美国科学60秒 SSS 2013-06-06

（单词翻译）

 　　Most birds don't have penises. Good, I have your attention. Of course, female birds don't have penises. But, neither do most males. Now researchers have unravel¹ the genetics behind the dispearance of this anatomical apparatus³. Their penetrating⁴ insights are in the journal Current Biology. Only 3% of avian species have males with obvious organs of intromission. Ducks and other water fowl⁵ are fairly well endowed. But most birds have just rudimentary gear, if anything at all. To get rid of this penile puzzle, researchers compared the embryos⁶ of chickens and ducks. Both types of birds start to develop a member. But in chickens, the so-called 'genital tubercle' shrinks before the little guys hatch. It's because of a gene² called Bmp4. Bmp4 basically tell cells to comitte suicide. This form of cell death keeps birds from sprouting⁷ teeth, and in developing male chickens, it causes the phallus to regress. Why this happens is still a mystery. It may allow females birds to be more selective about their mates. Because males with external equipement tend to impose it where they will. Whereas those without have to let the ladies roll the roost.