The Hunters and the Hunted

Download Subtitles

Transcript

0:00:53 > 0:00:56The forests of the world, whether the jungles of Asia

0:00:56 > 0:01:01or the tropical rainforests of South America or woodlands in Europe,

0:01:01 > 0:01:06haven't really changed in their essentials for 50 million years.

0:01:06 > 0:01:11Then, as now, there were ferns and flowering plants

0:01:11 > 0:01:14and tall trees with broad leaves. Leaves, in fact, everywhere,

0:01:14 > 0:01:19sprouting and falling season after season, century after century.

0:01:19 > 0:01:21Dinosaurs had fed on leaves.

0:01:21 > 0:01:24Some of the biggest were plant-eaters.

0:01:24 > 0:01:26But when the dinosaurs disappeared,

0:01:26 > 0:01:28for reasons we still don't understand,

0:01:28 > 0:01:31these forests were left empty of any large creatures.

0:01:31 > 0:01:34There were just birds in the trees,

0:01:34 > 0:01:37insects and a few small reptiles and amphibians.

0:01:37 > 0:01:42And they stayed empty for several hundred thousand years.

0:01:42 > 0:01:44That may seem a very long time,

0:01:44 > 0:01:47but in geological time it's really quite short.

0:01:47 > 0:01:51And then, amongst those small creatures,

0:01:51 > 0:01:52there were warm-blooded,

0:01:52 > 0:01:55inconspicuous little animals that fed on insects.

0:01:55 > 0:01:58They'd been around a long time.

0:01:58 > 0:02:01They had been in the forests with the dinosaurs,

0:02:01 > 0:02:03but with the dinosaurs gone,

0:02:03 > 0:02:05those creatures began to develop

0:02:05 > 0:02:10ways of raiding this untapped larder of leaves.

0:02:10 > 0:02:12And their descendants are still at it.

0:02:30 > 0:02:34One or two have become extraordinarily specialised.

0:02:34 > 0:02:36The three-toed sloth in South America

0:02:36 > 0:02:39eats only the leaves of the Cecropia tree.

0:02:39 > 0:02:42Hanging beneath the branches, no predators can reach it.

0:02:42 > 0:02:47And perhaps lulled by this security, it's fallen into a kind of torpor

0:02:47 > 0:02:51so that it's totally unable to move any faster than this.

0:02:52 > 0:02:58Others became nimble and agile acrobats - the monkeys.

0:02:58 > 0:03:01But the leaf-eaters didn't have everything their own way.

0:03:07 > 0:03:10There were also hunters in the forest,

0:03:10 > 0:03:14moving silently and stalking alone.

0:03:41 > 0:03:45These duels are also played out at night.

0:03:47 > 0:03:50From the beginnings of the mammals' history they'd been able,

0:03:50 > 0:03:53with the help of warm bodies, to remain active

0:03:53 > 0:03:56even when the warming sun had gone down.

0:03:56 > 0:03:58A great proportion of them have never lost the habit

0:03:58 > 0:04:02and venture out only under the cover of darkness

0:04:02 > 0:04:05to nibble buds, bark and green shoots.

0:04:05 > 0:04:07This is a dormouse.

0:04:13 > 0:04:17On the woodland floor, a little hamster busily gathers food.

0:04:17 > 0:04:21It needs great quantities, for vegetation contains

0:04:21 > 0:04:24little nourishment in proportion to its bulk.

0:04:25 > 0:04:29But eating for hours on end out in the open can be dangerous,

0:04:29 > 0:04:31so the hamster stuffs all it can find

0:04:31 > 0:04:34into its cheek pouches as quickly as possible

0:04:34 > 0:04:38and then scampers back to the safety of its burrow.

0:04:41 > 0:04:44There it unloads its collection and eats it at leisure.

0:04:44 > 0:04:49Throughout the summer, it builds up immense stores down here,

0:04:49 > 0:04:51because soon another problem will face it,

0:04:51 > 0:04:54as it faces many other vegetarians.

0:05:06 > 0:05:10The frozen forests can no longer provide sufficient food

0:05:10 > 0:05:12to sustain the army of vegetarians

0:05:12 > 0:05:15that gnawed and nibbled here throughout the summer.

0:05:26 > 0:05:30The dormouse deals with the problem by hibernating.

0:05:30 > 0:05:33Its blood cools to only a few degrees above freezing,

0:05:33 > 0:05:36and the motors of its body slow down and idle,

0:05:36 > 0:05:41driven only by the fat accumulated during the leafy days of summer.

0:05:44 > 0:05:49The wood mouse, too, lives off its fat, though it doesn't hibernate

0:05:49 > 0:05:53and manages to keep going by finding seeds and gnawing bark.

0:06:00 > 0:06:03This is the time when the old and the weak die,

0:06:03 > 0:06:06and only the strongest survive.

0:06:13 > 0:06:15Bigger creatures too, like deer,

0:06:15 > 0:06:20are driven to search for nuts and to strip bark from trees.

0:06:21 > 0:06:24But after months of hardship,

0:06:24 > 0:06:28the year eventually turns and the world becomes green again.

