Say you're in the swamp and a menacing, hungry reptile heads your way -- your dying wish is to know the particular species of said attacker. One way you could tell whether you're about to be eaten by an alligator or a crocodile doesn't even require you to look the approaching killer in the face.
Since alligators and crocodiles generally don't live in the same region, a good clue to their identities is their location. Whereas gators live mainly in freshwater swamps, lakes and slow-moving streams in the southeastern United States, South America and China, crocodiles have a wider range. Along with swamps and slow-moving rivers, crocs also tolerate saltier waters like mangroves and estuaries in Africa, North America, South America and Asia. The only place you'll find both species is the southern tip of Florida.
The main reason for this geographical separation has to do with biology. Crocodiles have well-developed salt glands on their tongue that expel large amounts of salt, enabling them to live in more saline waters. Alligators' salt glands don't function as well, leading them to stick to freshwater habitats.
Another way to identify your stalker is to take a good look at the shape of its jaw. Crocodiles have long, pointed snouts, shaped like a 'V,' whereas alligators have more rounded snouts, like the letter 'U' [source: San Diego Zoo]. The broader alligator jaw is stronger -- designed to withstand the pressure of cracking down on hard-shelled prey like turtles that are abundant in its particular habitat. The narrower crocodile jaw, while still capable of exerting a powerful punch, is ideal for a wider variety of prey.
While your eyes are focused on the jaw, go ahead and get a long look at the teeth too -- another good identifier. Because an alligator's lower jaw is slightly smaller than its upper jaw, its lower teeth are hidden when it closes its mouth. When an alligator flashes a closed-mouth smile, all you can see are its downward-pointing top teeth.
Crocodiles have toothier smiles: Since both jaws are roughly the same sizes, their upper and lower teeth interlock when they shut their mouths, giving you an eyeful of both upper and lower teeth. If you happen to see a prominent, fourth tooth protruding from its bottom jaw (which isn't visible with an alligator's grin), you'll know exactly which type of crocodilian is about to make you into a meal. Though their teeth look different, both species of crocodilians have a limitless number of pearly whites [source: Goodisman]. Unlike mammals that get one shot to replace their teeth, crocodilians can grow a lifetime supply of these dangerous little daggers.
Another way to differentiate between the two animals is by checking out their skin (or taking a closer look at your leather wallet or handbag). Both crocodiles and alligators have small pits called integumentary sense organs or dermal pressure receptors, which enable them to locate prey by recognizing the small pressure changes in water made by other animals. The pits, which look like small dots, cover almost every inch of a crocodile's body, but only the jaws of an alligator. Thus, it's easy to spot the differences between crocodile and alligator skin simply by looking for the tell-tale dots.
So whatever crocodilian is after you (actually an uncommon occurrence as the roles are usually reversed), you'll be confident in knowing exactly what toothy creature you're dealing with.