0:00:42 > 0:00:50When specimens of this came from Australia in the 18th Century, people didn't believe their eyes.
0:00:50 > 0:00:57They said it was a hoax - bits of different creatures
0:00:57 > 0:01:03crudely sewn together - but it's no hoax, it's a platypus.
0:01:03 > 0:01:07Yet in a way, those sceptics were right.
0:01:07 > 0:01:14The platypus is an extraordinary mixture of different animals - part mammal & part reptile.
0:01:14 > 0:01:18So it gives us some idea of how mammals developed.
0:01:35 > 0:01:40At first sight, it looks a regular mammal & has dense soft fur,
0:01:40 > 0:01:43a hallmark of mammals.
0:01:43 > 0:01:46When you handle it, it's warm.
0:02:05 > 0:02:08Its feet are webbed for swimming.
0:02:20 > 0:02:26The strange bill isn't hard like a bird's beak, but soft, rubbery & very sensitive.
0:02:26 > 0:02:33The platypus, with poor eyesight, uses it to find food - crayfish & other water creatures.
0:02:53 > 0:02:59When it breeds, it does something that separates it from all other mammals except one.
0:02:59 > 0:03:03In its nest in a burrow, it lays eggs.
0:03:04 > 0:03:07This links it with reptiles
0:03:07 > 0:03:12& entitles it to be regarded as the most primitive living mammal.
0:03:12 > 0:03:17What makes it doubly paradoxical is that when the egg hatches
0:03:17 > 0:03:25the baby's not left to find food itself like reptile babies, but is given food by the mother.
0:03:27 > 0:03:33The platypus, like all mammals, has in its skin to help deal with overheating, sweat glands.
0:03:33 > 0:03:42On the underside of the body these glands are especially big & produce a fatty sweat which is milk.
0:03:42 > 0:03:47It oozes from the skin & the young suck it from tufts of hair.
0:03:47 > 0:03:53There's no nipple so it hardly qualifies as a breast, a mamma,
0:03:53 > 0:03:57which gives mammals their name.
0:04:00 > 0:04:06Only one other mammal lacks a true breast - the echidna.
0:04:06 > 0:04:09It, too, lives in Australia,
0:04:09 > 0:04:15& it too, lays eggs - but the female doesn't deposit them in a nest, she carries them with her.
0:04:16 > 0:04:23They have sticky shells & become glued to the hair on her underside in a groove across her stomach.
0:04:28 > 0:04:30Each is no bigger than a pea,
0:04:30 > 0:04:34& after 10 days, it hatches.
0:04:42 > 0:04:48By now, glands near the groove are producing creamy milk.
0:04:51 > 0:04:56The baby echidnas remain inside the groove for the next eight weeks,
0:04:56 > 0:04:59steadily taking in milk and growing.
0:05:10 > 0:05:15When the spines develop, they're uncomfortable passengers & the mother puts them in a den.
0:05:17 > 0:05:21Eventually, they abandon milk & take to their adult diet.
0:05:22 > 0:05:24Ants.
0:05:26 > 0:05:31A long snout is, in evolutionary terms, a recent acquisition -
0:05:31 > 0:05:34a tool for food gathering.
0:05:34 > 0:05:40It houses a long sticky tongue with which the echidna flicks up its ants & termites.
0:05:40 > 0:05:44The animal defends itself by digging downwards.
0:05:44 > 0:05:47There's nothing visible but spines.
0:05:50 > 0:05:56Platypus & echidna are oddities - we've no fossil evidence
0:05:56 > 0:06:00to say where or when they developed,
0:06:00 > 0:06:07but it's a good guess, because of another kind of echidna which lives not far away in New Guinea,
0:06:07 > 0:06:11that the group originated in this part of the world.
0:06:11 > 0:06:17It's certain it's these creatures from which modern mammals came
0:06:17 > 0:06:22about 180 million years ago.
0:06:22 > 0:06:26And we can trace the ancestry of mammals even farther than that.
0:06:31 > 0:06:38Reptiles began 300 million years ago & with watertight skins & eggs
0:06:38 > 0:06:42and so they survived in the driest country.
0:06:42 > 0:06:44After 20 million years
0:06:44 > 0:06:49a group of hunting reptiles evolved called pelycosaurs.
0:06:49 > 0:06:56Reptiles can't generate heat in their bodies, so after a cold night
0:06:56 > 0:07:00they're sluggish in the morning.
