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Page last updated at 16:06 GMT, Monday, 6 September 2010 17:06 UK
Sloth locomotion secrets revealed
By Matt Walker
Editor, Earth News

Watch how a sloth 'walks' upside down (video courtesy of Mr Nyakatura et al)

Sloths live a uniquely upside down life, and a new study has revealed some of the secrets of how they do it.

Scientists have analysed how these odd-looking animals move around while hanging from trees.

Using various imaging techniques, the researchers have found that sloths actually walk upside down, but have a host of special adaptations that allow them to lead an inverted lifestyle.

Details of the study are published in the journal Zoology.

It is time people start to think about sloths differently

Evolutionary biologist Mr John Nyakatura

"They are the prototypical upside-down animals. They suspend with all fours from the branch," says evolutionary biologist Mr John Nyakatura of the Friedrich-Schiller University based in Jena, Germany.

He and colleagues have studied the locomotion of the two-toed sloth (Choloepus didactylus) to find out how its body is adapted to this unique lifestyle.

They filmed moving sloths, generating a series of videos, CT scans and images known as video radiographs, which revealed the mammal's bones.

Using these images, they created 3D animations that show in detail how a sloth's skeleton is adapted to moving upside down.

Mr Nyakatura feeds a sloth
Mr Nyakatura feeds one of his subjects

"Sloths surprisingly move very similar to right-side up mammals, except for being upside-down," PhD student Mr Nyakatura told the BBC.

For example, the angles of the limbs and joints are similar to other small or medium-sized mammals.

"The same muscles responsible to move the limb forward and backward during locomotion in 'normal' mammals are also responsible for this task in sloths," Mr Nyakatura adds.

"The pattern is conserved in evolution."

However, sloths do have a host of special adaptations, the studies reveal.

"They do some things uniquely, apart from the obvious," says Mr Nyakatura.


Sloths have comparably long arms, but very short shoulder blades and a very narrow chest.

The researchers argue that these features help to maximize mobility at the shoulder.

They also help these creatures navigate around the rainforest and between trees, a complex three-dimensional habitat, without being able to jump between gaps.

Sloths also have a special arrangement of muscles.

One of the pectoral muscles, those that human athletes would use to bench press a weight, for instance, descends down to a sloth's lower arm.

Video radiograph of a moving sloth
A video radiograph of a moving sloth

In other mammals, including people, it is usually attached at the upper arm near the shoulder joint.

This peculiar arrangement helps sloths carry their own body weight, which is always being dragged to the floor by gravity.

"And of course they have hook-like claws and other things as well," says Mr Nyakatura.

These adaptations all help sloths lead their unusual lifestyle.

"Actually this strategy opened a niche for them," explains Mr Nyakatura.

Sloths move so slowly because of their low-calorie food.

Being able to survive just on leaves means they can avoid the need to seek out high-calorific food such as meat of fruit, more energetic behaviour that in turn might attract the attentions of predators.


"They are specialized in deliberate, energy efficient, cryptic behaviour.

"People should probably stop thinking of them as deficient animals," says Mr Nyakatura.

"It is time people start to think about sloths differently. They are not just lazy and slothful. They evolved a different strategy to cope with everyday demands."

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