- 7 weeks ago
see more on:
https://realtv.mytvchannel.org/index.html
https://realtv.mytvchannel.org/index.html
Category
📚
LearningTranscript
00:00.
00:30when specimens of this creature first reached Europe from Australia at the very
00:48end of the 18th century people refused to believe their eyes they said it was a
00:55hoax bits and pieces of different creatures rather crudely sewn together but it's no hoax
01:02it's a platypus and yet in a way those early skeptics were right the platypus is the most
01:09extraordinary mixture of different animals it's part mammal and part reptile and so it can give
01:15us some idea of how the first mammals developed
01:25at first sight the platypus looks like a regular mammal it has after all dense soft fur which is
01:41one of the hallmarks of mammals and when you handle it you can feel that its body is warm
01:46its feet are webbed for better swimming rather like an otter's
02:10the strange bill is not hard like a bird's beak but soft rubbery and very sensitive and the
02:27platypus which has poor eyesight uses it to find its food crayfish and other water living creatures
02:33but when it comes to breed it does something that separates it from all other mammals except one
02:59in its nest deep in a burrow it lays eggs it's this that links the platypus with the reptiles
03:07this that entitles it to be regarded as the most primitive living mammal but and this is what makes
03:15it doubly paradoxical when the egg hatches the young platypus is not left to find food by itself
03:22like reptile babies are but is provided with food by the mother the platypus like all mammals has in
03:30its skin as part of the mechanism for dealing with overheating sweat glands and on the underside of the
03:35body these sweat glands are especially big and produce a kind of fatty sweat which is milk it simply
03:44oozes from the skin and the young suck it from tufts of hair there is no nipple so it hardly qualifies as a
03:52breast or a mamma to give it its technical name which is the feature that gives the mammals their name
04:01only one other mammal lacks a true breast and this is it the echidna it too like the platypus lives in
04:09australia and it too lays eggs but the female doesn't deposit them in a nest she carries them
04:15around with her they have sticky shells and become glued to the hair on her underside in a temporary
04:21groove that develops across her stomach each is no bigger than a pea and after 10 days it hatches
04:39by now glands on either side of the groove in the mother's stomach are producing rich creamy milk
04:53the baby echidnas remain inside the groove for the next eight weeks steadily taking in milk and growing
05:09the baby lion when the spines develop the youngsters must become pretty uncomfortable passengers and
05:15their mother deposits them in a den eventually they abandon milk and take to their adult diet ants
05:26the long snout like the platypus's bill is in evolutionary terms a recent acquisition
05:32a specialized tool for food gathering it houses a long sticky tongue with which the echidna
05:38flicks up its ants and termites. The animal defends itself by the simple
05:43expedient of digging vertically downwards so that within a minute or so
05:46there's nothing to be seen but spines. The platypus and the echidna are
05:54isolated oddities. We've virtually no fossil evidence to tell us where or when
06:00they developed but it's a reasonable guess because there's another kind of
06:04echidna which lives not far north of here in New Guinea that the group did
06:09originate in this part of the world. It's absolutely certain however that they do
06:13represent a group of primitive creatures from which more modern mammals
06:18developed about 180 million years ago. And of course we can trace back the
06:25ancestry of mammals even farther than that.
06:29The reptiles had first developed about 300 million years ago. They had watertight
06:37skins and shelled eggs and so they could survive in the driest country. After
06:43some 20 million years a group of swift moving hunting reptiles evolved called
06:48pellicosaurs. Reptiles can't generate heat within their bodies and that means that
06:55after a cold night they are very sluggish and slow to get going in the
07:00morning. Pellicosaurs dealt with that difficulty by developing great sail-like
07:06fins along their backs with which to catch the first rays of the morning sun so
07:11that they could get out hunting really early. In places like this it's easy to
07:16imagine some 12-foot species of pellicosaur like Dimetrodon lying basking on the
07:21rocks in the early morning sun. And it's been calculated that with the aid of their
07:27fins they could raise their body temperature some six degrees inside an hour
07:32whereas without them it would take nearly three hours. But those fins were stopgap
07:39devices only. Later species of pellicosaurs managed to do without them and that's
07:44probably because they were able to generate heat internally and their teeth
07:49provide evidence which supports that idea.