0:07:00 > 0:07:03Pelycosaurs dealt with that
0:07:03 > 0:07:07by developing sail-like fins on their backs
0:07:07 > 0:07:13with which to catch the first rays of the sun, so they could get out hunting really early.
0:07:13 > 0:07:18In places like this, it's easy to imagine some 12-foot species
0:07:18 > 0:07:24like dimetrodon lying in the sun.
0:07:24 > 0:07:27It's been calculated that with the fins,
0:07:27 > 0:07:31they could raise their body temperature 6 degrees in an hour.
0:07:31 > 0:07:36Without them, it would take nearly three hours.
0:07:36 > 0:07:43Those fins were stop-gap devices only & later species did without them.
0:07:43 > 0:07:47That's probably because they were able to generate heat internally -
0:07:47 > 0:07:51and their teeth support that idea.
0:07:57 > 0:08:04Dimetrodons' teeth, as those of most reptiles, were spikes which did no more than grip a victim.
0:08:04 > 0:08:08But generating heat in the body requires a great deal of energy.
0:08:08 > 0:08:15So a warm-blooded animal must eat more food than a reptile, & digest it fairly rapidly.
0:08:15 > 0:08:22Changes in the teeth of successive generations of pelycosaurs suggest that's just what they did.
0:08:22 > 0:08:26The spikes changed to tools for butchery.
0:08:26 > 0:08:28Daggers appeared on the sides of the upper jaw
0:08:28 > 0:08:30for slitting open the hide of prey.
0:08:30 > 0:08:36Knives for slicing the meat, & grinders for crunching bones.
0:08:36 > 0:08:39Most reptiles shed teeth as they become worn,
0:08:39 > 0:08:41throughout their lives.
0:08:41 > 0:08:45The teeth of these, not only became specialised but permanent.
0:08:45 > 0:08:51The upper & lower ones meshed to give a highly efficient bite.
0:08:51 > 0:08:59As they generated heat internally they also needed a coat of hair to conserve it - mammals had arrived.
0:09:01 > 0:09:07The acquisition of warm blood brought more advantages to these creatures than speed of movement.
0:09:07 > 0:09:15That you'd have seen had you been able to walk through forests of 180 million years ago at night.
0:09:22 > 0:09:29The first true mammals appeared at a time when reptiles ruled the world.
0:09:29 > 0:09:38But solar-powered animals had one major disability - at night when it was cool, they became sluggish.
0:09:38 > 0:09:48It left the field to any creature that could be active at night - & mammals could do just that.
0:09:50 > 0:09:58The ancient mammals were small, nocturnal insect hunters relying on smell to find food.
0:09:58 > 0:10:05In fact, they were probably very similar to present day shrews & hedgehogs, though they laid eggs.
0:10:05 > 0:10:10Warmth was the key to their survival & ultimate success.
0:10:10 > 0:10:15Since they alone could hunt in the cool of the night they didn't have to face competition with reptiles.
0:10:15 > 0:10:21So those primitive mammals were able to live right through the age of the dinosaurs
0:10:21 > 0:10:26& be poised to inherit the world when the reptiles finally declined.
0:10:26 > 0:10:33The problem of keeping warm was one that didn't just face adults - it also faced eggs & embryos.
0:10:33 > 0:10:38Mammals developed three ways of dealing with that.
0:10:38 > 0:10:43Primitive ones incubated eggs as the platypus does today.
0:10:44 > 0:10:48Others developed better methods
0:10:48 > 0:10:53The opossum that lives in North & South America is one - it doesn't lay eggs,
0:10:53 > 0:10:56but gives birth 12 days after mating.
0:11:08 > 0:11:16There may be 20 of them, the size of bees & the only well-formed features are the front legs.
0:11:16 > 0:11:23With these, they haul themselves through hair on the mother's belly on the 1st journey of their lives.
0:11:26 > 0:11:31At last they reach a pouch in the belly - inside are 13 nipples
0:11:31 > 0:11:36& each fastens to one for milk.
0:11:36 > 0:11:43If more than 13 are born, the last to reach the pouch will find no vacant nipple & die.
0:11:43 > 0:11:51The Latin for pouch is marsupium, & this gives a name to those who reproduce in this way - marsupials.
0:11:52 > 0:11:54This is the woolly opossum.
0:11:54 > 0:11:57Its babies are sufficiently well-grown to have left the pouch,
0:11:57 > 0:12:01but they still cling to their mother and return to the pouch for drinks
0:12:09 > 0:12:15There are about 70 kinds of opossum in the New World & most live in South America.
0:12:15 > 0:12:20Some are small as mice, others as big as domestic cats - there's even an amphibious one.
