us vital information about how sloths rely on the envoronmental conditions for all aspects of their energy usage, and we will need to take this into account as we consider the impact of a warming climate on the sloths survival in the future.

The Great Sloth Run – the first ever 5K race to benefit sloth conservation!

We are excited to announce that, together with Pangols and Sloth Relief & Shelters, we are launching the first ever 5K race to benefit sloth conservation – The Great Sloth Run!


You can compete to win, or you can simply channel your inner sloth and go as slowly around the course as you like – as long as you make it to the finish line! Participants will receive a race T-shirt and a race medal, in addition to gifts from sponsors in the packet pick up. There will be prizes for first second and third place, in addition to a prize for the best sloth costume! The race is capped at 250 individuals so it’s probably best to sign up first and nap later!

This 5K will be held in Arlington, Virginia on November 3rd. The race will begin at 11:30 AM at Bon Air Park and participants will run along a section of the Bluemont Trail. Don’t worry if you aren’t in the area, we also have a limited virtual option for up to 50 people where you will still be able to receive a medal and a T-shirt in return for your participation. However, if you are going for this option we do encourage you to sign up quickly as places are limited and you may not receive your goodies in time for the event.


Here is how you can get involved:

1. If you would like to participate in the official run in Virginia, simply register here at Race Entry.

2. Virtual runners can purchase their participation package here.

3. Register to fundraise on Givebutter.

4. Start collecting donations – the more you fundraise for the sloths, the better! We are asking for people to try to raise $200 to start at the official event.

5. Check out the event page on the Pangols website!

6. Also, check out the Facebook event and reguster your attendance!


Good luck and see you on November 3rd!

Sloth Island, Guyana: in pursuit of the pale-throated sloth!

Hiding deep within the remote jungles of South America, there is an elusive and commonly overlooked species of sloth clinging onto survival. The pale-throated sloths are perhaps the most striking of all the sloth species, with a large bright yellow face, white throat and mottled fur, and yet they also receive the least attention. While the maned sloths and pygmy sloths are famous for their critically endangered conservation status, and the brown-throated sloths are the most commonly seen species, the pale-throated sloths have slipped under the radar for decades. They have also bypassed the attention of the scientific community, with not a single scientist currently researching any aspect of their ecology or behaviour. With this in mind, SloCo Founder and Executive Director Dr Rebecca Cliffe and award-winning wildlife photographer Suzi Eszterhas embarked on a mission to locate and photograph the pale-throated sloths in order to shine a spotlight on the plight of this mysterious species.



The first big challenge when planning this adventure was deciding where to go to find the sloths. This species can only be found on the east coast of South America, ranging from Venezuela through Guyana, Suriname, French Guyana and into Northern Brazil. These regions receive very little tourism, and so finding an area where the sloths can be found reliably in the wild in relatively high densities (and not just at the tops of the tallest rainforest trees where photographs become impossible) was certainly challenging. After doing a lot of research, everyone that we spoke to unanimously directed us to the same place; Sloth Island in Guyana. This appropriately named island is located in the vast Essequibo river (the 3rd largest river in South America) and was named by early Guyanan settlers who discovered that it was home to a particularly impressive abundance of sloths. The island is now privately owned by a fascinating man called Raphael who has constructed an idyllic Eco-lodge and vowed to protect the pristine rainforest that covers the interior of the island.

