Crocodile (reptile)

I

Introduction

Crocodile (reptile), large meat-eating reptile with long, strong jaws and a powerful, muscular tail. Crocodiles are air-breathing land animals but are well adapted for life in water, where they hunt, mate, and keep cool. Some crocodiles may reach more than 6 m (20 ft) in length and weigh more than 1,000 kg (2,200 lbs). Crocodiles live in tropical and subtropical regions and are found on all continents except for Europe and Antarctica.

The term crocodilian refers to all members of the order Crocodylia, which includes alligators, caimans, and gavials as well as crocodiles. Crocodilians have backbones and belong to a group of animals known as archosaurs. Crocodilians are more closely related to birds and the extinct dinosaurs than to lizards and turtles. Crocodilians have a more advanced physiology than other living reptiles and are also capable of complex behaviors such as caring for their young.

II

Crocodilians

Crocodilians are often called “living fossils” because they have changed very little in their appearance and habits for tens of millions of years.

A

Evolution of Crocodilians

Modern crocodilians are survivors from a much more varied group of archosaurs that lived during the Mesozoic Era, often called the Age of Reptiles. During the Triassic Period, two groups of archosaurs appeared, one related to crocodiles and another related to dinosaurs and birds. The crocodile-related archosaurs were the dominant land animals during much of the Triassic Period while dinosaurs remained fairly small in size. Many of these close relatives of crocodiles were highly active animals that walked erect on four limbs instead of sprawling, with some forms that moved bipedally on two hind legs. The direct ancestors of modern crocodilians were lightly built, fast-moving predators that lived on land and ran on four long legs.

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After a mass extinction at the end of the Triassic Period about 200 million years ago, dinosaurs became the dominant land animals. Only the ancestors of true crocodilians survived out of the many types of crocodile-like archosaurs that once existed. During the Jurassic and Cretaceous periods many unusual types of crocodilians evolved. Some resembled modern crocodilians and lived an amphibious lifestyle around water; a few grew to enormous size, reaching up to 12 m (40 ft) in length. Other crocodilians became marine animals, evolving paddles for limbs and flattened tails with a fin on the end. A number of types of crocodilians lived entirely on land, including one group that ate plants and could chew food the way mammals do.

Crocodilians survived another great mass extinction 65 million years ago—the event that wiped out the dinosaurs, flying reptiles (see Pterosaurs), and giant marine reptiles at the end of the Cretaceous Period. During the Cenozoic Era that followed, mammals became the dominant animals on Earth. Crocodilians remained highly successful predators in tropical regions, living in rivers, lakes, and swamps, and along coasts—habitats where they are still found today. Some extinct Cenozoic crocodilians grew to enormous size, rivaling the giants from the Mesozoic Era.

B

Characteristics of Crocodilians

Modern crocodilians are well-adapted as predators, with few natural enemies. They are amphibious, spending much of their time in water, where they swim with rhythmic strokes of the tail. The powerful tail is sometimes used to capture prey, sweeping it from shallow to deeper water, where it can be devoured more easily. As the crocodilian floats almost completely submerged, its protruding nostrils and eyes and a portion of its back are the only parts visible as it stalks its prey. On land, crocodilians move quickly in a belly crawl but can also gallop and walk mammal-like on all four legs.

Bony plates, called osteoderms, form a kind of armor in their thick skin. Their teeth, about 30 to 40 in each jaw, are set into sockets in the jawbones and interlock when the mouth is closed. In crocodiles, the fourth tooth on each side of the lower jaw protrudes when the mouth is closed; in alligators, these teeth are not visible. The jaws of crocodilians are powerful enough in closing to crush the bones of small animals, but so weak in opening that they can be held together by hand.

Crocodilians are physiologically the most advanced of all living reptiles. In some ways their internal anatomy resembles that of birds, including a four-chambered heart. Their blood also contains a special type of hemoglobin that can carry more oxygen than in other animals, allowing them to remain under water for long periods without breathing. They also have very active immune systems that quickly heal wounds exposed to bacteria-rich waters. Some individuals are thought to live up to 100 years.

Crocodilians have well-developed senses. Their hearing is comparable to that of birds and mammals, and functions well both in air and under water. Their eyesight is also keen; their eyes have a special retinal layer that reflects light to improve vision at night. Sense organs in scales on their heads and around their jaws have special pressure sensors that detect vibrations in water, allowing them to locate prey in total darkness. Some crocodilians also have pressure sensors along their bodies.

Crocodilians are cold-blooded like other modern reptiles, and their body temperature depends on the environment. They often bask in the sun out of water to raise their body temperature. Because of their large size, however, some crocodilians are also at risk of overheating and may enter water or gape their jaws to cool off. Crocodilians bury themselves in mud to estivate or hibernate. In warm regions they are dormant during droughts; in colder regions, during winter.

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