07:56Dimetrodon's teeth like those of most reptiles were simple spikes which did
08:02little more than grip a victim. But generating heat within the body and
08:06requires a great deal of energy and so a warm-blooded animal must eat much more
08:11food than a normal reptile and digest it fairly rapidly. Changes in the teeth of
08:17successive generations of pellicosaurs suggest that that is just what they
08:21did. The simple spikes changed into specialized tools for butchery. Daggers
08:27appeared on either side of the upper jaw for slitting open the hide of the prey,
08:31knives for slicing the meat and grinders for crunching bones. Most reptiles shed
08:38their teeth as they become warm sporadically throughout their lives but the
08:42teeth of these creatures not only became specialized but permanent and the upper
08:47lower ones meshed accurately to give a highly efficient bite. And since these
08:52creatures generated their heat internally they would also have needed a
08:56coat of hair in order to conserve it. The mammals had arrived. The acquisition of
09:02warm blood brought more advantages to these little creatures than
09:06straightforward speed of movement. As you would have seen had you been able to
09:10walk through the forests of 180 million years ago at night.
09:14The first two mammals appeared at a time when the reptiles ruled the world. But those great solar-powered animals had one major disability. At night when it got cool they became sluggish.
09:17And that left the field open to any creature that could be active at night. And the mammals, with their warm blood, could do well.
09:24Just that.
09:25The ancient mammals were small, nocturnal insects.
09:26The ancient mammals were small, nocturnal insects.
09:28They were small, but they were small, and they were small, and they were small.
09:29The most true mammals appeared at a time when the reptiles ruled the world. But those great solar-powered animals had one major disability. At night when it got cool, they became sluggish.
09:39And that left the field open to any creature that could be active at night. And the mammals, with their warm blood, could do just that.
09:51The ancient mammals were small, nocturnal insect hunters that relied chiefly on their sense of smell to find their food.
09:58In fact, they were probably very similar to present-day shrews and hedgehogs, although they may have laid eggs.
10:04Warmth was the key to their survival and ultimate success. Since they alone could hunt during the cool of the night, they didn't have to face competition with the reptiles.
10:15And so those primitive mammals were able to live right through the age of the dinosaurs and be ready, poised to inherit the world when the reptiles finally declined.
10:26The problem of keeping warm, of course, was one that didn't just face adults. It also faced eggs and embryos.
10:33And the mammals developed three ways of dealing with that.
10:38The primitive ones incubated their eggs as the platypus does today.
10:43Other mammals developed more efficient methods.
10:48The opossum that lives in North and South America is one.
10:52It doesn't lay eggs, but gives birth to its young only twelve and a half days after mating.
11:09There may be as many as twenty of them. They're only the size of bees and almost their only well-formed features are their front legs.
11:16With these tiny limbs, they haul themselves through the hair of their mother's belly on the first and the most hazardous journey of their lives.
11:24At last, they reach a pouch in their mother's belly. Inside are thirteen nipples. Each of the tiny infants fastens onto one and begins to take milk.
11:36If more than thirteen babies were born, the last to reach the pouch will find no vacant nipple and die.
11:43The Latin for pouch is marsupium, and this gives the name to the whole group of mammals that reproduce in this way. They're marsupials.
11:52This is the woolly opossum, and its babies are sufficiently well grown to have left the pouch, but they still cling to their mother and return to the pouch for drinks.
12:01There are about seventy different kinds of opossum in the New World. Most of them live in South America. Some are as small as mice, others as big as domestic cats.
12:19There's even an amphibious one, the yapok, which has webbed hind feet and eats fish.
