Mutualism of the Month: Sloths, moths, and a garden of algae

The brown-throated three-toed sloth Bradypus variegatus feeding high up in the jungle canopy. Photo courtesy of wikimedia.org.

The brown-throated three-toed sloth Bradypus variegatus feeding high up in the jungle canopy. Photo courtesy of wikimedia.org.

Every month we showcase a relationship between two or more species or groups of species that can be considered a mutualism: a relationship where both members benefit. This month’s mutualism is the relationship between the brown-throated three-toed sloth Bradypus variegatus, a group of moths, and some algae that grows on the fur of the sloth. 

Sloths, a relative of anteaters, are arboreal herbivores, one of just 10 mammal groups that live permanently in trees and feed strictly off leaves. This kind of lifestyle requires unique adaptations to overcome the relationship between the low-quality food (low calorie/low nutrient) and the size of the animal. Sloths are folivores, spending most of their time eating the leaves of a limited selection of tree species or resting in the forest canopy. In order to maintain this lifestyle sloth metabolism has slowed down to such a rate that they require only half the expected calories of a mammal of similar size. They also maintain relatively low body temperatures for mammals (86–93°F or 30–34°C) to reduce energy consumption.

Two sloth moths crawling on a brown-throated three-toed sloth. Adult moths live out their entire lives within the sloth's fur. Photo courtesy of flickr.com via the user "take-nothing-but-photos".

The sloths are relatively safe within the tree canopy, but once a week the three-toed sloth must come down from the tree tops to defecate. This is a risky move, opening the sloth up to forest floor predators such as jaguars. The journey is also energetically costly (about 8% of their energy budget!). So why does the sloth do it, why not just let it rip in the canopy?

One species of sloth moth, Cryptoses choloepi. Photo courtesy of darnis.inbio.ac.cr.

One species of sloth moth, Cryptoses choloepi. Photo courtesy of darnis.inbio.ac.cr.

Sloth fur harbors a diverse ecosystem of insects and algae including some species which are exclusively found in sloth fur. When the sloth climbs down for its weekly defecation female moths fly out from the fur and lay their eggs in the fresh sloth dung. After the larvae of the moths grow up, they emerge from the dung as adult moths and fly up into the canopy to locate another sloth and so the cycle continues.

In return for transportation down to a provided nursery for the moth’s young, the moths die within the fur of sloth. As their dead bodies decompose, they release nitrogen and other essential nutrients into the fur. The nutrients, in combination with water from frequent rain events (sloths live in rainforests), provide all that is required for algae to grow on the fur of the sloth.

The three-way relationship between sloths, moths, and their algae is complex and dependent on all players within the relationship. "Three-toed sloth" photograph courtesy of wikimedia.org, "Sloth moth" photograph courtesy of tumblr.com via the user  "take-nothing-but-photos", and "Sloth fur algae" photograph courtesy of pitt.edu.

This algae is not only digestible, but it’s lipid-rich (rich in fat). The algae supplements the sloth’s nutrient-poor diet of leaves and this may explain how sloths have become successful arboreal herbivores. This beneficial relationship has locked the three-toed sloth into a trade-off that requires the sloth to face increased predation in order to preserve the three-way mutualism. In order to keep the moths alive, and therefore its algae alive, it must take a trip down the tree risking its life in order to continue the cycle.

If you’d like to read the original scientific publication, check it out here.

Don’t forget to check out the previous installments of mutualism of the month below and stay hungry!

More Mutualisms of the Month