Why Are Sloths Green? An Ecosystem in their Fur

The fur of our wonderful sloth friends come in a variety of colors, from a gray-beige color to a dark reddish brown. But we also sometimes see sloths in a color rarely seen in mammals:  green.

Why are sloths green?  The answer is simply that there is algae growing in their fur. It’s not common for animals to have plants growing on them, so let’s explore how it is that sloths have other lifeforms using their fur as a habitat.

Sloths can turn green because of two of their main characteristics:

  1.  their long fur, and 
  2.  their slow movement.
By derivative work: Stevenj (talk)Original photo by Leyo. - Two-toed sloth Costa Rica.jpg, CC BY 2.5, https://commons.wikimedia.org/w/index.php?curid=4372587

We love the long, luxurious locks that sloths have. That hair is also coarse with long grooves or cracks, depending on the species. That texture on the sloth’s fur makes it ideal for collecting moisture and serves as a place for organisms to nest or take root. Obviously sloths tend to live in one of the wettest places on earth, the rainforest, where it rains often and mists often fill the air

The most extensive and conclusive study about the symbiotic relationship of sloths and algae was done by a team led by Milla Suutari of the University of Helsinki. Their findings were published in the peer-reviewed journal BioMedical Center Evolutionary Biology. You can review the article here

When scientists studied sloths, they found that most species had algae in their hair. The ones that didn’t have algae primarily were sloths from dryer climates. The other sloths without algae were baby sloths. It seems, though, that once those sloths get older the algae appears. The curious thing is that the algae in the young sloths is genetically related, as in the direct offspring of the algue on the mama sloth. So, the same sloth families have the same algae families.

This genetic pattern actually might go back millions of years. Some of the species of algaes found on sloths are found nowhere else. And different sloth species have their own unique species of algae. So, what that means is when the three-toed sloth and two-toed sloths took different evolutionary paths, so did species of algae which got passed down from mother to child year after year over eons. This is an amazingly unique circumstance in biology.

So, what else does this mean for sloths? After millions of years together, the sloth and the algae provide each other with certain benefits. The algae gets a home for which it is perfectly adapted to (the sloths fur) and which collects and holds moisture that the algae needs. 

Here is a stunning video from National Geographic of a sloth with green algae-covered fur:

Benefits for the sloth

The benefits for the sloths are even more remarkable. In a simple sense, having the green algae in their fur serves a camouflage. Remember, a sloth spends most of her time sleeping in a tree, and when awake she is still kind of a sitting duck for predators (or maybe a ‘slouching sloth’?), but that slothy slowness combined with the green plant color provided by the algae actually becomes a defense for the sloth—they end up simply looking like part of the forest and predators leave them alone.

As cool as that camouflage aspect of the sloth-algae relationship, it is not the most amazing. Although researchers are still working to confirm this, scientists have theorized that the algae (and/or other organisms living in the sloths’ fur) excrete nutritional substances that the sloth absorbs through their skin. If this can be confirmed, it would possibly show that the sloths’ health is improved by having the green algae grown in their fur. The reason why this hypothesis exists is that the sloth’s diet normally consists of only tough leaves from the trees they live in, leaves which do not have some vital nutrients for an animal like sloths. Those same nutrients, lipids and amino acids, are present in the algae. 

So, now we know how and why sloths can be green, as well as the unique benefits of that symbiotic relationship the sloth has with algae, but if algae can grow in a sloth’s fur, what else might we find there?

Other sloth companions

Perhaps just as remarkable and certainly linked to the algae colonies on the sloths are a number of species of moths, a few of which, just like the algae, live only in the sloths’ fur and nowhere else on Earth. Certain moth species only leave the fur of the sloth to find the poo that sloths have left on the group. That is where they lay their eggs and where the moth larvae spend the first part of their lives. (OK, kind of gross, but its nature).

Once the moths can fly, they take to the air to find a new home/sloth. Among the long hairs of our favorite lazy forest creatures, the moths nest, mate, and feed. The algae and other organisms growing in the sloth’s fur serve as a buffet for the moths. As a home, the sloth offers the moths security by scaring off the birds that prey on insects. So this is quite a good deal for the moths. What do the sloths get out of it?

Remember the sloths benefit from the algae. Well the algae need not only the moisture in the sloths’ fur, but also nutrients like nitrogen to grow. Guess where the nitrogen comes from?  The moth’s poo.  (There is so much poo in nature!). So, these 3 creatures have evolved to live closely together and provide benefits for each other, forming a biological pyramid in which the sloth is the most important piece.


Beyond these two examples, different types of fungi, bacteria, beetles and a number of parasites are found commonly living on sloths. While there are some unusual and maybe unappetizing aspects of this (poo everywhere!),it is quite amazing and shows us how truly special sloths are as animals and the vital part the play in the ecology of the rainforest.

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