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Sloths:  Chew Your Food Slowly

by Joe E. Meisel
© 2006 Ceiba Foundation for Tropical Conservation

There’s a new diet craze that’s the talk of the jungle, guaranteed to make you lose weight! Are you ready? Just eat leaves, nothing but green leaves, all the time -- it’s the All Leaf Diet! Wait, you say, even vegetarians get to eat fruits, nuts and other produce. How long could I survive on a diet of leaf salad? I’ll have to spend a lot of time eating, a lot of time chewing, and a lot of time digesting -- just to gain enough energy for the weakest of movements. There’s hardly any protein in leaves! I’ll wear my teeth down to tiny nubs in a matter of weeks!

Well, this extreme diet pretty much sums up the lifestyle that sloths have adopted. But, unlike you and me, they’ve managed to master a few tricks that let them prosper while eating leaves, leaves and nothin’ but leaves. In fact, sloths are so unique in their adaptations that they are practically the only mammals that can survive this diet. Their singular success means they have few competitors, and explains why sloths have remained mostly unchanged for thousands of years.

Sloths belong to the primitive mammal family Xenarthra, along with other relicts like anteaters and armadillos. Members of the family are characterized by simple peg-like teeth, or in some cases, lack teeth entirely. Sloths all have extremely durable, blunt teeth that they use like anvils to pulverize their leafy lunch. They have long limbs tipped with heavy claws that allow them to climb through the canopy, always hanging upside-down below the branches. Sloths even have fur that sheds water from the middle of the belly towards the back -- so they don’t get soaking wet -- unlike nearly all other mammals whose fur hangs down from the middle of back.

The easiest way to understand sloths is to think of them as cows in trees. Okay, without the mooing! For starters, both animals chew their food very slowly. This mechanical digestion helps them to extract more nutrients from the leaves. Sloths have a ruminating stomach like cows, that relies on bacteria to break down the tough cell walls present in all plant material. Their intestines are unusually long, again to get as much out of their food as possible. And even though all leaves may look alike to us humans, they most certainly are not: sloths have to vary their menu in order to get an adequate supply of essential nutrients, so they switch from one tree to the next every day or two.

The most important adaptation sloths have made to their all-salad diet is to slooooooow down. They take being casual to a whole new level: compared to sloths, even chess players are bundles of high energy! The reason, however, is simple. If your food supply is nutrient-poor (especially lacking in protein), you can’t afford to be highly active. So sloths have succeeded by being so mellow that they minimize the amount of energy they burn. In fact, compared to other mammals of similar size, a sloth’s metabolism is about half of what you would expect! Part of this achievement is due to their high skeleton to muscle mass ratio (less active cells to feed); they also allow their body temperature to drop at night, almost as low as their surroundings, to further conserve energy.

Because there are plenty of green leaves in the forest canopy, and because potential predators mostly prowl the ground, sloths spend nearly all their lives up in the trees. They eat, sleep, travel, find partners, mate, and raise young (even nursing them!) all in the canopy. About the only reason they ever come near the ground is to use the bathroom. They climb down their tree to just above the ground, and use their stubby tail to scrape a small depression, into which they defecate and urinate. Because their metabolism is slow, they need to do this very infrequently -- only about once per week! Imagine a sloth elementary school: “teacher, I have to go to the bathroom” ... “Melvin, you already went this week!”

Have you heard the old chestnut about rolling stones gathering no moss? Well, sloths don’t exactly roll, and in fact they do gather moss! Okay, actually it’s algae, but you get the picture. Because sloths travel high in the canopy, near the sunlight (only about 2% of which reaches the forest floor), they make a perfect planter for algae. Algae confer some camouflage to their host sloths, by turning their coats a lovely shade of green, and thus protecting them from predators! Although I mentioned that most predators are terrestrial, sloths are routinely hunted by huge harpy eagles that spot them from high above, and pluck them from the canopy. It isn’t easy being green, though, and sloths have a special adaptation that seems to help: each of their hairs is marked with a long groove, which appears to give algae a secure footing to get established.

There are other organisms that have developed mutualisms with sloths. Most famous among these are the pyralid moths, whose adults live on the backs of sloths and feed on the algae growing there. Even more amazing, these moths lay eggs in sloth dung (deposited at the base of a tree), in which their larvae later hatch out (and feed!); when the adult moths emerge, they fly up into the trees to find a new sloth host, and the cycle starts over. There numerous other arthropods that live on sloths, including flies, mites, and at least three separate genera of beetles: researchers once found over 900 beetles living on a single sloth!

Because sloths are slow moving, and not given to brawling, it is thought that they can live in rather high densities. This is a difficult assumption to test, since their lack of movement and green coloration make them very difficult to spot. Still, careful surveys on Barro Colorado Island show that between five and eight sloths coexist per hectare (two or three per acre). In another published account from Suriname, when rising waters from the Afobaka Dam flooded a forested valley, boaters rescued over 2,000 sloths that had climbed to the treetops and simply started swimming.

