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  • When the platypus first came to the attention of European scientists in 1798, not everyone

  • was convinced the creature before them was real. Some thought a prankster had mashed

  • together separate parts of different animals to create a fake - a not uncommon occurrence

  • at this boom time of naturalist discovery. But the platypus was a very real animal, and

  • one that confused anatomists for some time. A creature with fur, a bill and webbed feet,

  • that lays eggs, and can secrete venom? Was this a mammal? A duck? Some sort of furry

  • reptile? These European scientists were asking the

  • same questions that the aboriginal people of Australia had asked a long time before

  • them. There are several Aboriginal stories about the origins of the platypus, one of

  • which tells of a union between a duck and a water rat.

  • In science terms, we have ultimately classed the platypus as a mammalor more specifically,

  • a monotreme: an egg laying mammal - of which there are only two kinds of animals in the

  • world. And this, along with its other rather reptilian traits, has made scientists scratch

  • their heads for a long time. Where does the platypus fit exactly, on the tree of life?

  • As continents divided, and the branches of the tree of life diverged, the platypus seems

  • to have taken its own, very special route. But just like with all animal adaptations,

  • there has to be some purpose to all of the platypus's strangeness. But what evolutionary

  • question was this the right answer to?

  • If you look in any textbook, the definition of a mammal is a warm-blooded vertebrate animal

  • that has fur, secretes milk, and typically gives birth to live young.

  • Typically being the key word there. Platypuses are one of only two mammals that lay eggs.

  • The other is the only other monotremethe echidna, of which there are 4 species.

  • When it's time to lay the eggs, female platypuses dig their burrows, crawl in and seal themselves

  • up. Here they lay their eggs, curling them between body and tail, until they hatch 10

  • days later. From here, the platypus acts like most other mammals, nursing their young on

  • milk for three to four months until they are capable of swimming solo.

  • To understand how and why this egg laying ability exists in a mammal, we need to wind

  • back the clock a long long time.

  • Around 340 million years ago, the first amniotes appeared on earth, in the form of small lizard-like

  • creatures. Amniotes are four legged vertebrates that are defined by the membrane, or amnion,

  • that protects the embryo during development. The amniotic egg was an evolutionary invention

  • that first allowed reptiles to colonize dry land. Fish and amphibians must lay their eggs

  • in water and therefore cannot live far from water. But thanks to the amniotic egg, reptiles

  • can lay their eggs nearly anywhere on dry land.

  • Soon, amniotes spread far and wide around Earth's land and became the dominant land

  • vertebrates. And around 315 million years ago, they split into the two major groups

  • of four legged vertebrates which still exist today.

  • One branch contains the modern reptiles and birds. The other included the mammal-like

  • reptiles, from which modern mammals later evolved. This branch of mammals eventually

  • developed to have the amniotic egg grow inside the mother's womb, giving rise to internal

  • pregnancies. But one branch of mammals did not follow suit.

  • The monotremesor egg-laying mammalssplit off from the mammalian lineage around 200

  • million years ago. They never gained the ability to have an internal pregnancy, and never lost

  • their egg laying ability.

  • Genome sequencing of platypus sex cells has shown there are a large number of shared genes

  • between platypuses and birds.

  • In particular, the platypus retains copies of the vitellogenin gene, which codes for

  • egg protein that is a precursor to egg yolk, which in turn helps sustain growing embryos.

  • Platypuses have fewer copies of the gene than birds and reptiles, but most mammals don't

  • have the gene at all. [4] This means the platypus has the ability to lay eggs, but that their

  • young are perhaps less reliant on egg protein than birds and reptiles. This makes sense

  • when we remember that platypuses also feed their young via lactation after hatching.

  • Their laying of eggs is a sort of hangover from their reptilian ancestors, and for a

  • while, it served them well. The monotremes were the dominant mammals on what is now the

  • continent of Australia for a long time. That is, until they got swept aside by the

  • arrival of their marsupial cousins.

  • Marsupials originated in what is now South America, and migrated over to what is now

  • Australia, via the supercontinent Gondwana, around 70 million years ago. Their bodies

  • were more efficient at locomotion, and their internal pregnancies meant they could better

  • protect their young, and thus they outcompeted the monotremes on almost every front.