0:12:20 > 0:12:26The yapok - it has webbed feet & eats fish.
0:12:27 > 0:12:33When a mother goes for a swim, she closes the opening to her pouch to prevent the babies drowning.
0:12:33 > 0:12:39But they need to breathe, so she only swims a few minutes at a time when she has young.
0:12:40 > 0:12:45Mouse opossums are like the earliest marsupials.
0:12:46 > 0:12:50Fossils of similar creatures have been found in rocks that also
0:12:50 > 0:12:55contain the bones of dinosaurs.
0:12:58 > 0:13:02They live now, as they must have done then, by feeding at night
0:13:02 > 0:13:06on worms, insects and small reptiles like lizards.
0:13:12 > 0:13:16A larger one lives in the dank scrub of the High Andes - the rat opossum.
0:13:16 > 0:13:22It's a ferocious hunter with fangs on its lower jaw with which it stabs its prey.
0:13:22 > 0:13:25It, too, has an ancient ancestry.
0:13:25 > 0:13:32It doesn't even have a pouch, but its young hang from the mother's teats.
0:13:37 > 0:13:46Fossils resembling those primitive marsupials are found in America, dating back 60 million years.
0:13:46 > 0:13:55That makes them the oldest marsupial fossils known, older than any found elsewhere on Earth.
0:13:55 > 0:14:00So it's reasonable to assume marsupials originated here.
0:14:00 > 0:14:06If that's so, how did they get to Australia, where they flourish in the greatest numbers today?
0:14:06 > 0:14:10This tree may provide the answer -
0:14:10 > 0:14:17it's growing in the bleak lands of Patagonia on the tip of S America.
0:14:17 > 0:14:23It's a kind of beech related to the European beech & called the southern beech.
0:14:23 > 0:14:28It's a tree with a long ancestry & was growing here
0:14:28 > 0:14:35when marsupials first appeared & it seems likely they lived in forests like this.
0:14:39 > 0:14:44It's only relatively recently that scientists have demonstrated beyond all doubt
0:14:44 > 0:14:51that the continents aren't static but have drifted slowly over the globe for millions of years.
0:14:51 > 0:14:56To go back to when marsupials appeared in South America, is to return to a time
0:14:56 > 0:15:03when that continent wasn't joined with North America but fitted along the west coast of Africa.
0:15:05 > 0:15:11Australia and Antarctica were also joined, and they lay beside the east coast of Africa.
0:15:11 > 0:15:16Forests of southern beech grew in many parts of this land mass.
0:15:16 > 0:15:24But as it split & drifted apart, so the pieces carried the forests & marsupials that lived in them.
0:15:24 > 0:15:28The middle part drifted over the Pole & was covered in snow & ice,
0:15:28 > 0:15:31so forests & inhabitants died out.
0:15:31 > 0:15:34In the eastern fragments, they flourished -
0:15:34 > 0:15:36for that was Australia.
0:16:02 > 0:16:09Here in Australia, these ancient beautiful trees, the southern beech, still grow -
0:16:09 > 0:16:14as they once did in Antarctica & still do in South America.
0:16:14 > 0:16:20Evidence of the one-time unity of those three continents back in geological time.
0:16:20 > 0:16:24With them in these forests grow other ancient plants -
0:16:24 > 0:16:28tree-ferns & cycads. Living in holes in the trunks
0:16:28 > 0:16:34& in leaves on the floor are small, warm-blooded, furry creatures
0:16:34 > 0:16:40that bear their young the same way as American opossums - marsupials.
0:16:40 > 0:16:47The Australian marsupials fared better than their American cousins for S America continued to drift.
0:16:47 > 0:16:55Eventually it came into contact with N America & advanced mammals in that continent invaded south.
0:16:55 > 0:17:02The S American marsupials couldn't face the competition & many became extinct.
0:17:02 > 0:17:09Australia was different - this huge island continent has remained cut off from the rest of the world,
0:17:09 > 0:17:14& here they have remained the dominant mammals.
0:17:14 > 0:17:18They've branched out into many forms & a great number of them
0:17:18 > 0:17:20are active at night.
0:17:26 > 0:17:34Some are similar to S American opossums and, indeed, are known as possums.
0:17:43 > 0:17:51There are mouse-size ones too & like the Americans, the female carries her young clinging to her.
0:18:11 > 0:18:16This is the smallest marsupial of all - it looks like a mouse, but it's very different.
0:18:16 > 0:18:19It doesn't gnaw seed, but hunts insects,
0:18:19 > 0:18:22and will unhesitatingly tackle really big ones.
