Sloths Like It Hot: ambient temperature modulates food intake in the brown-throated sloth (Bradypus Variegatus)
During this study we measured exact levels of food intake in three-fingered sloths and investigated how these levels were affected by changes in the ambient temperature. We discovered that sloths actually eat surprisingly little on a daily basis (73.5 g dry weight of leaves per day) – that is three times less than the amount eaten by the similarly sized howler monkey!
Furthermore, we found that the amount of food consumed is unbelievably consistent between individuals. Over the course of 5 months, my three study sloths consumed a total of 61.3%, 60.0% and 61.3% of food provided respectively – less than a 1.5% difference!
Finally, we discovered that the levels of food intake actually increase on hotter days, which is in stark contrast to what we expect for most other mammals (they tend to eat more on colder days). Sloths really do seem to do everything backwards! If you would like more information, you can read and download our full publication here.
The Pygmy sloths of Panama are considered to be one of the world’s most critically endangered mammals with the last official population count identifying only 79 individuals. Beyond the original description of the species we still know almost nothing about these little sloths with scientific rigour. We don’t know how many of them remain, we don’t know enough about their diet and habitat needs, we don’t understand when and how they reproduce, and we still don’t really know why and how they became dwarfed on their small island.
What Are Pygmy Sloths?
Pygmy sloths are a dwarfed version of the brown-throated three-fingered sloths found on mainland Panama, but this species is endemic to the remote Isla Escudo de Veraguas. This tiny island, which only formed 9000 years ago, is located 17.6 km off the coast of mainland Panama and covers just 4.3 km2. The pygmy sloths were first described as a distinct species in 2001 based on morphological differences in body size (they are reportedly 40% smaller than the mainland species), but due to a lack of research we still have no genetic information on the status of these animals. Isla Escudo is very difficult to get to, and much of the mixed forest coving the interior of the island remains unexplored. The Pygmy sloths are most commonly found inhabiting and feeding from red mangrove thickets, but these trees are often logged and now only constitute 0.024% of the total island area (1.67 ha). No one really knows whether these little sloths use the dense mixed forest covering the interior of the island, or whether they feed from anything other than red mangrove leaves. With the number of remaining pygmy sloths suspected to be in decline, it is becoming essential that we work towards better understanding the ecology of this amazing species.
In addition to the Pygmy sloths, there are also known to be four other nearby islands in the Bocas del Toro archipelago that support three-fingered sloths smaller in size due to their confinement on an island. Despite being a similar size to the pygmy sloths, these island sloths are still classified as Bradypus variegatus (the mainland species), but the genetic status, health and ecology of these isolated populations are poorly understood.
Sloth populations across South and Central America, like many other animals, are under threat from unprecedented levels of habitat loss. There is no way to escape the fact that 1-2 acres of rainforest land are cleared every single second. That means that in the time it has taken you to read this sentence, 8 more acres of rainforest have disappeared. Think how many sloths (among all of the other wildlife) would have been living in those 8 acres? It’s quite frankly alarming to say the least – up to 137 plant, animal and insect species are lost every single day due to rainforest destruction.
But what are the driving factors behind the massive amounts of deforestation? And what can we do about it? Can we even make a difference as individuals? The answer is yes.
What if I told you that you could personally save 30 sq. ft. of rainforest from being chopped down, 1,100 gallons of water, 45 lbs of grain and 20 lbs of CO2 every single day? And all you have to do is change the food that you eat.
It might come as a surprise that animal agriculture is responsible for up to 91% of 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%. Even more surprising is that fact that nobody really knows about it. This issue receives virtually no media attention. How are we supposed to make a difference if we aren’t aware of the real problems? We have all heard about the terrible effect that the palm oil industry is having on the Indonesian rainforest – but animal agriculture is shockingly responsible for 5 times the amount of deforestation when compared to palm oil (26 million rainforest acres: 136 million rainforest acres).
The average American will nosh their way through an astonishing amount of meat each year (209 lbs / 95 Kgs), and the rest of the world is not far behind. There is no question that the current levels of meat and dairy consumption are completely unsustainable. As a population, we simply cannot continue in the way that we are doing. But what is the solution? Are we supposed to go completely vegan? And is that option sustainable in itself? I guess this is down to personal preference.
After having the issue of animal agriculture drawn to my attention, and having looked into it in more detail, the only sustainable way forward that I can see is to switch to a much more, if not entirely, plant-based diet. I strongly believe that the idea of vegetarianism / veganism doesn’t need to be strictly black and white. I think we can be “mostly vegetarian” or “mostly vegan” – flexibility and moderation are key. As long as we are drastically reducing our current level of meat and dairy consumption then we will be heading in the right direction.
