Apart from a weekly trip to the base of its tree for a toilet break, a sloth lives in the jungle canopy 24/7, and for much of that time it’s upside down, says Sheena Harvey. To enable it to live comfortably in this manner it has three powerful claws on each back foot and a very peculiar digestive system.
Thanks to the specialised tendons in its arms and legs, that lock it to a branch, the animal can hang pretty much motionless for hours, even maintaining the position when it’s fast asleep. What sets it apart from other tree-dwelling mammals that have to stay reasonably upright is the odd arrangement of its food pipe and stomach.
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Sloths consume a huge number of leaves that remain in their digestive systems for up to 30 days before they are full processed. That’s how long it takes the gut bacteria to break down and ferment the tough vegetation. Their large four-chambered stomach therefore holds a huge volume of brewing leaf mash, so much so that it accounts for about two-thirds of the sloth’s body weight.
Think of a compartmentalised bag full of semi-solid liquid, hanging upside down with an open narrow pipe leading out at the bottom. You would automatically assume that the pressure of the contents would force the semi-digested food out of the pipe, ie, the sloth’s mouth. But it doesn’t happen.
The reason is that the sloth’s oesophagus doesn’t stretch in a straight line from its stomach to its mouth as it does in most animals, it loops round, like the U-bend in a toilet. If you’re spending 30 days digesting your meals you don’t want to accidentally lose all that nutrition just because gravity has taken over.
Also, sloths are prone to falling out of trees, especially in disputes with other sloths where the primary objective seems to be to knock their opponent off their perch. They don’t seem to hurt themselves in these falls, and they don’t lose their lunch, as might happen with a heavy blow and a straight path from stomach to mouth.
The downside to having a physiology that prevents a vomiting episode is that sloths also can’t burp. With the rate of their food processing being slow and continuous, they don’t get to fart much either. So excess air ingested with the leaves, and gas formed by the fermentation process, are absorbed into their bloodstream and expelled through their skin or when they breathe out.
There is, however, an added advantage of this complicated digestion process. When sloths come down from their lofty upside-down world for a swim, which they very much enjoy, they have a lot of natural buoyancy, thanks to all that trapped gas. It can only be imagined that the looped oesophagus helps with the effects of any nausea from bobbing up and down in the river, too.