0:06:28 > 0:06:30Among the leaves that sprouted

0:06:30 > 0:06:33during the springs of some 25 million years ago,

0:06:33 > 0:06:38there was a sudden increase in a particular kind - grass.

0:06:38 > 0:06:41The spread of grass was probably triggered

0:06:41 > 0:06:42by a drying of the climate.

0:06:42 > 0:06:47It may appear to be a simple kind of plant, little more than leaves,

0:06:47 > 0:06:50but it is a complex and specialised one.

0:06:50 > 0:06:54It has tiny flowers that rely on the wind for pollination.

0:06:54 > 0:06:57Its leaves grow not from the tip, like most other plants,

0:06:57 > 0:07:00but from the bottom, close to the ground.

0:07:00 > 0:07:04So when a fire sweeps over the plains, the leaves may burn,

0:07:04 > 0:07:09but new ones will sprout from the root stocks almost immediately.

0:07:09 > 0:07:12Similarly, when animals nibble the top part,

0:07:12 > 0:07:13the bottom continues to grow,

0:07:13 > 0:07:17providing a never-ending supply of succulent food.

0:07:26 > 0:07:30Leaf-eaters from the forest soon moved out onto the plains

0:07:30 > 0:07:35to gather this new and bountiful supply of sustenance.

0:07:39 > 0:07:42Leaves are not, however, easy to digest.

0:07:42 > 0:07:45To extract their nourishment,

0:07:45 > 0:07:48they have to be worked on by the digestive juices for a long time.

0:07:48 > 0:07:52And rabbits make this happen in a most surprising way.

0:07:55 > 0:07:58Having nibbled a stomachful above ground,

0:07:58 > 0:08:01the rabbit retreats to its burrow.

0:08:02 > 0:08:06Here it excretes special mucus-covered pellets,

0:08:06 > 0:08:09but the grass in them is only half-digested.

0:08:09 > 0:08:13As each pellet emerges, the rabbit immediately swallows it,

0:08:13 > 0:08:15so that eventually all its food

0:08:15 > 0:08:18passes through its digestive system twice.

0:08:33 > 0:08:38These creatures also live entirely on plants.

0:08:38 > 0:08:41They're buffalo, and I'm in North America.

0:08:41 > 0:08:44Like all vegetarians, they have teeth

0:08:44 > 0:08:47that are specially modified for the job.

0:08:47 > 0:08:50Those at the front here are nippers,

0:08:50 > 0:08:53which shear off the grass or the browse.

0:08:53 > 0:08:56Then, at the back, there are these grinding molars,

0:08:56 > 0:08:58a great battery of them.

0:08:58 > 0:09:02They're open-rooted, so they keep on growing as the enamel wears down.

0:09:02 > 0:09:04They have these ridges,

0:09:04 > 0:09:07which help to break down the walls of the cellulose in the plant,

0:09:07 > 0:09:11and, also, the jaw can be moved from side to side

0:09:11 > 0:09:13to help in the grinding process.

0:09:21 > 0:09:25Gathering sufficient grass to sustain an animal this size

0:09:25 > 0:09:28takes a long time - up to nine hours a day.

0:09:28 > 0:09:30But even that battery of grinding teeth

0:09:30 > 0:09:33doesn't solve the problem of digesting grass.

0:09:33 > 0:09:37And the buffalo also has to give its meals a double treatment,

0:09:37 > 0:09:41though in a rather neater way than that used by the rabbit.

0:09:41 > 0:09:45The chewed grass goes down to a large chamber, the stomach,

0:09:45 > 0:09:47that serves as a fermentation vat

0:09:47 > 0:09:51and contains a particularly rich brew of bacteria

0:09:51 > 0:09:53and single-celled creatures.

0:09:53 > 0:09:57These actively swimming little organisms are so small

0:09:57 > 0:10:00that a thousand million of them could get into a teaspoon.

0:10:00 > 0:10:03The rectangular slabs are fragments of leaves.

0:10:03 > 0:10:08No mammal can digest the cellulose walls of plant cells,

0:10:08 > 0:10:11but these micro-organisms can.

0:10:11 > 0:10:14They produce a ferment which dissolves the cellulose,

0:10:14 > 0:10:18changing it into a substance that the buffalo can absorb.

0:10:18 > 0:10:22But the bodies of the microbes also contain valuable protein.

0:10:22 > 0:10:24This, too, will be digested,

0:10:24 > 0:10:28but not until the half-digested mash, or cud,

0:10:28 > 0:10:31is brought up a lump at a time...

0:10:34 > 0:10:37..and given a second chewing.

0:10:42 > 0:10:46One mouthful goes down for a second time...

0:10:47 > 0:10:51..and up comes another.

0:10:51 > 0:10:53Many grass-eaters chew the cud like this,

0:10:53 > 0:10:56and a very convenient technique it is, too.

0:10:56 > 0:10:58It can be done away from the open pasture,

0:10:58 > 0:11:01lying concealed and comfortable in the shade,

0:11:01 > 0:11:06or with the head held high if there is a need to keep watch for danger.

0:11:12 > 0:11:17So, many leaf-eaters from the forest found food on the plain,

0:11:17 > 0:11:21and out of the forest, too, in pursuit of them, came the hunters.

