High in a Central American rainforest, a shaggy shape hangs from a branch. For an hour it barely moves. A harpy eagle glides past overhead and does not notice it. For a sloth, doing almost nothing is a skill.
Sloths are slow because they live on leaves, a food that gives very little energy. Their key adaptations save energy: a metabolism only 40 to 45 percent of what is expected for their size, a body temperature that drifts with the weather, hooked claws for hanging without effort, and greenish fur that hides them, says the Smithsonian’s National Zoo.
Key takeaways
- Why slow: leaves are low in energy and hard to digest, so sloths save energy at every step.
- 3 key adaptations: a very low metabolism; long, hooked claws for hanging; algae-tinted fur for camouflage.
- Record holder: three-toed sloths have the lowest field metabolic rate measured for any mammal (Pauli et al. 2016).
- Myth buster: wild three-toed sloths slept under 10 hours a day in a brainwave study, not 15 to 20.
- Most at risk: the pygmy three-toed sloth, Critically Endangered and found on one small island in Panama.
Why are sloths so slow?
Because their food gives them so little energy. Sloths eat mostly leaves, which are tough and low in nutrients. Their food stays in the gut for about a month while it is broken down, says the Smithsonian. Moving slowly, keeping a low body temperature and resting a lot let them live on this poor diet.
Scientists think this is why so few mammals live in treetops eating only leaves. A 2016 study by Jonathan Pauli and colleagues measured how much energy wild sloths use. It found that three-toed sloths had the lowest field metabolic rate recorded for any mammal, and argued that leaf-eaters in trees are “tightly constrained” by energy.
What are the main adaptations of a sloth?
Nearly every sloth adaptation saves energy or avoids being noticed. They have a slow metabolism, a body temperature that rises and falls, long curved claws that hook over branches, a stomach with several chambers for digesting leaves, a flexible neck, and fur that grows algae. They also swim well and come down to the ground only rarely.
| Type | Adaptation | How it helps | Key number |
|---|---|---|---|
| Structural | Long, curved claws | Hook over branches so hanging takes little effort | 8–10 cm (3–4 in) on two-toed sloths |
| Structural | Grooved hairs that hold algae | Green tint for camouflage in the canopy | Two kinds of blue-green algae (two-toed sloth) |
| Structural | Extra neck bones (three-toed sloths) | Turn the head far without moving the body | About 10 neck vertebrae; turns up to 270° |
| Structural | Stomach with several chambers | Breaks down tough leaves | Food stays in the gut about a month |
| Physiological | Very low metabolism | Uses less energy | 40–45% of the expected rate |
| Physiological | Changing body temperature | Saves energy at night and in rain | 30–34 °C (86–93 °F) |
| Behavioural | Moving slowly and rarely | Saves energy; avoids catching a predator’s eye | Home range about 4 hectares (10 acres) |
| Behavioural | Toilet trip about once a week | Limits risky trips to the ground | Once a week |
Sources: Smithsonian’s National Zoo (two-toed sloth); Animal Diversity Web (brown-throated three-toed sloth). Our guide to behavioural adaptations explains the three types.
Structural adaptations
These are body parts. A sloth’s long, curved claws hook over branches, so it can hang for long periods with little effort. Three-toed sloths have about 10 neck vertebrae, more than most mammals, and can turn their heads up to 270 degrees, says Animal Diversity Web. That lets them reach leaves without shifting their whole body.
Physiological adaptations
These are about how the body works. Two-toed sloths run at only 40 to 45 percent of the metabolic rate expected for their weight, and their body temperature drifts between 30 and 34 °C (86 and 93 °F), dropping at night and in wet weather, says the Smithsonian. Letting the body cool saves the energy most mammals spend staying warm.
Behavioural adaptations
These are things sloths do. They move slowly, rest a lot, and come down from the trees only to change trees, find food or go to the toilet, which they do about once a week. Pauli’s 2016 study found three-toed sloths moved even less than two-toed sloths and let their temperature vary more.
Picture it: running on low-power mode
Think of a phone on low-power mode: the screen dims and background apps stop, so the battery lasts longer. A sloth does the same. A two-toed sloth weighs 4 to 8 kg (9 to 17 lb), says the Smithsonian, about as much as four to eight one-litre bottles of water. Yet its body runs at well under half the energy expected for an animal that size, and its meal takes about a month to pass through.
Why is sloth fur green?
Because algae grow in it. Each hair has grooves that hold blue-green algae, which give the fur a greenish tint that helps hide the sloth in the canopy, says the Smithsonian. The fur is a whole tiny ecosystem, with moths living in it too.
A 2014 study by Pauli and colleagues found that three-toed sloths carry more moths and more algae than two-toed sloths. The sloths ate algae from their own fur, and the algae were easy to digest and rich in fat. The researchers suggested this “algae garden” adds extra nutrition to a poor diet.
Why do sloths come down from the trees to poop?
Nobody is completely sure, but there is a leading idea. Three-toed sloths climb down about once a week to defecate, which is risky and uses energy. Pauli’s team proposed that the trip helps moths lay eggs in sloth dung. The moths then move into the sloth’s fur, add nitrogen, and help the algae grow.
