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Creation 38(4):34–37, October 2016

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The sloth—slowest mammal on Earth



Hanging upside down and motionless, high in a tree canopy, is not a good way to attract attention, but this is exactly the lifestyle the solitary sloth prefers. Designed for living in the treetops, sloths spend nearly all of their life hanging from branches aided by a powerful grip from their prominent 8–10 cm (3–4 in) long claws on each foot. Sloths weigh about 4–8 kg (10–20 lb), and are commonly found in the lowland tropical rainforests of Central and South America.

The sloth has a round head, a short snout, small eyes, long legs, tiny ears, and a stubby tail. Their fur is long, coarse, and light brown in color.

Sloth families

There are only two families of sloths, living or extinct: one is Bradypodidae (three-toed sloths), of which there is only one genus alive today, with four species. The other is Megalonychidae (two-toed sloths), of which also only one genus is still around, represented by two species. The toe number refers to only the functioning digits on their forelimbs, as both groups have three on their hind limbs. Three-toed sloths alive today have short stubby tails, whereas two-toed ones are tailless.

All living sloths are nocturnal, and get most of their water from their food. Three-toed sloths are folivores (a subset of herbivores), meaning they eat mostly leaves, though also shoots and some fruit.1 However, two-toed ones are omnivores, i.e. like humans, they eat a wide variety of foods. This includes the foliage and sometimes the fruit of the Cecropia tree, as well as algae, fungi, small animals such as small lizards, bird eggs, and carrion.2

The three-toed sloth can turn its neck up to 270° in either direction, because it has three extra vertebrae compared with the two-toed sloth. Sloth hair grows away from the extremities (hands and feet) to provide protection from the elements while it hangs upside down.

The three-toed sloth emits a high-pitched call that resonates through the forests as “ahh-eeee”. Because of this sound, it is sometimes called an ai (plural ais pronounced ‘eyes’).3

Sloth lifestyle

Sloths are thought to live about 10–20 years. The name ‘sloth’ is an old term for ‘laziness’, because it moves so little, and extremely slowly at that—about 15–30 cm (6–12 in) per minute.4 With very low metabolic rates of less than half that expected for a mammal of similar size, sloths maintain low body temperatures of 30–34° C (86–93° F) when active, and still lower while resting. The slowest mammal on Earth, the sloth sleeps 15 to 20 hours a day in captivity, much more than in the wild.5 Sloths will often remain motionless even while awake, raising the question of whether captive ones really do sleep that much.


Recently, however, Smithsonian scientists, using miniature devices that measure brain waves, found that wild sloths do indeed sleep six hours a day less than ones in captivity, on average.6

Sloths may live in the same tree for years, descending only to defecate or occasionally forage nearby. Once sloths are down from the tree however, they become an easier target for predators or indigenous people who hunt them for meat. Sloths have even been known to retain their grip and remain suspended in a tree after they have died.

Sloths mate and give birth while hanging in the trees. Three-toed sloth babies cleave to their mothers for the first nine months of their lives as they travel among the trees. On land, sloths’ weak hind legs and their long claws hinder movement. As National Geographic’s site states:

“They must dig into the earth with their front claws and use their strong front legs to pull themselves along, dragging their bellies across the ground. If caught on land, these animals have no chance to evade predators, such as big cats, and must try to defend themselves by clawing and biting.”7

They sometimes fall from rainforest trees into rivers where they swim efficiently and gracefully with their long arms.

Extinct giant ground sloths

There are many extinct groups of sloths, most of them being ground-dwellers. They were often considerably larger than the two modern groups of (tree-dwelling) sloths, though all are thought to be related to them. For example, the giant ground-dwelling sloth of the genus Megatherium (Greek; ‘great beast’) was the size of a modern elephant. With a likely weight over five tonnes, it measured over 6 m (20 ft) in length. The Megalonyx (Greek: ‘great claw’), at about half that size, was still a formidable creature. During the Ice Age, Megatherium lived in Central and South America while Megalonyx lived in North America. Rather than hang upside down from trees, giant sloths probably stood on their hind legs, able to bend branches down in order to reach their food.

Another extinct large sloth was the aquatic Thalassocnus, about 2 m (6–7 feet) long. Its leg bones were especially dense to help with diving.

