Slowing It Down: Sloths and their Symbionts


This is Aladdin, a male brown-throated three-fingered sloth that fell from a tree after losing a fight with another male. He sustained minor injuries from the fight, but otherwise seemed very healthy. Hair samples were collected from his back and a tracking collar was put around his neck so that we can find him again when I return to Costa Rica this summer!

Sloths have skyrocketed to favorite animal status for many Americans due to adorable videos, viral memes, and spotlight roles in commercials and films like Zootopia.  But why are they so popular? Is it their cute faces with permanent smiles?  Or their relatable lazy” demeanor (I prefer to call them energy-efficient!”)?

I’m a Masters student in the Hom Lab and was drawn to sloths because of their unusual behavior and ecology. There are two types of sloths two-fingered and three-fingered that diverged about 40 million years ago and display unique convergent evolution. Both sloth types have incredibly low metabolism, altered anatomy for efficient life in the canopy of trees, interesting bathroom habits (ahem), and a diverse array of microbes and arthropods living in their fur. They also have unique cracks in their hair that seem to aid in algal growth, giving the sloths a greenish color. Although historically thought of as a camouflage benefit for sloths, there seems to be a serious lack of evidence, showing green fur benefiting sloths in this way. Other scientists claim that theres a three-way mutualism between sloths, algae, and moths that live in their fur. The idea is that fur-residing moths lay their eggs in sloth poop when the sloth descends to the base of a tree once a week to relieve itself; then they fly back up to a sloth and deposit nutrients on the sloth fur to fuels algal growth; the sloth presumably benefits from all this by eating their fur algae as a nutritional boost but sloths don’t lick themselves or show any behaviors that can be interpreted as eating algae from their fur, so I’m not so sure about this part.
But I’m determined to answer some of the lingering questions regarding the sloth hair microbiome.  In January, I traveled to Costa Rica to study brown-throated three-fingered and Hoffmann’s two-fingered sloths in collaboration with Sam Trull at The Sloth Institute of Costa Rica ( I climbed trees, caught sloths, and collected hair samples to begin to answer questions like: do sloth-associated algae also grow on the leaves of the trees that the sloths may be eating? Does the composition of the hair microbiome change depending on the season ? Does a greener” sloth mean a healthier sloth?
Sure, sloth’s are cute and highly relatable, but their biology is super fascinating and strange as well!  Perhaps the next time you see a sloth on your Facebook feed or in a commercial, you’ll appreciate them for their unique biological characteristics that make them true wonders of nature.