Sunset at Sloth Island, Essequibo river, Guyana

Before our arrival at the lodge we were told that we would be met by a Guyanan man from the ‘Macushi’ tribe called Claude, who is the caretaker of the island and apparently a master sloth-spotter. Claude did not disappoint. Within 30 seconds of landing on the island he had already pointed out our first sloth; a young male sleeping in a tree right outside the front door of the lodge. It was a good sign of things to come. The male sloth remained in the garden of the lodge for a full week, regularly coming down to within a few meters of the ground in the afternoons to seek out shade from the scorching sun – a perfect position for photographs! Because much of the island floods at high tide, much of our sloth searching in the dense forest was restricted to a wooden canopy walkway, raised about 6 feet off the ground. This didn’t hamper our sloth spotting opportunities however, as Claude proceeded to point out 13 different sloths over the course of 5 days, including two females carrying tiny babies. One of these females blessed us with her presence every single day, hanging out peacefully right above the walkway and descending to just above head height to feed on vines in the afternoons. This unfortunately always coincided with the aforementioned male sloth (who also descended from the canopy in the afternoons) and triggered a lot of chaotic sprinting back and forwards through the jungle carrying heavy camera equipment! We thought that our biggest challenge would be finding the sloths, but it actually turned out to be choosing which sloth to prioritise and getting the camera set up in the right place to catch the rapidly unfolding action (really quite ridiculous considering we were working with the worlds slowest mammal)! In one instance we were following a female sloth with a baby and didn’t realise that there was a second male sloth hanging out in the exact same tree until she quite literally climbed right up to him. Sloth Island really did live up to its name.

We spent 12 continuous hours waiting for a sloth to wake up – and then three sloths all appeared at once


Exactly why there are so many sloths on the island compared to the mainland remains a bit of a mystery, although it is likely that the island represents a sort of safe haven due to a lack of land predators (although the breath-taking harpy eagle – one of the largest species of eagle in the world and the sloths main predator – can regularly be seen scouring the skies around the island)! Though the island is famous for its sloths, we were regularly surrounded by all sorts of wildlife – from waking up to the sounds of howler monkeys and dodging piranhas in the river to watching spider monkeys leap through the canopy and ending the day with thousands of Amazonian parrots filling the sky. Guyana represents one of the few remaining areas of the world where nature dominates over human disturbance. With a human population of only 750,000 (virtually all of which live on a narrow strip of land by the ocean) and very little tourism, this means that over 80% of the country is still covered in undisturbed rainforest. Guyana really is an undiscovered and unparalleled paradise for nature lovers. To top it all off, a historic agreement made between Norway and Guyana represents a perfect example of how developed and developing countries can work in partnership to save the worlds tropical forests. Norway pledged to provide financial support of $250 million in exchange for Guyana halting deforestation over a 5-year period.5
While tourists flock in their thousands to countries like Costa Rica (which has a primary forest cover of only 3.5%) to see sloths and other tropical wildlife, the truly wild countries like Guyana remain well off the beaten track. If you are planning a vacation and want to see sloths in the wild, or just want to embrace your wild side and experience the jungle exactly how nature intended it, there is no better place than Sloth Island in Guyana!

To learn more about Sloth Island and to book your visit, check out their website by clicking here.


Is Costa Rica really the best place in the world to see sloths?

If you want to see sloths in the wild, a quick google search will definitively point you in the direction of Costa Rica. With its abundance of wildlife (boasting 5% of the worlds biodiversity), assorted ecosystems (ranging from the cool montane cloud forests to the humid tropical lowland rainforests) and a relatively well-developed infrastructure, approximately two million tourists flock to this tiny Central American country every year. And for good reason; you are almost guaranteed to see a sloth. But these animals are found in numerous countries throughout South and Central America – so what makes Costa Rica different? Why does this little country appear to have so many sloths? And why are they so easy to see? The truth is perhaps surprising.

Brown-throated Three-toed Sloth Wild sloth climbing trees in forest at Sanctuary Aviarios Sloth Sanctuary, Costa Rica
A brown-throated sloth reaching to cross a gap between two trees. Copyright Suzi Eszterhas

Sloths are perfectly adapted for life high up in the canopy of ‘primary’ rainforests. Primary rainforests are defined as those forests that are in pristine condition, having never been disturbed by humans. They typically have very tall trees (that can reach over 250 feet in height) forming an upper canopy, followed by several layers of understory canopy. In this environment, the trees all connect and so the sloths can travel easily throughout their home-range without having to come down to the ground very often. Sloths have thrived in this continuous and undisturbed habitat for over 64 million years, and they are continuing to thrive in regions where this ecosystem remains.

