Happy Sloth Week - Here's Ten Facts About Sloths

Christian Mehlführer, via Wikimedia Commons

The name “sloth” was given to members of the Megalonychidae and Bradypodidae families to describe six species of animals that move very slowly. But aside from their lack of urgency, how much do you really know about these arboreal creatures?

In honor of Sloth Week, here are ten facts about sloths that you may not have known:

Sloths have an extremely slow metabolism

This one seems fairly obvious, but it is worth mentioning exactly how slow the metabolism actually is. As a frame of reference, it takes the average human around 12-48 hours to ingest, digest, and eliminate waste from food. Sloths can take 30 days to completely digest a single leaf. They rarely like to leave the safety of trees, though they will come down in order to relieve themselves. Fortunately for them, they can hold it all in and only evacuate their bowels once a week. Sloths can lose about a third of their body weight every time they defecate and urinate.

Sloth fur contains fungi and algae, for good reason

Sloths move incredibly slowly at less than three meters per minute, so their top line of defense against predators (eagles, jaguars, snakes, and poachers) is just hoping they aren’t seen. Sloth fur has two layers: an inner layer closest to the sloth’s skin that is short, fine, and provides warmth, and a coarse outer layer that has cracks, allowing for the growth of algae and fungi, which is fantastic news for the sloth. In this case, being filthy and covered in algae gives the fur a green tint, acting as an effective camouflage up in the trees.

As a bonus, researchers recently discovered that some fungi within sloth fur could have applications for fighting off certain parasites, cancers, and bacteria. 

Their fur acts as a home for many insects

While there is a population of hematophagous insects that feed on the sloth itself, including ticks and mites, there is also a coprophagous guild of insects which have a commensal relationship with the sloth and feed on the fungi and algae in sloth fur. The coprophagous guild can have around nine different species of beetles, mites, and moths, with many members of each species. One sloth was found to have 980 individual Trichilium adisi beetles living within its fur.

The five species of sloth moths find a host and ride around with it, waiting until the sloth relieves itself (more on that later). After the sloth defecates, the moth lays her eggs in the dung. The larvae remain in the dung until they have reached the adult stage, where they will then find their own sloth to live on and continue the life cycle. As the moths die, they decompose within the fur, feeding the algae and fungi.

Wild sloths don’t sleep as much as we thought

After studying captive sloths, scientists determined that the animals slept up to 15-18 hours every day. However, a study published in 2008 utilized electroencephalogram (EEG) recordings to monitor wild sloths’ brain activity. They found that the average wild sloth sleeps about 9-10 hours a day; not much more than the average human. The researchers noted that “sleep in the wild may be markedly different from that in captivity,” and called for more research to explore all of those differences. Lack of threat of predation while in captivity is an obvious difference, though diet and other factors could be at play also.

Sloths have changed considerably over time

As we know them today, all six sloth species are arboreal creatures who grow to be 50-60 cm (20-24 in) in length. Ancient sloths were ground dwellers, and some species were up to 6 meters (20 ft) long and weighed over 5 tons; similar to an African elephant. These ancient sloths first appeared about 35 million years ago and went extinct around 10,000 years ago, after humans spread throughout the Americas.

Even two-toed sloths have three toes

Stay with me here; I know that sounds counterintuitive. While two-toed sloths indeed have two digits on their forelimbs (arms), they have three digits on their hind limbs (legs). Realistically, a better name for these sloths would be “two-fingered sloths.” It appears that the confusion came when the names were translated into English, as the Spanish word for digit (dedo) can be used to mean finger or toe. 

They are surprisingly adept at swimming

Aside from defecating, sloths will typically only leave the trees to take a swim. During the rainy season, sloths will sometimes drop from a tree into the water and then swim to another tree. Using a stroke that sort of looks like a doggy paddle/breaststroke combination, sloths can swim up to three times faster than they move on land. Their incredibly slow metabolism affords them another advantage here, as they can slow their heart rate down to less than a third of its normal rate, allowing it to hold its breath underwater for over 40 minutes.

Specialized anatomy allows them to hang upside down

For the most part, sloths prefer to be hanging upside down. They are able to eat, mate, and give birth while hanging upside down, without the weight from their internal organs and waste products pressing on their diaphragm, affecting their breathing. Researchers discovered that sloths’ internal organs are actually anchored on the abdomen, keeping the weight away from the diaphragm. If this weren’t so, breathing would be much more energetically taxing, and could not be supported by their slow metabolism.

Their parental love isn’t unconditional

Many parents in the animal kingdom will go to great lengths to protect their young. Sloth mothers do care about their offspring, at least to an extent. For the first 3-8 months of a baby sloth’s life, it clings to its mother, even after it has been weaned. While holding onto her young, a female sloth will hiss and swipe their long claws at a potential predator, in order to keep her baby safe. 

The limit to that devotion appears to end if the baby should slip and fall. Sloths are fairly sturdy animals, and the fall alone won’t kill most baby sloths. The real danger comes from the fact that the mother rarely comes to retrieve her fallen, crying baby. Realistically, she might not be able to get to her baby and get back up into the safety of the tree before a predator comes, so she doesn’t bother. It has also been suggested that perhaps the mother sees a potential defect in the baby and is just cutting her losses and letting it go. 

Fortunately, sometimes the abandoned baby sloths are found by compassionate humans who find them before an eagle or jaguar. The orphaned sloths are temporarily taken into sanctuaries where they are rehabilitated and released back into the wild, as two-toed sloths have a very restricted diet and do not fare well in captivity. While the thought of a mother dropping her baby and leaving it to scream and die is an incredibly depressing thought, the noise that the sloths make is ridiculously adorable.

They break the rules on mammalian cervical vertebrae 

Of the estimated 5,400 species of mammals on the planet, nearly all of them have seven cervical vertebrae. Blue whale, possibly the largest animal ever? Seven cervical vertebrae. Kitti’s hog-nosed bat, the smallest known mammal? Seven. Giraffes? Seven. Humans? You guessed it. Two-toes sloths can have 5-7, making those (and manatees, who have 6) the only species to have fewer than the highly-conserved number of seven. Three-toed sloths typically have 8 or 9 cervical vertebrae. 

How these anomalies evolved is not entirely clear, and is still debated among biologists. For most other animals, straying from seven cervical vertebrae is a severe birth defect that can either result in stillbirth or the animal doesn’t live very long, which likely played a role in the demise of woolly mammoth. However, the incredibly slow metabolism of the sloth may have made it less risky to have a variable number of vertebrae. For three-toed sloths, the extra vertebrae allow it to turn their heads up to an astonishing 270 degrees.


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