Crocodiles

01 July 2010 | Fact sheet

What is the difference between a crocodile, an alligator and a gharial? Can crocodilians run? What’s the link between temperature and crocodilians’ sex and why do crocodilians attack people?

1. What are “crocodilians”?

The term "crocodilians" refers to members of three families: the Family Crocodylidae (crocodiles), Family Alligatoridae (alligators and caimans) and Family Gavialidae (gharial). The term "crocodiles" refers only to crocodiles.

2. What is the difference between a crocodile, an alligator and a gharial?

The main difference between the three families is associated with the head, particularly the jaws and skull. Although they function in the same way in all crocodilians, different bones compose certain structures in different ways. Some external characteristics of the head make it possible to see these differences. 

Alligators
Alligators tend to have broad, ‘shovel-shaped’ snouts. The upper jaw is so broad that when the jaws are closed many of the teeth of the lower jaw fit into sockets along the edge of the expanded upper jaw. The 4th tooth on the lower jaw, which in all crocodilians is enlarged, fits into a socket in the upper jaw of the alligator when its jaws are closed, so that its tip is hidden.

Crocodiles
In contrast, the upper jaw of crocodiles is not so broad and when they close their jaws, the enlarged 4th tooth on the lower jaw is clearly visible.

But when in water, crocodiles and alligators seem identical: they both have a ‘minimum exposure’ posture, in which only the eyes, the cranial platform (overlying the brain), ears and nostrils lie above the surface.

Gharials
Compared to alligators and crocodiles, a gharial has a very long and narrow snout. However, this does not mean that the whole head is longer than that of other crocodilians: gharials simply have a greater proportion of the head allocated to the snout. As a result, the head length of a 3 m long gharial is not very different from the head length of a Saltwater Crocodile of the same total length.

3. Can crocodilians run?

Some crocodilians, such as the Australian Freshwater Crocodile, gallop when they need to move quickly on land, which allows them to jump over rocks and logs. When crocodiles gallop, their front limbs go out and forward while the back limbs push the body forward. The tail tends to move up and down rather than from side to side. The maximum speed that crocodiles can reach when galloping is about 18 km/h, although they become exhausted before they have covered 100 m.

4. What’s the link between temperature and crocodilians’ sex?

The sex of crocodilians is determined by the incubation conditions, particularly the temperature. Incubation at 30°C or less gives exclusively females, incubation at around 31°C gives both sexes, whereas incubation between 32°C and 33°C gives mostly males. Incubation at temperatures above 33°C gives males in some species, whereas in others, the sex reverts to females.

5. Why do crocodilians attack people?

Eight of the 23 crocodilian species are known to carry out unprovoked attacks on people. There can be various reasons behind this.

Crocodilians may attack people when hunting for food, as people are well within the size range of prey that can be taken by them. They can also attack to defend their territory against intruders or to defend their nest or young. Sometimes crocodilians hurt people by mistake, when in fact the attack is directed at dogs or other animals.

The Nile Crocodile is responsible for more attacks on humans than any other crocodilian species. After lions and hippos, the Nile Crocodile causes the highest number of wildlife-related fatalities in Africa. It is difficult to estimate the total number of attacks per year as many non-fatal attacks go unreported, but since the year 2000, it is believed to have exceeded 300 per year.

Source and more information: IUCN Crocodile Specialist Group