0:11:53 > 0:11:57The serval is considerably bigger than a domestic cat.

0:11:57 > 0:11:58It hunts rats and mice

0:11:58 > 0:12:03and must catch about a dozen each day in order to survive.

0:12:05 > 0:12:08Flesh-eaters need quite different teeth from vegetarians.

0:12:08 > 0:12:11Instead of grinders and pulpers

0:12:11 > 0:12:15they require the armoury of the butcher's shop.

0:12:15 > 0:12:18A lion has two pairs of fangs at the front,

0:12:18 > 0:12:23daggers to stab the prey and grip it unrelentingly as it struggles.

0:12:23 > 0:12:28At the front, nipping teeth to pick off strands of meat.

0:12:28 > 0:12:30At the back, the cutters.

0:12:30 > 0:12:34Self-sharpening blades which mesh onto one another so accurately

0:12:34 > 0:12:38that they can shear through hide, tendons, even bones.

0:12:38 > 0:12:43These are why lions and cats chew with the sides of their mouths.

0:12:43 > 0:12:46With such weapons around, it's hardly surprising

0:12:46 > 0:12:49one of the most pressing concerns of leaf-eaters on the plains

0:12:49 > 0:12:52is to keep out of the way.

0:12:52 > 0:12:56One of the ways to do that is to go underground.

0:12:56 > 0:12:58These are the waste tips of the mole rat,

0:12:58 > 0:13:02a creature that has foregone the lush leaves of the grass

0:13:02 > 0:13:05and specialised instead on eating the roots.

0:13:24 > 0:13:28It tunnels industriously a few inches below the surface,

0:13:28 > 0:13:32nipping off the grass roots from beneath.

0:13:32 > 0:13:36Eyes are no use underground, and the mole rat has become totally blind,

0:13:36 > 0:13:38the furry skin of its head

0:13:38 > 0:13:41having completely covered the vestiges of its eyes.

0:13:41 > 0:13:43It finds its way around by touch,

0:13:43 > 0:13:47using lines of bristles growing on either side of its head.

0:13:47 > 0:13:50It happily scuttles along its dark tunnels,

0:13:50 > 0:13:53not only forwards, but backwards, like a tram.

0:13:53 > 0:13:57For burrowing it uses predominantly its large and powerful gnawing teeth

0:13:57 > 0:14:00and its shovel-shaped snout.

0:14:02 > 0:14:05It excavates enormously long tunnels beneath the turf

0:14:05 > 0:14:07and guards them energetically

0:14:07 > 0:14:11to protect its supply of roots and bulbs growing down from the ceiling.

0:14:17 > 0:14:20If a mole rat meets a stranger in its tunnel,

0:14:20 > 0:14:22there is likely to be trouble.

0:14:34 > 0:14:37Having established by smell that they are rivals,

0:14:37 > 0:14:41their first reaction is to build a wall between the two territories.

0:14:51 > 0:14:55If they still run into one another, then they fight.

0:15:11 > 0:15:16In the spring, particularly large mounds appear above their runs,

0:15:16 > 0:15:20thrown up by the females as they excavate their breeding chambers.

0:15:20 > 0:15:24Each of these underground mansions has larders stocked with bulbs,

0:15:24 > 0:15:28special lavatories and passages to the tunnels where the males live.

0:15:28 > 0:15:33Here the young, blind like their parents, are born and reared.

0:15:33 > 0:15:37Mole rats are the most dedicated of underground dwellers.

0:15:37 > 0:15:40Other inhabitants of the plain, like these prairie dogs,

0:15:40 > 0:15:43are rather more confident about life.

0:15:50 > 0:15:53Prairie dogs are also burrowers,

0:15:53 > 0:15:56but they spend much of their time not below ground, but above it.

0:16:00 > 0:16:03They actively farm their fields.

0:16:03 > 0:16:06If a plant they don't like to eat, such as sage,

0:16:06 > 0:16:08takes root on their land, they will cut it down

0:16:08 > 0:16:12and so make room for more of the plants that they do like.

0:16:12 > 0:16:14If one patch of pasture gets overgrazed,

0:16:14 > 0:16:17they abandon it and let it lie fallow

0:16:17 > 0:16:19while they feed on another patch.

0:16:26 > 0:16:30They live in huge towns many thousands strong.

0:16:30 > 0:16:34These great communities are made up of groups of about 30 animals,

0:16:34 > 0:16:36which all know one another personally

0:16:36 > 0:16:39and often have interconnected burrows.

0:16:39 > 0:16:43When these neighbours meet, they kiss and groom one another.

0:16:46 > 0:16:49They exchange many kinds of signals.

0:16:49 > 0:16:53A citizen declares his ownership of a burrow like this.

0:16:53 > 0:16:55SCREECHING

0:17:05 > 0:17:08They also bark warnings when they spot danger,

0:17:08 > 0:17:11such as eagles or coyotes on the hunt.

0:17:11 > 0:17:13BARKING

0:17:15 > 0:17:18So, as the open plains spread through the world,

0:17:18 > 0:17:20the animals that left the forests to graze there

0:17:20 > 0:17:23developed different ways of digesting grass

0:17:23 > 0:17:28and of protecting themselves in this dangerously exposed environment.