12:26When the yapok mother goes for a swim, she can close the opening to her pouch to prevent her babies from drowning.
12:34But they still need to breathe, so she can only swim for a few minutes at a time when she has young.
12:41The mouse opossums are very like the earliest marsupials of all, and fossils of similar creatures have been found in rocks that also contain the bones of dinosaurs.
12:56They live now, as they must have done then, by feeding at night on worms, insects, and small reptiles like lizards.
13:06A slightly larger one lives in the cool, dank scrub of the high Andes, the rat opossum.
13:16It's a ferocious hunter with forward-pointing fangs in its lower jaw, with which it stabs its prey.
13:23It, too, has a very ancient ancestry. It doesn't even have a pouch, but its young hang to their mother's teats like a cluster of berries.
13:31Fossils closely resembling those primitive marsupials have been found here in America, dating back some 60 million years.
13:46That makes them by far the oldest marsupial fossils known, much older, in fact, than any that have been found elsewhere in the world.
13:54So it seems reasonable to assume that the marsupials originated here in America.
14:00But if that's so, how on earth did they get to Australia, where they flourish in the greatest numbers today?
14:06Well, this tree may provide evidence for the answer.
14:10It's growing here in the bleak lands of Patagonia, on the southern tip of South America.
14:16And it's a kind of beech related to the European beech, and called, in fact, the Southern beech.
14:23It's a tree with a very long ancestry.
14:26It was growing here when the marsupials first appeared.
14:31And it seems likely, in fact, that they lived in forests not unlike this.
14:36It's only relatively recently that scientists have demonstrated beyond all doubt that the continents are not static,
14:46but have been drifting slowly over the surface of the globe for many millions of years.
14:51To go back to the period when the first marsupials appeared in South America
14:55is to return to a time when that continent was not connected in any ways to North America,
15:00but fitted alongside the west coast of Africa.
15:03Australia and Antarctica were also joined,
15:08and they lay beside the east coast of Africa.
15:11The forest of Southern beech grew over many parts of this great land mass.
15:16But as it split and drifted apart,
15:18so the separate pieces carried with them the beech forests and the marsupials that lived in them.
15:24The middle section drifted over the pole and became covered in snow and ice,
15:28so that the forests and their inhabitants died out.
15:31In the eastern fragment, however, they flourished, for that was Australia.
15:36It's Australia
15:37And here in Australia these ancient
16:07beautiful trees, the southern beech, still grow, just as they once did in Antarctica
16:12and still do in South America, living evidence of the one-time unity of those three great
16:18continents back in geological time. And with them in these forests grow other ancient plants,
16:24tree ferns and cycads, and living in holes in their trunks and scurrying around on the
16:29beach leaves on the floor are small, warm-blooded, furry creatures that bear their young in just
16:36the same way as the American opossums, marsupials. The Australian marsupials fared much better
16:43than their American cousins, for South America continued to drift. And eventually it came
16:49into contact with North America and more advanced mammals from that continent invaded south. The
16:56South American marsupials couldn't face the competition, and many of them became extinct.
17:02But Australia was very different. This huge island continent has remained cut off from the rest of
17:09the world, and here the marsupials have become and remained the dominant mammals. They've branched out into
17:17many, many different forms, and a great number of them are active at night.
17:21Some are very similar to their South American cousins, the opossums, and indeed are known in Australia as
17:34possums.
17:35There are mouse-sized ones here too, and like their American relations, the female carries her young,
17:50clinging to her.
18:12This is the smallest marsupial of all. It may look like a mouse, but it's very different. It doesn't
18:17gnaw seeds, but hunts insects, and will unhesitatingly tackle really big ones.
18:21There are about two dozen kinds of these mouse-sized marsupials, and their reproductive techniques are, in principle, the same as any others in the group, but because they're so small, they're so small.
18:26The process is extremely difficult to observe.