In conclusion, I don’t think any human can claim to be an environmentalist (or even particularly environmentally friendly) whilst regularly eating meat. There is no denying the facts, and unfortunately a meat based diet has become an unsustainable option. However, I also don’t believe that we all have to become strictly vegan in order to make a difference. I personally am going to follow a mostly vegetarian / vegan diet moving forwards – I would see myself as a hypocrite to dedicate my life’s work to sloth conservation, whilst significantly contributing to the destruction of their habitat every day with my personal food choices.
Of course there are other, smaller, drivers of deforestation which also play a significant role in the loss of sloth habitat. One in particular that I know has a huge negative impact on sloth populations in Costa Rica is the fruit industry. Huge mono-culture plantations of bananas, pineapples and watermelons now cut the once continuous rainforest into smaller and smaller fragments. Furthermore, these plantations are regularly sprayed with toxic pesticides well known to cause a whole host of diseases in both humans and wildlife. We have seen this first-hand in Costa Rica with baby sloths being born with birth defects all too frequently. The only advice that I can give on this issue is to buy organic fruits from small local farmers (avoid the big companies as much as possible – we are looking at you Dole, Chiquita, Del Monte).
It certainly isn’t an easy issue to tackle, but in order to safeguard the future of wild sloth populations, and indeed much of the life on this planet, we are going to have to make some drastic changes. Of course we can recycle and reduce our energy / water consumption on a daily basis, but this alone will no longer be enough.
If you would like more information on the topics that I have raised in this blog, or if you need some additional motivation to move away from the meat, I would highly encourage that you research the issue of animal agriculture further.
Here are some links and scientific publications that might be useful for you:
Sloths are known to spend a significant amount of their time hanging vertically upside down, yet no one has ever discovered how they manage this. If we were to stand on our heads for several hours a day, we would probably end up with a lot of problems. In this publication, we solve part of the mystery!
Three-fingered sloths have multiple unique fibrinous adhesions that effectively anchor their abdominal organs against the lower ribs. These evolutionary ‘coat hangers’ support the weight of the sloths stomach and bowel whilst the animal is hanging inverted, preventing the lungs from being squashed and facilitating breathing. Offering further support to this hypothesis, we discovered that the kidneys were bound by connective tissue against the pelvic girdle with absolutely no mobility. This unusual configuration means that they too are exempt from contributing to the abdominal weight.
We used ‘Daily Diary’ data loggers on both wild and captive sloths to determine the proportion of time spent inverted. Combining the hang-angle preference information with ventilation rate data, we use an energetics based model to estimate that the presence of the adhesions could reduce a sloths energy expenditure by 7- 13% when hanging upside down!
Of course, these adhesions don’t solve all of a sloths upside-down problems. Indeed, we are discovering that both two and three-fingered sloths have a whole array of fascinating anatomical features that allow them to hang inverted for up to 90% of the day. These include circulatory adaptations that prevent the blood rushing to the head, and a highly specialised oesophagus that allows them to swallow whilst upside down – just to name a few!
How did you first discover the adhesions?
Discovering the internal adhesions was actually quite a slow process. The Sloth Sanctuary has been rescuing sloths for over 20 years and during that time, they have had to perform many necropsies on sloths that didn’t survive. At first, because sloth anatomy was still poorly understood, the adhesions were assumed to be scar tissue from previous injuries. It was only when we began to document the exact location of the attachments that we discovered they were a general feature of sloth anatomy. It was then a case of working backwards and figuring out what the adaptive benefit could be. Interestingly, two-fingered sloths (Choloepus) actually have more of these internal adhesions than the three-fingered sloths – probably because they spend a higher proportion of time hanging upside down.
Why do you think the complications of hanging upside down have never been considered previously?
I am sure that someone somewhere has briefly pondered this issue in the past, but it has never been followed up or solved. I think this is partly due to the difficulties of studying sloths in the wild and the lack of reliable scientific data documenting their natural behaviours. The typical sloth hanging position (horizontal) and sleeping position (sitting vertically) don’t result in any gravity based problems. It takes a good amount of time spent observing them in the wild to see that they do hang vertically upside down, and more importantly, they need to. Sloths need to eat the young, tender leaves that grow on the outer tips of branches because they are easier to digest and often have a lower toxin content. In order to reach these new leaves, sloths frequently have to hang inverted.
Why are these adhesions so important for sloths?