0:17:28 > 0:17:32During this period, South America, where I am now,

0:17:32 > 0:17:35became isolated as a gigantic island.

0:17:35 > 0:17:39The land bridge of Panama sank beneath the sea.

0:17:39 > 0:17:41Cut off from the rest of the world,

0:17:41 > 0:17:44the inhabitants of these grasslands, the pampas,

0:17:44 > 0:17:50developed into forms that to our eyes seem very extraordinary indeed.

0:17:51 > 0:17:55One looked like a cross between a camel and an elephant.

0:17:55 > 0:17:58There were huge grazing beasts bigger than rhinos,

0:17:58 > 0:18:01and an armoured animal the size of a small car

0:18:01 > 0:18:05that trundled about beneath a great dome of bone.

0:18:05 > 0:18:07These vegetarians were preyed on

0:18:07 > 0:18:10by gigantic flightless birds with beaks like hatchets

0:18:10 > 0:18:14and hunters with sabre teeth that looked like tigers

0:18:14 > 0:18:17but bore their young in pouches, like kangaroos.

0:18:17 > 0:18:19But about five million years ago,

0:18:19 > 0:18:22the land link with North America was re-established

0:18:22 > 0:18:25and different creatures from the north moved south.

0:18:25 > 0:18:28As the populations mixed,

0:18:28 > 0:18:32northerners and southerners competed for the same food and territory.

0:18:32 > 0:18:33There were winners and losers,

0:18:33 > 0:18:37and most of the strange South Americans disappeared.

0:18:37 > 0:18:39But in this gigantic cave in Patagonia,

0:18:39 > 0:18:41on the southern tip of the continent,

0:18:41 > 0:18:46tantalising evidence has been found of a really dramatic survival.

0:18:46 > 0:18:49At the end of the 19th century a German came down here

0:18:49 > 0:18:51to settle and to ranch cattle and sheep.

0:18:51 > 0:18:55And this cave lay on his estancia.

0:18:55 > 0:18:59When he came to explore it, he found, at the back,

0:18:59 > 0:19:01behind that line of boulders,

0:19:01 > 0:19:07a pile of the most extraordinary bones, skin and dung.

0:19:07 > 0:19:10He hung a piece of the skin on one of the posts

0:19:10 > 0:19:12that marked the boundary of his property.

0:19:12 > 0:19:17And there, a few years later, a Swedish traveller noticed it.

0:19:17 > 0:19:21He sent it to the Natural History Museum in London,

0:19:21 > 0:19:27and there they identified it as belonging to a giant ground sloth.

0:19:29 > 0:19:33This animal had been known for some time from its fossilised bones.

0:19:33 > 0:19:36But the remains in the cave were not fossilised

0:19:36 > 0:19:38and seemed extraordinarily fresh.

0:19:38 > 0:19:41Could the animal still be alive somewhere?

0:19:41 > 0:19:44Why were there such huge piles of dung in the cave?

0:19:44 > 0:19:46Was it possible that the line of boulders

0:19:46 > 0:19:49was the remains of a wall built by men

0:19:49 > 0:19:53to pen the animals in the cave, like enormous cattle?

0:19:53 > 0:19:57For a long time, nobody knew the answers to those questions.

0:19:57 > 0:20:01But recent excavations have at least cleared up some of them.

0:20:01 > 0:20:04A few years ago, these bits of bone were dug up here.

0:20:04 > 0:20:07This is a bit of the jaw, and this of the hip.

0:20:07 > 0:20:11From tests on them and their position in the ground, we now know

0:20:11 > 0:20:17that the giant ground sloths were here up to about 5,000 years ago.

0:20:17 > 0:20:21The same excavations have also shown that the Indians were here

0:20:21 > 0:20:23between 8,000 and 10,000 years ago.

0:20:23 > 0:20:29So it is indeed possible that the Indians hunted those huge animals.

0:20:29 > 0:20:32But that line of boulders, I'm afraid,

0:20:32 > 0:20:35is no more than a natural fall of rock from the ceiling.

0:20:35 > 0:20:38And the piles of dung behind it are no mystery either.

0:20:38 > 0:20:42We now know that many animals habitually use the same dung hills,

0:20:42 > 0:20:47and maybe the sloths came in here during the winter to keep warm.

0:20:47 > 0:20:50But I'm afraid the animal is really now extinct.

0:20:50 > 0:20:54You can't hide a creature twice the size of a cow

0:20:54 > 0:20:57in the bleak emptinesses of Patagonia.

0:20:57 > 0:21:01So we've missed our chance of seeing it.

0:21:01 > 0:21:05But it was quite a close thing, in geological terms, at any rate.

0:21:06 > 0:21:09The South American giants may have gone,

0:21:09 > 0:21:13but some less conspicuous but equally bizarre creatures remain.

0:21:13 > 0:21:15There are many kinds of armadillos,

0:21:15 > 0:21:18trotting over the pampas and foraging in the forests,

0:21:18 > 0:21:22diminutive descendants of the huge extinct ones.

