A few days ago, our group was given the incredible opportunity of riding a canopy crane operated by the Smithsonian Tropical Research Institute high up above the forest canopy at Fort Shermin, Panama.
While exploring the canopies, we were fortunate enough to find a brown-throated three-toed sloth (Bradypus variegatus) mother and her baby clinging to the limb of a tree and foraging on nearby leaves. This encounter reminded me of the curious symbiotic relationship these creatures have with the organisms that call sloth fur their home.
Of all the mammals we have encountered in the tropical forests of Panama thus far, the two- and three-toed sloths are among the most fascinating in terms of their life history and behavior. Both two- and three-toed sloths are remarkably slow-moving animals with low metabolic rates as a consequence of their folivorous diets. Since they subsist almost entirely on hard-to-digest leaves filled with fiber, cellulose, and toxic secondary compounds, sloths have evolved a sedentary life-style to allow ample time for the microbiota in their numerous lengthy guts to release the scarce nutrients found in leaves and make them available to the sloths. In fact, studies have suggested that sloths may have the slowest mammalian digestive rate, with an average of 16 days required to pass a meal entirely! However, this intimate and drawn-out relationship with microbes is not the only symbiosis in which sloths partake.
In a fascinating triumvirate of mutualistic adaptations, a group of moths, known as sloth moths (Lepidoptera: Pyralidae), obligatively inhabit the fur of some sloth species and use sloth feces as an egg laying substrate. However, until recently scientists were unsure of why three-toed sloths descend from the canopy to the ground to defecate before returning to the treetops. A new paper in the Proceedings of the Royal Society posits an explanation that is driven by another denizen of sloth fur, algae (Trichphilus welckeri). By presenting evidence of the algae feeding both sloth moths and the sloth itself, while also gaining nutrients from decomposing sloth moths, Pauli et al. 2015 have shown a possible explanation for the strenuous and seemingly unnecessary weekly trip to the toilet some sloths make. In turn for providing moths with an oviposition substrate, nutrients for their larvae, and safety from predators, sloths are able to optimize algae production and supplement their nutrient-low diet with algae from their own fur. Some may ask, then, why would the sloths not simply eat the moths and forgo the algae altogether? I don’t think a sloth could even close its mouth fast enough to catch anything faster than algae!
Though often viewed as odd, awkward animals, sloths clearly demonstrate the incredible elegance with which creatures have managed to survive through evolutionary time.
Thanks for your attention and I hope you enjoyed reading!