0:18:41 > 0:18:50There are two-dozen kinds of this size & reproductive techniques are much as others in the group,
0:18:50 > 0:18:55but because they're so small, the process is extremely difficult to observe.
0:18:55 > 0:19:02By giving this expectant mother a nest floored by glass, we can film a birth for the first time.
0:19:02 > 0:19:0730 days after mating, she licks the birth canal & minute, worm-like young,
0:19:07 > 0:19:13smaller than a grain of rice, will emerge immediately after the birth fluids,
0:19:13 > 0:19:16and within three seconds, squirm across to the pouch
0:19:16 > 0:19:20a few millimetres in front. First the birth fluids pour out.
0:19:20 > 0:19:24There's the first one, and there's the second.
0:19:24 > 0:19:27Here's that crucial moment slowed down.
0:19:27 > 0:19:31The opening to the pouch is that dark patch above the middle of the picture.
0:19:31 > 0:19:34The young come from the birth canal.
0:19:34 > 0:19:36There.
0:19:39 > 0:19:41And it's gone.
0:19:44 > 0:19:47The female may produce 6-8 young in a single batch.
0:19:55 > 0:20:03When they arrive, the babies are so small, the mother seems almost unaware of their existence.
0:20:03 > 0:20:07But they grow fast & soon are quite a burden.
0:20:13 > 0:20:19Eventually, the pouch can't hold them & they hang beneath like squirming pink grapes.
0:20:25 > 0:20:28They don't let go of the teats until they're 56 days old,
0:20:28 > 0:20:31and they'll go on suckling milk sporadically
0:20:31 > 0:20:34for many days after that.
0:20:40 > 0:20:46When they're 3-4 months old, they are independent & join the parents
0:20:46 > 0:20:48hunting insects in the night.
0:21:48 > 0:21:53Larger marsupial hunters seek larger prey - this is the quoll,
0:21:53 > 0:21:55as big as a cat.
0:22:05 > 0:22:08It has a sensitive nose and acute ears to help it find food.
0:22:17 > 0:22:19A marsupial mouse.
0:22:35 > 0:22:37Bigger still, the size of a corgi,
0:22:37 > 0:22:40the Tasmanian devil.
0:22:48 > 0:22:51The quoll has found some carrion.
0:22:53 > 0:22:56It doesn't stay long when the devil appears.
0:23:13 > 0:23:15Here's a second.
0:23:24 > 0:23:28Devils devour everything - skin, bones, the lot.
0:23:43 > 0:23:47Not so long ago, there was an even bigger marsupial hunter -
0:23:47 > 0:23:50The thylacine.
0:23:50 > 0:23:57The last recorded living one died in a zoo in 1933 & today, the species may be extinct.
0:23:57 > 0:24:05The resemblance between this & the wolf of the northern hemisphere is remarkably close.
0:24:05 > 0:24:08The processes of evolution even on different stocks,
0:24:08 > 0:24:12produce similar creatures for similar ways of life.
0:24:12 > 0:24:16The thylacine & the wolf are both swift-running flesh-eaters,
0:24:16 > 0:24:19so they look much the same.
0:24:19 > 0:24:23There are other parallels between marsupials & other mammals/
0:24:23 > 0:24:26The numbat has an elongated nose
0:24:26 > 0:24:29& a long tongue, like a pangolin,
0:24:29 > 0:24:34so it's no surprise to find that, like a pangolin, it feeds on ants and termites.
0:24:40 > 0:24:42One of the closest parallels of all
0:24:42 > 0:24:46appears in the eucalyptus trees of Australian forests at night.
0:24:52 > 0:24:57This little marsupial is a sugar-glider, and with good reason.
0:25:12 > 0:25:19In both appearance & acrobatic skill, it's indistinguishable from the flying squirrel of N America.
0:25:19 > 0:25:22Both have a wide flap of skin between their legs
0:25:22 > 0:25:26that catches the air and enables them to guide great distances.
0:26:12 > 0:26:18Evolution in Australia hasn't always produced such parallels to its products elsewhere.
0:26:18 > 0:26:25This, for example, is the koala - it lives in trees, eats nothing but a few special leaves.
0:26:32 > 0:26:39Sloths in S America do the same & are equally fussy about leaves - so are some monkeys in Africa.
0:26:39 > 0:26:43But neither of them looks like the koala,
0:26:43 > 0:26:46which has an Australian charm all of its own.
0:26:58 > 0:27:02The wombat - a cousin of the koalas.