Hairy ecosystem

The sloth’s furry body would need a good shampooing by human standards, but it serves as an important habitat for colonies of symbiotic cyanobacteria (formerly called blue-green algae) that turn the sloth’s fur green, especially during the rainy season. The thick, coarse outer hairs of a sloth have transverse cracks where these grow and flourish. The algae are species-specific, and are passed down by close physical contact and licking from mother sloth to her offspring. The algae form a symbiotic relationship with the mammal—giving the sloth camouflage and nutrients in exchange for water and a place to grow. Roundworms and moth larvae, hiding from predators of their own, also flourish in the hairy ecosystem that sprouts while sloths hang upside down from trees.

Within this veritable zoo of flora and fauna, 84 isolates (separate viable organisms) of fungi have been obtained from laboratory cultures.8 A research team from Panama and the United States took samples of coarse outer hair from the lower backs of nine sloths living in Soberanía National Park near the Panama Canal in 2011. It was found that some of the fungi are active against parasites that cause malaria, MCF-7 human breast cancer cells, and a range of pathogenic bacteria. Eight other fungi inhibit Trypanosoma, a type of unicellular protozoon that causes Chagas disease. It is believed that future medicines for humans may come from these microbes.9

Sloth origins

A sloth’s fur can turn green due to the colonies of symbiotic cyanobacteria living within it.

Secular biologists, who claim that the common ancestor of today’s two sloth genera lived 35–40 million years ago, admit the evolutionary history of the two living sloth groups is not well known. Interestingly, though, they believe that despite the extensive similarities between the two genera, each is much more closely related to some of the ground-dwelling sloths (all now extinct—see box above) than to the other living sloth genus. Thus, they are forced to consider living sloths as stunning examples of ‘parallel evolution’—repeat evolution of similar traits, by sheer chance, in multiple lineages.10

Obviously, original design—with built-in variation potential—is a much more reasonable explanation. It is actually not inconceivable that living sloths descended from ground-based ones through adaptation by natural selection. This is capable of producing considerable variation, including new species, limited to within each kind. (It was likely involved in the diversification of the Genesis kinds on the Ark into numerous descendant species, each carrying less information than the ancestor population, and so less able to vary or diverge further.)

Much more needs to be learned to be sure of this, or the number of original sloth kinds—or even whether the two living sloth groups are the same kind. Their overlapping distribution (Central/South America, even sometimes inhabiting neighbouring trees), suggests they are, though no cross-genus hybridization has been reported to try to confirm this.

It is already clear, though, that the evidence concerning these wonderful animals, extremely well equipped for their upside-down environment, is utterly consistent with Genesis—the creation of the ‘ancestor sloths’ on Day 6 of Creation Week, at the hand of God.

Posted on homepage: 13 August 2018

References and notes

  1. Ledbetter, J., Hanging with the Sloths; perezosoproductions.com, 2005. Return to text
  2. Mitten, M., Animal Corner; animalcorner.co.uk, 2004. Return to text
  3. Three-toed Sloth, National Geographic; animals.nationalgeographic.com, accessed 1 June 2016. Return to text
  4. The issue sometimes raised by biblioskeptics, of how sloths could migrate from Ararat in the time available, is addressed in Chapter 17 of CMI’s Creation Answers Book, available here. Return to text
  5. Bradford, A., Sloth Facts: Habits, Habitat & Diet, livescience.com, 21 May 2014. Return to text
  6. Smithsonian Tropical Research Institute, Wild three-toed sloths sleep 6 hours less per day than captive sloths, first electrophysical recording shows, ScienceDaily, 14 May 2008. Return to text
  7. Two-toed Sloth, National Geographic; animals.nationalgeographic.com, accessed 1 June 2016. Return to text
  8. Richmond, B., How sloth fur is going to revolutionize medicine; motherboard.vice.com, 11 July 2014. Return to text
  9. Higginbotham, S. et al., Sloth hair as a novel source of fungi with potent anti-parasitic, anti-cancer and anti-bacterial bioactivity, PLOS ONE 9:e84549, 15 January 2014 | doi:10.1371/journal.pone.0084549. Return to text
  10. Gaudin, T.J., Phylogenetic relationships among sloths (Mammalia, Xenarthra, Tardigrada): the craniodental evidence. Zoological Journal of the Linnean Society 140(2):255–305, 2004 | doi:10.1111/j.1096-3642.2003.00100.x. See also Switek, B., The sloth’s evolutionary secret; wired.com, 3 January 2012. Return to text

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