STRANGE THINGS ARE HAPPENING IN THE TREE TOPS

by Jack Ewing

He wasn't supposed to do it. I was sure that Dorothy was mistaken, because that kind of thing just doesn't happen. As a matter of fact it completely cramped my style and ruined the story I had already begun. But let me back up a bit and explain.

Dorothy, Bill and I were about as high as a 12 story building, on a platform suspended in the crown of an enormous rain forest tree called a "camarón" (Licania operculipetala.) We were observing a male three-toed sloth (Bradypus variegatus,) which was hanging from the upper branches of another tree about 40 meters away. We knew for sure that it was a male, because its back was turned toward us, and we could see the swath of short hair and the wide black stripe between his shoulders, this feature being absent in females. Dorothy was watching him with her binoculars while Bill waited his turn, and I was talking about sloths. I was just getting to the part about the strange toilet habits of the three-toed sloth, who for some unknown reason always descends to the ground to urinate and defecate. I must admit that parts of that last statement are arguable, namely the words "unknown" and "always."

The doubt surrounding "always" is easier to deal with, so let's take that first. The reason I said that the three-toed sloth always comes to the ground to urinate and defecate is because I had never seen a sloth defecate from the canopy, none of the other guides at Hacienda Barú National Wildlife Refuge had ever seen a sloth defecate from the canopy, and every biological text I have ever read says the sloth comes to the ground to urinate and defecate. Nowhere have I ever read or heard anything that would indicate that sloths ever defecate from the canopy.

So just at that moment Dorothy, looking through her binoculars says, "Oh look, he pooped."

I opened my mouth with the intention of blurting out some idiocy such as, That's not possible. But instead, thanks to the powers that be, I shut my mouth and raised my binoculars to my eyes just in time to see a second black mass fly out of the back end of the sloth.

"Oh, look!" exclaimed Dorothy, "He did it again."

I kept watching for a long time in hopes of observing this miracle again, but two quick blasts had apparently cleared the sloth's lower intestines. Eventually, I lowered my binoculars and pondered the situation. I even explained the reasons for my confusion to Dorothy and Bill. We had just witnessed an extraordinary event. I later consulted with all six of Hacienda Barú's naturalist guides. None had ever seen a sloth defecate from the canopy. I searched through books such as: Costa Rican Natural History, Neotropical Rainforest Mammals, and Mammals of Costa Rica. None mentioned any exceptions to the normal toilet behavior of the three-toed sloth. Finally I consulted a couple of biologist friends, experts on mammals, neither of whom had ever seen a sloth defecate from the canopy. Yet the fact remains that Dorothy, Bill and I clearly observed this aberrant act, and from that moment forward, I desisted from using the word "always," when referring to the sloth's weekly habit of coming to the ground to do its stuff. Since I have seen them defecate on the ground at least fifty times and only seen it once from the canopy, I will have to say "almost always" from now on.

"So," you ask, "why did you say that the sloth comes to the ground to urinate and defecate for ‘unknown reasons?' Doesn't anybody have a theory that explains why?"

Oh yes, theories abound. They come in many different shapes and sizes. They go from the absurd to the barely plausible. A couple are even possible. But before we look at the theories it is important to understand a few of the basics about sloths. First, the sloth is relatively safe when it is up in the canopy. Not many predators can harm an adult three-toed sloth while it is up there. It's predators would be limited to ocelots and tyras -- black weasel-like animals weighing about 8 kilograms. When it descends to the ground, however, it is vulnerable to many other predators including coyotes, peccaries, pumas, domestic dogs, snakes, etc. Additionally it must expend a lot of energy climbing to the ground and back up to the canopy. Any theory that attempts to explain the reasons for the sloth's toilet habits must explain what advantage the sloth gains from this behavior, and that advantage must outweigh the increased risk and energy expenditure involved in climbing to the ground and returning to the canopy. With that in mind, let's have a look at some of the different theories.

The most ridiculous I have ever heard, came from a biologist guide who was leading a group of British bird watchers. Upon spotting a three-toed sloth in the top of a tree, he explained to his visitors that the sloth comes to the ground to go to the toilet because it hangs upside down in the trees, and if it defecated from the tree tops it would get poop all over itself. I held my tongue because it wasn't my place to speak up. He was in charge of the group, and I had no right to dispute his ideas in front of his clients. The flaw in his explanation is obvious. I have seen lots of sloths come down the tree to defecate, and not one came down head first. I have often photographed them holding onto a tree trunk in a vertical position with the rectum pointing downward.