Costa Rica does not have an unusually high sloth population. In fact, experts predict that sloth numbers in Costa Rica are falling rapidly. While there is no official population count (for scientists to count an animal, it must first be seen – something which sloths are very good at avoiding), rescue centres across the country are receiving sloths at an alarmingly fast rate. Sloths are incredibly slow to reproduce, having only one baby every 3 years and facing a natural mortality rate in infants of approximately 60%. Consequently, the sloths are being wiped out faster than they can reproduce.

The simple reason for this is that Costa Rica does not have enough healthy forest left – particularly primary rainforest – for the sloths to inhabit. In fact, as of 2005, only 3.5% of the country was covered by primary rainforest (compared to an average of 40-50% for other South and Central American countries). This is astonishing. Since the 1960s Costa Rica’s primary forests have been wiped out for agriculture and cattle ranching (not helped by the United States offering Costa Rican farmers millions of dollars in loans to produce beef). In the 1990’s Costa Rica had one of the worst deforestation rates in the entire world, with only 26% of the country having any sort of tree cover left. Realising the detrimental effect that this was having on ecosystems and wildlife, the Costa Rican government implemented some impressively rigorous conservation strategies to protect the remaining forests. As a result of these efforts, Costa Rica is now the only tropical country in the world that has managed to actually stop and reverse deforestation – the amount of forested land is increasing year by year, albeit very slowly. While this is certainly beneficial, the loss of the ancient primary forests can never be reversed, and it is the true primary forest that the sloths need in order to thrive.

Change in forest cover in Costa Rica between 1940 – 2005

But why, then, are sloths so easily seen in Costa Rica? Sloths inhabiting their preferred habitat – primary rainforests – will be very difficult to spot. In this environment, sloths are perfectly camouflaged and become invisible hiding in the dense foliage at the top of gigantic trees. But in Costa Rica the sloths don’t have this option. They are being forced to exist in increasingly urbanised environments, and here they cannot hide! While Costa Rica may appear to be a green and eco-friendly country on the whole, the secondary growth forest that covers much of the land is sparse and fragmented in comparison to the dense vegetation found in the primary rainforests. If you look closely at the tree cover in many regions, you will notice that the trees rarely connect or overlap at the top. This is manageable for species such as monkeys that can traverse gaps by jumping, but it is detrimental to sloths which are forced to descend to the floor and crawl across the ground. As a result, while attempting to navigate these disturbed areas, the sloths are very easily seen while crossing on the ground or hanging from a power line or isolated tree at the side of the road. Furthermore, as travel becomes more accessible and increasing numbers of tourists flock to Costa Rica because of its notorious sloth viewing, the infrastructure is being developed to meet the demand. For example, roads that were once pot-hole riddled dirt tracks have been transformed into 80 km/h highways, and the government have just revealed plans to expand the only major highway that links the Caribbean coast to the capital city (route 32) which is now predicted to create a 50-meter gap between the trees on either side of the road. When considering the speed at which development is occurring, it hardly seems surprising that sloths are being attacked by dogs, poached / harassed by humans, run over by cars and electrocuted on the power lines in record numbers. This equally explains why Costa Rica also has more wildlife rescue centres per square mile than any other country in Central or South American. All of the wildlife is struggling, but the sloths are facing a particularly vulnerable future.


Size (miles 2) Population

% Forested

% Primary Forest

Costa Rica

20,000 5 million 52 3.5


83,000 750,000 77 48


3.2 million 207 million 62 50
Panama 31,000 4 million 58


Colombia 441,000 48 million 59


Peru 496,000 32 million 54


So what can we do about it? Sadly, there is no way to replace the lost primary rainforest, and the knock-on consequences for wildlife are going to be seen for the foreseeable future. However, the negative impacts are certainly being minimised by the implementation of rigorous conservation policies by the government. When faced with a booming population and tourism industry exerting pressure on the infrastructure, managing to maintain a reverse in the levels of deforestation is an impressive feat. However, there are also things that we can all do on an individual level to help. For example:

  • Practice sustainable tourism – if you do decide to visit Costa Rica, try to be a responsible tourist. This includes:
    • Respect the personal space of all wildlife. Leave at least a 3-meter gap between yourself and any animal you come across and definitely do not touch / harass the animal for a photo opportunity. Try to spread this message to anyone that you see getting too close to an animal.
    • Stay in eco-friendly accommodation. Try to find places that utilise renewable energy, protect the natural resources on the property and use sustainable materials.
    • Take care while driving. The new roads may make it easy to drive quickly, but be wary that an animal may be trying to cross the road just around the corner…
  • Buy Eco-friendly products – Always try to support local, organic farmers. If you are buying fruit, do not support the large, mono-culture companies that are responsible for so much of the deforestation and pollution in tropical countries. This includes Dole, Chiquita and Del Monte.
  • Eat less meat – Cattle ranching is the leading cause of deforestation in Costa Rica and animal agriculture is responsible for up to 91% of all Amazon destruction. That includes clearing land for both cattle grazing and to make space for the vast crop plantations for livestock feed. Furthermore, livestock and their by-products are accountable for 51% of all greenhouse gas emissions worldwide – in comparison, the entire transport sector (including cars, airplanes, and boats) account for just 13%.
  • Support small non-profit conservation organisations that work on the ground – Be careful who you donate money to. Many of the large organisations have such big overhead costs that virtually all donation money ends up in the pocket of the employees. Try to find smaller charities that are working on the ground to protect wildlife. In Costa Rica some good organisations include:
  • Expand your horizons – If you are planning a trip to South or Central America with the hopes of seeing sloths in the wild then potentially consider visiting countries other than Costa Rica. For example, Guyana has an incredible 77% forest cover, a human population of only 750,000 people and no need for any animal rescue centres because the wildlife isn’t facing major disturbance. SloCo founder Dr Rebecca Cliffe recently returned from a trip to Guyana to observe pale-throated sloths in the wild with incredible success. We recommend staying at the Sloth Island Nature Resort – a pristine island located in the Essequibo river, named hundreds of years ago after settlers discovered it was home to a particularly high density of sloths. During her 6-night stay on the island Rebecca was able to observe 13 different sloths, including close encounters with two wild females carrying babies.

Starving to death on a full stomach

There is a reason why sloths are only found in neotropical rainforests – because the weather is warm, humid and relatively stable all year round. Sloths are unable to regulate their own body temperatures like most mammals, and instead are completely reliant on the environmental conditions. This means that when it is too hot or too cold, sloths have to modify their behaviour accordingly. On hot days, sloths move very little (because movement generates even more body heat). They drop down to the lower canopy, find some shade, spread out their limbs and wait for the temperature to cool down. When it is too cold, sloths either have to warm their body temperature by basking, or they have to just sit still, curl in a ball and wait for the sun to come out.

Copyright, Suzi Eszterhas

Although the tropics do sometimes experience temperature extremes (cold depressions and heat waves), temperatures typically stay within a very narrow range. For example, during my sloth studies I set up a remote weather station and monitored the conditions for 3 continuous years. The temperature on the Caribbean coast of Costa Rica never dropped below 23℃, even in the middle of the night. However, this is no longer the case. During the end of 2017 and the beginning of 2018, the temperature in the Caribbean has plummeted. In December we experienced our first tropical storm on record, and temperatures frequently fluctuated below 20℃ for weeks on end. Species that live in the tropics have evolved over millions of years in a climatically very stable environment, and as such, they do not have the flexibility to cope with extreme, rapid weather changes. This is particularly the case for sloths.

