鳄鱼分为3个完全不同的科：鳄科（鳄鱼）、鼍科（短吻鳄和凯门鳄）以及食鱼鳄科（食鱼鳄），共有23种鳄鱼。
为了简单起见，我们可以说它们最根本的区别在于头部的形状。短吻鳄和凯门鳄的吻硕大且呈“U”字型，而鳄鱼的吻则较窄且呈“V”字型。
另一个有趣的差别是，鳄鱼的舌头上有可以将体内多余盐分分泌出来的盐腺。短吻鳄和凯门鳄的舌部结构基本相同，但却起不到任何作用。这就是为什么咸水鳄鱼可以在咸水中生活数日、甚至数周，而短吻鳄和凯门鳄却几乎只能栖息在淡水中。
此外，鳄鱼的牙齿比短吻鳄和凯门鳄都大，而且鳄鱼也更喜欢炫耀自己的牙齿。还有什么显著差别？凯门鳄比鳄鱼更重，体形也更为健硕；但是要注意了：凯门鳄却没有那么迅捷——换句话说，没有鳄鱼那么危险。
What is the difference between an alligator and a crocodile？That’s kind of a trick question.“Crocodile”is a common name for an order，or large group，of animals called crocodilians.Alligators，caimans，and gavials are all crocodilians—but so are crocodiles.
Alligators and crocodiles are reptiles that have four short legs and a long tail.These animals look alike but one way to tell the difference is to look at their mouths.When a crocodile’s mouth is closed，you can see its bottom teeth.An alligator’s teeth don’t show when its mouth is closed.
The Chinese alligator lives on the Yangtze River in China.Because people have hunted it too much，this alligator is considered endangered.
The gavial is a member of the crocodile family and a close relative of the alligator.It lives in northern India.
The caiman is related to alligators and crocodiles.It is smaller than most other crocodilians，reaching a length of about 9feet (3meters).
Alligators and crocodiles can move silently through the water with just their eyes showing，looking for prey.They attack with lightning quickness.Alligators and crocodiles eat fish，turtles，frogs，birds，and other animals.
Alligators and their relatives are cold blooded reptiles.They lie in the sun to keep warm，as this alligator is doing in a Georgia swamp.
One way to tell crocodilians apart is by the shape of their skull.This picture shows，from left to right，the skulls of a crocodile，an alligator，a caiman，and a gavial.
How Are They Different？