0:21:24 > 0:21:26There are some very odd rodents here, too.

0:21:26 > 0:21:31These are capybaras. They find safety in water.

0:21:31 > 0:21:34Like a hippopotamus, their eyes, ears and nostrils

0:21:34 > 0:21:36are all on the top of the head.

0:21:36 > 0:21:37And for the same reason -

0:21:37 > 0:21:40so that the capybara can lie in the water

0:21:40 > 0:21:43fully aware of what's going on around it,

0:21:43 > 0:21:47but with practically all its body hidden beneath the surface.

0:22:07 > 0:22:11Capybaras are the largest rodents to be found anywhere in the world.

0:22:11 > 0:22:14A male grows to be three feet long, a metre or so,

0:22:14 > 0:22:19and they're the descendants of an even bigger extinct ancestor.

0:22:21 > 0:22:23They move around in large family groups

0:22:23 > 0:22:26and are excellent swimmers from an early age,

0:22:26 > 0:22:31though sometimes the young find even easier ways of getting around.

0:22:47 > 0:22:50They eat virtually nothing except leaves of one sort or another,

0:22:50 > 0:22:55either water plants or the grasses of the river bank.

0:23:00 > 0:23:04The viscacha is the South American equivalent of the prairie dog,

0:23:04 > 0:23:08though it's very much bigger, about the size of a badger.

0:23:12 > 0:23:15At dusk it comes up from its sleeping quarters

0:23:15 > 0:23:20and surveys the world before starting on its nightly labour

0:23:20 > 0:23:22of nibbling grass for hours on end.

0:23:30 > 0:23:34The mara, another South American rodent, has very long legs,

0:23:34 > 0:23:38for it finds safety not by burrowing underground, but by running.

0:23:38 > 0:23:41So when it browses, it's always on the alert,

0:23:41 > 0:23:44nervously watching for danger and so highly strung

0:23:44 > 0:23:47that it will race away at the crack of a twig

0:23:47 > 0:23:50or the faint whiff of a dangerous scent.

0:23:52 > 0:23:54This little leaf-eater

0:23:54 > 0:23:58appeared some 50 million years ago in North America.

0:23:58 > 0:24:00It was no bigger than a spaniel,

0:24:00 > 0:24:03and it had four toes on its front legs and three on the back.

0:24:03 > 0:24:07Like the mara, it sought safety in speed.

0:24:07 > 0:24:10The longer your legs, the longer strides you can take

0:24:10 > 0:24:12and the faster you can run.

0:24:12 > 0:24:14Over generations these creatures

0:24:14 > 0:24:18increased the length of their legs by rising up on their toes.

0:24:18 > 0:24:20After millions of years some developed

0:24:20 > 0:24:24which carried their main body weight on the middle toe alone,

0:24:24 > 0:24:27and the side toes barely touched the ground.

0:24:27 > 0:24:30Their continuously growing nail on the middle toe

0:24:30 > 0:24:32became thick to reduce wear - a hoof.

0:24:32 > 0:24:36Eventually the side toes disappeared altogether.

0:24:36 > 0:24:38These were the early horses,

0:24:38 > 0:24:41and they spread right across the northern hemisphere

0:24:41 > 0:24:42and down into Africa.

0:24:50 > 0:24:54Zebras, with their long legs jointed to a stiff backbone,

0:24:54 > 0:25:00can gallop at speeds of up to 40 miles, 65km an hour.

0:25:00 > 0:25:02They run in groups for the safety in numbers.

0:25:02 > 0:25:05It's more difficult to take an animal by surprise

0:25:05 > 0:25:06if it's in a group,

0:25:06 > 0:25:10and a swirling mass, like this, makes a very confusing target.

0:25:10 > 0:25:13Other grass-eaters on the plains

0:25:13 > 0:25:17find protection from hunters in different ways.

0:25:17 > 0:25:21The rhinoceros has a hide as tough as any mammal's,

0:25:21 > 0:25:24proof against the sharpest claws and teeth.

0:25:24 > 0:25:27Its sheer bulk makes it very formidable, too.

0:25:28 > 0:25:32But that also helps it with the universal problem for browsers -

0:25:32 > 0:25:35how to digest cellulose.

0:25:36 > 0:25:39The rhinoceros doesn't chew cud.

0:25:39 > 0:25:44Instead it keeps its food in its belly for a very long time indeed,

0:25:44 > 0:25:47so that the bacteria have plenty of time to work on it.

0:25:47 > 0:25:52To do that, you need a very large belly in which to store the food,

0:25:52 > 0:25:55and if you are to carry a large belly, you must be big.

0:25:56 > 0:26:00Only one creature on the plains is much bigger than the rhino.

0:26:09 > 0:26:12The elephant is the largest land animal alive,

0:26:12 > 0:26:16and its huge size makes it virtually invulnerable.

0:26:16 > 0:26:19No hunter is big enough or powerful enough

0:26:19 > 0:26:22to pull down a full-grown elephant.

0:26:22 > 0:26:25It's so big, it hardly gets any shade

0:26:25 > 0:26:28from any of the trees occasionally studding the plains,

0:26:28 > 0:26:32and out in the baking sun it's in danger of getting overheated.