18:41By providing this expectant mother with a nest floored by glass, it was possible to film a birth for the first time.
18:48Thirty days after mating, she licks the opening of the birth canal. Her minute worm-like young, smaller than a grain of rice, will emerge from it immediately.
18:54immediately after the release of the birth fluids, and within three seconds, squirm across to the pouch a few millimeters in front.
19:01First, the birth fluids pour out. There's the first one, and there's the second.
19:07And there's the second.
19:09And here's that crucial moment slowed down. The opening to the pouch is that dark patch just above the middle of the picture. The young comes out of the birth canal.
19:22There.
19:24And here's that crucial moment slowed down. The opening to the pouch is that dark patch just above the middle of the picture.
19:30The young comes out of the birth canal. There. And it's gone.
19:44The female may produce six to eight young in a single batch.
19:56When they first arrive in her pouch, the babies are so small that their mother seems almost unaware of their existence.
20:02But they grow fast, and within a week or two, they become quite a considerable burden.
20:12Eventually, the pouch can no longer hold them, and they hang beneath her like squirming pink grapes.
20:19They don't let go of the teats until they're 56 days old, and they'll go on suckling milk sporadically for many days after that.
20:32When they're three or four months old, they become independent, and can join their parents hunting insects in the Australian night.
20:48Inp типos.
20:49The
21:48Other larger marsupial hunters seek larger prey.
21:53This is the quoll, as big as a cat.
22:05It has a sensitive nose and acute ears to help it find its food.
22:17A marsupial mouse.
22:19Bigger still, the size of a corgi dog, the Tasmanian devil.
22:40The quoll has found some carrion.
22:51But it doesn't stay long when the devil appears.
22:56Here's a second.
23:13Like hyenas, devils devour pretty well everything. Skin, bones, the lot.
23:28Not so long ago, there was an even bigger marsupial hunter.
23:46These are the jaws of the thylacine.
23:49The last recorded living individual died in a zoo in 1933,
23:54and today the species may well be extinct.
23:56The resemblance between this marsupial and the wolf of the northern hemisphere,
24:01which does not rear its young in a pouch, is remarkably close.
24:04The processes of evolution, even when working on different stocks,
24:08tend to produce similar creatures to fit similar ways of life.
24:12The thylacine and the wolf are both swift-running flesh-eaters,
24:16and consequently they look very much the same.
24:19There are several other such parallels between marsupials and other mammals.
24:23The numbat has an elongated nose and a long sticky tongue like a pangolin,
24:28so it's no surprise to find that, like a pangolin, it feeds on ants and termites.
24:33One of the closest parallels of all appears in the eucalyptus trees of Australian forests at night.
24:53This little marsupial is called a sugar glider, and with good reason.
25:02In both its appearance and its acrobatic skill,
25:15it's almost indistinguishable from the flying squirrel of North America.
25:19Both have a wide flap of skin between their legs that catches the air
25:24and enables them to glide great distances.
25:27,,
25:29,,
25:30,,
25:32,,
25:34,,
25:36,,
25:40,,
25:42!!!!
25:43Evolution in a
26:13Australia has not always produced such close parallels to its products elsewhere.
26:19This, for example, is the koala.
26:21It lives in trees and eats nothing but the leaves of just a few particular kinds.
26:32Sloths in South America do much the same and are equally fussy about their leaves,
26:37and so are some monkeys in Africa.
26:39But neither they nor the sloths look like the koala, which has an Australian charm all
26:46of its own.
26:47The wombat is a much less specialised cousin of the koalas, and it lives entirely on the
27:05ground.
27:05It, too, is a vegetarian, but it's very much less selective about what it eats.
27:17If you had to pick a northern hemisphere version of this creature, it might well be the marmot,
27:22both greys and both dig burrows for themselves.
27:25There are several kinds of wombat.
27:31This is another, the hairy-nosed.
