0:01:01 > 0:01:05This is one of the coldest places on Earth - the high Arctic.
0:01:05 > 0:01:10Here, the temperature drops to 50 degrees below freezing.
0:01:10 > 0:01:16If I didn't have all this specialist clothing on, the cold would kill me in minutes
0:01:16 > 0:01:21and yet there are animals that live here all the time.
0:01:21 > 0:01:26And one of the most remarkable is hunting just over there.
0:01:28 > 0:01:30An arctic fox.
0:01:30 > 0:01:37The only reason that it and I don't freeze solid is that we're both mammals,
0:01:37 > 0:01:43and have the mammal's ability to use our food to heat our bodies. We're warm-blooded.
0:01:43 > 0:01:47The reason that it is more at home up here than I am
0:01:47 > 0:01:53is it has more of another mammalian characteristic, hair, than I have.
0:01:53 > 0:01:56Its body is insulated with fur.
0:01:56 > 0:02:03Warm-bloodedness is one of the key factors that have enabled mammals to conquer the Earth,
0:02:03 > 0:02:08and to develop the most complex bodies in the whole animal kingdom.
0:02:15 > 0:02:22In this series, we will travel the world to discover just how varied and how astonishing mammals are.
0:02:24 > 0:02:29We go to Africa - where the mammals are at their most spectacular.
0:02:29 > 0:02:32Here the plains are thronged
0:02:32 > 0:02:35with specialist grass eaters.
0:02:35 > 0:02:39And there are other mammals here too, with different tastes.
0:02:45 > 0:02:50Some hunting mammals have become the fastest creatures on Earth...
0:02:52 > 0:02:56..and those they hunt have had to respond or die.
0:03:01 > 0:03:06Some mammals have become fearsomely strong and aggressive.
0:03:11 > 0:03:13They fight for mates.
0:03:20 > 0:03:23They fight for food.
0:03:25 > 0:03:29Some even have to fight for a place to live.
0:03:29 > 0:03:34Wherever you go, you find a bewildering variety of mammals.
0:03:37 > 0:03:40Some are miniatures -
0:03:40 > 0:03:42a few inches long.
0:03:42 > 0:03:44Others are massive.
0:03:50 > 0:03:55And the biggest of those on land are dwarfed by those in the sea.
0:03:55 > 0:03:58I can see its tail -
0:03:58 > 0:04:02just under my boat here. And it's coming up.
0:04:04 > 0:04:07There! The blue whale!
0:04:07 > 0:04:12It's the biggest creature that has EVER existed on the planet.
0:04:19 > 0:04:23Mammals are as at home in the water
0:04:23 > 0:04:25as they are on land.
0:04:26 > 0:04:29Some lounge around on the surface...
0:04:32 > 0:04:36..others prefer to do so on the beach.
0:04:37 > 0:04:41We will go underground to track them,
0:04:41 > 0:04:44and up into the tops of the tallest trees.
0:05:07 > 0:05:12Mammals have even taken to the air and challenged the birds.
0:05:16 > 0:05:23In some places, they congregate in astronomical numbers. They thrive almost everywhere.
0:05:23 > 0:05:29And how they do so depends, as does so much in the life of mammals, on what they eat.
0:05:29 > 0:05:33Between them, they tackle everything that's edible.
0:05:44 > 0:05:47Some are very particular about their food.
0:05:47 > 0:05:52Others will simply take the best of whatever's around at the time.
0:05:53 > 0:05:56On top of the menu, right now, is salmon.
0:06:01 > 0:06:04We will look at the lives of our closest relatives...
0:06:06 > 0:06:08LAUGHS
0:06:12 > 0:06:16..and they will lead us to ourselves...
0:06:17 > 0:06:20..perhaps, the most successful variation
0:06:20 > 0:06:23of the mammal's winning design.
0:06:34 > 0:06:41To catch a glimpse of the very beginnings of the mammalian dynasty, we must travel to Australia.
0:06:51 > 0:06:55I'm looking for one of the most ancient of all mammals.
0:06:55 > 0:07:01It's so ancient, it shares at least one characteristic with reptiles.
0:07:01 > 0:07:06It's a very elusive creature, but here, in South Australia,
0:07:06 > 0:07:14there's a population that have been fitted with radio transmitters, and I can track them with this aerial.
0:07:14 > 0:07:17And I've got a very strong signal.
0:07:27 > 0:07:33At first glance, you might think that this mammal is some sort of hedgehog
0:07:33 > 0:07:36or perhaps a porcupine,
0:07:36 > 0:07:40but actually it's weirdly different
0:07:40 > 0:07:44from a hedgehog, a porcupine or almost any other kind of mammal.
0:07:48 > 0:07:51It's an echidna.
0:07:51 > 0:07:55And you can tell that it's a mammal because it's got hair.
0:07:55 > 0:07:58And only mammals have hair.
0:07:59 > 0:08:04Indeed, some of its hairs have been enlarged and strengthened
0:08:04 > 0:08:07and then turned into big spines,
0:08:07 > 0:08:10which give it such an effective armour.
0:08:12 > 0:08:15This hair helps to keep the echidna warm,
0:08:15 > 0:08:20making sure that it doesn't lose valuable body heat to the cold air.
0:08:33 > 0:08:39The fuel with which the echidna and every other mammal generates that heat is, of course, food.
