雅思考試的每一部分都不是容易的，聽力，口語，閱讀，寫作，都是一點(diǎn)點(diǎn)的積累，然后運(yùn)用之。閱讀題最是展現(xiàn)你詞匯量，邏輯理解的時候了，平時的閱讀積累就顯得尤為重要，多看多讀多練習(xí)。下面是有出國留學(xué)網(wǎng)為你整理的《雅思閱讀考前必看：Male ostrich》，希望對你的雅思考試有所參考價(jià)值，考出一個好的成績。
Mystery of the male ostrich's erection solved
Getting an erection is no problem for male ostriches – but maintaining it for more than a few seconds is a different matter. The birds may have inherited their underpowered genitalia from their dinosaur ancestors.
The penis is a vital piece of equipment for many vertebrates. Most male reptiles and mammals have them, and two of the most ancient groups of birds have them too, suggesting that dinosaurs, from which birds have descended, also possessed the male member. The majority of male birds don't have one, though: the reason why is something of a mystery.
The ostrich belongs to one of those two ancient, penis-bearing bird groups. It is a palaeognath, a group that also includes emus, kiwis and the near-flightless tinamous. The other endowed group is the Galloanserae, commonly known as fowl, which includes chickens, pheasants and ducks.
Many of these birds' penises are well understood, but the penises of ostriches and their relatives remained a mystery. So Richard Prum of Yale University and Patricia Brennan, now at the University of Massachusetts Amherst, obtained a dead ostrich and three emus to find out more.
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In both species they found spongy organs called paralymphatic bodies at the base of the penis. That tells us that the birds' penises fill up with lymphatic fluid to make an erection, like all other bird penises studied – and unlike reptiles and mammals, which use blood for the job.
That is bizarre, Brennan says, because the body keeps lymphatic fluid under lower pressure than blood. As a result, birds can't maintain an erection. "Copulation is very brief compared with mammals and reptiles," Brennan says. "It's a matter of a few seconds."
Brennan says she doesn't know why such an inefficient system would evolve, especially as it must have evolved from the more evolutionarily ancient blood-based system. However, all birds with penises produce erections this way, so the condition must already have existed in the last common ancestor of modern birds.
That last common ancestor was alive in the early Cretaceous, at least 130 million years ago. It is not clear now much further back down the bird evolutionary tree these inefficient penises might be found, but it is at least possible that the trait first appeared in the dinosaurs from which birds evolved.
Gregory Erickson of Florida State University in Tallahassee says it makes sense that dinosaurs had penises, but so far the fossil record has remained silent on the matter. It would take a dinosaur fossil with exceptional soft tissue presentation to establish whether dinosaur sex was as brief as the birds' or as leisurely as mammal and reptile couplings.
A dinosaur penis would be far from the oldest in the fossil record, though: that crown is currently held by a tiny crustacean found in rocks 425 million years old.