#231 Why do sloths move so slowly?

I learned this today. Sloths move so slowly because they have an extremely low metabolic rate.

There are six species of sloth that are divided into two different types of sloth: three-toed sloths and two-toed sloths. Both of these groups of sloths are from different families that are very distantly related. They have both evolved in parallel to be extremely similar.

There used to be more species of sloth. There was a sloth called megatherium that lived during the early Pliocene, which was about 5.3 million years ago. This was a ground sloth and one species of them was the size of an elephant. They became extinct about 12,000 years ago.

Sloths grow to be about 60 to 80 cm long and they can weigh between 4 and 7 kg. They have very poor vision and hearing, so they rely on their sense of smell and touch.

Sloths spend the majority of their time hanging upside down in trees. Their bodies and claws are adapted to hanging and can’t really support their body weight. They have only about 30% of their body as muscle, compared to about 45% for most mammals. They cannot walk, so they drag themselves along the ground.

Because they spend so long hanging upside down, their internal organs have evolved attachments to hold them to the sloth’s rib cage. Sloths only come out of the trees once a week to defecate, so about a third of their body weight is stored urine and feces. If their organs weren’t attached to their ribs, when hanging upside down the weight would press on their lungs and they wouldn’t be able to breathe. They have evolved attachments to cope with this. They have also evolved fur that goes in the opposite way to most mammals. With almost all mammals, the fur points from the head down towards the extremities. With sloths, it points in the opposite direction to keep rain off them when they are upside down. They also let moss grow in their fur to act as camouflage.

So, why do sloths move so slowly? There are several reasons. Here’s why.

Firstly, sloths are unable to see. They are completely colorblind and pretty much totally blind in daylight. Because of this they cannot take the risks of moving quickly through the trees. One wrong jump and they would plummet to their deaths. They move steadily, testing the strength of the branches as they go.

Secondly, they don’t have any energy to spare. They are existing almost on the edge of starvation. This is caused by the speed of their metabolism, the size of their stomachs, and their low-calorie diet.

Sloths have a four-chambered stomach that isn’t very large. This wouldn’t be a problem if their metabolic rate was faster. Their metabolic rate is about half that of other mammals the same size. It can take a week or even longer to digest one leaf. Because of this and the small size of their stomach, they are constantly full of digesting food. It is not possible for them to eat more. They are severely limited by this. On top of this, sloths live on leaves that don’t have a very high calorific content. It takes them ages to digest leaves and they cannot get much energy out of those leaves. Some animals have evolved to cope with low-calorie leaves by eating far more of them. Sloths have evolved to use less energy.

Thirdly, sloths don’t have enough muscle mass to move quickly. Because they don’t get enough energy from their food, they have evolved to need less energy. Muscles require energy, so fewer muscles makes more sense. However, fewer muscles means they move more slowly. For the same reason, sloths cannot regulate their body temperature as much as other animals. Keeping a steady body temperature requires energy that the sloths don’t have. Instead, they bask in the sun to warm up and their temperature fluctuates greatly.

Fourthly, sloths can’t see predators because of their poor eyesight. They cannot dodge an incoming eagle or jaguar. However, those animals hunt by sight, so the slow speed of the sloth makes them very hard to see. That, coupled with the moss on their coat, makes slow speed a very good survival tactic.

So, the slow speed with which sloths move is an evolved strategy to cope with low energy. Some animals evolve to cope by eating more. Sloths have evolved to cope by eating less. And this is what I learned today.

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Photo by Denys Gromov: https://www.pexels.com/photo/brown-sloth-climbing-the-tree-branch-4835276/

Sources:

https://en.wikipedia.org/wiki/Sloth

https://www.worldwildlife.org/stories/why-are-sloths-slow-and-six-other-sloth-facts

https://www.bbc.com/future/article/20190828-why-do-sloths-move-so-slowly