雅思阅读：Mystery of the male ostrichs erection solved

雅思考试主要是通过对考生听、说、读、写四个方面英语能力的考核，综合测评考生的英语沟通运用能力，实现“沟通为本”的考试理念。对于雅思考生来说，也有很多考试难点和政策盲区需要帮助解答。今天雅思无忧网小编准备了雅思阅读：Mystery of the male ostrichs erection solved，希望通过文章来解决雅思考生这方面的疑难问题，敬请关注。

　　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. 　　Fast action 　　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.

以上就是雅思无忧网为您准备的访问雅思无忧网(https://www.yasi.cn/)，了解更多雅思考试新消息，新动态。

雅思培训
免责声明：文章内容来自网络，如有侵权请及时联系删除。