0:08:39 > 0:08:47On a cold winter's day like this, the echidna spends most of its time searching for its next meal.
0:08:47 > 0:08:52Although echidnas have good eyesight and excellent hearing,
0:08:52 > 0:08:56it's their sense of smell which guides them to food.
0:08:56 > 0:08:58They sniff out insects and grubs,
0:08:58 > 0:09:04and get at them by ripping open the nests and tunnels with their claws.
0:09:07 > 0:09:13That beak-like snout pokes into holes, and then out comes a long sticky tongue
0:09:13 > 0:09:19that flicks into cracks and crevices to lick up whatever's worth eating.
0:09:26 > 0:09:30Echidnas are particularly fond of ants and termites,
0:09:30 > 0:09:33and will even climb trees to find them.
0:09:44 > 0:09:51This particular female has an unusually healthy appetite because she's about to breed.
0:09:51 > 0:09:58And the way she does so is the reason why the echidna is such a truly weird mammal.
0:10:03 > 0:10:08The echidna doesn't give birth to live babies. She lays an egg.
0:10:08 > 0:10:13It's hidden in her fur in a shallow depression on her underside.
0:10:13 > 0:10:15It's no bigger than a marble.
0:10:15 > 0:10:19Inside it, a young echidna is slowly developing.
0:10:23 > 0:10:29After her baby hatches, she carries it around on her underside for about 50 days,
0:10:29 > 0:10:35until it begins to develop spines. She then deposits it in a burrow,
0:10:35 > 0:10:39where it stays and grows for nearly seven months.
0:10:39 > 0:10:43But how does she feed it during this long time?
0:10:43 > 0:10:50To answer that question, we need to find the only other egg-laying mammal in the world today.
0:10:50 > 0:10:52And it too lives here in Australia.
0:11:13 > 0:11:15Just surfacing beside me here
0:11:15 > 0:11:19is one of the most extraordinary animals.
0:11:24 > 0:11:31So bizarre that, when specimens of it were first sent from Australia to Europe,
0:11:31 > 0:11:34people thought it must be a fake.
0:11:35 > 0:11:38But it's not.
0:11:38 > 0:11:40It's real... It's alive...
0:11:40 > 0:11:43It's a platypus.
0:11:47 > 0:11:51That bill looks as though it should belong to a duck,
0:11:51 > 0:11:55but it's not hard like a bird's beak - it's rubbery.
0:12:02 > 0:12:07Like the echidna, the platypus feeds on small invertebrates,
0:12:07 > 0:12:09but it looks for them underwater.
0:12:11 > 0:12:17Once it's collected a mouthful, it takes them up to the surface and grinds them to a pulp.
0:12:17 > 0:12:23It doesn't have teeth, but horny plates inside the bill do the job.
0:12:25 > 0:12:28But how does it find that food?
0:12:28 > 0:12:34Underwater, it closes its white eyelids tight, so it can't see anything.
0:12:37 > 0:12:41But it has a remote-sensing device - its bill.
0:12:41 > 0:12:46As it sweeps it from side to side like a metal detector,
0:12:46 > 0:12:50sensors in it pick up the infinitesimal electric currents
0:12:50 > 0:12:54that are given off by ALL living things.
0:13:00 > 0:13:07There were very few other mammals on Earth 100 million years ago when the first platypus appeared,
0:13:07 > 0:13:11but there was another animal hunting in rivers.
0:13:15 > 0:13:17Birds.
0:13:17 > 0:13:24As the platypus grubs around on the riverbed, it attracts fish, which the cormorant then snaps up.
0:13:29 > 0:13:33Water birds are among the most ancient bird families,
0:13:33 > 0:13:37so this could be a scene just after the death of the dinosaurs,
0:13:37 > 0:13:44when a new kind of animal had appeared on Earth - one with warm blood and fur.
0:14:05 > 0:14:07The platypus has had enough.
0:14:07 > 0:14:11She's heading back home for her breeding burrow.
0:14:11 > 0:14:16For there, at the end of a tunnel that may be 20 yards long,
0:14:16 > 0:14:20safe in a leaf-lined nesting chamber, she's laid an egg.
0:14:23 > 0:14:28Exactly what goes on inside her nest no-one really knew.
0:14:28 > 0:14:34No-one had even succeeded in breeding platypus in captivity until very recently,
0:14:34 > 0:14:41and, certainly, no-one at all had ever seen inside an occupied platypus's nest...until now.
0:14:41 > 0:14:48We have bored, very carefully, a hole into the nest that lies below here and inserted this tube.
0:14:48 > 0:14:55This is an optical probe with a little light on the end, and I can manipulate it like this...
0:14:57 > 0:14:59..so that I can scan it.
0:15:01 > 0:15:07If I then insert that inside this tube, I'll be able to see something
0:15:07 > 0:15:10that no-one has ever seen before.
0:15:13 > 0:15:16Aha. That's her in close-up.
0:15:16 > 0:15:21There's her eye, her ear. It looks as though she's seen us.
0:15:27 > 0:15:31Yeah, she's... She's nibbling it.
0:15:34 > 0:15:37Oh, not worth eating.
0:15:37 > 0:15:41She doesn't seem particularly disturbed by it.
0:15:42 > 0:15:44But has her egg hatched?
0:15:46 > 0:15:51I think that quivering may have something to do with feeding.
0:15:54 > 0:15:58I'll move the camera and see what's going on.