0:26:32 > 0:26:36Flapping its ears helps considerably in cooling the blood,

0:26:36 > 0:26:39as it passes through the veins of the ears.

0:26:44 > 0:26:47The elephant manages to live on what is probably

0:26:47 > 0:26:50the poorest diet of any mammal.

0:26:52 > 0:26:55Although it welcomes leaves when it can get them,

0:26:55 > 0:27:00it also eats the most fibrous browse of all - twigs, bark, even branches.

0:27:00 > 0:27:03But again its huge intestines allow it

0:27:03 > 0:27:06to give this roughage prolonged chemical treatment.

0:27:06 > 0:27:09Our food takes about a day to pass through our bodies.

0:27:09 > 0:27:12An elephant's takes two and a half days.

0:27:12 > 0:27:14For most of that time, the browse,

0:27:14 > 0:27:16having been mashed by the elephant's molars,

0:27:16 > 0:27:19is stewing in the digestive juices

0:27:19 > 0:27:23and bacterial broth of its gigantic gut.

0:27:28 > 0:27:31Forest antelopes also moved out into the open country,

0:27:31 > 0:27:35lured by so much readily available vegetation.

0:27:37 > 0:27:39The little dik-dik resembles

0:27:39 > 0:27:42the early forest-living antelopes in many ways.

0:27:42 > 0:27:44Like them, it's small, only a foot high,

0:27:44 > 0:27:48which is convenient for moving in thick vegetation.

0:27:48 > 0:27:49And it lives in pairs,

0:27:49 > 0:27:53for the shaded floor of the tropical forest is poor in leaves

0:27:53 > 0:27:57and can't sustain a dense population of leaf-eaters.

0:27:57 > 0:27:59So the antelopes living there

0:27:59 > 0:28:03mark and defend their precious pastures against rivals.

0:28:08 > 0:28:12The male deposits musk from a gland beneath his eye on twigs

0:28:12 > 0:28:16by poking the tips actually in the gland.

0:28:17 > 0:28:19The scent proclaims that

0:28:19 > 0:28:21the territory belongs to a particular pair,

0:28:21 > 0:28:25which will remain here together throughout their lives.

0:28:25 > 0:28:28Another male reads the signs.

0:28:28 > 0:28:31Smell is very important to the dik-dik.

0:28:31 > 0:28:34They have other scent glands between their hooves,

0:28:34 > 0:28:37which probably mark the trails.

0:28:37 > 0:28:40Since the pair never stray from this one patch of land,

0:28:40 > 0:28:43they know it intimately, and that's important in defence.

0:28:43 > 0:28:46When danger approaches,

0:28:46 > 0:28:49they know the best escape routes, the best corners in which to hide.

0:28:58 > 0:29:01Impala inhabit more open country.

0:29:01 > 0:29:03They no longer live in pairs,

0:29:03 > 0:29:06but have formed herds for safety's sake.

0:29:06 > 0:29:09It's very difficult to take them by surprise.

0:29:13 > 0:29:15When they are attacked,

0:29:15 > 0:29:18the herd works together to baffle their assailants.

0:29:23 > 0:29:26As the animals suddenly leap in all directions,

0:29:26 > 0:29:29a hunter may well hesitate in deciding which it should follow,

0:29:29 > 0:29:34and that may make the difference between a killing and an escape.

0:30:01 > 0:30:06Impala still prefer country containing bushes and trees,

0:30:06 > 0:30:09but some grazers spend all their lives exposed

0:30:09 > 0:30:14out on the open plain, where there is no cover of any kind.

0:30:25 > 0:30:29The wildebeest form some of the greatest herds of all.

0:30:29 > 0:30:32Tens of thousands of animals move across the plains,

0:30:32 > 0:30:36sometimes making huge journeys to follow the rains

0:30:36 > 0:30:38and find the newly springing grass.

0:30:38 > 0:30:41In vast assemblages like these,

0:30:41 > 0:30:44there are no pairs of males and females.

0:30:44 > 0:30:45During the rutting season

0:30:45 > 0:30:48the dominant bulls will set up small stamping grounds,

0:30:48 > 0:30:51which the females will visit one after the other.

0:30:51 > 0:30:54So a single bull will service many cows

0:30:54 > 0:30:58and then lose touch with them as individuals

0:30:58 > 0:31:01when they return to the anonymity of the herd.

0:31:01 > 0:31:04Not surprisingly, this immense concentration of animals,

0:31:04 > 0:31:09this vast reserve of meat, attracts the attention of hunters.

0:31:12 > 0:31:16The cheetah relies on speed in a straightforward chase.

0:31:16 > 0:31:19It's said to be the fastest runner in the world,

0:31:19 > 0:31:23capable of reaching 70 miles, 110km an hour.

0:31:23 > 0:31:25Its legs are not as long as an antelope's,

0:31:25 > 0:31:27but it increases their effective length

0:31:27 > 0:31:29with a backbone that's extremely flexible,

0:31:29 > 0:31:33so that it can take huge strides.

0:31:34 > 0:31:36But it can't sprint like this for long.

0:31:36 > 0:31:39If it doesn't catch its prey within a quarter of a mile,

0:31:39 > 0:31:42it has to give up exhausted.

