SSS 2009-01-13
时间：2019-01-08 作者：英语课 分类：Scientific American(一)月
This is Scientific American's 60-Second Science. I'm Karen Hopkin. This will just take a minute.
A bird in flight is a thing of beauty. Even their takeoffs and landings usually look effortless. But pterodactyls? Well, that’s another story. Scientists have long assumed that pterodactyls, also called pterosaurs, flew just like birds, and launched themselves using their hind 1 legs. Now a biologist at Johns Hopkins says that can’t be true. Instead, he says that pterosaurs needed all four limbs for liftoff.
Modern birds use their legs to launch and their wings to stay aloft. Once they’re in the air, their hind limbs are essentially 2 payload, carried along for the ride. That arrangement presents a problem. A bird’s legs have to be muscular enough to get Tweety off the ground, but not so big that they drag him down. That limits how big a bird can be. Some pterosaurs, on the other hand, were the size of a giraffe. And looking at the bones of three different pterodactyl species, the Hopkins scientist concludes that there’s no way those legs were strong enough to get that bulk airborne. Instead, he says a pterosaur used all four limbs, leapfrogging forward on its knuckles 3, to propel itself into the air. Maybe not a thing of beauty. But it must have been something to see.
Thanks for the minute for Scientific American's 60-Second Science. I'm Karen Hopkin.
- The animal is able to stand up on its hind limbs.这种动物能够用后肢站立。
- Don't hind her in her studies.不要在学业上扯她后腿。
- Really great men are essentially modest.真正的伟人大都很谦虚。
- She is an essentially selfish person.她本质上是个自私自利的人。