Sloths are beloved animals on the internet. And I totally get it: they are so cute, slow, and funny. Even the inhabitants of Caracas, who have the privilege to see them frequently around the city, do not get tired of them. Besides their cuteness, they are really interesting from an evolutionary point of view for having adaptations that allow them to have a very specialized way of living. They are strictly arboreal and have high fidelity to only a few trees, with a very specific diet comprising only certain types of leaves. Since they live hanging in the canopy almost all the time, one day I faced a truly important question: How do sloths poop?
It turns out that they do it on the ground, around once a week. But if you think about it, it is strange. Why would an animal, that lives all the time in the canopy, come down just to defecate? Not only it is a loss of energy to climb down and up the 15-30 meters of a Cecropia tree, but by doing so they are also more exposed to predators. This silly question about the pooping strategy was getting deeper, but I was not expecting that it would lead me to a fantastic chain of mutualism.
A sloth is like a moving ecosystem. The dense fur retains water from the rain, which turns into a green coloration after algae, fungi, and other microorganisms grow in it. Also small arthropods use the sloth's fur as a home, like the Pyralid moths (Cryptoses spp.). Once the sloth climbs down the tree, these moths reproduce and insert their eggs in the sloth's droppings, which later serve as food for the larvae to grow into adulthood. The behavior of going to the ground then assists the life cycle of the Pyralid moths, but what is the benefit for the sloth? The presence of moths is correlated to higher concentrations of nitrogen in the fur, either because they die and decompose or because they transport it directly from the feces. In the same sense, these higher quantities of nitrogen appear to benefit the growth of algae communities, producing larger biomass of algae in the sloth's fur. To end the chain of beneficial interactions, the sloth consumes the algae. These small snacks have carbohydrates and lipids that can bring additional nutrition, especially in a constrained herbivorous diet as the Three-toed sloths.
Phylogenetic analysis has shown a codivergence between the algae and the Three-toed sloths (Bradypus spp.), meaning that the relationship pattern between closely related species of algae matches the pattern of the Bradypus species. On the contrary, this match was not found in the Two-toed sloths (Choloepus spp.), whose diet is more flexible and might not depend on the algae for extra nutrition. The authors of the study suggest that this intimate mutualism in the Bradypus species played a role in the speciation of the algae through coevolution, a process where they not only live together but actually have a reciprocal influence on their evolution. Although more research is needed to disentangle the complex associations between sloths and its symbionts, every piece of information gathered shows an unimaginable panorama of interconnections that promote more and more questions, even if the starting point was a silly curiosity.
📑 References and further info:
Fountain ED, Pauli JN, Mendoza JE, Carlson J, Peery Z. (2017). "Cophylogenetics and biogeography reveal a coevolved relationship between sloths and their symbiont algae". Molecular Phylogenetics and Evolution 110: 73-80.
Pauli JN, Mendoza JE, Steffan SA, Carey CC, Weimer PJ, Peery MZ. (2014). "A syndrome of mutualism reinforces the lifestyle of a sloth". Proceedings of the Royal Society B 281: 20133006.
📸 Pictures: Irina Formoso.
Young Three-toed sloth (Bradypus variegatus). Caracas, Venezuela. 2015.