Sloth

Life in the Slow Lane

The sloth is a candidate for nature’s most misunderstood animal. Saddled with a name that is also one of the Seven Deadly Sins, the world’s slowest-moving mammal has been eternally condemned for its lethargic lifestyle. The illustrious French aristocrat and naturalist, Comte de Buffon, certainly had a miserable opinion of this idle leaf-eater. He disliked the sloth’s small eyes, goofy face, long nails and coarse, untidy fur. But most contemptible of all in Buffon’s view was the sloth’s laziness. 

‘Sloths are the lowest term of existence in the order of animals with flesh and blood; one more defect would have made their existence impossible.’

_{^{Comte de Buffon}}

A Slothful Existence

Sloths are the world’s only inverted quadrupeds. They spend most of their time hanging upside down from branches using long, gangly limbs, gripping branches with their hooked claws in such a manner that no effort whatsoever is required. This lifestyle demands only half the muscle mass needed to prop up an upright existence, and sloths, therefore, have much less muscle tissue than other animals of a similar size and weight. This means they move extremely, often mesmerisingly, slowly. The explorer William Dampier, who became the first person to circumnavigate the world three times, claimed that a sloth would take ‘up to eight or nine minutes to move three inches forward’ – even when he whipped them. This was obviously a deeply disparaging exaggeration, as we now know that the average cruising speed of a sloth is, at 0.2 mph, slightly faster.

There are six living species of sloth. All hail from the rainforests of South and Central America and can be split into two groups, both of which have been given derogatory names: Choloepus (meaning ‘crippled’) and Bradypus (‘slow-footed’). They are more commonly known as the two-toed and three-toed sloths, respectively, but the difference is actually in the number of fingers they possess, not their toes. The two genera are actually as genetically different as cats and dogs, having diverged from a common ancestor – probably a type of ground sloth – some 40 million years ago. They then went along separate branches of the evolutionary tree, both becoming adapted to an arboreal, slow-motion, upside-down way of life completely independently of one another. Such a lifestyle must clearly have its benefits if it evolved twice.

Of the two varieties, the three-toed, with its distant, permanent, Zen-like smile and a large belly, is by far the more slothful. During a 24-hour period, a three-toed sloth spends around 8-10 hours sleeping, looking like a bundle of withered leaves or a hairy hammock dangling beneath a branch. This may seem like a surprisingly small amount for what is commonly referred to as ‘the world’s laziest animal’, but even when it is awake it often hangs there in complete immobility, doing virtually nothing. Apart from its open eyes, which stare blankly into space, it may give little indication that it is awake, or even alive. But this seemingly meditative state is crucial for energy conservation.

The three-toed sloth is a surprisingly fussy, fastidious eater and will only take leaves from a few particular kinds of tree, primarily Cecropia. These are virtually unobtainable outside their natural range, which is why you will almost never encounter this particular variety of sloth in a zoo. The two-toed sloth, however, has a much more varied diet – it enjoys a wide range of leaves, vegetables and even fruits – and is therefore considerably easier to care for in captivity. It is also somewhat livelier, ranging more widely in search of food. And, despite its cuddly appearance, it can be surprisingly bad-tempered. This makes for a slightly more interesting exhibit than the chronically lethargic three-toed sloth, although it is still by no means the most dazzling zoo animal to behold.

But why has the sloth evolved to move so slowly? Why has it become so extremely sparing in the expenditure of energy? The answer is down to what it eats: leaves.

Slow and Steady

Leaves are not very nutritious. The cellulose within them is remarkably indigestible. The sloth, like all mammals that eat leaves in bulk, maintains a flourishing culture of bacteria in its digestive system that can break cellulose down. Most mammals that eat leaves must consume vast quantities of them just to maintain a standard mammalian body temperature and go about their daily business. But the sloth, rather than eating more, simply does less. The leaves that it eats provides the animal with just 160 calories a day – the equivalent of a packet of crisps. It is now physically incapable of speed, even if its life is in mortal danger.