Sadly, as a result of the cold temperatures, we are seeing a huge increase in the number of sloths that are being brought into rescue centres because they are starving to death. This is not because they can’t find enough food (there are still plenty of leaves on the trees), but rather because they can’t digest the food that they are eating. Sloths rely on a range of symbiotic gut bacteria and microbes in their stomach to break down the tough cellulose in the leaves that they eat, and these microbes are temperature dependant. When the environmental temperature drops, so does the sloths body temperature, and if it drops too low then the bacteria and microbes die. In this situation, the sloth can eat the same amount of leaves as normal but can starve to death on a full stomach because they can’t extract any nutrients. The only way to save these animals is to replenish the gut bacteria. This can be done by feeding with probiotics and (in extreme cases) faecal matter from a healthy sloth – something that rescue centres fondly refer to as a ‘poop cocktail’. But the real problem that we need to address is the changing climate. While some people may shake their heads, there is no way to deny that the weather patterns in the tropics are becoming progressively more unstable. Unfortunately, there is also no easy fix. It is going to require a collective effort by the human population to reverse the already apparent effects of climate warming. All we can really do is each do our best on an individual level to reduce our carbon footprint. Eat less meat, utilise public transport, walk or ride your bike instead of driving, recycle and try to cut down on your use of products that negatively impact the environment (this includes plastics as well as products that stem from the monoculture plantations that are responsible for a huge proportion of deforestation – palm oil, mass produced fruit etc.).
It may seem overwhelming, but as the famous anthropologist Margaret Mead once said: “Never doubt that a small group of concerned citizens can change the world. Indeed, it’s the only thing that ever has”.


The Male Sloth Speculum


We often receive reports of sloths that have been severely burnt or attacked with machetes as they are spotted with what appears to be a horrifying injury on the upper back. What looks like an open wound or blood-stained fur, however, is in fact completely normal. It is a little known fact that all male three-fingered sloths develop this striking and totally unexpected feature called a ‘speculum’ at sexual maturity.


The speculum consists of a mid-dorsal brightly coloured patch of fur that develops on all male Bradypus sloths (with the exception of B. torquatus, the Maned sloths) at sexual maturity. The speculum ranges from yellow to dark orange in colour with a prominent black stripe running down the centre and has additional markings displayed in a pattern unique to each individual. The hair within the speculum is short and downy, covered in an oily secretion, and has a musty scent. While the presence of the speculum has occasionally been acknowledged in the scientific literature, there has never been any speculation as to its purpose. For a cryptic animal such as the sloth, which relies entirely on avoiding detection by predators (to the extent that they have developed a symbiotic relationship with green algae growing in their fur to aid camouflage), the development of a highly conspicuous feature appears counterproductive. Sloths are one of the main prey items for the harpy eagle, a bird with highly developed colour vision and which can consequently clearly distinguish between the green rainforest canopy and the bright orange colouration of a sloth speculum. It is hypothesised that these brightly coloured markings are linked to sexual selection, with fitter males developing larger, more conspicuous speculums which in turn give off a more pungent scent and are favoured by females. The scent is likely of high importance since recent research suggests that all sloths have a rare condition called ‘rod monochromacy’ which renders them completely colourblind.

Dr. Rebecca Cliffe

Copyright. Suzi Eszterhas
Copyright. Suzi Eszterhas

Gifts That Give Back

Shopping for the sloth lover in your life? Look no further! We can provide you with the perfect gift that will not only bring a smile to someone’s face, but will save a sloth at the same time! You can adopt your very own sloth, or we now have our best-selling 2018 sloths wall calendar and stunning coffee-table book available for purchase online and from Amazon, with proceeds going to support our sloth conservation efforts!


SLOTHS 2018 wall calendar ($14.99 / £10.99)

Sloth calendar cover
Beautiful, educational and heart-warmingly cute, SLOTHS 2018 is the second official calendar for the Sloth Conservation Foundation (SloCo): an organisation dedicated to saving sloths in the wild through research and conservation initiatives. By purchasing this calendar, you are directly supporting important conservation efforts to safeguard a future for these amazing animals. Each month features stunning, full-colour images of sloths in the wild, taken by world-renowned wildlife photographer Suzi Eszterhas. Here, she captures some of the most precious and intimate moments shared between mother and baby sloths in their natural habitat while fascinating captions written by sloth researcher and SloCo founder Rebecca Cliffe offer surprising insights into the biology and behaviour of these unusual animals. Did you know that sloths have more ribs than any other mammal? Or that the modern tree sloths we see today evolved from extinct giant ground sloths, some of which were over 6 meters in height! This inspiring calendar is complemented by the inclusion of a 12-month yearly planner.

