Remember the old stories about how sloths are so slow that algae grows on them? No? Well, it's true!
A new paper in BMC Evolutionary Biology reports on DNA sequences from sloth fur. Suutari et al. looked at all six sloth species and found that one third of the recovered sequences were green algae. They also found ciliates, apicomplexans, dinoflagellates, fungi, euglenas, arthropods, and red algae. One green alga (found in four of the six species) was apparently a species of Trichophilus, and Hoffmann's two-toed sloth (Choloepus hoffmanni) and the pale-throated three-toed sloth (Bradypus tridactylus) had unique Trichophilus species that were different from other sloths. The maned three-toed sloth (B. torquatus) had typical environmental algae in its hair and no Trichophilus.
More nifty symbiosis. I suppose the sloths could be getting some kind of camouflage from the relationship, but I'm not sure what the algae would get out of it. Maybe just a substrate to grow on? Now that we know what's growing on sloth hair, I'm sure future research will tell us more.
Suutari et al. 2010. Molecular evidence for a diverse green algal community growing in the hair of sloths and a specific association with Trichophilus welckeri (Chlorophyta, Ulvophyceae). 10:86.