Why Do Sloths Have Green Fur? Algae, Moths and Camouflage

Sloth fur is not green by itself

A sloth’s own hair is grayish-brown to beige. The brown-throated sloth, for example, has a pale face, a dark brown throat and a shaggy grayish-brown coat. The green comes from living organisms on the hair, not from pigment.

How green a sloth looks varies a lot. Sloths in very wet forests and during the rainy season can be distinctly green, while others look almost plain brown or gray. Very young sloths usually have little or no algae, and very old individuals can lose much of theirs as the outer layer of the hair wears away.

Special hair made for algae

Sloths have two layers of fur: a short, soft underfur and a long, coarse outer coat. The outer hairs are unusual:

• Three-toed sloths have hairs with many tiny cracks running across them.
• Two-toed sloths have hairs with long grooves running along their length.
• Absorbent: these cracks and grooves soak up water like a sponge when it rains, so the hair stays damp.

In a humid rainforest, this wet, sunlit hair is an ideal place for algae to grow. Sloth hair also grows from the belly toward the back, the opposite of most mammals, so rain runs off when the sloth hangs upside down.

What kind of algae live on sloths?

Several kinds of green algae have been found on sloths, but some are special. A 2010 study using DNA analysis found a diverse community of algae in sloth hair and a close link between three-toed sloths and an alga called Trichophilus welckeri. Some of these algae have so far been found only in sloth hair, which suggests they depend on sloths. Researchers think the algae are passed from mother to baby while the young sloth clings to her.

How green fur helps sloths hide

The most obvious benefit is camouflage. Sloths are slow and cannot run from danger, so their best defense is to avoid being seen. Their main predators, such as the harpy eagle, jaguars and ocelots, hunt by sight.

A motionless sloth with greenish-brown fur looks remarkably like a clump of leaves, moss or a termite nest. Combined with its slow, deliberate movements, the green tint makes a sloth very hard to pick out in the canopy. You can read more about this strategy in why are sloths so slow?

A tiny ecosystem: moths, fungi and more

A sloth’s coat is a whole miniature world. Besides algae, sloth fur can contain:

• Sloth moths: small moths, such as Cryptoses choloepi, that live as adults in the fur. More than 100 have been counted on a single three-toed sloth.
• Fungi and bacteria: many kinds, some of which have shown antibacterial or other chemical activity in laboratory tests.
• Small invertebrates: mites, ticks and beetles.

Sloth moths have a remarkable life cycle. When a sloth climbs down to the ground to defecate, about once a week, female moths leave the fur and lay their eggs in the fresh dung. Their caterpillars grow up in the dung, and the new adult moths fly up into the trees to find a sloth. More about this weekly trip is explained in how often do sloths poop?

Do the algae feed the sloth?

A 2014 study led by Jonathan Pauli proposed that sloths, moths and algae form a three-way partnership. In this idea, the moths bring nutrients, especially nitrogen, into the fur as they live and die there. These nutrients help the algae grow. The sloths then eat some of the algae, perhaps by licking it from their fur, gaining a small extra supply of fats and other nutrients that their leaf diet lacks.

The researchers found that sloths carrying more moths had more nitrogen and more algae in their fur, and that algae from sloth fur were easy to digest and rich in fats. This could help explain why three-toed sloths keep making the risky trip to the ground. Other scientists are not convinced, and the question is still being studied.

Two-toed vs three-toed sloths

Both groups of living sloths can have green fur, but they differ in detail:

Two-toed sloths, such as Hoffmann’s two-toed sloth, have a different hair structure and generally carry far fewer moths, which fits the idea that the moth partnership matters most for three-toed sloths.

A walking home for other species

Algae grow on sloths mainly because of their special, water-holding hair and the humid forests they live in, not simply because sloths move so little. Certain algae and moths depend so heavily on sloths that they are rarely, if ever, found anywhere else, making each sloth a walking home for other species.