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Sloth

life science Maturity 5-7 Next article →

Sloths are unique Neotropical mammals belonging to the suborder Folivora. They are famous for their extremely slow movement and deliberate lifestyle. Today, most sloths are arboreal, meaning they live in trees. They spend most of their lives hanging upside down in tropical rainforests. These animals are part of the superorder Xenarthra, which evolved in South America around 60 million years ago. This group also includes anteaters and armadillos. Sloths are most closely related to anteaters, forming the order Pilosa.

Three-toed sloth crossing road in Costa Rica
Three-toed sloth crossing road in Costa Rica

A sloth's slow pace is a result of its unique biology. They have a very low metabolic rate, which is less than half of what is expected for a mammal of their size. This low metabolism allows them to survive on a low-energy diet of leaves. Their body temperature is also quite low and can change with the environment. This trait is called being heterothermic. When they are resting, their temperature can drop as low as 24.4 degrees Celsius. This state is known as torpor. Their slow movement also serves as a defense mechanism. It helps them avoid detection by predators like hawks and cats that hunt using sight.

Bradypus pygmaeus
Bradypus pygmaeus

The sloth's body has developed amazing ways to work with its environment. Their shaggy coats have grooved hair that hosts symbiotic green algae. This means the algae live on the sloth and help the sloth in return. The algae provide camouflage by making the sloth blend into the trees. They also provide nutrients to the animal. These algae even support sloth moths, with some species living only on sloths. Physically, sloths are quite different from other mammals. Most mammals have seven cervical vertebrae, or neck bones. However, two-toed sloths have five to seven, while three-toed sloths have eight or nine.

Two-toed sloth Costa Rica - cropped
Two-toed sloth Costa Rica - cropped

There are six living species of sloths divided into two genera. The first genus is Bradypus, known as the three-toed sloths. This group includes the common brown-throated three-toed sloth. It also includes the maned three-toed sloth and the critically endangered pygmy three-toed sloth. The pygmy sloth lives only on the small island of Isla Escudo de Veraguas near Panama. The second genus is Choloepus, known as the two-toed sloths. These are slightly larger than the three-toed variety. While they have only two digits on their forelimbs, they still have three toes on their rear limbs.

Bradypus pygmaeus
Bradypus pygmaeus

History shows that sloths were once much more diverse. During the Pleistocene Epoch, many species of ground sloths lived on land. These ancestors were not tree-dwellers but lived on the ground. Some were massive, such as the Megatherium, which could reach the size of an elephant.

Ground sloth size comparison
Ground sloth size comparison

Other groups, like the Nothrotheriidae, included the genus Thalassocnus. These were semiaquatic or even fully aquatic sloths. They evolved to be bottom feeders of seagrasses over a span of 4 million years.

Megatherium americanum Skeleton NHM
Megatherium americanum Skeleton NHM

Most of these large ground sloths became extinct around 12,000 years ago. This occurred during the Quaternary extinction event. Scientists suggest that the arrival of humans may have contributed to this loss. Evidence shows that humans hunted, cooked, and ate ground sloths. Climate change at the end of the last ice age might also be a factor. Other sloths, like those in the Greater Antilles, survived longer. They did not go extinct until humans settled those islands about 6,000 years ago.

Megalonyx wheatleyi skeleton & restoration
Megalonyx wheatleyi skeleton & restoration

The evolution of modern sloths is a great example of convergent evolution. This happens when different species evolve similar traits because they live in similar ways. The two living genera, Bradypus and Choloepus, evolved similar body shapes to live in trees. However, they are actually distantly related. DNA evidence shows they belong to different families. This shows how the environment can shape animals in similar ways over millions of years.

San Diego Paramylodon
San Diego Paramylodon
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🖼️ Images & Media (10)
File:Three-toed sloth crossing road in Costa Rica.jpg
Three-toed sloth crossing road in Costa Rica.jpg
File:Megatherium americanum Skeleton NHM.JPG
Megatherium americanum Skeleton NHM.JPG
File:Nothrotheriops shastensis La Brea.jpg
Nothrotheriops shastensis La Brea.jpg
File:Megalonyx wheatleyi skeleton & restoration.jpg
Megalonyx wheatleyi skeleton & restoration.jpg
File:San Diego Paramylodon.jpg
San Diego Paramylodon.jpg
File:MC Drei-Finger-Faultier.jpg
MC Drei-Finger-Faultier.jpg
File:Ground sloth size comparison.png
Ground sloth size comparison.png
File:Two-toed sloth Costa Rica - cropped.jpg
Two-toed sloth Costa Rica - cropped.jpg
File:Bradypus pygmaeus.jpg
Bradypus pygmaeus.jpg
File:SlothDWA.jpg
SlothDWA.jpg
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