4.1 Morphology and anatomy
4.1.1 Fur
Sloth fur is very important to their thermoregulation.
Sloth fur has two distinct layers: a soft, downy undercoat, which is important for warmth, and a coarse outer coat for camouflage. The hair grows in the opposite direction to every other mammal, parting on the stomach and pointing toward the back. This peak of hair is called the drip tip, and it makes it easier for water to run off their body in heavy rain. Sloths groom their fur daily to keep it in good condition.
Sloth hair is specially adapted to allow the sloth to be a miniature, mobile ecosystem. Just as the forest is home to sloths, each sloth is home to a symbiotic variety of plants and animals.
Sloth fur contains micro-cracks which trap moisture for over 80 different kinds of algae and fungi. This algae turns the sloth fur green over time and helps the sloth blend into the rainforest canopy.
Additionally, their fur provides a habitat for a number of different invertebrates, including five different species of moths known as sloth moths (Pyralidae). These moths as well as many of the algae and fungi species found in sloth fur are not found anywhere else on the planet. They only live in sloth fur!
Sloths have a mutualistic relationship with the ecosystem living in their fur – this means that both the sloth and the organisms living on them benefit from the relationship. Sloths provide a home for their symbionts and the algae provide sloths with coloration and camouflage.
Although it was once believed that sloths eat the algae from their fur, scientists now know that this doesn’t happen.
Some of the fungi found on sloths have antibacterial properties and have been found to be active against some parasites and cancers. This might help sloths to resist certain diseases, and may also help other animals, including humans.
No one knows for sure what benefit the moths provide for sloths, but the sloths do not seem to mind them. The moths might eat the algae and fungi from the sloth’s fur (although this is not known for certain), and they lay their eggs in the sloth’s feces.
4.1.2 Teeth
Unlike human teeth, which have two layers (the inner dentin and the outer enamel), sloths have only the inner dentin. Without the protective enamel layer, sloth’s teeth absorb tannins from the leaves they eat and often turn black.
Sloths don’t have to worry about losing teeth as easily as humans because they are what are known as “hypsodonts”, which means their teeth grow continuously throughout their lives.
These continuously growing teeth without enamel are a characteristic shared by all Xenarthran mammals.
Three-fingered sloths have only a set of small, peg-like cheek teeth that are used for chewing leaves. Two-fingered sloths have the addition of large, sharp frontal teeth called ‘caniniforms’ (named because they resemble the typical canines of other mammals). The slight overbite causes the top pair to grow in front of the bottom pair and constantly rub against each other. Every time a two-fingered sloth opens its mouth their teeth self-sharpen!
4.1.3 Claws, fingers and nails
Sloths have long hook-like fingers and toes that allow them to hang suspended from a branch without using any energy. They can eat, sleep, and give birth in this position, sometimes even remaining like this after death.
Sloths’ claws are actually formed by the elongated and curved distal phalange bones protruding from their limbs. These bones are covered by a sheath of the same material that makes up our fingernails and hair (keratin).
The nail grows continuously. In wild sloths the length of their nails is maintained through constant use; the nail is worn down while the sloth is hanging and climbing.
Captive sloths must have their nails trimmed regularly so that their fingers and toes do not become overgrown and disfigured.
If they get broken or damaged, sloths can actually regrow their claws thanks to their low metabolic rate (in a similar way to when reptiles regrow their limbs).
However, the claws will rarely regain their original shape, often growing back deformed. In the wild, this can put the sloth at a great disadvantage as their claws are vital for life in the canopy.
4.1.4 Sloth vision and sense of smell
All sloths have a condition called rod monochromacy—very rare among mammals– meaning that they completely lack cone cells in their eyes.
Since cone cells are what allow animals to see in color, sloths are color blind. They also don’t see very well in dim light and are completely blind in bright daylight.
Instead of using sight as a primary sense, sloths have an excellent sense of smell.
4.1.5 Stomach
Sloth’s stomachs are very large and have four chambers, the same as a cow. The partially digested leaves can account for up to 37% of a sloth’s weight!
A sloth’s esophagus does not go in a straight line from mouth to stomach, but instead has a loop in it. This enables the sloth to eat while hanging upside down without having gravity pull the food back out.
This loop also keeps sloth from being able to vomit—so they have to be very careful not to eat anything that makes themselves sick!
4.1.6 Fibrinous adhesions
Sloths have unique attachments inside their bodies that help anchor their organs against their lower ribs. This helps their organs stay comfortably in place while hanging upside down, making it easy for the sloths to breathe. The presence of these adhesions reduces the amount of energy that sloths use each day by 7-13%.