This is a hypothesis supported by one detailed study, not a settled fact. It links sloths, moths and algae in a chain of partnerships, which the researchers called “a syndrome of mutualism”.
Can sloths swim?
Yes. Despite moving slowly in trees, sloths are surprisingly competent swimmers, says the Smithsonian, and Animal Diversity Web describes three-toed sloths as capable swimmers too. Swimming lets them cross rivers and flooded forest, and water takes the weight off their bodies.
How long do sloths sleep?
Less than many people think. Wild brown-throated three-toed sloths living in a tropical rainforest canopy slept about 9.6 hours a day, over 6 hours less than had been reported in captivity, found a 2008 study by Niels Rattenborg and colleagues. It was the first study to record the brainwaves of wild sloths as they slept.
Many sources still say sloths sleep 15 to 20 hours a day. Those figures come from animals in zoos and labs. The study’s authors noted that age may have played a part, but said sleep in the wild “may be markedly different” from sleep in captivity. It is a good example of a number repeated long after better data came along.
Are sloths endangered?
Most sloths are not, but one is in serious trouble. The brown-throated three-toed sloth is listed as Least Concern, says Animal Diversity Web. The pygmy three-toed sloth is Critically Endangered. It lives only on Isla Escudo de Veraguas, a 4.3 km² island off Panama, where a 2011 survey counted just 79 animals.
That survey, published in PLOS ONE in 2012, found most pygmy sloths in mangrove thickets that cover a tiny part of the island, and reported people cutting mangroves. Even common sloths are losing forest: the Smithsonian says two-toed sloths are in serious danger of losing habitat to logging.
What role do sloths play in the rainforest?
Sloths are food for big predators: harpy eagles, spectacled owls and wild cats all hunt them, says Animal Diversity Web. Their fur also hosts algae and moths, so each sloth is a small moving habitat. Their weekly dung piles at the foot of trees become nurseries for those moths, linking the canopy to the forest floor.
Myths about sloths
Myth: Sloths are slow because they are lazy.
Fact: Slowness is an energy-saving strategy that suits a diet of leaves. Why people believe it: the word “sloth” also means laziness in English.
Myth: Sloths sleep 15 to 20 hours a day.
Fact: Wild three-toed sloths slept about 9.6 hours a day in a 2008 study. The higher numbers came from captive animals.
Myth: Being slow makes sloths helpless.
Fact: Moving little and looking green helps sloths go unnoticed in the canopy. They are still hunted by eagles, owls and cats, but stillness is part of their defence.
Test yourself
1. Why do sloths move so slowly?
Their leafy diet gives them very little energy, so they save energy by moving slowly and keeping a low metabolism.
2. What makes a sloth’s fur green?
Algae growing in grooves in its hairs.
3. How often do sloths come down to go to the toilet?
About once a week.
Where to next?
➡️ Go wider: what behavioural adaptations are⬆️ Go deeper: are slow animals really “dumb”?🔀 Go sideways: the camel, another master of saving energy and water
Frequently asked questions
How slow is a sloth?
Very slow, but we could not find a well-measured top speed in a scientific source, so we have not given one. What has been measured is energy use: three-toed sloths have the lowest field metabolic rate recorded for any mammal, and they moved less than two-toed sloths in a 2016 study.
What is the difference between two-toed and three-toed sloths?
The names refer to the number of clawed toes on the front feet. Two-toed sloths weigh 4 to 8 kg, says the Smithsonian. Three-toed sloths are more specialised leaf-eaters: they use even less energy, carry more algae in their fur, and have extra neck vertebrae.
How long do sloths live?
In human care, two-toed sloths have a median life expectancy of about 16 years, and the oldest at the Smithsonian’s National Zoo lived to about 49, it says. Lifespans in the wild are harder to measure, so reliable figures are scarce.
Are sloths good pets?
No. Sloths are wild animals with very particular needs for diet, warmth and space. If you want to see sloths, visit an accredited zoo or a responsible wildlife reserve, watch them from a distance, and never pay to hold one for a photo.
Sources
- Smithsonian’s National Zoo & Conservation Biology Institute. Two-toed sloth.
- Pauli, J. N. et al. (2016). Arboreal folivores limit their energetic output, all the way to slothfulness. American Naturalist 188: 196-204.
- Pauli, J. N. et al. (2014). A syndrome of mutualism reinforces the lifestyle of a sloth. Proceedings of the Royal Society B 281: 20133006.
- Rattenborg, N. C. et al. (2008). Sleeping outside the box: electroencephalographic measures of sleep in sloths inhabiting a rainforest. Biology Letters 4: 402-405.
- Animal Diversity Web, University of Michigan. Bradypus variegatus: brown-throated three-toed sloth.
- Kaviar, S., Shockey, J. and Sundberg, P. (2012). Observations on the endemic pygmy three-toed sloth, Bradypus pygmaeus, of Isla Escudo de Veraguas, Panamá. PLOS ONE 7: e49854.
Sources checked 12 October 2026.
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