What’s the difference between a crocodile and an alligator?
By Jack Ashby, on 24 October 2013
Crocodiles and alligators are big nasty predators. All of them. (Except the ones that are small lovely predators). If you see one swimming towards you then be concerned. Whilst considering your impending doom, you may wish to ascertain the correct taxonomic position of the beast. Here’s a quick guide to help you tell the difference between crocs and gators…
Before that, I should explain that there are 23 members of the order Crocodylia, which contains both the crocodile family (Crocodylidae) and the alligator family (Alligatoridae), as well as the gharial (the sole member of the family Gavialidae). When I say “crocodile” I am referring to members of Crocodylidae, not all members of Crocodylia, otherwise there wouldn’t be much point to this post.
Things to ask to work out whether you are being eaten by a crocodile or an alligator…
1) Where are you?
Conveniently, there is a global pattern as to what lives where. If you are in the Americas the chances are it’s an alligator, while if you are somewhere else it’s probably a crocodile. As this is zoology and not physics, life has ensured that this isn’t a solid rule – there are three species of crocodile in the Americas (the American crocodile Crocodilus acutus, Morelet’s crocodile Crocodylus moreletii and the Cuban crocodile Crocodylus rhombifer), and one species of alligator in the Old World (the Chinese alligator Alligator sinensis).
If you forget which continent you’re on or near, the kind of habitat you’re in doesn’t help. Both families have species that live in both fresh and salt water.
2) Can you see the teeth on the bottom jaw?
The shape of the jaw is the most distinguishing feature for comparing crocs with gators:
a) Alligators have relatively STRAIGHT jaws – the teeth are in a straight line.
a) Crocodiles have relatively WAVY jaws – the teeth aren’t in a straight line.
b) Alligators mostly have WIDER snouts (more “U” shaped).
b) Crocodiles mostly have NARROWER snouts (more “V” shaped).
c) The fourth tooth on a crocodile’s lower jaw is OUTSIDE the top jaw (because their jaws are wavy).
c) All the teeth on an alligator’s lower jaw are INSIDE the top jaw (as their lower jaw is SMALLER than their upper jaw, which is STRAIGHT).
3) How porous is the skin?
Crocodylians have pores in their skin which connect to pressure sensors called integumentary sense organs (so they know where prey is underwater when it moves. This is AS WELL AS the transparent second eye-lid goggles they have (nictitating membranes). They are proper nifty predators). These appear as little pits on the scales.
In alligators they are ONLY FOUND AROUND THE JAWS.
In crocodiles (and gharials) they are FOUND ALL OVER THE BODY.
Let’s apply this to some non-standard crocodylians:
Caimans are relatively small crocodylians from South and Central America (though the black caiman is 4 metres). So apply Q1 and it gives a good clue. They have straight, wide jaws and the lower jaw sits inside the upper jaw, with no 4th lower tooth protruding. Therefore caimans are alligators.
b) False gharials
False gharials are a long-snouted species from Malaysia down to Borneo. They have convergently evolved to look like the gharial from the Indian sub-continent (hence the name). The narrow spiky-toothed jaw is a classic tool for catching lots of fish, a feature seen popping up in vertebrates wherever fish-eaters do. Just look at dolphins and the dinosaur Baryonyx.
This skull may hide whether the snout is straight, but that tell-tale fourth tooth is sticking out proudly from the lower jaw. False gharials are crocodiles.
So there you have it. Obviously any attack on a human by a crocodylian is tragic. But hopefully we can agree it would be worse if they didn’t know what kind of crocodylian it was.
[UPDATE: This article was updated on 28/10/13 to include Morelet’s crocodile in the list of American crocodiles]
Jack Ashby is the Manager of the Grant Museum of Zoology.