Crocodylia

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Crocodilians, along with birds, are the only survivors of the once-prevalent Archosauria. During the Mesozoic (245-65 million years ago) this group, including dinosaurs and other reptiles, dominated life on all continents and in the oceans. Most or all of crocodilians' adaptations had already evolved by the late Triassic (about 200 million years ago). Crocodilians are the most advanced surviving reptiles; many of their features are more similar to mammals or birds than to other reptiles.

There are only a few members, and all modern crocodilians have adapted to a semi-aquatic life, although as recently as 3000 years ago there may have been a terrestrial crocodilian species on New Caledonia. Some crocodiles may venture into larger bodies of fresh or salt water, but all must lay their eggs on dry land. Most crocodilians live in the tropics. The only exceptions are the American alligator ( Alligator mississippiensis) and the Chinese alligator ( Alligator sinensis), and they still cannot tolerate climates colder than temperate climates. No crocodilians venture out of lowlands; it is speculated that none ever lived above 1000 meters (3260 feet) above sea level.

There are three families of Crocodylia, with 23 species total.

All crocodilians have a similar body shape, with a head held horizontally in front of the body, four legs which project from the sides, heavy scales which function as armor, and a heavy muscular tail. Their front feet have five separate toes and their rear feet have four partially-webbed toes. Their eyes are on the top of their head, close together to allow for binocular vision (the field of vision of the two eyes intersects, to provide more accurate depth perception in front of the animal). The nostrils are crescent-shaped and valvular, and set at the end of the snout, which allows breathing even when the animal is almost entirely submerged.

Crocodilians range in size from Cuvier's dwarf caiman, which only grows to about 1.5 meters (five feet) long, to the Indopacific crocodile, which grows to 7 meters (23 feet) long. Male crocodilians are larger than females.

Their skin is covered with non-overlapping scales composed of the protein keratin and often studded with bony plates called scutes. (Lizard scales are similar, but lack the bony plates.) The scales are shed individually, so crocodilians do not molt (shed their skin all at once) like snakes do. This skin does not provide much insulation, preventing crocodilians from inhabiting cooler climates.

Crocodilians' normal gait is with their bodies off the ground; their ankles swivel to allow their legs to be almost underneath their body, making their gait resemble mammals'. No other reptiles move in this way. Crocodilians can also run by simply speeding up their walk; in smaller crocodilians this may change the gait into a "gallop" in which the animal appears to be bouncing; some may acheive speeds of 10 miles per hour (17 kph). Crocodilians can also move like lizards, moving one foot at a time with their bellies scraping on the ground; they also use this mode of movement when sliding down a river bank when frightened and sometimes occurs when they are running and their legs get out of sync.

Crocodiles swim with back-and-forth movements of their tail. Most of the time crocodilians cruise slowly through the water, holding their legs against their body to reduce drag. They are also capable of great bursts of speed, including a "tail walk" like dolphins do, in which their head and body are held vertically out of the water.

All crocodilians lay eggs in nests made out of plant material and/or mud; the nests may dry so hard that hatchlings would be trapped inside without help. Adults, especially mothers, often guard nests. All eggs in a nest hatch at the same time, and the entire brood leaves the nest at once. Adults will respond aggressively to hatchlings' distress cries, and mothers (and fathers, in some species) may attend hatchlings for several weeks. Crocodilians from the same brood stay together at least while they are young; in some species family groups stay together much longer.

Crocodilians' senses of smell, sight, and hearing are well developed. Their ears are covered by flaps which close to prevent water from entering them. Crocodilians' eyes are immobile spheres covered by three eyelids: the third eyelid, the nictitating membrane, is transparent, but protects the eye from water. They have vertical, cat-like pupils which dilate to allow them to see well in the dark. A layer of tapetum at the back of their eyes greatly increases their ability to see at night as well; this also makes their eyes glow in the dark. Crocodilians do not see well underwater.

Crocodilians have an elongate open-ear canal. They have two openings in their skull behind their eye sockets; it has been suggested that these openings help the attachment or functioning of crocodilians' powerful jaw muscles.

Their thoracic and abdominal cavities are separated by a muscular diaphragm, used in breathing. Their nostrils close when the animal dives; the nostrils are separated from the mouth by a bony palate (like mammals have, although other reptiles don't) and a valve in the back of the mouth. This allows crocodilians to breathe when their entire body except their nostrils are submerged, and also when holding prey.

Crocodilians' hearts have four chambers like mammals and birds, but there is a pore between the left and right ventricles which allows some mixing.

Crocodilians are poikilothermic (cold-blooded), and can only regulate their internal body temperature by arranging for their environment to warm them. Temperate species will bask in the sun during the day to raise their body temperature, returning to the water to cool off; they mostly hunt at night, leaving the daytime for basking in the sun. Opening their mouths can also cool them off, since the large exposed wet surface allows much evaporation. Tropical species may avoid the hot sun by remaining under water or mud during the day. Some crocodilians also estivate (sleep out the summer). Alligators, which live in temperate regions, may remain completely submerged except for their nostrils when the air is very cold in the winter. After feeding, crocodilians tend to seek more heat, as it speeds digestion.