0:15:58 > 0:16:02Yes. And there it is - its milk.
0:16:02 > 0:16:08Milk is the perfect food. It provides the growing youngster with everything it wants,
0:16:08 > 0:16:15and only mammals produce milk. In most mammals, of course, it comes from a nipple,
0:16:15 > 0:16:20but in this very primitive mammal, it simply oozes through the skin.
0:16:26 > 0:16:29She's leaving.
0:16:30 > 0:16:32Off she goes.
0:16:32 > 0:16:34The end of her furry tail.
0:16:37 > 0:16:40But what's that among the leaves?
0:16:41 > 0:16:45And there it is. Yes. That's her baby.
0:16:48 > 0:16:50I'll zoom in on it.
0:16:50 > 0:16:57Now, you can see it. A tiny little grub-like creature. It's naked and blind.
0:16:57 > 0:17:03On its bill is a tiny spike. That's an egg tooth that it used to cut its way out of its shell,
0:17:03 > 0:17:08in just the same way as reptiles and birds do.
0:17:08 > 0:17:11It can only be a few days old.
0:17:13 > 0:17:18The platypus and echidna are the only mammals alive that lay eggs -
0:17:18 > 0:17:24living links with the egg-laying reptiles from which mammals are descended.
0:17:24 > 0:17:29They're both so well-adapted to their ways of life
0:17:29 > 0:17:33that they're still very successful and are widespread in Australia.
0:17:33 > 0:17:38That's an achievement - for they've been around for 100 million years,
0:17:38 > 0:17:41as the fossil evidence makes clear.
0:17:41 > 0:17:45Most of that evidence is just tiny fragments,
0:17:45 > 0:17:50but at Riversleigh, in Northern Australia, it's a different story.
0:17:53 > 0:18:0050 million years ago, Australia was much wetter than it is today, and just here was then a swampy area.
0:18:00 > 0:18:05The bones of animals that died in or around those swamps
0:18:05 > 0:18:12became buried in limey mud at the bottom of the pools and are now preserved in limestone.
0:18:14 > 0:18:21This rock is full of bone. Here's the rectangular boney plate from the back of a crocodile.
0:18:21 > 0:18:24The rest of it looks as though it's bird bone.
0:18:24 > 0:18:30But the limestone in which these bones are embedded is so hard,
0:18:30 > 0:18:37that the only way to get them out is to put the whole block in a bath of acid for a few weeks.
0:18:37 > 0:18:44The limestone then dissolves away, and what is left is sometimes the most extraordinary bones -
0:18:44 > 0:18:46beautifully preserved.
0:18:46 > 0:18:49This is the skull
0:18:49 > 0:18:53of an extinct platypus - about 15 million years old.
0:18:53 > 0:18:58It's been called Obdurodon, which means enduring tooth,
0:18:58 > 0:19:03because unlike today's platypus, which has no teeth,
0:19:03 > 0:19:10this one still has them. They're the empty sockets of the molars. They're two little premolars.
0:19:10 > 0:19:16But what was this place like 15 million years ago, when Obdurodon was alive?
0:19:35 > 0:19:39The night sky would have been full of the calls of animals
0:19:39 > 0:19:43in the surrounding lush tropical forests.
0:19:43 > 0:19:48Obdurodon would have spent much of its time swimming in pools.
0:19:48 > 0:19:53But in the trees there were other mammals of a rather different kind.
0:19:53 > 0:19:55Marsupials.
0:19:55 > 0:20:01There were many different kinds of possums - very similar to those alive today.
0:20:01 > 0:20:09Down on the ground, though, there were less familiar creatures - like this large marsupial leaf eater.
0:20:09 > 0:20:13Nothing like it is alive today.
0:20:17 > 0:20:24There were great numbers of small mouse-sized animals which, judging from their teeth, ate insects.
0:20:24 > 0:20:27Others had a taste for flesh.
0:20:34 > 0:20:36Preying on these small animals -
0:20:36 > 0:20:40- a marsupial lion... - LION GROWLS
0:20:41 > 0:20:46..which was certainly big enough to make a meal of an unwary Obdurodon.
0:20:52 > 0:20:58As the millions of years passed, Australia began to dry out.
0:20:58 > 0:21:03The rainforests retreated and were replaced by grassy plains.
0:21:03 > 0:21:07And as the landscape changed, so did the marsupial mammals.
0:21:07 > 0:21:12They thrived and diversified into many different species,
0:21:12 > 0:21:14and are still abundant today.
0:21:14 > 0:21:19They differ from the platypus and echidna in the way they reproduce.
0:21:19 > 0:21:23Instead of laying eggs, they produce young
0:21:23 > 0:21:28without protective shells. And this grey kangaroo is about to do so.
0:21:31 > 0:21:36Out comes not a shelled egg, but a tiny underdeveloped little worm.
0:21:56 > 0:21:59It weighs less than a lump of sugar,
0:21:59 > 0:22:02it has no back legs, but it has forelegs,
0:22:02 > 0:22:07and they are just strong enough to pull it through its mother's fur.
0:22:07 > 0:22:11It's started on an extraordinary journey.
0:22:11 > 0:22:16To survive, it must get to a pouch higher up on its mother's belly.
0:22:16 > 0:22:21Instinctively, this tiny living particle climbs upwards
0:22:21 > 0:22:26against the pull of gravity and towards the smell of the pouch.
