The answer is no. This delicate issue arose in our video and subsequent discussion of sloths: the beast has the odd habit of defecating on the ground, at the base of the tree it inhabits. That means a looooong, slow climb down from the branches. And, of course, you’ve seen from that video how awkward sloths are on the ground, so the descent exposes it to all sorts of predatory dangers. Fortunately, sloths digest their food (leaves) as slowly as they move, so they have to make the round-trip only about once a week.
Here’s an Attenborough video showing sloths at the loo, with His Holiness noting that the behavior is a complete mystery:
Given the difficulty and time involved in the trip, and the helplessness of sloths on the ground, why do they do it? By “why”, of course, I mean what were the advantages of any genes that produced this behavior? I’m assuming here that this behavior is genetically based rather than simply learned, which seems a reasonable supposition. I can think of four evolutionary explanations:
- It reduces predation from above. Eagles and other aerial predators are said to detect the sound or appearance of droppings, using them to cue in on the sloth as prey. By hiding its toilet at the base of the tree, it makes itself less liable to aerial attack. I don’t find this very plausible given that aerial predators hunt visually, and I don’t see how they could detect a sloth more readily when it’s defecating from a branch. Indeed, it seems like it would be more likely to detect a sloth climbing down the tree.
- It reduces predation from below. This seems more likely to me than alternative #1. Terrestrial predators like jaguars can hear droppings striking the forest floor, and seem likely to be able to associate them with prey above. Cats, for example, are known to climb trees to take sloths (if you’re not squeamish you can see a video of that here.) On the other hand, couldn’t a cat smell sloth droppings and use them as a way to hunt?
- It fertilizes the sloth’s tree. I’ve heard this bandied about, but it doesn’t seem credible. Sloths hang out in big trees, and I’m not sure that a sloth with genes to move to the base and deposit fertilizer would really gain a reproductive advantage. That presumes that that such fertilizing would make such a substantial difference in the tree’s output of leaves that the sloth would wind up better fed and have more offspring. In addition, I’m not sure (though perhaps a reader can tell me) whether sloths remain in the same tree for years, as is required by this hypothesis.
- It’s a way to attract mates. Creating your own personal dung pile may be the equivalent of expelling pheromones, alerting sloths of the opposite sex that you’re up above. I know nothing about sloth mating, but given their lassitude and site-fidelity, surely locating a mate—the most important behavior in outcrossing species—is subject to strong selection. How do you find another sloth two trees over? By sniffing the base of the tree.
The last explanation seems the best to me, though of course the reproductive advantage of sloth toiletry could have involved more than one of these factors. And in principle these theories are testable. We could see, for example, how sloths manage to find each other at mating time. We could also do mock-defecation studies from branches, using model sloths, to see if the noise attracts predators.
I’m pretty sure, however, that nobody is doing these studies . .
Feel free to offer your own explanations: it’s sloth evolutionary psychology!