0:31:46 > 0:31:48And this time, the gazelle has won.

0:31:54 > 0:31:57Young wildebeest are often taken by cheetah.

0:32:13 > 0:32:16This cheetah is one of a pair of males

0:32:16 > 0:32:21strolling through the herds, as if selecting their meal for the day.

0:32:43 > 0:32:46Oddly, perhaps, the wildebeest seem little concerned.

0:32:46 > 0:32:49Perhaps they know from the way the cheetahs are behaving

0:32:49 > 0:32:53that they're not about to attack yet.

0:33:04 > 0:33:08One male makes a tentative run.

0:33:08 > 0:33:09Bending his supple spine,

0:33:09 > 0:33:13he can cover an astonishing 23 feet, seven metres,

0:33:13 > 0:33:15in a single bound.

0:33:20 > 0:33:23It may be that by chivvying them in this way

0:33:23 > 0:33:25the cheetahs are trying to pick out

0:33:25 > 0:33:28the animal that is just a little slower than the others,

0:33:28 > 0:33:30just a little more vulnerable.

0:34:44 > 0:34:47His jaws are clenched on its windpipe,

0:34:47 > 0:34:50and the wildebeest dies throttled.

0:34:51 > 0:34:55The other male comes to share the catch.

0:35:20 > 0:35:22Cheetahs, like most hunting species,

0:35:22 > 0:35:25rarely tackle prey larger than themselves.

0:35:25 > 0:35:30But lions are not so restricted. They are social hunters.

0:35:30 > 0:35:31They're the biggest,

0:35:31 > 0:35:34the heaviest of all the hunters on the plains,

0:35:34 > 0:35:38and they live in groups, usually about 15-strong,

0:35:38 > 0:35:41though sometimes there may be over 30 in a pride.

0:35:49 > 0:35:51They're not as fast as the cheetah,

0:35:51 > 0:35:53about 35 miles an hour is their top speed,

0:35:53 > 0:35:57and out in the open they're no threat to the herds.

0:35:57 > 0:35:59The zebras can keep their distance.

0:35:59 > 0:36:03Provided they've got this amount of a start in a chase,

0:36:03 > 0:36:05the lions will never catch them.

0:36:09 > 0:36:12The lions' only chance is to get really close

0:36:12 > 0:36:17and then rely on their spectacular acceleration.

0:36:17 > 0:36:20Now one lioness begins to stalk,

0:36:20 > 0:36:25keeping low and almost invisible in the tawny, sun-withered grass.

0:36:33 > 0:36:35The zebra spot her.

0:36:44 > 0:36:47Ahead there are other members of the pride.

0:36:47 > 0:36:50In their panic, the zebra run towards them,

0:36:50 > 0:36:52and now they take up the hunt.

0:37:01 > 0:37:05But though she's got hold of it, she can't overpower it by herself.

0:37:05 > 0:37:08The others come to her aid.

0:37:18 > 0:37:24Now there is food, not just for one hunter, but for the whole pride.

0:37:24 > 0:37:26Lions, hunting in groups like this,

0:37:26 > 0:37:30kill on average once in every three attempts.

0:37:37 > 0:37:40But lions do not always work in groups.

0:37:40 > 0:37:43Sometimes a lioness will set out by herself.

0:38:20 > 0:38:24Only one in five of these solitary hunts is successful.

0:38:24 > 0:38:27Scavenging is a much easier way of getting meat.

0:38:27 > 0:38:29Hyenas killed this wildebeest,

0:38:29 > 0:38:31but a lioness, so much bigger than they are,

0:38:31 > 0:38:35can chase them off if there are only a few of them.

0:38:52 > 0:38:57Hyenas are small, and they can only run half as fast as the cheetah,

0:38:57 > 0:39:00but they make up for that by hunting as a pack.

0:39:00 > 0:39:01They have enormous stamina

0:39:01 > 0:39:04and can keep up a good speed for a long time,

0:39:04 > 0:39:07harrying and wearing down their quarry.

0:39:13 > 0:39:18When a pack finally closes in on a wildebeest, there is no escape.

0:40:08 > 0:40:13Few creatures can defeat hyenas as long as they work as a team.

0:40:13 > 0:40:17A determined pack can even rob a lioness of her kill.

0:40:33 > 0:40:37She gives up. Perhaps she'd had enough anyway.

0:40:59 > 0:41:03So the pack and teamwork wins again.

0:41:08 > 0:41:11Sometimes, though, pack disputes with pack.

0:41:11 > 0:41:14This kill was made on the border between the hunting grounds

0:41:14 > 0:41:16of two neighbouring packs.

0:41:16 > 0:41:18The squabble surges this way and that,

0:41:18 > 0:41:22as each gains temporary control of the carcass.

0:41:42 > 0:41:46The females of the pack have their own dens where they raise the cubs.

0:41:46 > 0:41:49A pack may contain as many as 80 animals.

0:41:49 > 0:41:53They communicate with one another in the most comprehensive way,

0:41:53 > 0:41:56using sound, gesture and smell.

0:41:57 > 0:42:00The tail is particularly eloquent.