0:22:30 > 0:22:37After about three minutes, it reaches the lip of the pouch and clambers down to safety inside.
0:22:39 > 0:22:44There, it clamps its tiny mouth on its mother's nipple
0:22:44 > 0:22:47and takes its first meal of milk.
0:22:47 > 0:22:53As it grows, the ingredients of the milk coming from the nipple change
0:22:53 > 0:22:59to ensure that the infant gets exactly the nutrients it needs
0:22:59 > 0:23:01for each stage of its development.
0:23:03 > 0:23:10By the time it's nine months old, it's getting a bit cramped - it's time to enter the outside world.
0:23:10 > 0:23:13It's almost like a second birth.
0:23:20 > 0:23:25He's a little unsteady at first, but Mum offers a helping hand.
0:23:33 > 0:23:35Now, he's known as a joey.
0:23:53 > 0:23:58It's all a bit much for one day, and he heads back to mother's pouch.
0:23:58 > 0:24:03It will be another year before he's fully independent.
0:24:12 > 0:24:17Other marsupials have taken to the trees - koalas.
0:24:20 > 0:24:23They too have pouches.
0:24:28 > 0:24:35Indeed, it's the Latin word marsupial, meaning pouch or purse, that gives the whole group its name.
0:24:35 > 0:24:38When a koala joey emerges,
0:24:38 > 0:24:42it clings tight to Mother for several days
0:24:42 > 0:24:45before it risks going solo.
0:24:56 > 0:25:00Koalas feed on gum tree leaves - eucalyptus.
0:25:00 > 0:25:05But they're hardly an ideal food. They're tough, indigestible
0:25:05 > 0:25:08and full of unpleasant chemicals.
0:25:08 > 0:25:14The youngster learns from Mother how to pick the palatable leaves.
0:25:14 > 0:25:19But even these contain little nourishment, so koalas eat a lot
0:25:19 > 0:25:23and spend almost all their waking hours doing so.
0:25:24 > 0:25:29And when they're not feeding, they conserve energy - they go to sleep.
0:25:42 > 0:25:46Only koalas can live on a diet of these particular gum leaves.
0:25:46 > 0:25:51Australia seems to be full of difficult diets in awkward places,
0:25:51 > 0:25:56but there are marsupials that can deal with almost every one of them.
0:25:56 > 0:26:04The vast continent of Australia stretches from the temperate and sometimes chilly south,
0:26:04 > 0:26:06right up into the tropics.
0:26:11 > 0:26:14In the centre there are dry sun-baked deserts,
0:26:14 > 0:26:18where it's only too easy to die from thirst.
0:26:23 > 0:26:28There are mountain ranges, which in winter are crested with snow.
0:26:28 > 0:26:32But the mammalian characteristics of warm blood and insulating fur
0:26:32 > 0:26:36enables marsupials to cope with almost anything.
0:26:36 > 0:26:41The wombat has fur so thick that it can remain active throughout winter,
0:26:41 > 0:26:45even in the coldest parts of Australia.
0:26:46 > 0:26:48It feeds on grass and other plants,
0:26:48 > 0:26:54and the strong front limbs, with which it digs itself burrows,
0:26:54 > 0:26:58are equally good at clearing away snow to find food.
0:27:06 > 0:27:13Its pouch opens backwards, so that the youngster doesn't get a face full of snow, as Mum digs.
0:27:19 > 0:27:22Numbats live in woodland.
0:27:22 > 0:27:25But, even there, it can get quite cold at nights,
0:27:25 > 0:27:30and this family are warming themselves in the early morning sun.
0:27:35 > 0:27:38Fur needs to be kept in prime condition,
0:27:38 > 0:27:41if it's to function as an insulator,
0:27:41 > 0:27:44so grooming is essential.
0:27:48 > 0:27:55These dry eucalyptus forests may look unpromising as a source of food,
0:27:55 > 0:27:59but there are plenty of termites.
0:27:59 > 0:28:03Numbats have just the right equipment to collect them.
0:28:11 > 0:28:17That spectacular tongue has to be kept well-anointed with sticky saliva,
0:28:17 > 0:28:23and numbats spend some time making quite sure that it is.
0:28:30 > 0:28:35With gear like that, a numbat can collect 20,000 termites in a day.
0:28:38 > 0:28:45This creature's ancestors might also have used their tongues to collect insects,
0:28:45 > 0:28:49but the mammal tongue is a highly-adaptable instrument.
0:28:49 > 0:28:54and now, the honey possum uses it to gather pollen and nectar.
0:28:54 > 0:28:58It's one of the most specialised feeders of all mammals.
0:28:58 > 0:29:01Its tongue has a brush on its tip,
0:29:01 > 0:29:05which soaks up nectar from even the deepest flowers.
0:29:09 > 0:29:16These boulders are home to a less-fussy marsupial, which will collect whatever food is around.
0:29:16 > 0:29:20At the moment, there's an unusual delicacy -
0:29:20 > 0:29:23these moths sheltering from the summer sun.
0:29:23 > 0:29:28The mountain pygmy possum might be small, but it has a huge appetite.
0:29:31 > 0:29:35Moths provide a fast-food snack, high in energy-rich fat,
0:29:35 > 0:29:42and the pygmy possum will eat as much as it can, and put on fat to see it through leaner times.