Click here to buy on Amazon (US)

Click here to buy on Amazon (Europe)

Click here for international delivery 


SLOTHS: Life in the Slow Lane ($20 / £25)

Sloth expert Rebecca Cliffe and wildlife photographer Suzi Eszterhas have teamed up once again to produce this stunning coffee-table book which takes readers on an immersive journey through the jungles of South America to 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. Proceeds from the sale of this book go towards supporting the work of the Sloth Conservation Foundation.


Click here to buy on Amazon (US)

Click here to buy on Amazon (Europe)

Click here for international delivery 



Adopt a sloth today and receive an official personalised adoption e-certificate, full biography and history profile and high-resolution digital image of your adopted sloth, perfect for printing! You can choose between making a one-off payment or small monthly instalments, with 100% of the proceeds going towards sloth conservation! It’s a double win!

Click here to choose which sloth you would like to adopt! 


Do you think you are stronger than a sloth?

Do you think you are stronger than a sloth? Probably. Sloths are often perceived as being simple, lazy animals that do little other than sleep all day. In fact, you might find it downright insulting if somebody suggested that a sloth could beat you in an arm-wrestle. However, you may be surprised to know that sloths are, in fact, incredibly strong. Despite having 30% less muscle mass compared to other similar-sized mammals, we assume that sloths are weak, and that is just another example of humans underestimating what it really means to be a sloth.

Brown-throated Three-toed Sloth Bradypus variegatus Mother with newborn baby (less than 1 week old) climbing tree Aviarios Sloth Sanctuary, Costa Rica
© Suzi Eszterhas

If you have ever watched a sloth move through the trees, you will know that they can execute extraordinary slow-motion acrobatics that would make any gymnast jealous. A sloth can easily suspend its entire body weight from a single limb, holding it at a 90-degree angle for over ten minutes. They can hold the crucifix position, suspended between two tree branches for extraordinary lengths of time and their grip strength can withstand the force of a harpy eagle trying to rip them from the tree. However, underneath that shaggy coat of fur, sloths are surprisingly skinny with particularly unimpressive ribbon-like muscles (kind of like a junior high student in a track suit in the weight room). So how, then, do they manage to perform such great feats of strength and stamina?

To answer this question, we collaborated with muscle physiology expert Dr Michael Butcher and his team from Youngstown State University and the Sloth Sanctuary of Costa Rica. The resultant research paper has just been published in the Journal of Mammalian Evolution. By meticulously dissecting, mapping, weighing, measuring and sampling each of the 52 muscles that can be found in the arm of a sloth, we were able to shed some light on the sloth’s mysterious movement. As the technical terminology in the research paper can be difficult to understand or interpret for anyone who hasn’t been trained in physiology, we will simplify and summarise the main findings below.

Brown-throated Three-toed Sloth Bradypus variegatus Male Aviarios Sloth Sanctuary, Costa Rica *Digitally removed highlight in background
© Suzi Eszterhas

The muscles that sloths use to grip and produce a pulling motion are much more prominent than those that produce a pushing motion. Muscle tissue requires a lot of energy to produce and maintain, and this arrangement means that the sloths muscle mass is concentrated and limited only to the areas where it is most important. This simply means that sloths primarily use their arms to pull themselves upwards / along a tree branch, or to pull a branch towards the body for feeding or traversing a gap. They rarely have any need to push anything, and so maintaining muscles for this purpose would simply be a waste of energy. The arrangement of the individual fibres within the muscle also support this function, with the fibres becoming progressively more pennate (i.e. arranged at an angle rather than along the length of the muscle) towards the hand. When the fibres are more pennate, it means that they can produce more force, and in this instance, it would facilitate the strong pulling motion and incredible grip strength seen in sloths.

However, this research also threw out some surprises as not all muscles show the expected architecture. Instead, what appears to be happening is that the muscle groups responsible for movements are paired in unusual way so that muscles working together have opposing functions. I.e. one muscle group has properties that allow the joint to rotate quickly, while another muscle group working alongside it has properties to produce a lot of slow and steady strength. Muscles with opposing features working together in this unique way act as a trade-off and produces in the slow, deliberate and controlled movements shown by sloths.