For many mammals, an energy saving of 7-13% wouldn’t be particularly game changing. In fact, there are several groups of mammals (howler monkeys, for example) that do regularly hang inverted, yet don’t have any internal adhesions to support the organs. The difference with sloths however is a very tight energy budget. Because of their extremely slow metabolic rate and unusually low body temperature, it can take a sloth up to 30 days to digest a single leaf. Having a constantly full stomach means that they really can’t ingest very much on a daily basis and so their caloric intake is very low. Sloths have virtually no flexibility in their energy budget: they generate just about enough energy from their diet to move when and where required, but there is not much left in the tank afterwards. To a sloth, an energy saving of 7-13% is quite a big deal.
In addition, sloths can store up to a third of their body weight in urine and faeces before defecating – this means that the stomach and bowel contents make up an unusually large proportion of their body mass. With their limited energy supply, it would be energetically very expensive, if not completely impossible, for a sloth to lift this extra weight with each breath were it not for the adhesions.
A while ago, the internet went crazy over the story of a five-month-old baby girl and her best friend: a pet sloth. The story is accompanied by undeniably adorable photos and a video that has quickly gone viral, attracting attention from all over the world. I can’t deny that the photos are cute and that this catapulted sloths back into the media spotlight – but I fear that the negative repercussions may have outweighed the benefits on this one.
Firstly, sloths are wild animals – not domestic pets. Sure, the babies are undeniably cute, but these wild animals maintain their natural instincts. That means that they do not like to be petted, groomed, bathed… and they certainly do not enjoy being manipulated in an unnatural way. Sloths, unlike many animals, do not show obvious external signs of stress. Their natural response to fear or danger is to hold still, and as a result, it is difficult to tell when a sloth is scared or stressed. The animal may look perfectly happy to us – but the reality will probably be very different.
I have absolutely no doubt that this sloth photo shoot was staged with the sole purpose of creating a viral hit. But my biggest concern is that public exposure to these images will have negative repercussions for sloths in general. Whether this was intentional or not, the tone of the story effectively glamorises the concept of owning a pet sloth. From what I’ve seen, the standard response to the images seems to be: I want one! And that is where the problems begin.
The sad reality is, sloths that are sold as pets usually come from the wild. Even if the baby sloth was born in the US, it’s more than likely that the parents would have been taken from the wild several years earlier. Sloths are very slow to breed: the gestation period is 11 months, they only give birth to one baby at a time and this infant needs its mother’s milk for a long while after that. In the wild, baby sloths usually spend a full year with their mothers before reaching independence. This means that the sloths currently being held in captivity in the US cannot physically produce enough babies to meet the ever-increasing demand from people wanting pet sloths. So every year, hundreds of sloths are removed from the wild and shipped to the US from countries such as Venezuela and Ecuador where export laws are slack. These individuals are then forced to breed and the babies are sold into the pet trade at eye-watering prices.
“The media coverage of this pet sloth story absolutely portrays the image of a happy family and I fear that many people will buy into this fantasy.”
Because baby sloths are indisputably cute, this is what everybody focuses on. The sad reality of the situation is often overlooked. The media coverage of this pet sloth story absolutely portrays the image of a happy family and I fear that many people will buy into this fantasy. This doesn’t just pose a threat to the sloths, but can also endanger the sloth owners themselves. Sloths are not teddy-bears – they are wild animals and they have big teeth. I have worked with many sloths over the years, both wild and hand-reared, and they can all inflict serious injuries if scared or irritated. When they reach independence (at the age of about one year), even the most gentle of hand-reared sloths just do not want to be handled any longer. I suspect that many people are going to learn this the hard way and will find themselves with an expensive, hard-to-handle sloth that could live for up to 40 years.
At the end of the day, I can’t help but feel a little disappointed to see that as a society we can still find so much entertainment in the anthropomorphism and exploitation of wild animals. This is by no means an issue that is confined to sloths; indeed, many species are on the brink of extinction because of it. It’s also not a problem that is confined just to the pet trade; it is much bigger than that. It’s an issue of morality. I believe that the trouble lies in the way we, as humans, see ourselves as the dominant species, and the way in which we can morally justify our use of wild animals. I do believe that our attitudes are slowly changing, but education is unequivocally the key. As the environmentalist Baba Dioum once famously said:
“In the end we will conserve only what we love. We will love only what we understand. We will understand only what we are taught.”
The Sloth Sanctuary of Costa Rica have now received 9 orphaned baby sloths showing birth defects and other institutions such as the Jaguar Rescue Centre are reporting a similar trend. The affected sloths have all been C.hoffmanni infants originating from the Limon province on the Caribbean Coast of Costa Rica. The deformities have all been strikingly similar; missing fingers/toes, malformed ears, misshapen limbs and partial or full albinism. High numbers of birth defects like this in any population are a warning sign that something is seriously wrong. We suspect that the deformities are the direct result of either extensive habitat fragmentation, or the excessive use of pesticides for agriculture. However, before we can develop any targeted conservation strategies, we have to identify and fully understand the root cause of the problem – and that means completing the necessary genetic research.
