Monday, May 5, 2008

On Sloths

Heidi Rivera contributed this post

I did my term paper on sloths and I thought that I would share some interesting information that I learned while completing the paper starting with this short video.

As you can see from the video, current extant, tree-dwelling sloths that are put on land are very slow moving. Its hind limbs provide almost no help in propelling it forward and its long claws get in the way. On land, sloths have to dig their claws into the ground and drag their bodies with their stronger front limbs to move. They can move more easily on the ground if they have leaves, roots or branches to grasp. Although sloths have great difficulty moving on land, they are very efficient swimmers.


Sloths inhabit the lowland tropical forests of Central and South America. Bradypus variegates can be found in Central and South America from Honduras to northern Argentina. Bradypus tridactylus is found in South American, and Bradypus torquatus is confined to a small Atlantic forest of southeastern Brazil (National Geographic 1996). There current extant sloths can be found in the tropical canopies sunbathing, eating or resting. Their bodies are built for life in the treetops. They spend most of their time hanging from tree branches and are able to do so because of their long sharp claws and very strong grip. This grip is so powerful that dead sloths have been known to retain their grip and remain suspended from a branch. Sloths also sleep in trees and are known to sleep for 15 to 20 hours a day. In general, sloths are nocturnal, and when they are awake at night, they eat. They go down to the ground only once a week to urinate and defecate (Britton, 1941)


The two species of two-toed sloths are distinguished by the color of the fur on their throat. Choloepus didactylus has a dark colored throat whereas the Choloepus hoffmanni sloths have a pale gray to white colored throat (Crawford 1934). The four species of the genus Bradypus can also be distinguished from one another. B. variegates has brown fur on its throat and B. torquatus has dark fur on its throat. In comparison, B. tridactylus has pale fur on its throat. B. pygmaeus is distinguishable from others because it is significantly smaller in size. Both genera of sloths have long, thick hair that allows them to be camouflaged in their surroundings so that predators, such as jaguars and eagles have difficulty finding them. During the rainy season, some species of sloths develop an incrustation of symbiotic cyanobacteria on the surface of their hair. This offers the hosting sloth additional camouflage with its surrounding area, providing even more protection from predators (Muizon 2004).

Two-toed sloths are a little bigger than three-toed sloths. The two-toed adults are approximately 24 to 27 inches long, and can weigh about 17.6 pounds (National Geographic 1996). In comparison, the three-toed adults are approximately 23 inches long and on average weigh only 8.8 pounds (National Geographic 1996). Both species of sloths have a flat, short, rounded heads, large round eyes, a snub nose and highly reduced ears. The three-toed sloths have a short, stumpy tail whereas the two-toed sloth’s tail is absent or vestigial (Gaudin 2004).

Sloths are characterized by the number of long claws that they have on each front foot. The three-toed sloths have three long, curved sharp claws on all four limbs, while the two-toed sloth has two long claws on its front limbs and three claws on each of its hind limbs. Sloths have very long limbs, the front limbs being longer than the hind limbs. These long limbs are useful for suspending from tree branches, which they do for the majority of the day. Sloths are mostly nocturnal; however, they can be active in the day as well. They only spend about ten percent of their time moving, making them the world’s slowest mammal (Crawford 1934). Their lack of movement helps them to stay camouflaged with their surroundings and helps them to avoid any potential predators.

Another difference between two-toed sloths and three-toed sloths is the number of vertebrae they have. Most mammals have seven neck vertebrae but two-toed sloths have only six or seven (depending on the species) and three-toed sloths have eight or nine (depending on the species). Having eight or nine vertebrae allows these sloths to turn their head through a 2700 arc (National Geographic 1996). The benefit of their specialization is not fully understood.

I hope you guys enjoyed learning a little bit about one of my favorite animals!


Britton, W. S. 1941. Form and Function in the Sloth (Concluded). The Quarterly Review of Biology. The University of Chicago Press. Vol. 16:190-207

Crawford, C. S. 1934. The Habits and Characteristics of Nocturnal Animals. The Quarterly Review of Biology. The University of Chicago Press. Vol. 9: 201-214

Gaudin, T. J. 2004. Phylogenetic Relationships Among Sloths (Mammalia, Xenarthra, Tardigrada): The Craniodental Evidence. Zoological Journal of the Linnean Society. Vol. 140: 255-305

Muizon, D. C., H. G. McDonald, R. Salas, and M. Urbina, 2004. The Youngest Species of the Aquatic Sloth Thalassocnus and a Reassessment of the Relationships of the Nothrothere Sloths (Mammalia: Xenarthra) Journal of Vertebrate Paleontology. Vol. 24: 387-397

National Geographic. 1996. Sloths.


Steven Miller May 5, 2008 at 6:57 PM  

What about sloths did you do your term paper on? Their evolutionary history? The video said, on the road to San Jose... does that mean some can be found in California? If so, are they native because you indicated they are primarily in Central and South America.

Black Lotus May 5, 2008 at 8:20 PM  

That is fascinating. I have always thought that sloths were biologically interesting creatures for studying habitat adaption [i.e. the long arms and claws].

On a lighter note:


Madhu May 5, 2008 at 11:29 PM  

Steven - I believe the San Jose in question is probably in Costa Rica. I saw my first wild sloth in that vicinity some years ago!

Heidi Rivera May 5, 2008 at 11:35 PM  

This was just a random video I found on youtube and it says, "on the road to San Jose," because this sloth was found in the middle of the road while these people were on their way to San Jose, Costa Rica from Arenal, Costa Rica. So, it wasn't in San Jose, California. (Yes, Dr. Katti, you were right!) To answer your question Steven, I did my paper on the evolutionary history of sloths. However, while researching I learned a lot of interesting things about these animals and I hope that you guys found it interesting as well!


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