0:42:00 > 0:42:02Normally it's carried curved down.

0:42:02 > 0:42:05Erect like this, it's a sign of aggression.

0:42:05 > 0:42:09This female is unsure about her rival.

0:42:15 > 0:42:18Now she's happier.

0:42:20 > 0:42:24From a very early age, each hyena marks grass stems

0:42:24 > 0:42:26with scent from a gland beneath its tail.

0:42:35 > 0:42:37When members of the pack meet,

0:42:37 > 0:42:41they greet one another with extravagant smells and licks.

0:42:41 > 0:42:44Each animal knows its fellows individually,

0:42:44 > 0:42:49and each knows its place within the complex hierarchy of the pack.

0:42:54 > 0:42:58This elaborate social structure with leaders and followers

0:42:58 > 0:43:00and a highly effective system

0:43:00 > 0:43:02of communication, on which it's based,

0:43:02 > 0:43:05enables the pack to hunt most effectively as a team.

0:43:05 > 0:43:08So that the hyenas, small though they may be,

0:43:08 > 0:43:11are among the most effective killers on the plains.

0:43:11 > 0:43:15Indeed in some parts it's the hyenas, hunting at night,

0:43:15 > 0:43:19that are responsible for the majority of the kills.

0:43:28 > 0:43:32Lion society also has a well-defined structure.

0:43:32 > 0:43:34The females are the basis of the pride.

0:43:34 > 0:43:37There may be a dozen or so of them,

0:43:37 > 0:43:40and they are probably all sisters or half-sisters.

0:43:47 > 0:43:49They cooperate with one another,

0:43:49 > 0:43:53even, on occasion, suckling one another's cubs.

0:44:17 > 0:44:22These lionesses will remain together throughout their lives.

0:44:22 > 0:44:25The males come from other groups elsewhere to join them.

0:44:25 > 0:44:28But once the males are accepted as members of the pride,

0:44:28 > 0:44:30they take over much of the responsibility

0:44:30 > 0:44:32for defending the territory,

0:44:32 > 0:44:37roaring their claims of possession and fighting off intruders.

0:44:37 > 0:44:39The females do most of the hunting,

0:44:39 > 0:44:42but that doesn't take up much of their time.

0:44:42 > 0:44:44Meat is much more nourishing than grass.

0:44:44 > 0:44:48A lion can eat as much as 20 kilos at a sitting,

0:44:48 > 0:44:52so a single meal will last it two days or even longer.

0:45:09 > 0:45:12For the rest of the time there's not much to do

0:45:12 > 0:45:14except watch the wildebeest herds

0:45:14 > 0:45:16gathering their great quantities of grass.

0:46:21 > 0:46:25Eventually the time comes when more meat is needed,

0:46:25 > 0:46:27and the pride must hunt again.

0:46:29 > 0:46:31The male is not to be disturbed.

0:46:31 > 0:46:34He'll follow later when all the work has been done,

0:46:34 > 0:46:35and probably just as well.

0:46:35 > 0:46:39With a great mane, he's much more conspicuous than they.

0:46:55 > 0:46:57The wildebeest have come down

0:46:57 > 0:47:00to feed on the lush grass beside a marsh.

0:47:07 > 0:47:10The lionesses spread out in line.

0:47:44 > 0:47:48The herd won't run into the marsh that lies beyond.

0:47:48 > 0:47:50It's a barrier they can be trapped against.

0:47:59 > 0:48:02This lioness leaves the main group

0:48:02 > 0:48:05and walks off to the far flank of the herd.

0:48:08 > 0:48:11The others slowly advance on the wildebeest,

0:48:11 > 0:48:13which move nearer the marsh.

0:48:29 > 0:48:32Each lioness seems to keep a close watch on her companions

0:48:32 > 0:48:37as they advance together closer and closer.

0:48:50 > 0:48:52If the wildebeest get agitated,

0:48:52 > 0:48:55the lions simply sit and wait for them to settle again,

0:48:55 > 0:48:58which they do just a little nearer the marsh.

0:49:55 > 0:49:58Now the ambush is laid.

0:50:20 > 0:50:24Back and forth the wildebeest dash in panic and confusion,

0:50:24 > 0:50:27and the lionesses have time to select their prey.

0:51:19 > 0:51:22Once again jaws are clenched on a throat,

0:51:22 > 0:51:26and a wildebeest is throttled.

0:51:26 > 0:51:31By the time the others, including the male, arrive, it's dead.

0:52:04 > 0:52:06The hunt has produced two kills.

0:52:06 > 0:52:11That's more than enough meat for the whole pride.

0:52:36 > 0:52:40So the long duels between hunter and hunted fought out on the open plain

0:52:40 > 0:52:45led to a great development of teamwork and communication.

0:52:45 > 0:52:48One animal came out of the forest to hunt on the plains

0:52:48 > 0:52:49that I've not yet mentioned.

0:52:49 > 0:52:52That's a particularly interesting one to us,

0:52:52 > 0:52:54because it was our ancestor.

0:52:54 > 0:52:58To trace it from its origins, we'll have to go back into the forest,

0:52:58 > 0:53:01where its cousins still live.