0:29:46 > 0:29:50Only the indigestible wings are discarded.
0:29:50 > 0:29:57At other times, the pygmy possum lives on berries and seeds -
0:29:57 > 0:30:00picking them off with its nimble fingers.
0:30:05 > 0:30:09The striped possum has a particular taste for grubs.
0:30:15 > 0:30:19It lives in the few fragments of rainforest
0:30:19 > 0:30:23that survive in North-Eastern Australia.
0:30:30 > 0:30:33It's got all that's necessary to collect them -
0:30:33 > 0:30:40an excellent sense of smell, strong teeth to chew away the bark and a long sticky tongue.
0:30:50 > 0:30:54But perhaps the most challenging of all Australian environments
0:30:54 > 0:30:58is the arid hot desert at the continent's heart.
0:31:01 > 0:31:06There is little to eat, little to drink and few places to hide...
0:31:07 > 0:31:10..but marsupials have colonised this country, too.
0:31:19 > 0:31:24Everybody would recognise those as kangaroos...
0:31:25 > 0:31:31..but the kangaroos belong to a very big family - there are kangaroos, wallaroos and wallabies,
0:31:31 > 0:31:35big ones and small ones. These are red kangaroos -
0:31:35 > 0:31:42the biggest of the family. They're particularly at home in this dry country.
0:31:50 > 0:31:54This can be one of the hottest places on earth,
0:31:54 > 0:31:58so red kangaroos don't have to worry about keeping warm.
0:31:58 > 0:32:02Their problem is overheating. All mammals can sweat to lose heat,
0:32:02 > 0:32:05but water is in short supply here,
0:32:05 > 0:32:09and red kangaroos only do so when they are on the move.
0:32:09 > 0:32:16Instead, during the hottest part of the day, they make use of whatever shade they can find.
0:32:16 > 0:32:18Wiping saliva on their forearms
0:32:18 > 0:32:21helps to lose unwanted heat.
0:32:21 > 0:32:28Blood vessels are close to the surface of the skin - and as the saliva evaporates, the blood cools.
0:32:41 > 0:32:46They only feed in the morning and evening, when it's cooler.
0:32:46 > 0:32:53When they do, it's hard not to notice the extraordinary way by which they get about.
0:32:53 > 0:32:57The tail acts rather like a fifth leg,
0:32:57 > 0:33:02propping up the kangaroo as it swings forwards its huge hind limbs.
0:33:08 > 0:33:14It looks ungainly when they're moving slowly, but when a kangaroo senses danger,
0:33:14 > 0:33:19the advantage of these unusual proportions becomes very obvious.
0:33:28 > 0:33:30Hopping at full speed
0:33:30 > 0:33:33a kangaroo can outpace a racehorse.
0:33:33 > 0:33:39They're the only large mammals in the world that have developed this way of getting about,
0:33:39 > 0:33:43but it's a very efficient way of doing so.
0:33:45 > 0:33:51Tendons in the back legs act like giant springs - storing energy as the kangaroo lands
0:33:51 > 0:33:55and then releasing it to propel the animal forward.
0:33:55 > 0:33:58By recycling energy like this,
0:33:58 > 0:34:05kangaroos can quickly cover vast distances to escape predators or to search for food and water.
0:34:05 > 0:34:11It's not just out on the flat that hopping works well - some marsupials hop around on cliffs.
0:34:17 > 0:34:21The rock wallaby's key to success lies in its feet.
0:34:27 > 0:34:34The soles have thick corrugated skin - pads which give them a grip on every kind of surface.
0:34:34 > 0:34:39The wallaby can bounce about this difficult terrain with confidence.
0:34:49 > 0:34:52There's little to drink here,
0:34:52 > 0:34:59and though adults get the fluid they need from their diet, growing youngsters may find that difficult.
0:35:01 > 0:35:08This youngster is after an extra drink. Rock wallabies are able to bring up fluid from the stomach
0:35:08 > 0:35:15to ensure that their young don't go thirsty. It's a special adaptation to this arid environment.
0:35:36 > 0:35:43Grey kangaroos live out on the relatively well-watered grassy plains.
0:35:43 > 0:35:48They are among the most sociable of all Australian marsupials,
0:35:48 > 0:35:53but living in groups can lead to problems in getting on together.
0:36:00 > 0:36:04Last season's joeys are fast approaching independence,
0:36:04 > 0:36:09which means that their mothers will soon be ready to mate again.
0:36:21 > 0:36:27Males use their sense of smell to find out if a female is sexually available,
0:36:27 > 0:36:29and will court her for several days.
0:36:36 > 0:36:40Having found one who seems to be promising,
0:36:40 > 0:36:46a male stays close to her side to try and ensure that HE, and no other male, mates with her.
0:36:51 > 0:36:53The most dominant male
0:36:53 > 0:36:59is likely to be the one to father most of the next generation,
0:36:59 > 0:37:02and that is worth fighting for.
0:38:13 > 0:38:18Joeys also fight, but it's just play boxing -
0:38:18 > 0:38:22a way of learning skills that will be important in later life.
0:38:22 > 0:38:25But it's not always a fair fight.
0:38:28 > 0:38:34Fortunately, this little one still has Mother to see off the neighbourhood bully.
0:38:43 > 0:38:47Marsupials first appeared about 100 million years ago,
0:38:47 > 0:38:51towards the end of the age of the dinosaurs.
