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  • 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…

    An Australian freshwater crocodile. One of the smaller lovelier ones (a baby) (C) Jack Ashby

    An Australian freshwater crocodile. One of the smaller lovelier ones (a baby) (C) Jack Ashby

    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:

    Saltwater crocodile. Note the wavy jaw and the protruding fourth lower tooth (circled)

    Saltwater crocodile. Note the wavy jaw and the protruding fourth lower tooth (circled)

    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).

    Alligator family skull. Note the wider straighter jaw, with the lower jaw sitting inside the upper.

    Alligator family skull. Note the wider straighter jaw, with the lower jaw sitting inside the upper.

    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:

    a) Caimans
    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.

    Caiman skull - looking very alligatory

    Caiman skull – looking very alligatory

    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.

    False gharial skull. A straighter jaw, but the fourth lower tooth (circled) is protruding.

    False gharial skull. A straighter jaw, but the fourth lower tooth (circled) is protruding.

    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.

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    7 Responses to “What’s the difference between a crocodile and an alligator?”

    • 1
      Daniel wrote on 25 October 2013:

      If I remember correctly the Mugger Crocodile is an exception to the snout shape rule, possessing a broad, alligator-esque snout, although I think it still has a wavy jaw with that protruding fourth tooth. If in doubt you can always check its tongue for lingual salt glands on your way down. Well, it’s not as if you’ll be doing anything else is it?

    • 2
      Jack Ashby wrote on 25 October 2013:

      Thanks Daniel – the mugger is the reason that the point says “mostly” narrower snouts, but as you say they do have wavy jaws with a protruding fourth lower tooth.
      If only animals stop messing around with the rules. Someone should have a word.

    • 3
      facebook_lorna.steel2 wrote on 28 October 2013:

      Unfortunately, the position of the false gharials is still unresolved! Most writers tend to dodge the issue of the ‘tomistomine problem’- the sticky business of their affinities to either the crocs or true gharials. I’m hosting a French scientist next year (if he gets his Synthesys funding) who thinks he can get to the bottom of it once and for all, with the help of the fossil gharials, false gharials and crocs. Let’s wait and see , eh? Also, the early alligators looked more like crocs- they have the wavy jaw and the protruding tooth. So much for the rules!

    • 4
      Jack Ashby wrote on 28 October 2013:

      Thanks so much for that Lorna. It’s never easy is it! I suppose we should remember that taxonomy is an entirely human construct, and while utterly vital to our ability to understand the nature of things, it has very little to do with how the natural world works [Controversial?]
      Good luck with the funding, and send him our way if he does come – we have quite a few false gharials

    • 5
      facebook_lorna.steel2 wrote on 28 October 2013:

      Not controversial at all- it’s down to us silly humans liking to make rules! BTW I just remembered there’s a species of crocodile in Mexico too! But yes, in the Americas you are more likely to meet an alligator or caiman than a crocodile. I just came back from a trip where I got to hold a baby albino alligator, then I also helped feed its mother and some Cuban crocs, so your blog here was quite timely.

    • 6
      Jack Ashby wrote on 28 October 2013:

      Sounds brilliant! Thanks for the correction too. You’re quite right – Morelet’s crocodile (Crocodylus moreletii) is a Central American species. I’ll update the text now.

    • 7
      Specimen of the Week: Week 127 | UCL UCL Museums & Collections Blog wrote on 17 March 2014:

      [...] Bonus fact 6) If you aren’t sure how to tell the difference between an alligator and a crocodile, there just so happens to be a fog clearing explanation on our blog called What’s the difference between a crocodile and an alligator? [...]

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