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10.13: Alligators and Crocodiles

Difficulty Level: At Grade Created by: CK-12
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Crocodile or Alligator?

This picture is a crocodile, identified by it's V-shaped snout. Alligators have more of a U-shaped snout. Although crocodiles and alligators have a few differences, they are very much alike and belong to the same order, Crocodilia.

Alligators and Crocodiles

Crocodilia, containing both alligators and crocodiles, is an order of large reptiles. Reptiles belonging to Crocodilia are the closest living relatives of birds. Reptiles and birds are the only known living descendants of the dinosaurs.

The basic crocodilian body plan (Figure below) is a very successful one and has changed little over time. Modern species actually look very similar to their Cretaceous ancestors of 84 million years ago.

Nile crocodiles display the basic crocodilian body plan.

Characteristics of Crocodiles

Crocodilians have a flexible, semi-erect posture. They can walk either in a low, sprawled “belly walk,” or hold their legs more directly underneath them to perform the “high walk.” Most other reptiles can only walk in a sprawled position.

All crocodilians have, like humans, teeth set in bony sockets. But unlike mammals, they replace their teeth throughout life. Crocodiles and gharials (large crocodilians with longer jaws) have salivary glands on their tongue, which are used to remove salt from their bodies. Crocodilians are often seen lying with their mouths open, a behavior called gaping. One of its functions is probably to cool them down, but it may also have a social function.

Crocodilians are known to swallow stones, known as gastroliths, which help digest their prey. The crocodilian stomach is divided into two chambers. The first is powerful and muscular. The other stomach is the most acidic digestive system of any animal. It can digest mostly everything from their prey, including bones, feathers, and horns!

The sex of developing crocodilians is determined by the temperature of the eggs during incubation, when eggs are kept warm before they hatch. This means that the sex of crocodilians is not determined genetically. If the eggs are kept at a cold or a hot temperature, then their offspring may be all male or all female. To get both male and female offspring, the temperature must be kept within a narrow range.

Evolving More Complex Structures

Like all reptiles, crocodilians have a relatively small brain, but the crocodilian brain is more advanced than those of other reptiles. Because of their aquatic habitat, the eyes, ears, and nostrils are all located on the same "face" in a line one after the other.

The crocodiles have advanced sensory organs. They see well during the day and may even have color vision, and they also have excellent night vision. A third transparent eyelid, the nictitating membrane, protects their eyes underwater. The eardrums are located behind the eyes and are covered by a movable flap of skin. This flap closes, along with the nostrils and eyes, when they dive. This prevents water from entering their external head openings. Their jaws are covered with sensory pits, which hold bundles of nerve fibers that respond to the slightest disturbance in surface water. Crocodiles can detect vibrations and small pressure changes in water. This makes it possible for them to sense prey and danger even in total darkness.

Like mammals and birds, and unlike other reptiles, crocodiles have a four-chambered heart. But, unlike mammals, blood with and without oxygen can be mixed.


  • gaping: Lying with the mouth open, possibly to cool down.
  • gastroliths: Swallowed stones in the stomach that aid in digestion.
  • incubation: Keeping eggs warm before they hatch.
  • nictitating membrane: A transparent eyelid that protects the eyes.
  • sensory pits: Nerve fibers that respond to vibrations.


  • Crocodilians swallow stones, known as gastroliths, which help digest their prey.
  • The sex of developing crocodilians is determined by the temperature of the eggs during incubation.
  • The crocodiles have advanced sensory organs, including keen eyesight, eardrums, and sensory pits that detect disturbances in the water.


Use the resource below to answer the questions that follow.

  1. Where do American alligators live?
  2. What do alligators eat?
  3. What conditions have led to the recovery of the American alligator?
  1. In what kind of courting displays do saltwater crocodiles (Crocodylus porosus) engage?
  2. Do saltwater crocodiles have internal or external fertilization?
  1. What kind of maternal care do crocodile mothers show their young? When does this care begin?
  2. What do baby crocodiles do as soon as they hatch? How do they benefit from this behavior?
  3. Why is it important for mother crocodiles to guard their nests?


  1. How is the sex of alligators and crocodiles determined?
  2. What sorts of special sensory organs do crocodiles have?




Lying with the mouth open, possibly to cool down.


Swallowed stones in the stomach that aid in digestion.


Keeping eggs warm before they hatch.
nictitating membrane

nictitating membrane

Transparent eyelid that protects the eyes.
sensory pits

sensory pits

Nerve fibers that respond to vibrations.

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Difficulty Level:
At Grade
7 , 8
Date Created:
Nov 29, 2012
Last Modified:
Mar 23, 2016
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