雅思阅读:Mystery of the male ostrichs 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 dont 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 cant maintain an erection. Copulation is very brief compared with mammals and reptiles, Brennan says. Its a matter of a few seconds.
Brennan says she doesnt 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.
职称英语考试词汇题的解题方法
职称英语考试技巧职称英语通关四步答题法
职称英语考试单选题常用词对预测篇(4)
职称英语综合类考试考前每日一练(28)
职称英语阅读理解精选中英文语句学习(七)
职称英语卫生类考试考前每日一练(29)
职称英语考试词汇:容易写错的英文单词(5)
职称英语考试单选题常用词对预测篇(3)
职称英语考试必考的高频词汇(1)
职称英语考试高频词汇(14)
职称英语理工类考试考前每日一练(29)
职称英语理工类考试考前每日一练(24)
职称英语名师点评职称英语考试特点
职称英语卫生类考试考前每日一练(28)
职称英语应试攻略突击英语语法的七大捷径
职称英语考试单词的记忆法
职称英语考试必备词汇容易写错的英文单词(1)
职称英语综合类考试考前每日一练(30)
职称英语名师点评职称英语考试出题特点及重点
职称英语考试突击三类小题型解题的技巧小结
职称英语理工类考试考前每日一练(30)
职称英语综合类考试考前每日一练(24)
职称英语考试必考高频的词汇(8)
职称英语考前必备突击重点技巧全攻略
职称英语综合类考试考前每日一练(29)
职称英语考试必考的高频词汇(6)
职称英语卫生类考试考前每日一练(23)
职称英语理工类考试考前每日一练(26)
职称英语考试必考的高频词汇(11)
职称英语卫生类考试考前每日一练(30)
不限 |
英语教案 |
英语课件 |
英语试题 |
不限 |
不限 |
上册 |
下册 |
不限 |