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The sloth -

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David Attenborough: What would you like to be, people sometimes say, if you came back to earth as
an animal? Well the answer I give depends I suppose on the company. If it's a bit racy, I dare say
my mind would range over a number of mammals remarkable for the extravagance of their reproductive
techniques. But in more sober company, I usually find a safer answer; a sloth, I say. And why?
Because it spends most of its time hanging upside down from a branch in the tropical rainforest,
dozing. The sloth really is not only one of the laziest, but one of the strangest of mammals. It
spends most of its life asleep. But even when it's awake, it only moves very slowly. No matter what
the danger or the emergency, it is incapable of speed.

There are two main kinds defined by the number of toes that they have on their front legs. It's the
three-toed which has really slothful habits, and the one that interests me most. You're not likely
to have seen it in zoos, for unlike the two-toed which will eat all kinds of vegetables, the
three-toed is really picky and will only accept leaves and fruits of a few particular kinds, which
few zoos can supply. And in any case, it doesn't make an exactly dazzling exhibit, since it spends
most of its time in slumber.

It's about the size of a small slim upside down goat, if you can imagine such a thing. It's covered
in a shaggy coat of coarse untidy hair, blotched in grey, cream and brown. The hair on its head is
combed forward into a fringe; that on its body is parted down the middle of its belly, with the
hair running down its flanks towards its spine, so that as it hangs upside down the tropical rain
which it has to endure for several hours every day, does much the same.

The thick coat is itself a home for an abundant population of other creatures. There are moths of a
species that occurs nowhere else, and a particular kind of beetle. The hairs themselves have
grooves down them in which grows a special species of green alga. When the skins and skeletons of
this extraordinary topsy turvy beast reached Europe, artists given the task of illustrating the
species in natural history encyclopaedias, showed it standing upright on its four thin, rod-like
legs, even though the hooks on its feet and the fur on its body all pointed very obviously in what
you might call the wrong way. After all, the artists must have said to themselves, it would be too
absurd to show it as an animal that spent all its life upside down.

Because the sloth moves extremely slowly, there is little difficulty in catching one, once you've
spotted it, which is not easy because huddled in the crook of a branch or hanging beneath it, it
looks like a bundle of dead leaves. The first one I ever encountered in a forest in what was then
British Guiana and is now Guyana, was pointed out to me by one of the forest living Akawayo people
in whose village we were staying. The tree wasn't difficult to climb, I was younger and nimbler in
those days, and I was soon sitting astride a branch alongside it. The sloth looked at me amiably
from beneath its fringe with a distant but benign smile.

I was there making natural history films, and I thought I'd move the sloth to a branch where it
would be easier to film. So I hauled up a liana that dangled nearby and had no problem in
transferring the sloth's legs onto it one by one, so that eventually I was able to lower it to the
ground. We then took it back to the village and put it in an isolated tree just outside our hut,
the leaves of which were to the sloth's taste. There was little chance that it would stray, because
sloths on the ground are virtually helpless. Its legs are little more than hangers and they only
have slim, ribbon-like muscles that are just about strong enough to enable it to transfer its grip
from one branch to another, but quite incapable of supporting it standing upright. So on the ground
the sloth lies on its stomach and waves its legs around with a sort of vague swimming motion, and
just about succeeds in crawling very slowly forward, which is clearly doesn't like, because quite
apart from the awkwardness of it all, it's very vulnerable to attack from ground predators such as
jaguar.

The following morning, I went out to look for it as it hung from its branch and saw to my delight a
tiny, bedraggled baby sloth crawling very slowly from between its mother's hind legs and out onto
her belly. One of the Akawayo women in the village told me a story about baby sloths. A pair of
them, twins I suppose, were sitting on their mother's stomach, complaining that they were very
hungry. Their mother craned her head forward and said, clamber onto that branch, children, and I'll
go off and find some leaves for you. But you must promise that you won't stray from this spot, so
that I'll be able to find you again. The babies promised and away she went. A day passed and then
another. The babies got more and more hungry, until eventually on the fifth day one of the babies
whispered to the other, why don't we go and look for some leaves ourselves? Whereupon their
mother's voice came from the leaves nearby, I warned you, she said, if you don't stay where you
are, I won't go.

In reality though, there's not much whispering that goes on among sloths. They're only capable of a
faint bronchial wheeze and an occasional whistle. These sounds are probably only made to deter a
predator, and certainly not to communicate with one another, for sloths are virtually stone deaf.
It's said that an enthusiastic zoologist investigating the hearing ability of sloths discharged his
gun close to one. It slowly raised its head, blinked at the scientist, and then went back to sleep.
Their eyesight is not much good either. In fact, the only sense that seems at all acute is their
sense of smell. And it's that that they use to select their favourite leaves and fruit.

So how do these purblind, half deaf creatures find one another up in the branches when breeding
time comes round? Well, it seems to be connected to their rate of digestion. That, like everything
else, is extraordinarily slow. You might imagine that they would defecate with careless abandon, as
they just hang there 50 feet up in the forest, and allow their dung to fall to the ground. But not
a bit of it. When the time approaches, a sloth sets off on what may be quite a long journey to a
special tree in its territory. It slowly clambers down woods until it reaches the ground, where at
the bottom of this tree there is a big pile of sloth dung, to which all the sloths in the area have
contributed. Sloths have a communal loo. And there it hangs within a foot or so of the ground
beside the great smelly pyramid, while it adds its own contribution. Perhaps the midden is the one
and only conspicuous beacon in the sloth's dim and silent world, the only place where there's a
reasonable chance of getting close to another sloth, and to know that you're doing so. If that's
the case, then one would expect that sloth dung would be very pungent, and I can tell you, it
certainly is, even to the nostrils of a human being.

But what extraordinary quirk of evolution led sloths down into their dim and silent alley? Well,
raw leaves, no matter what salad enthusiasts may tell you, are remarkably indigestible. Cows, deer
and antelope deal with that as everyone knows, by giving a mouthful of good chew, sending it down
to the stomach, letting it stew there for a bit, and then bringing it back up again as cud, and
chewing it a second time. And even after all that, the bulk of the meal, the cellulose, is so
resistant to the digestive juices that it has no nutritive value and has to be ejected.

But out on the plains, an animal is very visible, and an obvious target for meat eaters like lions.
So plant eaters have to be either formidably, almost invincibly big, like buffalo or elephant, or
become such good runners that they can outpace most hunters. But up in the trees and well
camouflaged, an animal can chew away and take all the time it wants to digest its meals. If that
enables you to cope with particularly indigestible leaves that no one else wants, then you needn't
remain particularly active or strong or swift, and you can take all the time you like to digest
them. In short, you become slothful. There may be a moral somewhere in all this, but I confess I'm
not sure what it is.

Guests

David Attenborough

Broadcaster

Presenter

Robyn Williams

Producer

David Fisher

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