27:36It, too, is a burrower, and neither it nor any wombat, come to that, is exactly renowned
27:41for its darting intelligence or speed of reaction under distressing circumstances.
27:46Bandicoots look, at first sight, rather like rabbits, but the parallel here is not really
27:58close at all.
28:00Rabbits eat grass, bandicoots, insects and meat.
28:04The similarity between the ears and those of particularly long-eared rabbits, like the
28:09American jackrabbit, is due to the fact that both live in hot deserts and use their ears
28:13for cooling their blood.
28:18This Australian, the honeypossum, has no close equivalent at all elsewhere.
28:24It lives on nectar, which it gathers with a tongue that has a brush at the end.
28:29If you wanted to find a parallel for that, the nearest would be the brush tongue of nectar-feeding
28:34bats.
28:37When the southern supercontinent broke up, Australia was largely covered by forests.
28:43And those that remain still contain some very primitive marsupials.
28:47This little creature is a pot-a-roo, and in it you can see the beginnings of features
28:53that characterise the most famous of all the Australian marsupials, the kangaroos.
28:58For one thing, the pot-a-roo has a tendency to hop.
29:06For another, its young retain the habit of popping back to the pouch for a drink of milk
29:12until long after they're able to fend for themselves, and they have a marked preference
29:17for travelling that way, even when they're quite large.
29:19In more open woodlands, there are animals that have developed these two tendencies even
29:36further, wallabies.
29:39There are also about two dozen different kinds of them, and this one is known as a paddy
29:44melon.
29:53This odd wallaby has colonised the huge tropical island just north of Australia, New Guinea.
30:02Very few mammals of any kind live here.
30:04There's this creature and a kangaroo that, unbelievable though it may seem for an animal clearly designed
30:10for hopping, has taken to trees.
30:11The tree kangaroo seems just about the clumsiest climber of all tree-living creatures.
30:21The explanation seems to be that here in New Guinea, it's the only mammal that's managed
30:26to get up into the branches, and with no other creature competing with it for leaves, it's
30:31had no need to become any better adapted, it can get all it wants, just as it is.
30:40All in all, there are over 150 different kinds of marsupial in Australia and its offshore islands
30:46like New Guinea and Tasmania.
30:48But not so long ago, there were even more, and you can see the most spectacular evidence
30:54of their existence in Australia's caves.
31:09In 1969, a couple of zoologists came crawling down this narrow cavern in the limestone hills
31:32of Narrowcourt in South Australia, not far from Adelaide.
31:37They were the first people ever to come this way, and they were hoping that they might find
31:42a bone or two.
31:46What they discovered, another quarter of a mile farther on, exceeded their wildest imaginations.
31:52They discovered the greatest and most important deposit of bones ever found in a cave in Australia.
32:03It takes an hour and a half of crawling to reach this extraordinary gallery.
32:22Ancient streams have washed down thousands of bones and left them here,
32:43and they look so fresh that they might have been deposited only a few weeks back.
32:47But nearly all belonged to kinds of marsupial that have been extinct for thousands of years.
32:54This is a skull of a giant kangaroo that could browse up to a height of about nine feet above the ground,
33:02which is half as high again as any living kangaroo can do.
33:06It had a bulbous face with very big eyes and powerful high-crowned teeth,
33:14with which it could masticate really tough leaves.
33:22You could take this for the skull of a small rhino,
33:25but in fact it belonged to a giant wombat as big as an ox.
33:30Its teeth suggest that it chewed coarse vegetation.
33:33The most extraordinary skull in these caves is this.
33:45It belonged to a creature that was a kind of killer possum.
33:50It's popularly known as a marsupial lion.
33:53In life it was about the size of a leopard.
33:56Its four legs were shaped rather like those of a koala,
34:00except that it had on its thumb a vicious hooked claw with which it ripped apart its prey.
34:07But the most fantastic thing about it are its teeth.
