Why are sloths so slow? Sloths are slow because they live on a low-energy diet of leaves, digest it very slowly, have one of the lowest metabolic rates of any mammal and carry little muscle. Moving slowly saves energy and helps them hide from predators. Meet their cuddly cousins in our sloth stuffed animals collection.
Key takeaways
- Sloths have one of the lowest metabolic rates among mammals, about 40 to 74% of the value predicted for their body mass (The Sloth Conservation Foundation).
- Food can take from 157 hours to 50 days to pass through a sloth's digestive system (The Sloth Conservation Foundation).
- Sloths have about 30% of the muscle mass expected for a mammal of their size (The Sloth Conservation Foundation).
- Two-toed sloths sleep about 15 hours a day and usually climb down to defecate only once a week (Smithsonian's National Zoo).
- Slow movement is a survival strategy: jaguars, ocelots and harpy eagles hunt mainly by sight, so a still, green-tinted sloth is hard to spot.
Why are sloths so slow? The short answer
Sloths are slow because their whole body is built to save energy: they eat food that gives very little energy, so they spend as little as possible.
Slowness is not a flaw. According to The Sloth Conservation Foundation, sloths have survived for almost 64 million years by living this way. Several traits work together:
- A low-calorie diet made mostly of leaves.
- Very slow digestion in a large, multi-chambered stomach.
- A low metabolism and a body temperature that changes with the weather.
- Little muscle compared with other mammals of the same size.
- Camouflage that works best when the animal barely moves.
Each of these traits is explained below.
Sloths eat a low-energy diet of leaves
Sloths eat mostly leaves, which are low in calories and hard to digest, so their meals give them very little energy to spend on movement.
Leaves are everywhere in the rainforest canopy, but they are a poor fuel. Two-toed sloths also eat fruit and fresh green shoots, and occasionally insects or bird eggs, according to the Smithsonian's National Zoo.
Sloths also eat surprisingly little. The Sloth Conservation Foundation notes that howler monkeys, which share the same forests, eat about three times as many leaves per kilogram of body mass as sloths do. With so few calories coming in, a sloth simply cannot afford to rush.
Why do sloths digest food so slowly?
Sloths digest food slowly because breaking down tough leaves takes time, and their large, four-chambered stomach works at a very low pace.
The Sloth Conservation Foundation reports that food can take between 157 hours and 50 days to pass through a sloth. The Smithsonian's National Zoo says food stays in a two-toed sloth's digestive tract for about one month.
A sloth's stomach is almost always full. Its contents can make up to 37% of the animal's body weight, according to The Sloth Conservation Foundation. That is a lot of slowly fermenting leaves to carry around, and another reason a sloth keeps its movements to a minimum.
Sloths have one of the slowest metabolisms of any mammal
A sloth's metabolism runs at only 40 to 74% of the rate expected for a mammal of its size, so its body produces and uses energy very slowly.
Most mammals spend a lot of energy keeping a steady body temperature. Sloths do not. Two-toed sloths have a low body temperature of roughly 86 to 93°F (30 to 34°C), which drops at night, in wet weather and when they are inactive (Smithsonian's National Zoo). Their core temperature can change by about 10°C (18°F) over a single day (The Sloth Conservation Foundation).
Instead of burning energy to stay warm, sloths rely on behavior to manage their temperature, for example by moving into the sun (The Sloth Conservation Foundation). The Smithsonian's National Zoo adds that their reduced muscles mean they cannot shiver to stay warm.
Sloths can even slow their metabolism down further
A study published in the journal PeerJ and led by zoologist Rebecca Cliffe placed eight three-fingered sloths in temperature-controlled chambers. As the temperature rose, the sloths used more oxygen, but once it reached 86°F their energy use began to fall. Smithsonian Magazine described this as "the exact opposite of what typically happens in all other animals."
Sloths have less muscle than other mammals
Sloths have much less muscle than other mammals of the same size, which limits fast movement but keeps their energy needs low.
The Sloth Conservation Foundation puts it at about 30% of the muscle mass expected for a mammal of a similar size, and the Smithsonian's National Zoo describes sloth muscles as about half the relative weight found in most land mammals. Much of a sloth's apparent bulk is thick fur.
Less muscle does not mean weak. Sloths spend most of their lives hanging upside down, and their unusual muscle arrangement gives them surprising strength and resistance to fatigue. They are also surprisingly competent swimmers, according to the National Zoo.
Moving slowly helps sloths hide from predators
Sloths cannot outrun predators, so they avoid being noticed: slow, quiet movement and green-tinted fur make them hard to see in the canopy.