0:38:51 > 0:38:55Then, Australia was part of a great supercontinent,
0:38:55 > 0:39:02but as the millions of years rolled by, that continent began to split apart. One fragment drifted south -
0:39:02 > 0:39:09Antarctica. As it got closer to the South Pole, so it got colder, became covered with snow and ice
0:39:09 > 0:39:12and its animal inhabitants died out.
0:39:12 > 0:39:14A second part was Australia.
0:39:14 > 0:39:17It drifted north and got warmer.
0:39:17 > 0:39:20And here marsupials flourished.
0:39:20 > 0:39:24But there was a third part. It too drifted north.
0:39:24 > 0:39:28It too had a population of marsupials. And they're still there.
0:39:28 > 0:39:31That was South America.
0:39:35 > 0:39:39It may well have been in this region of the supercontinent
0:39:39 > 0:39:46that the marsupial mammals first appeared. Many died out, but there are STILL a lot of survivors.
0:39:46 > 0:39:49This is one of the most elusive of them.
0:39:49 > 0:39:53It lives in the streams of the Amazon forest
0:39:53 > 0:39:56and operates only at night -
0:39:56 > 0:40:02getting around in the blackness by feeling its way with its front paws and luxuriant whiskers.
0:40:02 > 0:40:05It's the yapok or water opossum.
0:40:09 > 0:40:11These pictures,
0:40:11 > 0:40:19taken with infra-red cameras, may well be the first time it's been filmed in its natural environment.
0:40:19 > 0:40:23It's hunting for fish and crustaceans.
0:40:23 > 0:40:28Its fur is so thick that its skin doesn't get wet.
0:40:28 > 0:40:32It has webbed feet to propel it through the water.
0:40:39 > 0:40:42It's too dark for even the sharpest eyes
0:40:42 > 0:40:44to see very much.
0:40:44 > 0:40:51The yapok relies on its acute sense of smell and hearing to locate its food.
0:40:51 > 0:40:58It swims with its arms apart, groping for its prey with its highly-sensitive fingers.
0:41:08 > 0:41:10It usually takes its catch
0:41:10 > 0:41:15to the shelter of nearby vegetation to devour it.
0:41:17 > 0:41:20But it doesn't only feed in the shallows.
0:41:20 > 0:41:23The yapok has a large territory,
0:41:23 > 0:41:27and there are many deeper pools in which to swim.
0:41:36 > 0:41:41Underwater, it swims with its eyes shut, like the platypus,
0:41:41 > 0:41:43and hunts entirely by feel.
0:41:46 > 0:41:52The female yapok can also shut her pouch, and does so with such muscular strength
0:41:52 > 0:41:59that water doesn't get in and drown her babies, though, no doubt, they must be close to suffocation
0:41:59 > 0:42:01after a few minutes of fishing.
0:42:04 > 0:42:07It's been a good night's hunting,
0:42:07 > 0:42:11and the yapok retreats to its burrow as day breaks.
0:42:21 > 0:42:25The yapok is the only aquatic marsupial in the world.
0:42:25 > 0:42:32Most of the marsupials in Central and South America live high in the canopy of the rainforest.
0:42:32 > 0:42:37Just how many there are up there no-one really suspected,
0:42:37 > 0:42:41until scientists started using cranes, like this one.
0:42:44 > 0:42:49Apparatus like this gives such easy access to this high canopy
0:42:49 > 0:42:53that it's now possible to get an accurate idea
0:42:53 > 0:42:57of just how rich wildlife is up here.
0:42:57 > 0:43:03While we might think of Australia as the land of the marsupials, in places, this rainforest
0:43:03 > 0:43:07may have more of them than any other kind of mammal.
0:43:10 > 0:43:13Most of them are strictly nocturnal.
0:43:13 > 0:43:19And though they are abundant, they, like everything else in these forests, can be difficult to spot.
0:43:19 > 0:43:23Many are similar to this woolly opossum -
0:43:23 > 0:43:29tree dwellers with few specialisations and a broad diet,
0:43:29 > 0:43:32which can include flowers, fruit and insects.
0:43:41 > 0:43:43These marsupial mammals, of course,
0:43:43 > 0:43:48reproduce in just the same way as their Australian relatives.
0:43:48 > 0:43:54They give birth to babies at a very early stage in their development.
0:43:54 > 0:43:57Their pouch isn't as well formed
0:43:57 > 0:44:00as that of a kangaroo or a koala,
0:44:00 > 0:44:04but their young survive, clinging to their mother's underside.
0:44:13 > 0:44:17Marsupial mammals dominate Australia,
0:44:17 > 0:44:21and flourish in the forests of Central and South America,
0:44:21 > 0:44:28but, alongside them, are living a different kind of mammal - a kind to which we ourselves belong.
0:44:28 > 0:44:34And it's only that kind that you find everywhere else in the world.
0:44:34 > 0:44:38The plains of Africa, for example, have an abundance of mammals,
0:44:38 > 0:44:41but not one of them is a marsupial.
0:44:41 > 0:44:45They all reproduce in a fundamentally different way.
0:44:45 > 0:44:49This wildebeest has nourished her baby within her
0:44:49 > 0:44:54by means of a remarkable organ on the wall of her womb - a placenta.
