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.TOEFL Listening Comprehension TranscriptNarrator
Listen to part of a university lecture in Biology.
Well, we''ve now studied the main subclass of mammals, the placental mammals, and also the subclass of marsupials- and now we ought to just quickly survey the third and smallest subclass, an odd little group called the ''monotremes''. The best-known monotreme, of course, is the duck-billed platypus, and I know you''ve all heard of that strange portmanteau animal. Well, the platypus is the only member of its family. There''s just a single species of ornithorhynchid- this family name just means ''bird-nosed''. The other family of monotremes are the echidnas or spiny anteaters, the tachyglossids. This family name means ''fast-tongued'', and there''re only four species of these. So altogether there''re just five species of monotremes.
The platypus lives only in the streams and ponds of eastern Australia, and the echidnas live in the forests of Australia and nearby New Guinea. So their ranges are very restricted. Nevertheless, a fossil monotreme found in Argentina suggests that the monotremes were once globally distributed.
These animals have a number of characteristics similar to reptiles, but it is important to realize that they''re no more closely related to reptiles than any other mammal group, and it''d be a mistake to consider them as more primitive. It''s just that after evolving from their therapsid ancestors, the monotremes broke off from the other mammals- about 150 million years ago- and went their own evolutionary way.
Platypuses, or platypi, look very different from echidnas, but these animals have several characters in common that separate them clearly from the placentals and marsupials. First, well, as their name says, they''re monotremes- they''re ''single-holed''. Their genital and excretory functions exit from the body from the same aperture, called a cloaca. Monotremes also have a low metabolic rate- that is, a lower body temperature- and they lay eggs. Like other mammals, though, they lactate, they produce milk, but they have no nipples. The females just secrete milk onto their skin, where the babies lap it up. The other things that make these animals mammals are that they''re covered in fur, they have a mammalian dental pattern, and they have a four-chambered heart.
Nevertheless, the duck-billed platypus is so bizarre-looking that when it was first discovered by Western scientists, at the end of the 18th century, they thought it was a hoax, a ruse- a real portmanteau animal, an animal made up by attaching a duck''s beak to a mole''s skin. Because the first specimen arrived in England by way of the Indian Ocean, scientists suspected that the creature was actually sewn together by Chinese or Japanese sailors, who were known for their skill at this kind of practical joke.
The platypus''s bill is like leather- it''s soft and flexible, and it''s sensitive both to touch and to weak electric fields. It uses both of these to find its food- crustaceans and other invertebrates- in the muddy waters it lives in. It has soft, thick, water-resistant fur, it has webbed feet, and a broad, flat tail- so it''s well-adapted to its aquatic existence, and it probably hasn''t changed much in the last few million years. And don''t try to pick one up- the males have poisonous spurs on their back legs that can be very dangerous!
Now, the echidnas- some recent DNA research has suggested that the echidnas evolved from a platypus ancestor relatively recently- only about 30 million years ago- so their evolution''s been more active than the platypus''s. Echidnas- or spiny anteaters, which is a much clearer name for them- sorry, I''m just used to calling them echidnas- anyway, they''re totally different-looking animals. They are stocky, sturdily-built guys with powerful claws and digging muscles. They live in forested country, where they dig for termites, ants and other invertebrates. Instead of a broad duck''s bill, they have a long, tubular, toothless snout and a long, extendible, sticky tongue- which accounts for their family name, tachyglossids, of course. Like the platypus, they also have electroreceptors to help locate food. Echidnas are covered with spines, and when they''re threatened, they erect these spines and roll into a prickly ball that''s very hard to attack.
And one little-known behavior of the echidnas is their ''love train''. Echidnas are normally solitary animals, but in breeding season, a female will attract several to a dozen males, who follow her around closely in single file for up to six weeks, before she finally chooses one to mate with. Then she lays her single egg in a temporary pouch that she develops.
Platypus populations seem to be holding their own, and echidnas sometimes wander across suburban gardens in Australia, but the New Guinea Long-beaked Echidna is in danger of extinction- it is a highly-sought-after prize for local traditional hunters. It''d be a shame to lose such interesting creatures from the earth, so I hope that efforts will be made by the Indonesian and Papua-New Guinean governments to protect these little guys.
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