Jaguars, ocelots and harpy eagles hunt mainly by sight, and a fast-moving animal is easier to spot (The Sloth Conservation Foundation). Sloths also have weak eyesight: all sloths have rod monochromacy, which means they are colorblind and see poorly in dim light.
Their fur helps too. Two species of blue-green algae grow in grooves in the hair of two-toed sloths, giving the coat a greenish tint that blends into the leaves (Smithsonian's National Zoo). If threatened, a sloth can still defend itself with its claws and teeth.
Are sloths lazy?
No. Sloths are not lazy; they are energy savers whose slowness is an adaptation, not a lack of effort.
Sloths do rest a lot. Two-toed sloths are mainly nocturnal and sleep about 15 hours during the day, waking at night to eat (Smithsonian's National Zoo). But resting is how they balance a tight energy budget. The Sloth Conservation Foundation sums it up: sloths are stealthy, not lazy.
Curious about other animals with surprising lifestyles? Read also: Llama vs Alpaca: How to Tell Them Apart.
Why do sloths climb down to poop once a week?
Sloths usually climb down to the base of a tree about once a week to defecate, and scientists think the habit supports a partnership with the moths and algae in their fur.
The trip is risky and costly in energy for such a slow animal. A study led by Jonathan Pauli of the University of Wisconsin-Madison and published in Proceedings of the Royal Society B offered an explanation, reported by Smithsonian Magazine:
- Sloth moths lay their eggs in sloth dung on the forest floor.
- The larvae feed on the dung, and the next generation of moths moves up into a sloth's fur.
- The moths add nutrients to the fur, which feed the algae growing there.
- The algae are rich in lipids, and the researchers suggest they may be a high-energy snack for the sloth.
Sloth slowness by the numbers
These figures sum up why a sloth's body favors saving energy over speed.
| Trait | Sloth | Source |
|---|---|---|
| Metabolic rate | 40 to 74% of the rate predicted for body mass | The Sloth Conservation Foundation |
| Digestion time | 157 hours to 50 days | The Sloth Conservation Foundation |
| Muscle mass | About 30% of what is expected for its size | The Sloth Conservation Foundation |
| Body temperature (two-toed sloth) | About 86 to 93°F (30 to 34°C) | Smithsonian's National Zoo |
| Sleep (two-toed sloth) | About 15 hours a day | Smithsonian's National Zoo |
| Trips to the ground to defecate | About once a week | Smithsonian's National Zoo |
Bring home a sleepy sloth
A sloth plush captures all the calm, slow-motion charm of the real animal, and it is happy to hang around all day. Here are three sloth picks from Mister Plush, in stock now:
- Hanging Sloth Stuffed Animal: a 15-inch brown and beige sloth with long arms that wrap around bedposts, backpacks or shelves, filled with 100% PP cotton, for ages 3 and up.
- Kawaii Sloth Plush: a round, light brown sloth with an embroidered face and blush cheeks, in 8.7, 11.8 or 15.4 inches.
- Sloth Pillow Plush: a huggable pillow-shaped sloth with floppy limbs, in 8.7 or 17.7 inches, made for naps and lazy lounging.
Looking for more cute companions? Explore our kawaii plush collection.
FAQ: why are sloths so slow
How long do sloths sleep?
Two-toed sloths sleep about 15 hours a day, according to the Smithsonian's National Zoo. They are mainly nocturnal: they rest during the day and wake at night to eat. Long rest periods help them save energy, because their leafy diet gives them very few calories to spend on activity.
Can sloths swim?
Yes. The Smithsonian's National Zoo describes two-toed sloths as surprisingly competent swimmers, even though they are nearly immobile on land. Sloths spend most of their lives hanging upside down in the trees, and they usually come down only about once a week, to defecate at the base of a tree.
How long does it take a sloth to digest food?
Digestion is extremely slow in sloths. The Sloth Conservation Foundation reports that food takes between 157 hours and 50 days to pass through, and the Smithsonian's National Zoo says food stays in a two-toed sloth's digestive tract for about one month. Their stomach is almost always full of slowly fermenting leaves.
Conclusion
So, why are sloths so slow? Because every part of their body, from their leafy diet to their low metabolism and light muscles, is designed to save energy, and staying still keeps them hidden from predators. Their slowness is a smart survival strategy that has worked for millions of years. Bring home your own laid-back friend from our sloth stuffed animal collection.
Sources
- The Sloth Conservation Foundation: Why Are Sloths So Slow?
- Smithsonian's National Zoo and Conservation Biology Institute: Two-toed sloth
- Smithsonian Magazine: Sloths Don't Just Live in Slow-Mo, They Can Put Their Metabolism On Pause
- Smithsonian Magazine: What Drives a Sloth's Ritualistic Trek to Poop?
