BBC Science Focus

The power of sloth A sloth’s lifestyle makes it the ultimate evolutiona­ry success story.

Why nature’s laziest animal is an evolutiona­ry success story


The sloth is a candidate for nature’s most misunderst­ood animal. Saddled with a name that speaks of sin, the world’s slowest-moving mammal has been eternally damned for its lethargic lifestyle. “These sloths are the lowest forms of existence,” proclaimed the great French naturalist Georges-Louis Leclerc, Comte de Buffon. “One more defect would have made their lives impossible.”

He couldn’t have been more wrong. The sloth is a supreme survivor that has graced our planet for some 64 million years. A survey of a Panama rainforest in the 1970s found that an incredible one-third of the total mammalian biomass was made up of sloths. The secret to the sloth’s success is its lackadaisi­cal nature. They are energy-saving icons, performing about 10 per cent of the physiologi­cal work of a mammal of similar size, and boasting a suite of ingenious adaptation­s that allow them to exist on as few as 160 calories a day.


Sloths are the world’s only inverted quadrupeds, hanging from the trees of the tropical rainforest­s of South and Central America. Their toe bones are not separately movable, but bound together by ligaments, which along with their curved claws act as efficient hooks for dangling upside down. Their muscles have also evolved to suit their lifestyle. They manage almost exclusivel­y with retractor muscles, like our biceps, which pull them along the undersides of branches.


Sloths subsist almost entirely on leaves. These are plentiful in the rainforest canopy, but are full of toxins and tough cellulose, making them hard to digest. To cope, sloths have evolved a four- chambered stomach, much like a cow’s, and employ a host of gut bacteria to digest the leaves. It takes sloths up to a month to break down a single leaf: if it happened any faster, their liver might not cope, and they’d be in danger of poisoning themselves.


Sloths have up to 10 neck vertebrae: more than any other mammal. Even the giraffe makes do with just seven. In 2010, scientists at the University of Cambridge discovered that these appear to have evolved from ribcage vertebrae that were co- opted over time into neck bones. The long neck allows nature’s couch potato to turn its head 270° and graze leaves all around it without wasting precious energy moving the rest of its body.


Sloths maintain a low core temperatur­e of just 28°C to 32°C, whereas most mammals rely on a toasty 37°C. Rather than keeping themselves warm by stoking their internal combustion engine with calories, sloths wear a dense coat, worthy of an Arctic animal. Energy from the sunshine is free, and sloths bask like lizards to soak it up. Like cold-blooded animals, they’re also able to withstand fluctuatio­ns in their body temperatur­e of several degrees throughout the day.


With an average cruising speed of just 0.3km/h, running from danger is not an option for the sloth. Instead, they avoid predation with their superb camouflage. Special grooves in the sloth’s coat collect water and act as hydroponic gardens for as many as 80 different species of algae and fungi (as well as a wealth of insects), giving their fur a greenish hue. Each sloth is in fact a slow-moving miniature ecosystem that blends in perfectly with the trees.


Sloths have evolved bands of tissue that anchor their guts to their lower ribs, preventing their massive stomach, which can make up as much as a third of their body weight in undigested leaves, from pressing down on their lungs. This adaptation makes breathing much less energy-intensive: researcher­s have estimated that the tissue fibres reduce a sloth’s energy expenditur­e by up to 13 per cent, which is a significan­t amount when you have such a low- calorie diet.

 ??  ?? Lucy Cooke is a zoologist and author of The Unexpected Truth About Animals (£8.99, Black Swan) and Life In The Sloth Lane (£9.99, Workman).
Lucy Cooke is a zoologist and author of The Unexpected Truth About Animals (£8.99, Black Swan) and Life In The Sloth Lane (£9.99, Workman).
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