‘Sloths are the lowest term of existence in the order of animals with flesh and blood,’ French naturalist Georges Buffon wrote in the eighteenth century. The man was clearly very irritated by these creatures. He disliked their small eyes, their goofy faces and their fur, which resembled dried, matted grass. Most of all, though, he disliked their laziness.
True, the sloth is a notoriously lazy animal. It is the only one named after one of the Seven Deadly Sins. During a twenty-four hour period, sloths expend energy for just 13% of that time. Though commonly believed to sleep for the rest, a lot of the time they are just sitting there unmoving, not doing anything whatsoever.
Sloths are certainly very adapted to a slow and pondering lifestyle in the rainforest canopy. A sloth hangs upside down for the vast majority of its life, gripping the branches with hooked claws in such a manner that no muscular energy whatsoever is expended. It has much less muscle tissue than other animals of a similar size and weight, so it only moves when absolutely necessary – and even then, extremely slowly. The question is, then, why have sloths evolved to move so slowly and incorporate so many strategies into saving energy?
The answer is down to what they eat: leaves. Leaves are not very nutritious at all and they provide very little energy. Since they do not digest easily, sloths have dealt with this by having large and specialised stomachs that take up most of their bodies. Even so, it can take a week or more for a meal to be fully digested. Most mammals that eat leaves must eat a great quantity of them just to maintain a standard mammalian temperature and go about their daily business; sloths, on the other hand, rather than eating more just do less.
Sloths have thus condensed their daily business into just two major activities: sleeping and, occasionally, eating. They may spend over eighteen hours a day asleep, so if a sloth ever reaches 30 years old then it will have spent around 25 of those years just snoozing. The sloth’s inactivity is the price it must pay for living entirely on leaves, but the one good thing about its diet is that it is unlikely to run out in a tropical rainforest.
A Sloth Success?
As if their near-immobility wasn’t bad enough, sloths also have poor senses. Their vision is very blurry and their hearing is equally bad – it has been reported that firing guns next to a sloth elicits little response. This probably means that the occasional noises that sloths make (guttural wheezes and whistles) are meant for deterring predators rather than communicating with each other. Their only decent sense is smell, which they use to find their favourite leaves.
It seems to us that the poor sloth is being punished, what with it being forced to live in a dim, muffled, slow-motion world, but it is actually a very successful animal. The only predator that regularly hunts it is the harpy eagle, which snatches them from the canopies as they bask in the sunlight. For most predators, though, a sloth doesn’t make for a very nutritious meal – they are little more than bags of half-digested leaves with hardly any meat on them. Not even man hunts the sloth on a regular basis, for if he shot one with a poison-tipped dart in the treetops, the animal would remain hanging from its branch even after death, forcing the hunter to climb up to retrieve it.
Over the course of history, many people have taken offence with the sloth’s inherent laziness. In addition to Georges Buffon mentioned above, the first Spanish explorers in the Americas claimed that sloths were ugly and useless. Charles Waterson, an eccentric English naturalist who 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’.
But it’s quite clear that, as much as some humans would hate to admit, laziness is a winning strategy.