  • And slowly, all but two, the echidna and the platypus, went extinct.

  • So the question is, why did the platypus and certain echidnas survive, when the rest did

  • not?

  • One hypothesis as to how the platypus persisted in the face of intense competition from the

  • marsupials is its ability to take to the water - a domain where the marsupials could not

  • follow. The echidna's earlier ancestor, too, is thought to have been semi-aquatic,

  • even though it is not any more. Marsupials could not colonize water environments

  • because when they are born, they have to live inside their mother's pouch for weeks to

  • suckle milk. The babies would drown if their mothers ever had to venture into the water.

  • But with their eggs secure in a nest, the platypus can happily stay in the water, avoiding

  • predation from the marsupials, and exploiting their very own environmental niche.

  • The platypus is an expert swimmer. Its ability to hunt below the water is down to a few,

  • key features of its physiology. The platypuses' webbed feet help power them

  • through the water, using their front feet for paddling, their back feet for steering.

  • And a feature that helps the platypus stay submerged for up to two minutes at a time

  • is its ability to become watertight where necessary. It has folds of skin that cover

  • its ears, and it can close its nostrils, too. And, despite its hunting prowess underwater,

  • it nearly completely closes its eyes when diving. Thanks to a 6th sense that almost

  • no other mammal possesses, it doesn't need to see to hunt.

  • One of the most distinctive parts of the platypus is its bill. It's iconic shape is wide and

  • flat, but unlike a duck's bill, the platypus bill is described as flexible, rubbery, and

  • a little fleshy. Its surface feels a bit like suede.

  • And the bill is the platypuses primary hunting tool. It can hunt with its eyes completely

  • closed because it is super sensitive in two key ways: it is mechanoreceptive and electroreceptive.

  • Mechanoreceptive means sensitive to external, mechanical stimulus, such as touch or pressure.

  • In the case of the platypus, its bill contains mechanoreceptors called pushrods. These are

  • columns of densely packed cells that move independently of the surrounding skin. When

  • pressure or a vibration is applied to the push rod, it triggers the nerve at the bottom

  • of the column. The pressure doesn't have to be largethe

  • slightest tremors can be felt through the water. The bill is so sensitive, it can detect

  • freshwater shrimp from a distance of 15-20 centimetres, simply by sensing movements in

  • the water

  • The other sensing ability of the bill is its electroreception. All animals emit electric

  • signals from their muscles moving, and the platypus bill can sense these electric fields

  • originating from their prey. The bill contains around 70,000 glands, that assist in the electroreceptive

  • function of the bill The mechanism of electroreception in the bill

  • is much like that of the elasmobranchs, like the hammerhead shark. The electrical currents

  • from the stimulus travel through the water, then through secretions from the glands in

  • the bill, which surround the nerve endings beneath the bill's surface. [9]

  • The 100,000 electro- and mechanoreceptors on the platypus bill are arranged in a beautiful

  • striped pattern, with bands of electro and mechanoreceptors alternating.

  • Electroreception is common in fish, but has only been found in three mammals to date:

  • the platypus, the echidna, and the Guiana dolphin.

  • Platypus bills have up to 70,000 electroreceptors, and those of long-billed and short-billed

  • echidnas have 2,000 and 400, respectively. You can see evolution in progress here. Since

  • moving back onto land, the electroreceptors of the echidnas are being selected against,

  • because such sensing ability is only useful in semi-aquatic environments

  • But this electroreception is not a remnant from an early fishier ancestor. It evolved

  • completely independently of electric fish, and is the answer two different lineages came

  • up with to a similar evolutionary pressure.

  • Electroreception isn't the only feature of the platypus that is rarely seen in mammals.

  • Next time you pick up a platypus, you'd be wise to keep its back legs pointing out

  • of reach. Male platypuses have spurs on the backs of their hind feet that connect to venom

  • glands in their abdomen. And that, while not deadly to humans, can have some pretty nasty

  • side effects: nausea, cold sweats and lymph node swelling, and immediate, excruciating

  • pain that can't be relieved through normal painkillers.