0:27:02 > 0:27:05It lives entirely on the ground.
0:27:11 > 0:27:16It, too, is a vegetarian, but is less selective about what it eats.
0:27:16 > 0:27:20If you had to pick a northern hemisphere version of this creature,
0:27:20 > 0:27:22it might well be the marmot.
0:27:22 > 0:27:25Both graze, and both dig burrows for themselves.
0:27:27 > 0:27:29There are several kinds of wombat,
0:27:29 > 0:27:32this is another - the hairy-nosed.
0:27:35 > 0:27:38It, too, is a burrower, and neither it nor any wombat, come to that,
0:27:38 > 0:27:42is exactly renowned for its darting intelligence
0:27:42 > 0:27:46or speed of reaction under distressing circumstances.
0:27:52 > 0:27:57Bandicoots look like rabbits, but the parallel isn't close.
0:27:58 > 0:28:03Rabbits eat grass, bandicoots eat insects & meat.
0:28:03 > 0:28:09The similarity between the ears & those of long-eared rabbits like the American jack rabbit
0:28:09 > 0:28:16is due to the fact that they both live in deserts & use the ears for cooling their blood.
0:28:17 > 0:28:23This Australian honey possum has no close equivalent at all elsewhere.
0:28:23 > 0:28:28It lives on nectar, which it gathers with a tongue that has a brush at the end.
0:28:28 > 0:28:34If you wanted to find a parallel, the nearest would be the brush tongue of nectar-feeding bats.
0:28:37 > 0:28:43When the super-continents broke up, Australia was largely covered by forests.
0:28:43 > 0:28:45Those that remain
0:28:45 > 0:28:50still contain primitive marsupials - this is a potoroo.
0:28:50 > 0:28:56In it are the beginnings of features that characterise the most famous of all the marsupials -
0:28:56 > 0:28:58the kangaroos.
0:28:59 > 0:29:03For one thing, the potoroo has a tendency to hop.
0:29:06 > 0:29:15For another, the young keep popping to the pouch for a drink after they can fend for themselves.
0:29:15 > 0:29:19They have a preference for travelling that way even when quite large.
0:29:31 > 0:29:38In more open woodlands, there are animals that developed these two tendencies further - wallabies.
0:29:38 > 0:29:43There are two dozen kinds - this is a pademelon.
0:29:52 > 0:29:58This odd wallaby has colonised the tropical island north of Australia - New Guinea.
0:30:01 > 0:30:05Very few mammals live here. There's this, and a kangaroo.
0:30:05 > 0:30:12Unbelievable though it seems for a creature designed for hopping, it's taken to the trees.
0:30:16 > 0:30:21The tree kangaroo seems the clumsiest climber of all tree-living creatures.
0:30:21 > 0:30:24The explanation is that in New Guinea,
0:30:24 > 0:30:30it's the only mammal that gets up in the branches & with nothing competing with it for leaves,
0:30:30 > 0:30:36it's had no need to become better adapted - it gets all it wants just as it is.
0:30:39 > 0:30:47All in all, there are 150 kinds of marsupial in Australia & islands like New Guinea & Tasmania.
0:30:48 > 0:30:51Not long ago, there were more.
0:30:51 > 0:30:56You can see the most spectacular evidence of their existence in Australia's caves.
0:31:23 > 0:31:36In 1969, two zoologists crawled down this narrow cavern in the hills of Naracoorte near Adelaide.
0:31:36 > 0:31:44They were the first people ever to come this way & they hoped they might find a bone or two.
0:31:45 > 0:31:52What they discovered a quarter of a mile farther on, exceeded their wildest imaginations.
0:31:52 > 0:32:00They discovered the greatest, most important deposit of bones ever found in a cave in Australia.
0:32:02 > 0:32:07It takes an hour & a half of crawling to reach this extraordinary gallery.
0:32:39 > 0:32:48Ancient streams washed thousands of bones & left them - so fresh it might have been a few weeks back.
0:32:48 > 0:32:53Nearly all belong to marsupials that have been extinct for thousands of years.
0:32:54 > 0:32:58A skull of a giant kangaroo
0:32:58 > 0:33:02that could browse to a height of 9 feet above the ground.
0:33:02 > 0:33:09That's half as high again as any living kangaroo can do. It had a bulbous face
0:33:09 > 0:33:14with big eyes & powerful high-crowned teeth,
0:33:14 > 0:33:17to masticate tough leaves.