You might remember that last year we ran a successful Indiegogo campaign to fund urgent research into the genetic health of wild sloth populations. As a result, we were able to collect data and hair samples from over 300 wild-born sloths (both two-fingered C.hoffmanni and three-fingered B.variegatus) originating from over 97 different regions in Costa Rica. This is a far larger sample size than has ever been studied before and it will undoubtedly provide us with the answers that we need.
Regardless of whether the observed deformities are due to habitat fragmentation or pesticides, one thing remains glaringly obvious – this is not a problem that will fix itself. In fact, with the human population booming to unprecedented levels, the problem is growing rapidly worse. In the 11 months since our first fundraising campaign ended, the Sloth Sanctuary of Costa Rica received an additional two deformed baby sloths, and the Jaguar Rescue Centre received their first orphaned juvenile showing partial albinism. Perhaps most concerning though is an email that we received from the Panamanian Government. They were requesting our help because of the sheer number of deformed baby sloths being found in their Cerro Ancón reserve:
In short, if we want to mitigate this problem, we must act now.
At the end of last year we were able to export the 300 hair samples and we recently just completed another successful Indiegogo campaign to fund the laboratory costs of genetic analysis. We now have everything that we need and the research is underway. We hope to have the initial results ready within a few months and then we can begin developing targeted conservation strategies to mitigate the problem.
Sloths are famous for their unusual and bizarre bathroom habits. Both two-fingered (Choloepus) and three-fingered (Bradypus) sloths will only defecate once a week, and for some strange reason they will only do it on the ground. For a sloth, this is a big deal. A sloths entire lifestyle is based around avoiding detection and using as little energy as possible. It takes a sloth an entire month to digest just one leaf, meaning that they don’t have much wiggle room when it comes to expending energy. Descending from the safety of the canopy to the forest floor is not only energetically very costly, it is also a suicide mission. Sloths are extremely vulnerable to predation when on the ground. As if that isn’t enough, these animals can store up to a third of their body weight in faeces; lugging that extra weight around is no easy task when you are a sloth. So why bother? There must be a huge selective advantage to this weird weekly routine – it should be obvious. Yet this remains one of the biggest mysteries surrounding sloth behaviour and one that scientists have struggled to explain for decades.
A blog post by Ed Yong offers up an interesting theory. A team of scientists from the University of Wisconsin suggest that this behaviour all revolves around the humble sloth moth. Sloth moths are a group of moths that are found exclusively in sloth hair. When a sloth descends to the forest floor to defecate, the female moths crawl off the sloth and into the faeces to lay their eggs. Here, the larvae feed and develop into adult moths before flying up into the canopy to find a sloth. Pauli et al suggest that the sloths benefit from having the moths, and so risk life and limb to maintain the relationship. They believe that the presence of moths facilitates the growth of algae in the hair, from which the sloth gains nutrients. It is certainly an interesting idea and one that crossed my mind when I first began working with sloths four years ago. Unfortunately, it doesn’t add up.
It is true that the sloths with the most algae tend to have more moths. But it is very unlikely that the sloths gain any benefit from the moths. The algae is a form of camouflage, but certainly isn’t essential to the sloths survival. For a start, sloths in captivity that are fed a natural diet but don’t have any algae are perfectly healthy. Blood analysis done at the Sloth Sanctuary of Costa Rica shows no difference between these captive animals and their wild, algae covered counterparts. Furthermore, a sloth has never been observed or documented licking the hair. I have spent countless days following sloths in the wild and observing every little movement (yes, movements are few and far between…) yet I have never seen anything that looks remotely like licking. Maybe the nutrients are absorbed through the skin? Perhaps. But if a sloth can survive without the algae, why risk everything to try and get it?
Could the moths benefit the sloth in other ways? It’s doubtful. Sloth moths don’t have mandibles in the adult form and so it is unlikely that they feed on the algae, dead skin or parasites. The moths essentially just use sloths as a handy form of transport. The moths need to find sloth poo in order to reproduce. The best way to find a fresh pile of sloth poo? Find a sloth and it will take you there! The moths have simply taken advantage of the sloths bizarre bathroom habits and found themselves a nice niche.
