Tree Sloths & Algae Symbiosis:

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The two main genera of sloths which have algae as its symbiotic partner include the genus Bradypus (three-toed sloth) as well as the genus Cholopeus (two-toed sloth).

Three-toed Sloths (Bradypus):

From G. Gene Montgomery's,...
From G. Gene Montgomery's,...

Three-toed sloths include four species named Bradypus variegatus, B. tridactylus, B. torquatus and B. pygmaeus. The fur of the genus Bradypus can be identified in comparison with other genera of the sloths as being brown or green, dependent upon the weather conditions. During the rainier seasons the color of the sloths fur becomes greener because the algae located within the fur starts to replicate when it is an optimal environment. This brown or green coloring is shown to be helpful because it allows the sloth to camouflage with the tree in which it hangs from. This animal is in fact nocturnal and therefore it is important to not be visible to prey on during the bright days.

The three-toed sloth lives in the middle layers of trees or even shrubs where in fact they will stay upside down while it sleeps during the day. This specific genus of sloths is found in the subtropics as well as the tropical regions of the Americas including Central and South America. Since they are dependent upon hanging from trees the three-toed sloth is most suitable in an environment that has many tress as well as being very humid near rivers or even the open jungles. Bradypus variegates is shown to be the most widespread of this genus.(Christine Chan)

Michael L. Smith....
Michael L. Smith....

Two-toed Sloths (Choloepus):

Very similar to the genus Bradypus this other genus Choloepus is faster as well as larger and therefore can survive captivity. Two-toed sloths include two species named Linnaeus's Two-toed Sloth (Choloepus didactylus) as well as Hoffmann's Two-toed Sloth (Choloepus hoffmanni). The fur of two-toed sloths is a grayish brown but like the Bradypus, when the algae begin to in fact grow, the fur will begin to become green. This allows the two-toed sloth to be camouflaged just like the three-toed sloth to the tree. This ultimately allows the Choloepus likethe Bradypus to become harder to prey on. The Choloepus is also nocturnal and will hang upside down while it sleeps and eats as well as does other daily routines. The sloth will become vulnerable when it is on the ground though, and not hanging from the tree, because it must walk on two legs which causes this animal to be very slow in comparison to other species in which prey on them.

The Choloepus is found through Central as well as South America. Again, they must be found within a tropical moist region which allows them to maintain their body temperatures. Choloepus are also found near rivers as well as open jungles ('Hoffman's Two-toed Sloths".)


The alga that has been found to be the counterpart to the symbiotic relationship with the sloths is called Trichophilus welkeri. This is a species of green algae which is contained within the phylum chlorophyta. These specific green algae have optimal conditions during rainy weather and therefore are shown to b
e most prevalent on the sloths fur during a rainy season. This type of algae then is extremely hard to locate and identify because it thrives at really high temperatures and therefore has a restricted occurrence.

According to the research done there were three different patterns of the algae that was in fact found in the fur of the different sloth species. In the brown-throated sloth, Bradypus variegates, and B. prygamaeus there was green algae that was found that none of the other species of sloths contained. The green algae were obviously found to be Trichophilus. The second pattern that was found in the Bradypus torquatus had a variety of algae belonging to the terrestrial type of algae. The Hoffmann’s two toed sloth and Bradypus tridactylus also hosted terrestrial green algae as well as Trichophilus.
When 71 different sloths were in fact studied the green algae was in fact found in 73% of all of the sloths. Due to the high specificity of Trichophilus it seems practical that it has a symbiosis with sloths to not only gain protection but also to live in the most optimal place in order to reproduce and live. The sloth provides the enviroment and the high temperatures that allow for the alga to grow and multiple. ("Molecular Evidence for a Diverse Green Algal Community Growing in the Hair of Sloths and a Specific Association with Trichophilus Welckeri.")