Brown-throated Three-toed Sloth Wild sloth climbing trees in forest at Sanctuary Aviarios Sloth Sanctuary, Costa Rica
© Suzi Eszterhas

While these findings explain how the sloths manage to move in the way they do, it does not explain how their muscles are able to resist fatigue or how they are able to support the body weight for such prolonged periods. It does tell you, however, to never get in a pull-up or arm wrestling competition with a sloth. The sloths surprising stamina is in fact linked to special metabolic properties of sloth muscles and is the topic of our next research paper due to be published this month!

~ Rebecca Cliffe 

Founder and Executive Director
** All sloths used for the completion of this research died of natural causes

Why did the sloth cross the road?

For many species, the construction of a road slicing through the forest is an unwelcome inconvenience. Animals that can leap, such as monkeys, may be able to traverse the gap in the forest canopy without too much difficulty. However, if the gap is too large, animals are forced to descend to the ground and take their chances dodging traffic. In this situation, being able to move fast is helpful. Unfortunately for sloths, leaping is impossible and speed is certainly not their forte. As a result, when a sloth encounters a road it is somewhat catastrophic.

Sloths are not very agile on the ground. Their long limbs and elongated hook-like digits are adapted to support the weight of their body when hanging upside down, suspended from a tree branch for prolonged periods – a feat that almost all other animals would struggle to do. But when sloths are on the ground, their body weight pushes downwards on their limbs with gravity and they are forced to support themselves in the exact opposite way. However, as awkward and ungainly as they may seem in this position, they are perfectly capable of walking – it just takes them a little while longer to reach their destination! Running is simply out of the question, and so when it comes to crossing a road, sloths cannot ‘dodge’ traffic. Instead, they slowly venture out into the oncoming cars and chance their survival on drivers stopping and waiting for them to pass. This may seem like a suicide mission, and indeed it is why so many sloths fall victim to road collisions across south and central America every day.


So why bother crossing the road in the first place?

Sloths have survived for millions of years in the undisturbed rainforest canopy. In this stable environment, they don’t need to be flexible in their behaviours. Due to their slow rate of digestion (taking 30 days to digest just one leaf) and low-calorie food choice (leaves), sloths have very little energy at their disposal. Therefore, in order to survive in a way which minimises energy expenditure, sloths have become the ultimate creatures of habit. Highly specific home-ranges and preferences for particular ‘modal’ trees are passed on from mother to infant, and a sloth will maintain this preference for the rest of its life. They know exactly which trees to feed from to acquire essential nutrients (while avoiding over-consumption of toxins) and they have no need to modify this pattern. Therefore, if a road is suddenly built which divides their home-range in half, a sloth will have no choice but to descend from the safety of the canopy and make the treacherous journey to the other side.


Sadly, it is not just the traffic that poses a threat to sloths in this situation. Poorly-insulated power lines are strung along every road in Costa Rica and electrocutions are one of the leading causes of sloth mortalities. Concrete drainage ditches that run alongside many roads are death traps for wildlife, and when on the ground in an urban environment, sloths have no way to defend themselves and are vulnerable to attacks from both dogs and humans.

What can we do to help?

Several organisations are working to construct wildlife canopy bridges across major roads which are successful in facilitating the movements of numerous species. Although sloths have been known to occasionally utilise these bridges, they do not have the energetic flexibility to travel long distances to find a crossing. If there is no canopy bridge in the immediate area where they need to travel, they will continue to cross the road on the ground. We need to ensure there are frequent bridges lining busy roads and that natural canopy crossings are maintained wherever possible. Power lines need to be better insulated, or in an ideal world, moved underground. But most importantly, we need to respect the remaining forest and try to minimise any further disturbance to the sloths’ existing habitat. This will not be easy – with the human population increasing at an exponential rate we are never going to stop encroaching on the rainforest. But perhaps we need to start compromising. If we want to see the survival of wildlife, we can no longer simply bulldoze everything in our path to make way for towns, farms and cities. We need to protect key areas, consider the habitat requirements of species and make sure that we conserve the essential components. We need to learn how to co-exist.

