A Sloth Climbs Down to Defecate Once a Week, and It Is the Riskiest Thing It Does

A three-toed sloth spends effectively its whole life in the canopy. It feeds there, sleeps there, mates there and gives birth there. It is built for hanging and is close to helpless on flat ground, where it cannot walk properly and has to drag itself along by its front claws.

Roughly once a week it climbs all the way down to the forest floor, digs a small hole at the base of a tree, defecates into it, covers it over, and climbs back up.

It is difficult to overstate how bad an idea this looks.

The Cost

The descent is slow, exposed, and takes place on the ground, which is where the things that eat sloths are. Studies of sloth mortality have attributed a very large share of predation deaths, on the order of half, to these trips.

That is the single most dangerous recurring event in the animal's life, undertaken voluntarily, roughly fifty times a year.

The energy cost is serious too. A sloth runs on an exceptionally slow metabolism because its diet is leaves, which are poor fuel and take a long time to process. Food can take days to weeks to pass through the gut. It does not have surplus energy for a weekly round trip up and down a tall tree.

And the obvious alternative is available. Two-toed sloths are much less committed to the ritual and will defecate from the branch. Plenty of other canopy animals simply let gravity handle it. So the three-toed sloth is paying a large and repeated cost for something with a free alternative, which means something is buying it.

The Fur Is an Ecosystem

The leading explanation involves what lives on the sloth.

Sloth fur is not ordinary hair. The strands have cracks and grooves running along them that trap moisture, and in the humid tropics they support growing algae, including species found essentially nowhere else. That is what gives wild sloths their greenish cast, and it is thought to help with camouflage.

The fur also hosts moths. Cryptoses species live in sloth coats, and their life cycle is tied to the descent: when the sloth goes down to defecate, female moths leave the fur and lay eggs in the fresh dung. The larvae develop there, and newly emerged adult moths fly up into the canopy to find a sloth.

Pauli and colleagues proposed that this is not incidental. Their argument is that the moths bring nitrogen into the fur, dead moths and their waste fertilise the algae, the algae grow better as a result, and the sloth then supplements its poor leaf diet by consuming some of that algae, which is more nutritious than the leaves. Under that model the weekly descent maintains the moth population, which maintains the algae, which feeds the sloth.

It is an elegant explanation and it is not settled. Later work has questioned how much algae sloths actually eat and whether the nutritional return could justify the mortality risk. Other proposals include communication, since a dung pile at the base of a tree is a scent marker that other sloths visit, and simple fertilisation of the trees the animal depends on. The honest position is that the behaviour is well documented and the reason for it is still argued about.

The Rest of the Numbers

While it is down there, the sloth may pass close to a third of its body weight in one go. A week of leaf digestion accumulates.

The slowness is not laziness, it is budgeting. Leaves provide very little energy, so a sloth runs at a metabolic rate far below what its body size would predict and moves at a speed that reflects that. Its muscle mass is roughly a third less than comparable mammals. It has extra neck vertebrae, up to nine in some three-toed species where almost every other mammal has seven, which lets it rotate its head very far without moving its body.

Sloths are also strong swimmers, and can move considerably faster in water than on land, which is a strange thing to be true of a canopy animal.

Sources

• Pauli, J. N., Mendoza, J. E., Steffan, S. A. et al. (2014). A syndrome of mutualism reinforces the lifestyle of a sloth. Proceedings of the Royal Society B.
• Cliffe, R. N., Haupt, R. J., Avey-Arroyo, J. A. and Wilson, R. P. (2015). Sloths like it hot: ambient temperature modulates food intake in the brown-throated sloth. PeerJ.
• Voirin, B., Kays, R., Wikelski, M. and Lowman, M. (2015). Why do sloths poop on the ground? In Treetops at Risk.
• Montgomery, G. G. and Sunquist, M. E. (1978). Habitat selection and use by two-toed and three-toed sloths. In The Ecology of Arboreal Folivores.