Pterodactyls' tough takeoffs
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 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 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, to propel itself into the air. Maybe not a thing of beauty. But it must have been something to see.
翼龙起飞其实不易
翱翔的飞鸟很优美。就连它们的起飞和降落通常看起来也毫不费劲。但是翼手龙也是这样的么？对翼手龙来说情形就不一样了。科学家们长期以来假设翼手龙（也叫翼龙）和鸟儿一样地的飞翔，也使用它们的后腿来起飞。现在一名来自约翰·霍普金斯（Johns Hopkins）大学的生物学家称，这种假设是不真实的。他说翼龙需要使用它们所有的四肢来起飞。
现代的鸟类使用它们的腿起飞并且凭借它们的翅膀飞在空中。一旦它们飞到空中，它们的后腿实质上在飞行过程中就成为了负担。这种情形就提出了一个问题：一只鸟的腿必须有足够的力量使它飞离地面，但是又不能太大以至于不利于飞行。这就限制了鸟的形体大小。不过，一些翼龙有长颈鹿那么大。通过研究三种不同翼龙物种的骨骼，这名来自霍普金斯大学的科学家得出结论：这些翼龙的腿不可能有足够的力量使其庞大的身躯飞起来。相反，他说翼龙使用它所有的四肢向前跑动，从而使自己飞到空中。这可能并不是一件赏心悦目的事情。但是这一定是一件了不起的事情。
Vocabulary:
Effortless: 毫不费力的
Pterodactyls: 翼手龙
Pterosaurs: 翼龙
Limb: 四肢
Liftoff: 起飞
Aloft: 在上面
Payload: 负荷；净载重量
Muscular: 肌肉发达的
Tweety: 又叫Tweety Bird，卡通篇里的一个虚构的鸟。
Giraffe: 长颈鹿
Bulk: 庞大；笨拙
Airborne: 在空中
Leapfrog: 蛙跳
Knuckle: 关节
Propel: 推进
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