Sloth and Algae Symbiosis

Most sloths fur will have the dirty brown coloration, however during the seasons where there is more rain the fur will become green because of the increased amount of the algae living within the fur. In the species of Bradypus tridactylus it has been found that the algae live between the cuticle scales. Over time this fur will age and this in fact will cause the hairs to change. The presence of the algae symbiont is revealed at an extremely young age for the sloth as narrow transverse lines. However, older hairs will show deeper transverse cracks and will have a wider more diverse algal colony. As the older hairs begin to age further there is the full exposure of the cortex because the cuticle has in fact broken down and therefore living algae is not present.

The genus Bradypus has been found to absorb the rain like a sponge and therefore can contain large amounts of water. This is the reason that the Bradypus shows the most evidence of the algae symbiont because it provides an extremely moist and wet environment which is beneficial to the algae.

The fur of Bradypus species has been shown to absorb water better than the genus Choloepus. In the two-toed sloths, Choloepus it has been found that rather than absorbing the water it causes the water to runoff the abdomen. The abdominal hairs which flow outward allows the water to runoff the sloths body and therefore allows the sloth to stay dry in a rather wet environment. Although algae do grow on Choloepus it is not as evident as in the Bradypus. This is primarily a direct result because the fur of the Choloepus is not as wet as that of the Bradypus and therefore the environment for the algae is not ideal.In the Choloepus it has been found that the algae live inside grooves found on the hair particles. It is still in question whether these are found on top of the hair particles or rather in the hair itself. Therefore, the amount of grooves do not change o
ver time as do the transverse cracks but are rather evident at an extremely young age and will continue to be there throughout the life of the sloth and hold the algae symbiont.

These cracks and grooves in both genera of sloths have shown to contain a variety of different organisms including cyanobacteria as well as moths, beetles and roundworms. All of these organisms have been found to have benefits to the sloth especially the algae.

Furthering questioning has come about now whether these cracks found within the hair particles of the sloth are there at birth or are created due to the presence of the algae. If the sloth is born with such cracks or grooves inside the hairs then it would be reasonable that the sloth has become better adapted to its environment by allowing the algae to easily intrude inside the particles. This reveals the sloth to encourage algal colonization. Therefore, in this case the alga does not harm the sloth in anyway but rather just becomes part of the sloth itself. However, if the cracks are caused by the algae then it would seem reasonable that the sloth might not benefit as much as the algae because it is not encouraging the algal colonization. The algae in this case would have to intrude into the hair particles and create these cracks which can be seen as a problem for the sloths fur in the long run. There is also some speculation that the sloth on top of becoming camouflage due to the presence of the algae can receive nutrient from the algal through diffusion along the outer portion of the hairs. The algae could also give the fur a specific texture so it becomes more beneficial for bacteria to live on the sloth as well. ("Sloth Biology: an Update on Their Physiological Ecology, Behavior, and Role as Vectors of Arthropods and Arboviruses")


It has been found that the algal symbiont is found in the sloth during the first month after birth. Studies have in fact been done that has found that the symbiont is passed directly from mother to offspring. This is because a known species of algae that has been found specifically on the mother sloth has also been shown on the new-born sloth. Therefore this symbiosis begins very early in a sloths life.This notion can also reveal the importance of the sloth to the algae. As soon as the algae recognize a safe environment they are able to move from mother to newborn and therefore the algae will have more room itself to reproduce. ("Molecular Evidence for a Diverse Green Algal Community Growing in the Hair of Sloths and a Specific Association with Trichophilus Welckeri.")