The second theory is the most widely accepted, a fact which never ceases to amaze me. This one says that by depositing its urine and dung at the base of the tree, the sloth is fertilizing its favorite trees. Sloths are classified as folivores, meaning they eat only leaves. This theory says that the sloth's bodily wastes deposited at the base of certain trees will increase the leaf production of those trees, thus benefiting the sloth. This theory reminds me of a perpetual motion machine. It would have us believe that a sloth can ingest leaves, extract all of the nutrients that are of any use to its body, expend an amount of energy equal to half a day's worth of food, while at the same time doubling its vulnerability to predation, deposit the waste fiber that is left over from the digestion process under the tree, and expect to get enough extra leaf growth to make the whole ordeal worth the trouble. What's more we are expected to believe that by making a hole with its stub tail at the base of the tree and depositing the dung and urine there, the fertilization value will be higher than if the corporal waste falls from above and is scattered over a larger area underneath the tree. My granddad was a dirt farmer with no formal education, but he knew better than that.

Quite a few years ago, I came across an article in Smithsonian Magazine (an issue from the late 1980s) which told of some US diplomats living in Surinam. They had a hobby of taking care of sloths that people brought to them. The sloths were victims of habitat destruction caused by development. Part of the regimen of sloth care included bathing them every couple of weeks. According to the article, each time the sloths were sprayed with water they would defecate. The US diplomats said that the local people believed that the sloth waits for a rain before defecating, in order to prevent a potential predator from locating it by the sound of the hard pellets of dung falling through the foliage. During a rain shower the sound would be masked by that of falling rain drops. In support of this theory I can say that at Hacienda Barú National Wildlife Refuge we see a lot more sloths coming to the ground to defecate during the dry season than we do in the rainy season. Also, the day when Dorothy, Bill and I witnessed the canopy poop, a light rain was falling. My only problem with this theory comes down to what is lost and what is gained. Although the act of coming to the ground would avoid alerting predators with the sound of falling feces, the sloth's very movement and its trip to the ground would make it more visible and vulnerable to other natural enemies. It seems that this would cancel any benefit gained by avoiding the sound of falling dung.

Another theory says that the habit of coming to the ground to defecate is a genetic memory that escaped being weeded out by natural selection. Sloths evolved several million years ago as terrestrial mammals. All of the species of giant ground sloths became extinct shortly after the end of the last ice age. Today's two-toed and three-toed tree sloths, both raccoon-sized arboreal mammals, diverged genetically from the ground sloths and came to live almost exclusively in trees. Since this has happened fairly recently in evolutionary terms, the genetic memory of urinating and defecating on the ground hasn't yet been selected out. I must agree that this theory is a real possibility.

Now it's my turn. I would like to throw my own hypothesis into the ring. I say hypothesis rather than theory because a theory must include some scientific study which has produced supporting evidence. My idea has none. Instead it is a mixture of common sense, logic and a gut feeling, but, for what it's worth, here it is.

Sloths have thick coats of hair that harbor many insects, primarily mites, beetles and moths. One researcher on Barro Colorado Island in Panama counted over 800 beetles on a single three-toed sloth. Among these insects there is a particular moth, which we will call the "sloth moth." The sloth moth leaves the sloth while it is urinating and defecating in a hole at the base of a tree. The moth lays its eggs on the dung. After the eggs hatch, the larvae eat sloth dung, grow, get fat and pupate. When the time is right, a mature sloth moth emerges and promptly flies into the canopy to find a sloth. There it lives, mates and returns to the ground to lay its eggs on sloth dung, thus perpetuating the cycle and the species.

With all that in mind, let's imagine that one day the three-toed sloth gets fed up and says; "Hey man, this is a lot of work. I'm getting tired of climbing up and down this tree just to go to the toilet. Not only that, but being on the ground scares the hell out of me. There's all kinds of nasty carnivores down there. Besides that, its no fun holding my bladder and bowel movements for a whole week. From now on, I gonna just let it fly from up here whenever I feel like it." What would happen to the sloth moth? It would become extinct, that's what. And, that is my point. Is the sloth moth so evolutionarily naive as to trust its entire existence to the whims of a sloth. I think not. I believe that the sloth moth performs some sort of service for the sloth, something so vital that it makes the well being of the moth a matter of survival for the sloth.

The service provided by the sloth moth is likely to be very complicated and may involve several other species. For example the moth may control the growth of algae on the sloth's hair, and this may somehow affect the population of mites. An out of control mite population can kill a sloth. On several occasions I have seen sloths scratch day and night for a week and then die, probably from a mange caused by rampant populations of mites. I grant that this is all pure speculation, but it makes as much sense as any other theory that has been offered. Perhaps some bright, young, ambitious biology student will decide to dedicate his or her life to unraveling the mysteries surrounding the sloth toilet riddle. Me? I'm having too much fun just watching sloths.

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