0:44:54 > 0:45:00It's a circular pad, rich in blood vessels, that is connected to her baby by the umbilical chord,
0:45:00 > 0:45:04through which she has fed her growing youngster.
0:45:04 > 0:45:09Blood vessels from the baby run up through the chords of the placenta,
0:45:09 > 0:45:13and pass so close to those of its mother,
0:45:13 > 0:45:20that they're able to absorb nutrient from her blood and carry it back to the unborn infant.
0:45:20 > 0:45:22But all this is about to change.
0:45:30 > 0:45:36Giving birth to such a large highly-developed baby places great strains on the mother.
0:45:39 > 0:45:43It's pretty traumatic for the baby, too.
0:45:46 > 0:45:50There's a great advantage in being born this way.
0:45:50 > 0:45:55There are plenty of animals around for whom a new-born calf
0:45:55 > 0:45:58would make a welcome meal.
0:45:58 > 0:46:04But this mammal baby, reared with the help of a placenta, is able to get to its feet
0:46:04 > 0:46:07within minutes of its birth.
0:46:16 > 0:46:21And while it's finding its balance, its mother is there to defend it.
0:46:35 > 0:46:38Now, the baby can be fed,
0:46:38 > 0:46:43in the same way as all mammal babies, with its mother's milk.
0:46:56 > 0:47:02Placental babies may still have months, even years to go,
0:47:02 > 0:47:05before they are fully independent.
0:47:05 > 0:47:09Those early months, when they were protected in their mother's body,
0:47:09 > 0:47:13have given these babies an invaluable start in life.
0:47:17 > 0:47:22So whether mammals lay eggs or give birth to live young,
0:47:22 > 0:47:26whether their babies develop in a womb or in a pouch,
0:47:26 > 0:47:30they've managed to live almost everywhere.
0:47:57 > 0:48:04The warm-blooded, furry, milk-producing, mammalian body, in all its multitudinous variations,
0:48:04 > 0:48:07really is a winning design.
0:48:27 > 0:48:30The duck-billed platypus seems to me
0:48:30 > 0:48:35just about the most extraordinary animal alive in the world today.
0:48:35 > 0:48:40I first tried to film it some 25 years ago for Life On Earth.
0:48:40 > 0:48:45We offered a generous grant to any scientist who could work out
0:48:45 > 0:48:49how we could peek inside the breeding burrow of a platypus.
0:48:49 > 0:48:54There were no takers. Everyone said it was quite impossible.
0:48:54 > 0:48:59This time, with new technology, we've managed to do just that.
0:49:00 > 0:49:05Europeans first encountered the extraordinary platypus in 1798.
0:49:05 > 0:49:11200 years later, we barely understand even the simplest aspects of its life.
0:49:11 > 0:49:18Piecing the evidence together has proved a fascinating detective story.
0:49:18 > 0:49:23Helping us unravel the mystery is platypus scientist Tanya Rankin.
0:49:23 > 0:49:29When the first platypus skin was sent to England, scientists thought it was a hoax.
0:49:29 > 0:49:35And they poked and prodded and jabbed at this thing
0:49:35 > 0:49:40thinking that it was a bill attached to a skin, but it was a real animal.
0:49:40 > 0:49:47Some scientists took it personally that there was this mammal that did not fit their classification.
0:49:47 > 0:49:53They had this rigorous idea of what a mammal, a reptile and a bird was,
0:49:53 > 0:49:56and the platypus was a bit of each.
0:49:56 > 0:50:00It took at least 100 years before it was confirmed that they laid eggs.
0:50:04 > 0:50:11Egg-layers like the platypus or echidna, and the possums, both have a quite extraordinary birth process.
0:50:11 > 0:50:1925 years ago, when filming Life On Earth, we may have failed to film the platypus birth,
0:50:19 > 0:50:25but we did make progress. For the first time ever, we filmed this -
0:50:25 > 0:50:30new-born opossums moving from the birth canal to the mother's pouch.
0:50:30 > 0:50:37But the birth itself happens so quickly, and the babies are so small, we thought we'd missed it.
0:50:37 > 0:50:44Only when we looked at the film frame by frame did we see the moment of birth in this Australian possum.
0:50:45 > 0:50:52But this is a scientific image, not a natural one. That was the challenge this time around.
0:50:53 > 0:51:01We've been getting pictures of what goes on in a breeding burrow or nest hole for some time.
0:51:01 > 0:51:08The standard way is to set up a breeding colony, and then provide them with an artificial nest hole
0:51:08 > 0:51:13in which you have preformed holes in which you can put your camera.
0:51:13 > 0:51:20But that wouldn't work with duck-billed platypus. Platypus had never been bred in captivity.
0:51:20 > 0:51:27It would have to be in the wild. One of the first people to do such a thing was Simon King,
0:51:27 > 0:51:34who worked with another creature that lives beside rivers and burrows holes in the bank.
0:51:34 > 0:51:37Not a mammal, but a bird - the kingfisher.
0:51:37 > 0:51:43How do the two compare? Kingfishers are brightly-coloured spectacular birds -
0:51:43 > 0:51:48not common, but very conspicuous. Not so, the platypus.
0:51:48 > 0:51:55Platypuses are really difficult to see in the wild. They're brown, they come out at dusk -
0:51:55 > 0:52:01very low profile in the water. So you could be walking past one and you wouldn't even know it.