A mutualism is defined according to Paracer in Symbiosis as “both partners benefit from the relationship.” This seems practical when thinking about the relationship between the sloth and the algae. In this symbiotic relationship the algae is benefiting from living within the sloths fur because it is getting protection and a safe environment in which to live. The algae is also at an optimal temperature in which it is able to grow and survive. The sloth on the other hand is benefiting because the green algae is allowing the sloth to become camouflaged to the trees in which they hang from. Therefore because the algae are present in the fur it allows the sloth to be harder to be preyed upon because they become harder to see.
Eliana Rose- Why Clouds Float
Eliana Rose- Why Clouds Float
On top of this many scientists have proposed other mechanisms in which could be shown to be beneficial to both of the organisms within this symbiosis. Sloths when feeding sometimes seem to eat the algae it which lay on their fur. Therefore for the sloths it is easy and uses only a minimal amount of energy in order to lick and eat its own fur to gain nutrients. Also some have proposed that the sloths also gain nutrients because their skin allows for them to absorb nutrients from the algae which lies on their skin. Therefore all together the sloth gains nutrients as well as a source of camouflage from the algae and the algae gains an ideal environment as well as safe environment so they can survive.

Although it can be seen as a mutualism in which both organisms benefit equally, some though will still think otherwise and feel as if the algae is benefiting more than the sloth. Some feel that the sloth, although some studies reveal otherwise, seem to not be using the algae as a source of camouflage and it just so happens because of the color of the algae. These scientists feel that the algae are just using the sloth as protections and that the sloths are not gaining anything from the algae. Therefore, for those that believe this, this symbiosis can be seen as a commensalism rather than a mutualism. A commensalism is when one organisms benefits, in this case the algae, while the other organism is unaffected, the sloth.

Further Studies:

Although some research has been done on this topic of the sloth and algae symbiosis there are still many questions that are left unanswered. These questions include research that should be done to study the actual nutrient exchange through the skin of the sloth and the algae. If this could be done then one could begin to understand the way in which the sloth actually benefits from this symbiotic relationship.
One could also understand the way in which the sloth benefits by investigating sloths that live without the algae symbiont and if they live as beneficial as a life of a sloth which does contain the algae. It has in fact been found already that the algae must live on the fur of the sloth and it is unique to the sloth because it needs a high temperature as well as extreme moisture. Since studies have already indicated that the algae is exclusive to the sloth one could begin to understand if camouflaging to the tree is extremely beneficial to the sloth.
Therefore with further studies one will be able to determine if this symbiosis is purely mutualistic and if both organisms benefit the same amount.

Questions for Answering:

-Is this really a symbiotic relationship classified as a mutualism?

-Do the sloths survive just as well when the algae is not present?

-Do any other animals or organisms use algae to obtain nutrients through their skin?

Norton Wu
Norton Wu

Fun Facts

*The genus Choloepus means lame foot, because they in fact one less toe on each of its front feet.

*Only mammal that can turn its head 180 degrees in both directions.

*The sloth is the world’s slowest mammal. 

*Although extremely clumsy on land, sloths are in fact very good swimmers.


-Chan, Christine. "Brown Throated Three-toed Sloth." San Francisco State University Department of Geography. 1999. Web. 10 Apr. 2011. <>.

-Gilmore, D.P, C.P. DaCosta, and D.P.F Duarte. "Sloth Biology: an Update on Their Physiological Ecology, Behavior and Role as Vectors of Arthropods and Arboviruses." SciELO - Scientific Electronic Library Online. Jan. 2001. Web. 10 Apr. 2011. <>.

-"Hoffman’s Two-Toed Sloth." RosamondGifford Zoo. 26 Mar. 2007. Web. 11 Apr. 2011. <>.

-Suutari, Milla, Markus Majaneva, David P. Fewer, Bryson Voirin, Annette Aiello, Thomas Friedl, Adriano Chiarello, and Jaanika Blomster. "Molecular Evidence for a Diverse Green Algal Community Growing in the Hair of Sloths and a Specific Association with Trichophilus Welckeri (Chlorophyta, Ulvophyceae)." BMC Evolutionary Biology. Bio Med Central, 2010. Web. 11 Apr. 2011. <>.

-"Symbiosis." Marietta College. 09 Apr. 2008. Web. 27 Apr. 2011. <>.