0:33:21 > 0:33:26You could take this for the skull of a small rhino,
0:33:26 > 0:33:33but it belonged to a giant wombat as big as an ox - its teeth say it chewed coarse vegetation.
0:33:38 > 0:33:44The most extraordinary skull in the caves is this -
0:33:44 > 0:33:49it belonged to a creature that was a kind of killer possum,
0:33:49 > 0:33:53popularly known as a marsupial lion.
0:33:53 > 0:33:59In life it was the size of a leopard, with legs like a koala,
0:33:59 > 0:34:07except it had on its thumb a vicious hooked claw with which it ripped apart its prey.
0:34:07 > 0:34:12But the most fantastic thing about it are its teeth.
0:34:14 > 0:34:21In the back of its jaws were teeth elongated to form shearing blades
0:34:21 > 0:34:25for slicing the flesh of its prey.
0:34:25 > 0:34:30Maybe it jumped from trees onto those giant kangaroos. Who knows.
0:34:33 > 0:34:43Despite the formidable armoury of teeth, all the marsupial lions became extinct 20,000 years ago.
0:34:43 > 0:34:47As did the giant kangaroos - why?
0:34:47 > 0:34:56Aboriginal man was in Australia by this time, but there's no evidence these creatures were overhunted.
0:34:56 > 0:35:02No, the reason is there was a change in climate which became extra dry about this time.
0:35:16 > 0:35:23That change in climate can be traced right back to a time 45 million years ago,
0:35:23 > 0:35:27when the continent first split away from Antarctica.
0:35:27 > 0:35:35After separating, Australia didn't stay still, but continued to drift towards the Equator.
0:35:35 > 0:35:44It's still going in that direction today & as fast as ever - about 5 centimetres a year.
0:35:44 > 0:35:56The effect on vegetation has been dramatic - lush forests changed to arid country, like this around me.
0:35:56 > 0:36:02One group of marsupials were quick to respond to the change - some are just over there.
0:36:10 > 0:36:16Out in open country, the small, wallaby-like marsupials, grew bigger.
0:36:16 > 0:36:22They hopped farther & became kangaroos - the marsupial equivalent of deer & antelope.
0:36:26 > 0:36:34With its huge hind legs & muscular counter-balance tail, red kangaroo, biggest of all marsupials,
0:36:34 > 0:36:40can bound 27 feet & leap over things 10 feet high.
0:36:45 > 0:36:54It's very hot, with temperatures as high as 45C, & kangaroos have a way of cooling themselves.
0:36:54 > 0:36:58They plaster their forearms with saliva - as it evaporates,
0:36:58 > 0:37:03it cools the blood running through capillaries just beneath the skin.
0:37:09 > 0:37:13The kangaroos also take advantage of the best shade they can find
0:37:13 > 0:37:15during the hottest part of the day,
0:37:15 > 0:37:19and scrape away the baking-hot surface soil to make a cooler,
0:37:19 > 0:37:22more comfortable bed for themselves.
0:37:37 > 0:37:40Out in the desert, food's always scarce
0:37:40 > 0:37:44& kangaroos will eat even the tiniest morsel of greenery,
0:37:44 > 0:37:49searching through the dry branches with their front legs to find something edible.
0:38:00 > 0:38:05The leaves of these bushes are very rough & tough on the teeth.
0:38:05 > 0:38:11The problem of tooth wear is something facing grazing animals all over the world.
0:38:11 > 0:38:18Antelope & deer solve it by having open roots to their teeth which grow throughout their lives.
0:38:18 > 0:38:21Kangaroos have a different solution.
0:38:21 > 0:38:27They have 4 pairs of molars on either side, but they move forward throughout their life.
0:38:27 > 0:38:35As they're worn down in front so the 4th one comes into play.
0:38:35 > 0:38:40The skull of an older animal - already the 1st molar has gone.
0:38:40 > 0:38:48The 2nd is so worn down that it's useless, & had the animal not died, it would have been shed.
0:38:48 > 0:38:53This process goes on throughout the animal's life.
0:38:53 > 0:39:00By the time it's 20, if it hasn't died for another reason, it'd die from starvation with no teeth.
0:39:09 > 0:39:17The red kangaroo has developed the marsupial reproductive method into a very efficient system indeed.
0:39:25 > 0:39:3333 days after fertilisation of the egg, the young, just an embryo, is expelled from the womb.
0:39:35 > 0:39:42The mother's cleaning up the birth fluids, not licking a path for her young as used to be thought.
0:39:42 > 0:39:50Indeed, she gives the feeble, blind creature no help at all - it must find its way by itself.
