How are sloths not extinct due to natural selection?

Sloths' slow movement may appear to be an evolutionary weakness, but it is actually one of their greatest strengths.

The long answer

Long before the sloths we know today inched along tree branches, there were giant sloths that roamed on the ground in North, Central, and South America. There were a great variety of ground sloths, the largest being Megatherium which stood 22 feet tall and weighed 14,000 pounds. These ground sloths moved around considerably more than modern sloths do and could walk on their hind legs, leaving their arms free for digging and defending itself from predators, presumably.

Elephant-sized sloths were driven to extinction likely because of a combination of climate change and humans. The theory goes that humans arrived and used tools to hunt the ground sloths. The sloths that ended up surviving used their claws for climbing up and out of reach of the humans. So that explains why today's sloths hang out so much, but what explains their slow movement?

For sloths, evolution was not a story of "survival of the fittest" but more like "survival of the least desirable". Sloths have extremely slow metabolisms which allows them to subsist on meager amounts of food. As a result, they move very slowly so as to not expend energy and also do not develop much muscle mass. Sloths' muscles make up only 25-50% of their total body weight, whereas most mammals are 40-45% muscle mass. It's simply not worth the hassle for most predators to hunt sloths – so most don't.

As an added bonus, sloths host blue-green algae in their fur which provides them with a green tint that allows them to camouflage.

Sloths as a whole may have gone extinct had it not been for the adaptations that they made to survive. Adaptations like unhurried and infrequent movement, hanging from trees, low muscle mass, and slow metabolism are precisely why they have lived on.

Book of the Week

​The Code Breaker: Jennifer Doudna, Gene Editing, and the Future of the Human Race by Walter Isaacson​

In the early 2000s, biochemist Jennifer Doudna was part of a team that made a stunning discovery: a way to rewrite the code of life using a new tool called CRISPR. Part biography/part deep dive into genetics, The Code Breaker is a fascinating journey into the world of DNA and shows us how CRISPR is changing the way we think about the human race. I found the ethics portion of the book to be the most fascinating. In a world wherein we can now change our DNA, where do we draw the line?

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Sources

Black, R. (2021, January 3). The sloth's evolutionary secret. National Geographic. Retrieved November 7, 2022, from https://www.nationalgeographic.com/science/article/the-sloths-evolutionary-secret

Holdrege, C. (1998). What does it mean to be a sloth? The Nature Institute. Retrieved November 7, 2022, from https://www.natureinstitute.org/article/craig-holdrege/what-does-it-mean-to-be-a-sloth

Mason, B. (2005, August 1). Humans drove giant sloths to extinction. Science. Retrieved November 7, 2022, from https://www.science.org/content/article/humans-drove-giant-sloths-extinction

National Science Foundation. (n.d.). Jefferson's Ground Sloth, Megalonyx jeffersonii. Illinois State Museum. Retrieved November 7, 2022, from https://iceage.museum.state.il.us/mammals/jefferson%E2%80%99s-ground-sloth

Sloth Sanctuary of Costa Rica. (n.d.). Extinct Giant Ground Sloths. Sloth Sanctuary of Costa Rica. Retrieved November 7, 2022, from https://www.slothsanctuary.com/about-sloths/giant-ground-sloth

Smithsonian Institution . (2020, October 18). Why are Sloths So Slow? And Other Sloth Facts. Smithsonian’s National Zoo & Conservation Biology Institute. Retrieved November 7, 2022, from https://nationalzoo.si.edu/animals/news/why-are-sloths-so-slow-and-other-sloth-facts