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
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.
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
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 Ceiba about arboreal
mammal conservation, and make a donation to help get this project off the drawing
board and into the field.