0:40:01 > 0:40:11The tiny baby - only one is born at a time - squirms its way to the pouch maybe for 5 minutes.
0:40:12 > 0:40:19Its forelegs are well formed to help it move forward, but its hind legs are no more than buds.
0:40:21 > 0:40:25The rim of the pouch - & safety.
0:40:33 > 0:40:40The mother's teat is considerably bigger than the baby which weighs less than a gram.
0:40:48 > 0:40:55Within a day of the young taking the teat, the mother produces another egg & will mate again.
0:40:55 > 0:41:05That fertilised egg will wait undeveloped until, in 235 days, the first baby leaves the pouch.
0:41:05 > 0:41:08Only then will the development of the next egg proceed.
0:41:17 > 0:41:22This system of production is so efficient that every female
0:41:22 > 0:41:30can reproduce 4 times in 3 years, & kangaroos have come to dominate the Australian countryside.
0:41:33 > 0:41:37Why should kangaroos hop? One suggestion is that
0:41:37 > 0:41:43as babies, they have to develop grasping forelimbs to haul themselves through the fur,
0:41:43 > 0:41:48and this character, being fixed so early, can't then be changed into one suited to running.
0:42:06 > 0:42:09Another explanation may be the position of the pouch.
0:42:09 > 0:42:19If large babies are to be carried at speed, perhaps it's easier with a torso inclined upwards.
0:42:19 > 0:42:25Whatever the reason, the kangaroo has brought the hop to a marvellous peak of power & grace.
0:43:11 > 0:43:14Hopping at full stretch,
0:43:14 > 0:43:17it can reach 50 kms an hour.
0:43:17 > 0:43:22Not as fast as an antelope, but a fair speed nonetheless.
0:43:22 > 0:43:24On the other hand,
0:43:24 > 0:43:32when moving in a more leisured way, the style proves to be an economic one.
0:43:32 > 0:43:37It demands far less energy than an antelope moving four-footedly at the same speed.
0:43:41 > 0:43:47The milk supplied by the female to the young varies as time passes.
0:43:48 > 0:43:54This well-grown youngster is not yet weaned, even though it nibbles grass now & then.
0:43:54 > 0:43:59The milk from the teat it's used throughout its life
0:43:59 > 0:44:04isn't the same as the liquid it drank when it first arrived in the pouch as a tiny worm.
0:44:05 > 0:44:08The ingredients change to meet changing needs.
0:44:13 > 0:44:17It'll continue taking milk after it's left the pouch for good.
0:44:17 > 0:44:21The mother by then will have another baby in her pouch.
0:44:21 > 0:44:27She'll be giving one kind from one nipple & a different mixture from another.
0:44:29 > 0:44:33The rearing of young in a pouch has its hazards,
0:44:33 > 0:44:39particularly that early journey to get there, but in some ways it brings advantages to kangaroos.
0:44:39 > 0:44:46If a female with a large youngster is chased, she'll often jettison her baby & so escape.
0:44:47 > 0:44:50No pregnant antelope has that option.
0:44:50 > 0:44:54A sustained drought, not uncommon, may make it difficult for her
0:44:54 > 0:44:57to produce sufficient milk -
0:44:57 > 0:45:02she may then discard the little babe without much trouble.
0:45:02 > 0:45:03When the drought is over,
0:45:03 > 0:45:12the egg in her womb is ready to start immediate development & the new baby's in her pouch in 33 days.
0:45:26 > 0:45:31It's a commonly held belief, that marsupials are primitive, backward mammals,
0:45:31 > 0:45:37with scarcely any improvement on the early egg-layers, the echidna & the platypus.
0:45:37 > 0:45:40It was the view of Charles Darwin.
0:45:40 > 0:45:47The fact is, today we recognise that many of them are extremely efficient organisms.
0:45:47 > 0:45:54It's true, their basic method of reproduction appeared very early in the development of the mammal.
0:45:54 > 0:45:58But many of them today have brought it to a high pitch of perfection.
0:45:59 > 0:46:03No other creature can compare with female kangaroos,
0:46:03 > 0:46:08which, throughout their maturity, continuously an almost without break
0:46:08 > 0:46:11have 3 young at different stages of development.
0:46:11 > 0:46:14One grazing & suckling,
0:46:14 > 0:46:16one within the pouch,
0:46:16 > 0:46:21one within the body itself, awaiting the best strategic moment
0:46:21 > 0:46:23in which to be born.