34:12In the back of its jaws it had these single teeth elongated to form great shearing blades,
34:22with which doubtless it sliced through the flesh of its prey.
34:26Maybe it jumped from trees onto those giant kangaroos.
34:30Who knows?
34:31But despite the formidable armory of teeth,
34:37all the marsupial lions became extinct between 20 and 18,000 years ago,
34:43as indeed did all the giant kangaroos.
34:46Why?
34:48Well, Aboriginal man had certainly reached Australia by this time,
34:52but there's no evidence that these creatures were overhunted.
34:56No, the reason seems to be that there was a change in the climate,
35:00which became extra dry about this time.
35:17That change in Australia's climate can be traced right back to a time some 45 million years ago
35:23when the continent first split away from Antarctica.
35:27Because after the separation, Australia didn't just stay still,
35:31but continued to drift slowly northwards towards the equator.
35:35Indeed, it's still moving in that direction even today,
35:38and as fast as it has ever done, which is about five centimetres a year.
35:44The effect on the vegetation has been dramatic.
35:47The lush, cool, wet forests changed into arid, open country like this around me in Central Australia.
35:56And one group of marsupials were quick to respond to this change.
36:00There's some of them just over there.
36:02Out in this open country, the small, wallaby-like marsupials grew bigger,
36:16hopped farther and faster, and became kangaroos,
36:19the marsupial equivalent of deer and antelope.
36:22With its huge hind legs and muscular, counterbalancing tail,
36:31a red kangaroo, the biggest of all living marsupials,
36:34can bound 27 feet, about eight metres,
36:37and leap over obstacles 10 feet, three metres high.
36:45It's often extremely hot out here,
36:47with temperatures as high as 45 degrees centigrade,
36:50and kangaroos have developed a special way of cooling themselves.
36:54They plaster their forearms with saliva.
36:57As this evaporates, it cools the blood,
36:59running through special networks of capillaries just beneath the skin.
37:10The kangaroos also take advantage of the best shade they can find
37:14during the hottest part of the day,
37:16and scrape away the baking-hot surface soil
37:19to make a cooler, more comfortable bed for themselves.
37:38Out in the desert, food is nearly always scarce,
37:41and kangaroos will eat even the tiniest morsel of greenery,
37:44searching through the dry branches with their front legs
37:47to find something edible.
37:49The leaves of these bushes are very rough and tough on the teeth.
38:05And the problem of tooth wear is something which faces grazing animals
38:10all over the world.
38:11Antelope and deer solve it by having open roots to their teeth,
38:15which grow throughout their lives.
38:18Kangaroos have a different solution.
38:20They have only four pairs of molars on either side,
38:24but they move steadily forwards throughout their life
38:27as they're worn down in the front,
38:29so this fourth one in this young animal here comes into play.
38:34This is the skull of an older animal,
38:37and already the first molar has gone,
38:40and the second one is so worn down here
38:43that it's virtually useless,
38:45and had the animal not died, it would have been shed.
38:48This process of moving forward goes on throughout the animal's life,
38:52so that by the time it's 15 or 20,
38:55if it hasn't died for any other reason,
38:56it would die from starvation,
38:58because it would have lost all its teeth.
39:10The red kangaroo has developed the marsupial reproductive technique
39:14into a very efficient system indeed.
39:1733 days after the fertilized egg started its development,
39:29the little young, scarcely more than an embryo,
39:32is expelled from the womb.
39:36The mother is cleaning up the birth fluids.
39:38She's not licking a pathway through her fur for the young,
39:41as used to be thought.
39:42Indeed, she gives the feeble, blind little creature
39:46no assistance at all.
39:48It has to find its way to the pouch entirely by itself.
40:02The tiny baby, and only one is born at a time,
40:05valiantly squirms its way towards the pouch,
40:08a journey that may take it up to five minutes.
40:11Its forelegs are sufficiently formed to help it move forward,
40:15but its hind legs are still no more than buds.