Pauli’s theory also leaves a few important questions unanswered. If the sloths were simply providing the moths with a place to reproduce, why would they need to return to the base of the same few trees? Wild sloths always poop in the same places. In fact, multiple poo piles can be found at the bases of certain trees. I have camera traps set on some of these places and we frequently see the sloths coming and going (admittedly, the sloths slowness occasionally struggles to trigger the camera traps!) Why wouldn’t the sloth just climb down any old tree? Why travel to these specific spots?
We believe that it is all about communication and reproduction. Pheromones present in urine and faeces can provide a lot of important information about the individual animal. If the sloths just let everything go from the canopy, these messages would be easily lost. We have heaps of data showing some really interesting patterns between a female’s oestrus cycle and the patterns of defecation. In the meantime, the mystery continues…
When you imagine a sloth, you probably think of a simple, lazy creature that does very little other than sleep all day. In fact, you might wonder how such an animal survives in the wild at all. Even the very name “sloth” in most languages translates as a version of lazy. In 1749 when sloths were first described in the scientific literature they were labelled as “the lowest form of existence” – it is little surprising that sloths have been subject to such profound speculation and misinterpretation; “sloths are slow because they eat leaves that drug them”; “sloths are so stupid that they mistake their own arm for a tree branch and, grabbing it, fall”; “if you cut the head off a sloth, the heart will continue to beat for 15 minutes……”. I have heard it all. But what does it really mean to be a sloth? Why are they so slow? And why does it work?
The answer is surprisingly simple: Being slow is an incredibly successful strategy for survival. In fact, being slow has helped sloths to survive on this planet for almost 64 million years. It is obviously a winning tactic. But in order to understand exactly what it is that makes them so slow and why it works so well, we have to look at the biology of these unusual animals in a little bit more detail. The first piece in the puzzle to understanding the sloths slow pace is their low calorie diet. Both two and three-fingered sloths have a predominantly folivorous diet, meaning that they feed mostly on leaves with a notably low caloric content. But eating a low-calorie diet doesn’t explain everything – there are plenty of mammals that are folivores that move at a normal pace – Howler monkeys for example. The difference lies in the sloths large, four-chambered stomach and extremely slow rate of digestion.
For the majority of mammals, digestion rate scales with body size, so larger animals should take longer to digest their food. Sloths appear to break this rule quite spectacularly. The exact rate of digestion remains unclear, but current estimations range from 157 hours to 50 days (1,200 hours) for the passage of food from ingestion to excretion! In general, most folivores will compensate for a low-calorie, leaf based diet by consuming relatively large quantities of food. For example, howler monkeys consume three times as many leaves per kilogram of body mass as sloths. So why don’t sloths just eat more? In accord with their slow rate of digestion, the sloths’ four-chambered stomach is constantly full, and so more leaves can only be ingested when digesta leaves the stomach and enter the small intestine. This means that food intake and energy expenditure are likely limited by digestion rate and room in the stomach. Indeed, the abdominal contents of a sloth can account for up to 37% of their ∼4.5 kg body mass! In short, this means that sloths can’t eat very much on a daily basis and therefore have barely any energy at their disposal.
In order to survive on such a limited diet, sloths have one of the lowest metabolic rates amongst mammals – estimated to be just 40–74% of the predicted value given for their body mass! This means that sloths are probably surviving on the very edge of their energy budget – and therefore everything that they do has to be constantly geared towards saving energy. An obvious example of such energy saving brilliance can be seen when we look at the sloths body temperature. Maintaining a stable core temperature is energetically very expensive, and sloths appear to have almost completely sacrificed this ability. Similar to many poikilotherms, they rely on behavioral methods of thermoregulation (basking etc.) and can exhibit daily fluctuations in core temperature of up to 10 ◦C. This fluctuation is in stark contrast to most endothermic mammals, which are able to maintain a constant core temperature of approximately 36 ◦C regardless of the ambient temperature.
In addition to a low and variable body temperature, sloths have also sacrificed muscle tissue. Although they might look quite large, most of a sloths visual mass comes from their coat of unusually thick fur (probably another method of maintaining body heat). Underneath all the hair, sloths are surprisingly skinny. Muscle tissue is metabolically expensive to maintain, and in order to save energy, sloths have just 30% of the muscle mass expected for a mammal of similar size. Despite this apparent deficiency, sloths have a highly unusual muscle arrangement which gives them surprising strength with a very high resistance to fatigue.
As a result of these energy saving adaptations, sloths physically don’t have the ability to move very fast. They can’t run away from predators like a monkey would and instead, they have to rely on camouflage. The sloths main predators (big cats – Jaguars, Ocelots; and birds – Harpy Eagles) all primarily detect their prey visually, and it is likely that sloths move at a pace that simply goes unnoticed – sloths move slowly to avoid being identified as prey. They aren’t lazy, they are stealthy.