Sexual maturity is reached once crocodilians reach a certain age and size (both are important: a crocodilian not big enough to become sexually mature may not, even if it is old enough). Crocodilians' sex is determined by the temperature of the environment at a critical stage in development; there is no X or Y chromosome, like birds and mammals have. Crocodilians continue to grow their entire lives, even after sexual maturity has been reached.

Crocodilians' teeth are mostly identical, thick-walled cones embedded in the outside of the jaw. Rear teeth are more cylindrical and blunter than front teeth. The teeth are not cemented into the jaw but simply attached to the outside, and can be broken off. Crocodilians can replace their teeth an indefinite number of times (as opposed to mammals, which only replace individual teeth once). Crocodilians do not have lips and their mouths leak when closed.

Their stomachs are the most acidic recorded for any vertebrate, allowing them to digest even the bones and shells of prey animals. Their digestion is also aided by a muscular gizzard containing stones to help break down food

All crocodilians have strong jaw muscles for biting and holding prey. They are all entirely carnivorous. Prey is not chewed or ground in the mouth: once it is impaled on the sharp teeth, it is swallowed whole. Crocodilians need not feed often. A study on Nile crocodiles indicated that they usually eat only about 50 full meals a year. While their staple food is always fish, large crocodilians may eat mammals; smaller ones' diet may include insects, tadpoles, frogs, snails, crabs, shrimps, birds and small fishes. Snakes, molluscs, turtles and bats may also be eaten. Crocodilians are opportunistic hunters; they eat whatever they can catch if they are hungry. Flying prey can be caught by the crocodilians leaping into the air with thrusts of their powerful tail. Some larger crocodilians may also eat humans.

Food is stored as fat in the animals' tails, backs, and elsewhere in the body; up to 60% of the food intake may be converted to fat. This, the fact that they're poikilothermic (discussed above), and crocodilians' opportunistic hunting strategy allow crocodilians to survive for long periods with no food at all: large crocodilians may be able to survive up to two years between meals.

Crocodilians hunt by lying concealed until some prey comes close to them. They are capable of very short bursts of high speed to catch prey, but cannot maintain speed to give chase if the initial attack is unsuccessful. These extreme bursts of speed also produce large amounts of lactic acid, which crocodilians are inefficient at removing; after an attack, they must rest to clean out their blood and replenish their oxygen supply.

Crocodilians are normally solitary animals, but plentiful food may bring many individuals together. Some species have been observed to hunt cooperatively. Even when large numbers congregate, they do not seem to fight over food. When large prey is caught, another crocodile may help to dismember it, so that the pieces are small enough to eat. Polygyny (males mate with more than one female) has been observed in all species studied.

Crocodilians communicate with each other by means of sounds, postures, motions, odors released by four scent glands, and by touch. Vocal sounds are made by forcing air through a voice box (larynx) in the throat. Young call to adults when in danger, but also are very vocal while being fed. Sounds (by the hatchlings themselves or by adults) also seem to keep young together. Adults also produce sounds to communicate with other adults. The most common adult sound is a loud, low roar which is repeated, and may be echoed by other adults. During mating, softer "purrs" are made. Threatened crocodilians may growl. Adults also grunt to signal to juveniles they will help, and may hiss while defending juveniles. They also communicate by slowly lifting their jaws off the ground with their mouth closed.

Adults are territorial, and mark their territory by loudly slapping their head down on the water or snapping their jaws on the surface of the water. Dominant animals tend to swim higher in the water; other crocodilians of the same species communicate their submission by swimming lower in the water. Dominant animals control access to mates, choice nesting sites, food, basking sites, and living space. During drought, territories are forgotten as crocodilians crowd into the smaller remaining inhabitable area, although hierarchies are still observed. In some species and in some areas, territories are only maintained by males or only during mating season.

Combat between crocodilians is rare, but does sometimes occur between animals of the same size competing for dominance. The two combatants line up next to each other facing opposite directions and bang the sides of their heads together; they also sometimes bite each other, but in either case they rarely cause any lasting damage.

References

Cogger, H. and R. Zweifel. 1998. Encyclopedia of Reptiles and Amphibians. Academic Press, San Diego, CA.

Ross, C.A., and Stephen Garnett. 1989. Crocodiles and Alligators. Facts on File, Inc. New York.

Rue III, Leonard Lee. 1994. Alligators & Crocodiles. Todtri Productions Limited. New York


Author: Danny Goodisman

Last updated 9 September 2002

Glossary

bilateral symmetry

having body symmetry such that the animal can be divided in one plane into two mirror-image halves. Animals with bilateral symmetry have dorsal and ventral sides, as well as anterior and posterior ends. Synapomorphy of the Bilateria.

ectothermic

animals which must use heat acquired from the environment and behavioral adaptations to regulate body temperature

motile

having the capacity to move from one place to another.