~ Rebecca Cliffe

Update: Sloth Research, Conservation and Life in the Slow Lane

2017 has been an eventful year so far with new sloth research as well as the launch of our coffee-table book – with proceeds going towards supporting The Sloth Conservation Foundation 

Brown-throated Three-toed Sloth Bradypus variegatus Male (covered in algae) Aviarios Sloth Sanctuary, Costa Rica

To begin, I have completed two more research papers that have both been submitted for publication (and are currently in the review process). The first one looks at the metabolic rate of three-fingered sloths and how this changes with temperature. We have found that the sloths are capable of doing something quite extraordinary when they get too hot – but I’m not able to give any further details away until the paper is officially published.

The second paper looks at the effect of temperature on the metabolic rate of two-fingered sloths. This time we look at how sloths from high altitude locations (where it is colder) have different metabolic adaptations to the climate. We also use climate change estimations to predict the impact that global warming will have on the energetics (and consequently the survival) of sloths in different regions. I will send links to both papers as soon as they are published, but for now that is all I can give away!

Brown-throated Three-toed Sloth Bradypus variegatus Rebecca Cliff, sloth biologist, releasing sloth wearing "sloth backpack" Aviarios Sloth Sanctuary, Costa Rica *Model release available

I am also still busy in the genetics lab. The analysis of the hair samples has taken far longer than expected (as always seems to be the case with sloths), but we are so close to the end now. We have most of the results already but there are just a few remaining samples that we have had to do repeat tests on. We are waiting to get the conclusive results through for those and then construction of the research paper can begin!

In the meantime, I have been busy writing the paper that encompasses all the work from the sloth backpack project. This includes all of the movement, activity, and behavioural data for all of the wild sloths that I studied for over 6 years. It has been a mammoth task to analyse the mountains of data and pick out exactly what is happening (imagine excel spreadsheets with over a million different rows….)! But I am glad to say it is almost complete. The biggest challenge has been trying to match up the sloth behaviour data with the corresponding weather data. It’s far too much to do manually, but with the statistical expertise of paeleo-ecologist Ryan Haupt from the university of Wyoming, we now have a solution.

sloco logo2

The Sloth Conservation Foundation 

My deadline for having all of this research finished is fast approaching as I have to hand in my PhD thesis in August. However, the sloth work does not end there. In august I am moving back to Costa Rica to work full time on the Sloth Conservation Foundation (SloCo). This will include continued scientific research as well as the implementation of our first targeted conservation strategies. These projects will be funded by the profits from our book and 2018 Sloth calendar and our adoptions/donations via our website. Our primary goals for 2018 are:

  • To initiate a project aiming to reduce the number of sloth electrocutions by funding the re-insulation of power lines in key areas
  •  To launch our first educational outreach program teaching children about sloths and the importance of protecting the rainforest ecosystem. We want to equip children with the knowledge and skills necessary to reduce their impact on wildlife in the future. In order to do this, we plan on employing and training local people to deliver classes in the community schools.
  • To improve rehabilitation and release programs by monitoring the long-term survival of hand-reared sloths that have been returned to the wild using radio-tracking equipment.


SLOTHS: Life in the Slow Lane 

Finally, I would like to end by telling you about our latest fundraising venture for SloCo. I have teamed up with world renowned wildlife photographer Suzi Eszterhas to produce a coffee-table book all about sloths. Pre-order copies are on sale now and for a limited time only come with a FREE signed print of an adorable sloth mom and baby
Getting the spectacular images for this book wasn’t easy… it took us on many wild adventures where we spent months on end in the jungle, were forced to go undercover, got stranded on a desert island and caught in tropical storms at sea. The book reveals some of the latest scientific discoveries and provides insights into the previously unknown habits of these unusual animals.

Click here to pre-order

Pygmy three-toed sloth Bradypus pygmaeus Isla Escudo de Veraguas, Panama

Lastly, I would like to thank you for your continued support. Without your help we wouldn’t be able to complete any of the research or conservation work that is so important to protect a future for sloths in the wild. I will provide a further update as soon as I can, but for now, it’s back to data analysis!

All the best,

Rebecca Cliffe

Images copyright Suzi Eszterhas