40:22The rim of the pouch and safety.
40:34The mother's teat is considerably bigger than the little baby,
40:37which at this stage weighs less than a gram.
40:48Within a day or so of the young fastening on the teat,
40:51the mother produces another egg in the womb and will mate again.
40:55But that fertilized egg will wait there without developing
40:58until, in 235 days' time, the first baby is sufficiently well grown
41:04to leave the pouch.
41:05Only then will the development of the next egg proceed.
41:09This system of continuous production is so efficient
41:22that every female can reproduce four times every three years.
41:26And kangaroos have come to dominate the Australian countryside.
41:29But why should kangaroos hop?
41:36One suggestion is that when they were newborn babies,
41:39they had to develop grasping forelimbs
41:41to haul themselves through the fur,
41:44and that this character, having been determined so early,
41:47cannot then be changed into one that's suited to running.
41:49Another explanation may be the position of the pouch.
42:10If large babies are to be carried in it at speed over rough ground,
42:14perhaps it's easier for mothers to do so
42:16with the torso somewhat inclined upwards.
42:20Whatever the reason, the kangaroo has brought the hop
42:23to a marvellous peak of power and grace.
42:46When it's hopping at full stretch, the kangaroo can reach speeds
42:51up to 50 kilometers an hour, which is not as fast as the swiftest antelope,
42:57but it still goes up to 50 kilometers an hour.
43:00When it's hopping at full stretch, the kangaroo can reach speeds
43:16of up to 50 kilometers an hour, which is not as fast as the swiftest antelope,
43:20antelope, but a fair speed nonetheless. On the other hand, when they're moving in a more
43:26leisured way, at about 25 kilometers an hour, the style proves to be a very economic one,
43:32demanding considerably less energy than an antelope moving four-footedly at the same
43:37sort of speed. The milk supplied by the females to their young varies as time passes. This
43:49well-grown youngster is not yet weaned, even though it nibbles grass every now and then.
43:55The milk it takes from the single teat which it has used throughout its young life is by no means
44:00the same as the liquid it drank when it first arrived in the pouch as a tiny worm. The ingredients
44:06have changed to match the youngster's changing needs. It will continue to take milk for some
44:15time after it's left the pouch for good. The mother, by then, will have another tiny baby
44:21in her pouch, so she will be supplying one kind of milk from one nipple and a different mixture
44:26from another.
44:30The rearing of young in a pouch has its hazards, particularly that early journey to get there,
44:36but in some ways it brings advantages to kangaroos. If a female with a large youngster in her pouch
44:43is chased, she will often jettison her baby and so escape. No heavily pregnant antelope
44:49has that option. A sustained drought, not uncommon in the Australian desert, may make it difficult
44:55for her to produce sufficient milk for the tiny young in her pouch. She may then discard
45:00the little babe without much trouble. When the drought is over, the fertilised egg, waiting
45:05within her womb, is ready to start immediate development and the new young can be in her pouch 33 days
45:11later.
45:26It's a commonly held belief that marsupials are very primitive and backward mammals. There's
45:32scarcely any improvement, if any, on those early egg layers, the echidna and the patipus.
45:37It's a view, after all, that was held by Charles Darwin. But the fact of the matter is that
45:42today we recognise that many of them are extremely efficient organisms. It's true, of course, that
45:49their basic method of reproduction appeared very early in the development of the mammals.
45:55But many of the marsupials today have brought that to a high pitch of perfection. As a result,
46:01no other creature can compare with these female kangaroos, which throughout their maturity,
46:06continuously and almost without break, have three young at different stages of development.
46:12One grazing and coming back to suckle, one within the pouch, and one within the body itself,
46:19awaiting the best strategic moment in which to be born.