0:52:01 > 0:52:06They live in similar places. This a typical platypus river,
0:52:06 > 0:52:11while Simon found his kingfishers in a Somerset peat cutting.
0:52:11 > 0:52:18When an adult bird flies into a hole carrying a fish, you know she's got young inside,
0:52:18 > 0:52:24but that's not possible with a platypus. Because a female feeds her young with milk,
0:52:24 > 0:52:32you can't tell whether one is a mother or not. Tanya needed technology to locate a nest burrow.
0:52:32 > 0:52:36I work with radio tracking - looking at their movements
0:52:36 > 0:52:41and what sort of habitat use they have of the river.
0:52:41 > 0:52:47I track them to their burrows during the day and find out where the nesting chambers are.
0:52:47 > 0:52:50Once Tanya had located a burrow,
0:52:50 > 0:52:54Mark Lamble handled the camera work.
0:52:55 > 0:52:59Birds are a joy. Once they have chicks, their bond is very strong
0:52:59 > 0:53:01and they'll return to the nest.
0:53:01 > 0:53:08With patience and care, you can use large-scale methods to look inside that nest.
0:53:12 > 0:53:16The platypus would be far more sensitive to disturbance.
0:53:16 > 0:53:21This meant that it was one turn at a time for Mark.
0:53:23 > 0:53:27It took 10 hours in the sun before they could insert the probe.
0:53:27 > 0:53:31The first burrow wasn't used as a nest.
0:53:32 > 0:53:37It's painstaking work. The final stages are similar for both teams.
0:53:37 > 0:53:42Finally, Simon filmed the behaviour he was looking for.
0:53:43 > 0:53:46And after three nests,
0:53:46 > 0:53:49I saw inside a burrow for the first time
0:53:49 > 0:53:52thanks to Mark and Tanya.
0:53:54 > 0:53:57It was an incredible experience,
0:53:57 > 0:54:05because it was something that had never been done before - to actually look inside a living, active burrow.
0:54:05 > 0:54:08It's so hard to describe. It was just incredible.
0:54:10 > 0:54:15And, finally, I saw the image I had waited 25 years to see.
0:54:15 > 0:54:17Ah!
0:54:18 > 0:54:23That little baby platypus, that we caught on camera,
0:54:23 > 0:54:28could not have been more than 3cm long - shorter than my thumb.
0:54:28 > 0:54:31The colour of it just amazed me.
0:54:31 > 0:54:37It never occurred to me that you'd get a magenta platypus. It was astounding.
0:54:37 > 0:54:41I learnt a lot about the workings inside a burrow.
0:54:41 > 0:54:48Even the structure of the nest, with the way the leaves lie, what they're made of,
0:54:48 > 0:54:52gave me a better understanding of the ingenuity of these animals.
0:54:52 > 0:54:59Although we've discovered a great deal about the platypus in recent years,
0:54:59 > 0:55:06we still don't fully understand the function of that extraordinary feature, its bill.
0:55:06 > 0:55:13It's rubbery, covered in skin with a good blood supply and a lavish network of nerves.
0:55:13 > 0:55:20The platypus brain has a larger area receiving nerves from the bill than either its eyes or ears.
0:55:20 > 0:55:22So what is the bill detecting?
0:55:22 > 0:55:29An early naturalist, Harry Burrell, always thought that platypuses had to use some sort of a sixth sense,
0:55:29 > 0:55:33because they close their eyes, ears and nostrils underwater,
0:55:33 > 0:55:37yet they can capture tiny prey, such as insects and shrimp.
0:55:37 > 0:55:42It wasn't until 1986 that scientists ran experiments with platypuses
0:55:42 > 0:55:48just using a nine-volt battery, to see if platypuses could sense that.
0:55:48 > 0:55:54They were astonished at the results. This sensory system has not been found in any other mammal.
0:55:54 > 0:55:57It's called electroreception -
0:55:57 > 0:56:02and the detectors are tiny pits on the bill.
0:56:02 > 0:56:06Magnified 1,000 times, this is what they look like.
0:56:06 > 0:56:08They're incredibly sensitive -
0:56:08 > 0:56:14detecting electrical currents that are given off by muscle activity
0:56:14 > 0:56:17and that carry very well in water.
0:56:23 > 0:56:28The tail flick of a shrimp can be picked up by the platypus.
0:56:28 > 0:56:34They use electroreception for hunting prey, but also for navigation underwater.
0:56:38 > 0:56:42They do this triangulation thing with the electrical sense
0:56:42 > 0:56:49and then this delayed physical movement of the water, and they pick that up with these bills
0:56:49 > 0:56:53and they manage to collect up enormous amounts of food.
0:56:53 > 0:57:00They can eat up to 25-30% of their body weight every night. It's tiny insects that they are picking up.
0:57:00 > 0:57:05The platypus can read a riverbed in a very different way to us.
0:57:05 > 0:57:10Even trying to visualise how the system works is a challenge.
0:57:10 > 0:57:16It shows that, far from a joke, the platypus is a unique animal
0:57:16 > 0:57:21that's developed some very special and very successful features
0:57:21 > 0:57:24at the dawn of the life of mammals.
0:57:27 > 0:57:32In the next programme of The Life Of Mammals, we meet insect hunters.
0:57:32 > 0:57:35These mammals race to conquer the planet,
0:57:35 > 0:57:43and they now include the most bizarre mammals ever to walk the Earth or to take to the sky.