字幕列表 影片播放 列印英文字幕 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 mammal – or 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 monotreme – the 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 monotremes—or egg-laying mammals—split 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.  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 large – the 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.  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 light – called 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 this one. 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