Why Are Sloths So Slow? Metabolism and Leaf Diet Explained

How slow is a sloth?

A sloth climbing through the canopy usually travels at about 0.24 km/h (0.15 mph), or a few meters per minute. Many wild sloths move less than 40 m (130 ft) in a whole day, and some stay in the same tree for days at a time. A brown-throated sloth may spend its entire life in a home range smaller than a few city blocks.

For comparison, a person strolling along walks about 20 times faster than a moving sloth. On the ground a sloth is even slower, because its legs cannot support its weight well and it has to drag itself forward with its arms.

A diet of leaves gives very little energy

The root of the answer is food. Sloths are folivores, which means leaf-eaters. Leaves are easy to find in a rainforest, and few other large animals compete for them high in the canopy, but they make a poor meal:

• Low in calories: most of a leaf is water and fiber, with little sugar, fat or protein.
• Hard to digest: the energy is locked in tough cellulose that only bacteria can break down.
• Full of defenses: many leaves contain toxins and tannins that the body has to cope with.

A sloth’s stomach has several chambers where bacteria slowly ferment the leaves. Food may take days to weeks to pass through the gut, and at any moment a large share of the animal’s body weight is leaves waiting to be digested. Because the energy trickles in slowly, a sloth can only spend it slowly.

A tiny metabolism and a cool body

Sloths have one of the lowest metabolic rates of any mammal. Their resting metabolism is roughly half of what would be predicted for a mammal of their size. A field study of wild brown-throated sloths published in 2016 found daily energy use lower than that measured for any other non-hibernating mammal at the time.

Part of the saving comes from body temperature. Most mammals keep their bodies at about 36–38 °C (97–100 °F), which costs a lot of energy. Sloths run cooler, at about 30–34 °C (86–93 °F) when active, and their temperature can drop further on cool nights. Like lizards, they warm up by basking in the sun in the treetops. The drawback is that sloths cannot survive cold climates, which is one reason they live only in tropical Central and South America.

Less muscle to power

A sloth carries much less muscle than most mammals. Muscle makes up only about 25–30 percent of a sloth’s body weight, compared with roughly 40–45 percent in many other mammals of similar size. Muscle is expensive tissue to maintain, so having less of it saves energy every hour of the day.

Sloths make up for this with clever anatomy. Their long, hooked claws let them hang from branches like a coat on a hanger, with almost no effort. Their arm muscles are built for strong, steady pulling and gripping rather than quick bursts of speed. They hang, eat, sleep and even give birth upside down, so they rarely need to hold their own weight up the way a walking animal does.

Slowness as camouflage

Moving slowly is not only about saving energy. It also helps sloths hide. Their main predators, such as the harpy eagle, jaguars and ocelots, find prey mostly by spotting movement. A sloth that barely moves, with grayish-brown fur often tinted green by algae, looks like a clump of dead leaves or a termite nest.

When danger comes, a sloth cannot run away, so it relies on not being noticed in the first place. If it is attacked, it can slash with its long claws, and its tough skin and dense fur give it some protection.

Two-toed vs three-toed sloths

There are two living groups of sloths, and both are slow, but in slightly different ways:

Two-toed sloths eat a more varied diet with more fruit, which gives them a bit more energy to spend. They are mostly active at night and generally a little more agile, especially on the ground. Three-toed sloths are the true specialists, with an almost pure leaf diet and the most extreme energy saving.

Are sloths really lazy? Myths and facts

The English word “sloth” means laziness, and many languages give sloths similar names, such as perezoso in Spanish and Faultier (“lazy animal”) in German. But a sloth is not lazy any more than a cactus is lazy for growing slowly. Its whole body is designed to live on very little.

Another common myth is that sloths sleep almost all day. Wild brown-throated sloths actually sleep about 9 to 10 hours a day; they just spend much of their waking time sitting still. And while sloths are the slowest mammals, they are not the slowest animals: many snails, sea stars and sea anemones move far less in a day.