46:24No, the fact of the matter is that the accidental isolation of the marsupials in Australia, brought
46:29about by the drifting of the continent some 45 million years ago, has given them a long,
46:35long time in which to weave variations on the basic model. And some of those variations are very
46:42efficient creatures indeed. While the marsupials were developing in Australia, another kind of mammal
46:49was coming to the fore in the northern hemisphere. Like the marsupials, its fossils dated back to the age of
46:55the dinosaurs. It was related to the American opossum. Like it, it was small and insect eating, but it
47:02differed in one crucial respect. And it may have looked something like this.
47:12This selenodon, a relative of the shrews, is a fair representative of them.
47:16And they developed a third technique of reproduction. The female doesn't lay a shelled egg like a
47:22platypus, nor give birth to a partially developed little worm like a kangaroo.
47:27She retains her young within her body and nourishes it with a placenta, a pad rich in blood vessels that
47:34is implanted on the wall of the womb and linked to the young by a tubular cord. It absorbs
47:40nutrients from the mother's blood and supplies them continuously to the growing baby.
47:46This innovation was bequeathed by the early insect eaters of the northern hemisphere to all their
47:52descendants, the majority of the mammals alive today. So none need give birth to their babies
48:00until they're very well developed.
48:09Baby rabbits develop within the mother for 28 days, twice as long as an opossum, a primitive
48:15marsupial of about the same size. They don't open their eyes until several days after birth, but
48:22when necessary, a young placental mammal can be ready for action almost as soon as it leaves its mother's body.
48:30A young wildebeest can run within minutes of its birth, though it's a little groggy at first.
48:46In some circumstances, it's important to condense suckling to as short a time as possible.
48:51These harp seals are very vulnerable out here on the ice. The sooner the pups can get to the safety of
48:57the sea, the better. So their mothers provide them with a very rich milk. In about three weeks,
49:02they double their weight, and can swim away and lead independent lives.
49:07They double their weight, and they double their weight, and they double their weight, and can swim away.
49:25Retaining the baby within the womb until it's fully formed seems an obvious way for the mammals to
49:32improve the care of their young. But in fact, it causes considerable problems in body chemistry.
49:40For one thing, the tissues of this little pup differ in a genetic way from those of its mother.
49:47They contain elements from the father. And so that means that it risks, when it's within the womb,
49:55rejection by the mother's body, just as a transplant does. Secondly, the young in the womb may be ejected if
50:04the mother produces another egg and comes on heat again. That problem doesn't face a baby marsupial,
50:11for in every case, its short development takes place within the period of the mother's sexual cycle.
50:19But a placental mammal, like this seal, has a much longer development. And it deals with the problem
50:25by producing from within the placenta a substance which actually suspends the mother's egg production.
50:32It manufactures other biochemical substances which suppress the production of the antibodies that
50:39cause the rejection of tissue, and so allows the developing young in the womb to remain there.
50:48So the placenta has had to become a chemical factory of great complexity.
50:59When the young is finally born, the placenta too is shed from the womb as the afterbirth.
51:04The body of a mammal, whether it's our own or a seal's, is extremely complex and takes a very long
51:17time to fully develop. These young seal pups around me were conceived almost a year ago, and until a few
51:27days ago when they were born, they were kept in the warmth and safety of their mother's body as she
51:33swam through the freezing polar seas. No marsupial could be reared in such a way because marsupial babies
51:43in a pouch need to breathe air. In fact, the placenta and the womb between them provide a degree of
51:53safety and a continuity of safety and a continuity of sustenance which is unparalleled in the animal
51:59world, and together they form one of the keys to the success of the placental mammals, which have
52:06in the end colonized all parts of the world, including even these bleak inhospitable ice flows.
52:22Sons of the
52:51ORGAN PLAYS
53:21ORGAN PLAYS
Recommended
53:21
|
Up next
54:26
53:44
52:55
25:12
23:24
44:12
58:38
29:08
47:47
48:55
49:24
49:13
48:29
49:23
49:14
49:29
49:02
49:16
48:37
48:49
52:51
52:46
51:39
Be the first to comment