Swimming, you ask? Yes, it’s true. In fact, sloths are amazingly graceful and agile swimmers. They’ve had to learn because their forested habitat usually is crisscrossed with streams and rivers, many of them wide enough to interrupt the forest canopy. So sloths have adapted to descend from the trees, slip into the water (where they float rather nicely), and use their long arms to swim with resolute strokes across to the other side. It’s quite a different story when the barrier they have to cross is a road, but we’ll get to that later.

All sloths in the world fall into two categories, the two-toed (Choloepus spp.) and three-toed (Bradypus spp.) varieties. These can easily be distinguished by, you guessed it, counting the toes (or more easily, the claws they bear). Apart from the toes, their characteristic markings make these sloths readily recognizable. Three-toed sloths have dark eye stripes that look a bit like sunglasses, and because of other markings their mouth always appears to be smiling; their bodies are often a mottled color. Males of the three-toed variety also can be identified, by the large tawny stripe down the center of their backs. Two-toed sloths in contrast have few facial markings, and are a bit smaller than their triple-clawed relatives. As a group sloths are restricted to the tropics, where they live in lowland rainforest, but they can also be found in dry tropical forests, secondary forest and other treed habitats.

Sloths have a fascinating system of reproduction and inheritance. A healthy sloth can live to be between 20 and 20 years of age, but reaches sexual maturity in about three years. Sloth infants, born after a six-month gestation period, nurse while clinging to their mother for approximately six weeks, at which time they are weaned onto solid food: green leaves. Because finding leaves with decent levels of nutrients -- and avoiding leaves protected by toxins -- is complicated, mothers train their young for up to six months in the art of assembling a menu of feeding trees. What happens next is unusual. Whereas among most animals (humans included) the young are raised within the adults’ territory, and leave to seek their own territory upon reaching maturity, sloths are quite different. Young sloths actually inherit their mother’s territory, and she is the one who emigrates to seek a new home.

For many people visiting the tropics, seeing a sloth is a high priority. There’s something about these creatures that appeals to us. Maybe it’s envy for their lazy lifestyle, maybe its their strict vegetarian ethic, maybe they’re just adorable. Whatever the reason, sloths are actually quite difficult to see in the wild. Their slow movements, green coloration and preference for the high canopy seem designed to render them invisible. I already mentioned that hiding from ravenous eagles has always been critical for sloths; unfortunately it turns out the camouflage they’ve adopted hides them from humans as well. So even though sloths are probably abundant in most intact forest you might visit, consider yourself lucky if you get a good look at a sloth.

Sadly, sloths these days are not uncommonly seen attempting to crawl across tropical roads and highways, often with predictably disastrous results. Because mothers bequeath their territories to their offspring, they must travel across the landscape to find a new home. If that landscape is broken up naturally by streams, they simply swim across; however, if that landscape is bisected by roadways, dispersing sloths face an extremely dangerous crossing, one they regularly fail to survive.

If you should happen upon a sloth crossing a road (yes, “to get to the other side”), there is an easy solution. You can simply pick up a sloth by grasping the fur in the center of its back, and carry it to a tree on the opposite side of the road. I’ve done this myself on several occasions, and it’s always a deeply satisfying experience. Just make sure that you do not grip the sloth too high up on its back: their long arms can reach behind their head and well below their neck, and their powerful claws can deliver a nasty wound (prone to infection by bacteria). But if you use gloves, and hold the sloth’s fur midway between its shoulders and its hips, you should have no problems.

Sloths are not alone in facing the dangers of road crossings. In the tropics, a great proportion of the animal biodiversity is wholly arboreal. These animals, from sloths to anteaters to monkeys and tree rats, all must descend from their home in the trees when attempting to cross a roadway. Thus, roads in forested tropical landscapes constitute a significant barrier to the mobility of arboreal animals. Researchers in Australia and elsewhere are experimenting with “animal overpasses” that allow these creatures to cross roads overhead, instead of through the path of onrushing automobiles. These efforts may lead to the invention of easily installed overpasses that can be used in

Sloths are unique and harmless animals that have made significant adaptations to thrive on a very low-nutrient food source. Apart from howler monkeys and a few other species, novertebrates can survive on this all-leaf diet. From their low muscle mass, long limbs and green fur to their peg teeth, symbiotic bacteria and low metabolism, sloths have become totally from most mammals in order to succeed in the treetops. Their success has been bought at a high price: no cavorting in the sun, no noisemaking, no running across open fields. So if you decide to embark on the All Leaf Diet, keep in mind it might slow you down a little!

Do you want to help? Please contact CFTC about arboreal mammal conservation, and make a donation to help get this project off the drawing board and into the field.