The three-toed sloth so assiduously avoids unnecessary exertion that it has condensed its daily business into just two major activities: resting and, occasionally, eating. Its low-calorie diet means that it spends very little energy heating its own body, averaging just 30-34°C when active, and even lower when resting. Energy from the sun, however, is free, so sloths take advantage of it by moving out onto the exposed crown of a forest tree to soak up the warmth, basking like a lizard. In the rainy season, a sloth’s metabolism – which is the slowest of any non-hibernating mammal – may slow down so much that it barely digests anything at all, and it can starve to death with a full stomach.

As if its near-immobility wasn’t bad enough, the sloth also has poor senses. Its eyes are small and its vision blurry. It shows very little sign of being able to hear much of what is going on around it, and it is even said that firing a gun next to a sloth elicits little response other than the creature turning slowly and blinking. They have dispensed with virtually all forms of vocal communication except one: an ear-piercing shriek that females use to attract males, which can travel long distances. Males are attuned to this sound – and perhaps only this sound, for they seem disinterested in every other noise. The sloth’s only decent sense is smell, which it uses to find its favourite leaves.

Sloths may be remarkably slow, but so, too, were some of the people who first studied these animals. The skins of sloths that were first sent back to Europe from the Americas, for example, caused considerable confusion. Artists were given the task of including them within natural history encyclopaedias but, having never seen a living specimen, they didn’t really know how to draw them. Many were depicted standing upright on all four wiry legs, even though their claws are clearly pointing in the wrong way for sloths to efficiently walk like that. Perhaps they thought an animal that spent its entire life hanging upside down from branches was too absurd to be true.

Over the course of history, many people have taken offence with the sloth’s apparent laziness. In addition to Comte de Buffon, the first Spanish explorers in the Americas claimed that sloths were ugly and useless. However, part of this may have had something to do with the fact that the first sloths encountered by Europeans were probably those collected by local indigenous people and simply placed on the ground, where, the ‘right’ way up, in a completely unnatural position for them, they would have been observed crawling pitifully along the ground. Charles Waterton, an eccentric English naturalist who is said to have imitated a dog at the dinner table and bit at the legs of his guests, wasn’t impressed either: he said of the animal, ‘The sloth is totally unfit to enjoy the blessings which have been so bountifully given to animated nature’.

A Sloth Success

We may think that the sloth, living as it does in a slow-motion, dim, muffled world, is being punished, but it is actually a very successful mammal. Other than the harpy eagle, sloths have few predators. This is partly because they move at a pace that rarely gets them noticed and are therefore hard to spot, and partly because – being little more than a stringy bag of half-digested, fermenting leaves – they don’t make for a very nutritious meal. Humans don’t hunt sloths on a regular basis either. Even if a person did shoot one with a poison-tipped dart in the treetops, the animal would probably remain hanging from its branch even after death, forcing the hunter to climb up to physically unhook it – hardly worth the effort.

Occasionally, sloths do fall from the trees, especially when they are young or during mating attempts, but they can survive plummets of up to 27 metres. Two-toed sloths possess 23 pairs of ribs – more than any other mammal – and this turns them into a sort of living wicker basket, which is said to enable them to bounce quite harmlessly when they hit the ground.

Although sloths are renowned for their slow pace, they are not just simple, lazy creatures that sleep all day. Their inactivity is the price they must pay for living entirely on leaves, but up in the trees and out of sight, they can take all the time they want to digest their meals. With few predators and almost no competitors, a sloth has no need to be strong or swift or even remotely active, and it has become one of the most successful animals in its habitat. Of the half dozen species alive today, only two – the maned and pygmy sloths (which we’ll be looking at in a later article) – are threatened. In some places, sloths are said to account for up to one-third of the total mammalian biomass. These animals are clearly survivors. And the very secret to their survival is, undoubtedly, their slothful nature.