  • Venomous mammals are now pretty rare. There are only a handful that we know of, such as

  • the slow Loris, the only known venomous primate, which uses venom to protect itself against

  • predators; or the American short-tailed shrew which uses its venom to immobilise its insect

  • prey. It's thought that with the development of teeth and claws, most mammals developed

  • much faster ways of killing prey than venom which needs time to take effect, and so venomous

  • capabilities became evolutionarily redundant. Of those mammals that kept their venom, the

  • very different methods of delivery suggest the ability evolved independently. Take those

  • slow lorises mentioned earlier: they can look pretty cute when they raise their arms in

  • the air as if they're asking for a hug, but don't be fooled, this is actually how

  • they access their venom. It's produced by glands in their armpits, which they lick.

  • The venom mixes with their saliva and settles into grooves in their teeth, ready to harm

  • anything the slow loris bites. The short tailed shrew also has grooves in its teeth, but its

  • venom comes ready made in the shrew's saliva. Both of which differ to the platypus, which

  • delivers its venom through it's spurs.

  • But interestingly, for the platypus, a recent study found that many of the proteins present

  • in its venom are the same as those found in reptile venom, even though the reptiles split

  • from mammals some 315 million years ago. Does this mean the venom is an evolutionary leftover

  • from the platypuses' reptilian ancestor? Or is it an example of independent, convergent

  • evolution of the venom? By sequencing the platypus genome, scientists

  • found that the platypus's ability to deliver venom is down to a duplication in a set of

  • reptilian genes that underwent the same duplication independently in snakes after reptiles and

  • mammals split in the evolutionary tree.

  • This suggests then, that platypus venom is an unlikely example of convergent evolution.

  • In this case, reptiles and the platypus developed similar venoms despite not having a common

  • ancestor for hundreds of millions of years. Scientists still don't know exactly why

  • platypuses have venom spurs, but it's thought they use them in mating practices, and to

  • defend territory and mates from other platypuses.

  • Despite its weirdness, we now understand that the platypus is not just a funny looking animal,

  • it is a highly adapted creature perfectly suited to its environment. Some of its bizarre

  • features are remnants from an ancient time, and others are more recent evolutionary inventions

  • that happen to be similar to ones in the fish, birds, and the reptiles. It's a wild and

  • unlikely mashup of traits that allows the platypus to sit on its very own branch of

  • the evolutionary tree. But we also know there's a lot more of the

  • puzzle to put together. Scientists are constantly stumbling across new, unusual findings about

  • this amazing animal. Just last year, researchers accidentally discovered that platypus skin

  • glows under UV lightcalled biofluorescence. Why? We still don't know. There are likely

  • many more secrets hiding within the platypus that we will continue to discover for years

  • to come. Australia's unique landscape and geography

  • made the platypus what it is today. Australia is literally teeming with biodiversity. It's

  • patchwork of different climates, from tropical forests, to hostile deserts, to tropical reefs,

  • along with its isolated nature, has given rise to some of the world's strangest, and

  • most iconic creatures. 80% of the plants and animals in Australia are unique to Australia,

  • found nowhere else on earth. Having grown up in America, Australia's animals seem

  • downright alien to me, and I love learning about their evolution, and their sometimes

  • wacky behavior. To immerse yourself in 50 minutes of the beautiful, weird, and wonderful

  • Australian continent, you should watch Hidden Australia on CuriosityStream. Believe me,

  • I have spent my life watching nature documentaries, and this one still showed me animals I never

  • knew existed. Like, what even is this guy.

  • This is such a fun film, and will make you daydream about getting to visit such an incredible

  • and diverse place.

  • CuriosityStream is a streaming platform with thousands of high quality documentaries like

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  • up to CuriosityStream you now also get a subscription to Nebula. Nebula is a streaming platform

  • made by me and several other educational YouTube content creators. It's a place where we can

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When the platypus first came to the attention of European scientists in 1798, not everyone

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疯狂生物学:鸭嘴兽(The Insane Biology of: The Platypus)

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    joey joey 發佈於 2021 年 07 月 07 日
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