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What's the difference between a crocodile and an alligator?

Gator
AMERICAN
ALLIGATOR
Crocodiles and alligators - two creatures that share many similarities. But what are the real differences between them? This is probably the most frequently asked question when it comes to crocodilians, and while the answer may appear straightforward the real truths lie in the details. Croc
SIAMESE
CROCODILE

Diagram comparing snout shape 1. Different families: There are three groups (families) of crocodilians: the alligatoridae, which includes the alligator and the caimans; the crocodylidae, which includes the "true" crocodiles; and the gavialidae, which contains only the gharial. So, the first difference is that alligators and crocodiles are actually in different families.

2. Shape of the jaw: The easiest way of telling apart crocodiles from alligators, however, is to look at their noses. Alligators (and caimans) have a wide "U"-shaped, rounded snout (like a shovel), whereas crocodiles tend to have longer and more pointed "V"-shaped noses. This is illustrated in the diagram to the left (C = alligator, D = crocodile). The broad snout of alligators is designed for strength, capable of withstanding the stress caused to bone when massive force is applied to crack open turtles and hard-shelled invertebrates which form part of their diet. Of course, alligators eat softer prey too, but hard-shelled prey are ubiquitous in their environment and it's a big advantage to be able to eat them. Conversely, the pointed snout of a crocodile isn't quite as strong as the alligatorine shape, but the crocodile is still capable of exerting massive biting power. Crocodile jaws can be thought of as being more generalised - ideal for a wide variety of prey. The full extent of the way jaw shape influences diet isn't particularly well studied in crocodilians, but it's obvious that a very thin nose like a gharial's is much better at dealing with a fish than a turtle! There are 23 species of crocodilians, though, and this simple broad vs. narrow rule doesn't always work.

3. Placement of teeth: In alligators, the upper jaw is wider than the lower jaw and completely overlaps it. Therefore, the teeth in the lower jaw are almost completely hidden when the mouth closes, fitting neatly into small depressions or sockets in the upper jaw. This is particularly apparent with the large fourth tooth in the lower jaw (see [A] in diagram on right). In crocodiles, the upper jaw and lower jaw are approximately the same width, and so teeth in the lower jaw fit along the margin of the upper jaw when the mouth is closed. Therefore, the upper teeth interlock (and "interdigitate") with the lower teeth when the mouth shuts. As the large fourth tooth in the lower jaw also fits outside the upper jaw, there is a well-defined constriction in the upper jaw behind the nostrils to accommodate it when the mouth is closed (see [B] in diagram on right). This constriction occurs at the boundary of the premaxilla and the maxilla in the upper jaw. Diagram showing arrangement of teeth

Lingual salt glands4. Lingual salt glands: Crocodiles and gharials also differ from alligators and caimans in having functioning salt glands on their tongue. Structurally, these are actually modified salivary glands, and while alligators and caimans also have these structures they appear to have lost the ability to use them for excreting significant amounts of salt. This makes crocodiles more tolerant to life in saline water, including sea water in some species. Moreover, it suggests that crocodiles have a more recent marine ancestry: the ability to migrate across wide marine bodies, and even live there for extended periods, would certainly explain their current wide distribution across different continents. If ancestral species could live in marine environments, this ability has not been completely lost in modern crocodiles. Species such as saltwater crocodiles (C. porosus) can survive for extended periods in tidal estuaries, around the coast, and even out to sea. Alligators and caimans have lost much of this osmotic ability to secrete excess salt through the tongue glands, and can only tolerate it for short periods of time, prefering to remain in freshwater areas when possible. However, it is not unknown for large alligators to find their way into tidal mangroves and very rarely into coastal areas.

Sensory pits on alligator jaw5. Integumentary sense organs: Both crocodiles and alligators have small, sensory pits dotted around the upper and lower jaws - take a close look on a photograph, and you'll see small, black speckles almost like unshaven stubble. These are capable of detecting small pressure changes in water, and assist in locating and capturing prey. These were originally called ISOs, or Integumentary Sense Organs, although recent research has renamed them DPRs (Dermal Pressure Receptors). Crocodiles have similar organs covering virtually every scale on their body, but alligators and caimans only have those around the jaws. Although it's been known for years that sense organs on the jaws are involved in pressure detection, nobody is quite sure what those organs covering the rest of the body in crocodiles actually do. They probably extend the sensory surface over the crocodile's entire body, but previous researchers have suggested they may assist in chemical reception, or even salinity detection. The confusion lies over why crocodiles have them, but not alligators and caimans. Regardless of their role, they're very good at telling apart crocodile skin from alligator skin. Crocodile and alligator skin wallets, handbags, boots etc are easy to tell apart - if the scales have a small spot or dimple close to the edge, you know the skin is from a crocodile and not an alligator or caiman. This is illustrated below - the alligator on the left does not have any sense organs, but the crocodile on the right does.

Neck of alligator
GULAR REGION (NECK) OF AMERICAN ALLIGATOR
 
Neck of crocodile
GULAR REGION (NECK) OF AMERICAN CROCODILE

Other differences: The above points are amongst the most obvious differences between crocodiles and alligators in terms of external appearance. However, each species is unique, and to list all the possible differences would be like comparing a jaguar with a lion. Differences in behaviour are also apparent. Most people regard crocodiles as more aggressive than alligators, and this is true of some species. For example, alligators are relatively docile next to saltwater crocodiles, but there are many species with many differerent kinds of behaviours and temperaments. A general rule that crocodiles are more aggressive than alligators just isn't possible to make. Alligators can often reach at least 14 or 15 feet in length, which is larger than some crocodile species, but not others. The largest crocodile species is the saltwater crocodile, which can get to at least 17 or 18 feet - some rare individuals exceeding 20 feet after many years. The African dwarf crocodile, as a contrast, doesn't grow larger than 4 or 5 feet.

The two images below show an exception to the "jaw shape" rule. The Indian mugger (Crocodylus palustris) breaks the crocodile convention of having narrow jaws - its jaws are superficially very similar in shape to those of an alligator, although the fourth lower tooth is still visible. When all the above criteria are considered, the mugger is definitely a crocodile. Always bear the details in mind when faced with general questions like what's the difference between crocodiles and alligators. As you can hopefully see, the simple answer is not always the most interesting!

American alligator
AMERICAN ALLIGATOR
 
Indian mugger
INDIAN MUGGER

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