Sloths ("the lazies" or "los perezosos” in Spanish) come in two essential “flavours”: the relatively upbeat, adventurous two-toed sloths (Choloepus spp.) and the true kings of the lazies, the three-toed sloths (Bradypus spp.). Both genera are tree-living leaf munchers (or arboreal herbivores if you're a zoological pedant like myself) found in the forests of Central and South America, and have evolved to avoid predation through camouflage rather than some other, more energetically costly, mechanism. They are also notorious for their incredibly bad personal hygiene, with pelts that are often discoloured green with algae ... and so filthy that they harbour their own diverse and unique microcosm of species.
Of the two genera, the three-toed sloths rest at the extreme end of the laziness scale. Individuals have a highly restricted home range and are strict folivores (leaf eaters), whilst two-toed sloths are relatively adventurous, both in terms of their diet and the distance they are prepared to wander. Another marked and much-discussed difference between the two sloth types relates to their toilet habits: two-toed sloths will quite happily defecate from wherever they happen to be hanging, but the three-toed sloths take a dangerous, weekly expedition down to a regular spot on the forest floor to empty their bowels. Why take all that trouble? The generally accepted explanation for this weekly toilet trip is that the sloths don't want to attract the olfactory attention of predators by pooping where they sleep. But a recent study published in the latest issue of the Proceedings of the Royal Society B describes a much more intricate explanation for this behaviour.
The forest floor is no place for a sloth. Another 2014 study (published in the Journal of Applied Ecology) demonstrated that over half of all recorded sloth predation events (aka sloth deaths in the jaws of predators) occurred close to or on the forest floor. So why does the three-toed sloth so regularly embark on such a dangerous expedition? And if the trip is so dicey surely there should be an evolutionary advantage that outweighs all the risk?
The authors of the new paper (entitled "A syndrome of mutualism reinforces the lifestyle of a sloth", catchy I know) hypothesised that there may be a significant but hidden nutritional benefit that's driving these hazardous ground-based toilet habits. And it's got something to do with all of those tiny hitchhikers living in the sloths' fur.
Both the pyralid moths and the green algae found in sloth fur are completely unique to their furry microcosm and interactions between these organisms and their slothful hosts have interested researchers for some time. The algae impart a slightly mouldy, green tinge to the sloth fur, which is thought to aid camouflage within the forest canopy. As for the moths, their place in this microcosm is far more tenuous. One theory is that this is an example of a commensalistic relationship, where the moth gains a safe place to live its adult life, but its presence doesn't affect its host.
“The new study points to a far more intricate relationship between the sloths and these tiny fur-dwelling hitchhikers.”
The new study points to a far more intricate relationship between the sloths and their residents. It suggests that the entire life cycle of the pyralid moth is inextricably intertwined with its host species of sloth. The larvae of the moth spend their days “coprophagically” (poo-munchingly) living within the sloth dung, emerging as fully grown moths and then flying into the canopy to colonise a new slothful host. This process allows nutrients (most notably nitrogen) from outside the sloth’s microcosm to be transported into the hair, providing essential ingredients for the life of some of the other fur-based residents. In addition, the decomposition of dead adult moths by species of fungi also living within the fur aids in increasing the overall nutrient content of this hairy "garden".
Much of these externally introduced nutrients allow the green algae that grows on sloth fur to thrive. A three-toed sloth's fur also has a specialised microstructure that invites algal growth: tiny cracks running across the hair filaments create living spaces for the cells of green algae and allow the hairs to soak up water like a sponge in order to hydroponically hydrate the algae. But why does the sloth need the algae so much?
By examining the stomach contents of sloths, the researchers suggest an interesting additional theory to explain the existence of the algae within the fur. Significant amounts of algae are ingested when the sloth grooms itself ... and it turns out these algal cells are particularly rich in digestible carbohydrates and fats (something that is largely lacking in the sloths' monotonous leafy diet). According to the new research, the primary function of the algae is to enhance the sloths' diet (camouflage is most likely a secondary advantage).
So it turns out that the unique, dangerous and laborious “Operation Defecation” performed by the three-toed sloth each week is likely to be a result of a complex evolutionarily driven symbiotic relationship between the moths, sloths and the algae, one that benefits all parties. The moths get a safe place to live and breed, the algae get a place to live and an influx of scarce essential nutrients, and, finally, the sloths get a boost of sugars and fats as a byproduct of their daily grooming. Nature, frugal in the extreme – as always!
Top header image: hollywoodsmile310, Flickr