0:46:24 > 0:46:32The isolation of the marsupials brought about by drifting continents 45 million years ago
0:46:32 > 0:46:37has given them a long, long time to weave variations
0:46:37 > 0:46:43on the basic model - and some of those variations are very efficient creatures indeed.
0:46:44 > 0:46:51While marsupials developed here, another mammal was coming to the fore in the northern hemisphere.
0:46:51 > 0:46:58Like marsupials, its fossils dated back to dinosaurs - it was related to the American opossum.
0:46:58 > 0:47:05Like it, it was small but differed in one respect & may have looked like this.
0:47:11 > 0:47:15This solenodon, a relative of the shrews, is representative of them,
0:47:15 > 0:47:19and they developed a third technique of reproduction.
0:47:19 > 0:47:26It doesn't lay eggs like the platypus nor give birth to a worm like a kangaroo.
0:47:26 > 0:47:31She retains her young in her body & nourishes it with a placenta -
0:47:31 > 0:47:37a pad, rich in blood vessels, that's implanted on the wall of the womb & linked to the young by a cord.
0:47:38 > 0:47:45It absorbs nutriments from the mother's blood & supplies them to the growing baby.
0:47:45 > 0:47:53This innovation was bequeathed by the early insect-eaters of the north to all their descendants -
0:47:53 > 0:48:01most of the mammals alive today - so none need give birth to babies until they're well-developed.
0:48:08 > 0:48:12Baby rabbits develop within the mother for 28 days -
0:48:12 > 0:48:16twice as long as an opossum, a primitive marsupial of about the same size.
0:48:17 > 0:48:24They don't open their eyes until several days after birth, but a young placental mammal
0:48:24 > 0:48:29can be ready for action on leaving its mother's body.
0:48:31 > 0:48:35A wildebeest can run within minutes of its birth -
0:48:35 > 0:48:37though it's a little groggy at first.
0:48:45 > 0:48:51Sometimes it's important to keep suckling as short as possible.
0:48:51 > 0:48:57Seals are vulnerable on the ice, the sooner pups can get to the safety of the sea, the better.
0:48:57 > 0:49:00So their mothers provide a very rich milk.
0:49:00 > 0:49:04In 3 weeks, they double their weight & can swim away
0:49:04 > 0:49:07and lead independent lives.
0:49:25 > 0:49:29Retaining the baby in a womb till it's fully formed
0:49:29 > 0:49:33seems an obvious way to improve the care of young.
0:49:34 > 0:49:39In fact, it causes problems in body chemistry.
0:49:39 > 0:49:43For one thing, tissues in this pup
0:49:43 > 0:49:50differ from those of its mother - they've elements from the father.
0:49:50 > 0:49:55So that means it risks, in the womb, rejection by the mother's body,
0:49:55 > 0:49:59just as a transplant does.
0:50:01 > 0:50:082nd, the young in the womb may be ejected if the mother produces another egg & comes on heat again.
0:50:08 > 0:50:13That problem doesn't face a baby marsupial - for its short development
0:50:13 > 0:50:23takes place within the mother's sexual cycle - but a placental mammal has a longer development.
0:50:23 > 0:50:30It deals with that problem by producing from within the placenta a substance which suspends
0:50:30 > 0:50:36the mother's egg production - it manufactures other substances.
0:50:37 > 0:50:45These suppress antibodies that cause rejection & so allow the young in the womb to remain there.
0:50:52 > 0:50:58So the placenta has had to become a chemical factory of great complexity.
0:50:58 > 0:51:04When the young is finally born, the placenta, too, is shed from the womb as the afterbirth.
0:51:09 > 0:51:13The body of a mammal, whether it's our own or a seal's,
0:51:13 > 0:51:19is extremely complex & takes time to develop.
0:51:19 > 0:51:26These seal pups were conceived almost a year ago.
0:51:26 > 0:51:29Until a few days ago, when they were born,
0:51:29 > 0:51:32they were kept in the safety of the mother's body
0:51:32 > 0:51:35as she swam through the freezing polar seas.
0:51:37 > 0:51:46No marsupial could be reared in such a way - marsupial babies in a pouch need to breathe air.
0:51:46 > 0:51:53In fact, the placenta & the womb between them provide a degree of safety
0:51:53 > 0:51:58and a continuity of sustenance unparalleled in the animal world.
0:51:59 > 0:52:05Together they form a key to the success of placental mammals,
0:52:05 > 0:52:11which have colonised all the world - including even these bleak,
0:52:11 > 0:52:13inhospitable ice floes.
0:52:40 > 0:52:45Subtitles by Red Bee Media Ltd