4.1.7 Veins and arteries
Specialized valves and sphincters within the sloth’s circulatory system ensure blood maintains constant pressure and direction, always flowing in the right direction at the right speed. This is important because sloths spend so much time hanging in unusual positions that they cannot rely on gravity to help blood flow.
4.1.8 Sloth Muscles
Sloths are incredibly strong– approximately 3 times stronger than the average human being when it comes to grip strength, despite having 30% less muscle mass than other mammals of their size. Muscle tissue is very metabolically expensive, but sloths are all about saving energy, so sloth muscles work very efficiently.
The muscles that sloths use to grip and produce a pulling motion are much more prominent than those that produce a pushing motion because sloths primarily use their arms to pull themselves upwards or to pull branches towards the body.
Sloth muscles are also predominantly made up of specialized slow-twitch fibers (also called type 1 muscles), and produce the slow, deliberate, and controlled movements shown by sloths.
Fun fact – sloths have 52 muscles in one arm!
4.1.9 Tail
Three-fingered sloths have small, stubby tails which are very strong – measuring approximately 6 – 7 cm (about 2.5 inches) long.
They use their tails as a brace while climbing and also to dig a hole before pooping.
Two-fingered sloths do not have a tail.
4.1.10 Sloth Moths
Sloth fur supports an entire mini-ecosystem, including several species of moths and beetles that are found nowhere else on earth!
A single sloth can host hundreds of moths which spend their entire life in the sloth’s fur, leaving only to lay eggs. These eggs are laid in the sloth’s feces when the sloth comes to the ground to poop.
After the eggs hatch the new moths either wait for the sloth to return, or fly up into the canopy to find a new sloth host.
Moth life-cycle:
Fun fact – A single sloth can host up to 950 moths and beetles within its fur.
4.2 Activity
4.2.1 Warm-blooded or cold-blooded?
Although sloths are mammals, they are what is known as Poikilotherms and do not regulate their temperature the way most warm-blooded animals do which helps them to save energy.
They live in tropical environments where it is warm all year round and maintain a thick coat of fur to assist in thermoregulation. The body temperature of sloths is very dependent on the surrounding environment, and if sloths get too cold they will stop digesting food and can die.
The average temperature of the three-fingered sloth is around 32.7℃ (91℉), compared to humans’ 36.5℃ (98.6℉). The sloth’s unusual muscle structure means that they cannot shiver to get warm.
As cold-blooded animals such as reptiles, sloths help control their body temperature by seeking warm and cool areas (such as basking in the sun or laying in the shade) or curling into a ball to conserve warmth.
Sloths’ core temperature may fluctuate up to 10℃ (18℉) over the course of the day.
4.2.2 Hanging
Sloth’s hands and feet are permanently curled into a hook-like arrangement and have very specialized muscles and tendons—this allows them to hang from any limb using almost no energy.
Sloths are so strong and so good at hanging onto branches that predators often cannot pull them off their trees.
Sloths are so well adapted to hanging from branches that they may continue to do so even after death. Wild sloths have sometimes been observed to die while hanging upside down on a tree branch and remain suspended by their long, curved fingers
4.2.3 Sloth Speed
Sloths are very, very slow. At top speed, a sloth can cover approximately 1 meter in 1.5 seconds (approximately 1.5 miles per hour), but for a sloth, this is the equivalent of a sprint– tiring them out quickly and burning lots of energy. Sloths only move this fast in response to immediate danger.
Typically, a sloth will move upside down through the treetops at an average speed of 1 meter every 3 seconds (or about 1 foot per second, just over ½ of a mile per hour).
Although they move slowly, sloths have incredible stamina, and can stay on the move for surprisingly long periods of time. This allows them to cover large distances and find new territory.
4.2.3 Sloth Intelligence
How intelligent are sloths? This is a very poorly studied topic. It is true that sloths have small brains compared to their body size, however scientists are now realizing that this actually doesn’t relate to intelligence at all.
The brains of sloths might be small but they are very much focused on the specific skills that they need for survival. For example, the section of the brain that controls forelimb movement is well developed for careful climbing, and the sloth’s spatial memory is particularly impressive.
Having a good spatial memory is important for sloths as they have poor eyesight. They navigate around their home ranges using their memory and sense of smell. However, the sloth’s social skills and problem-solving abilities are somewhat lacking, which often leads to them being labeled as “stupid”.
