Crocodiles

Crocodiles

Biology of crocodilians

Species of crocodilians

Crocodilians and people

Resources

The crocodile order (Crocodylia) consists of several families of large, unmistakable, amphibious reptiles: the crocodiles (Crocodylidae), gavials (Gavialidae), and the alligators and caimans (Alligatoridae). Although these animals look superficially like lizards, they are different in many important respects, and are believed by biologists to be the most highly evolved of the living reptiles.

Crocodilians are amphibious animals, spending most of their time in water but emerging onto land to bask in the sun and lay their eggs. Their usual habitat is in warm tropical or subtropical waters. Most species occur in freshwater, with only the saltwater crocodile being partial to marine habitats. Fish are the typical food of most adult crocodilians, but the biggest species will also eat large mammals, including humans. Younger crocodilians eat invertebrates and small fish.

Crocodilians are economically important for their thick, attractive hide, which can be used to make fine leather for expensive consumer goods, such as shoes, handbags, and other luxury items. Wild crocodilians are hunted for their hide wherever they occur, and in some areas they are also raised on ranches for this purpose. Crocodilian meat is also eaten, often as a gourmet food.

Most populations of wild crocodilians have been greatly reduced in size because of overhunting and habitat loss, and many species are endangered.

Biology of crocodilians

Among the more distinctive characteristics of the crocodilians are their almost completely four-chambered heart, teeth that are set into sockets in the jaw, a palate that separates the mouth from the nasal chambers, and spongy lungs. These animals also have a protective covering of partially calcified, horny plates on their back. The plates are notconnected with each other, and are set into the thick, scaly skin, allowing great freedom of movement. Crocodilians have a heavy body with squat legs and a large, strong, scale-ridged tail.

Sinusoidal motions of the powerful tail are used to propel the animal while swimming. The tail is also a formidable weapon, used to subdue prey and also for defense. Although they appear to be ungainly and often spend most of their time lying about, crocodilians can actually move quite quickly. Some crocodilians can even lift their body fully above the ground, and run quickly using all four legs; a human cannot outrun a crocodile over a short distance on open land.

Crocodilians have numerous adaptations for living in water. They have webbed feet for swimming

slowly and their nostrils, eyes, and ears are set high on the head so they can be exposed even while most of the body and head are below the surface. When a crocodilian is totally submerged, its eye is covered by a semi-transparent nictitating membrane, and flaps of skin seal its nostrils and ears against water inflow. Crocodilians often float motionless in the water, commonly with the body fully submerged and only the nostrils and eyes exposed. To accomplish this behavior, crocodilians regulate their body density by varying the amount of air held in the lungs. Also, the stomach of most adult crocodiles contains stones, to as much as 1% of the animal’s total body weight. The stones are thought to be used as buoyancy-regulating ballast.

Crocodilians are poikilothermic, meaning they do not regulate their body temperature by producing and conserving metabolic heat. However, these animals are effective at warming themselves by basking in the sun, and they spend a great deal of time engaged in this activity. Crocodilians commonly bask through much of the day, often with their mouths held open to provide some cooling by the evaporation of water. Only when the day is hottest will these animals re-enter the water to cool down. Most species of crocodilians are nocturnal predators, although they will also hunt during the day if prey is available.

Stories exist of birds entering the open mouths of crocodiles to glean leeches and other parasites. This

phenomenon has not been observed by scientists, although it is well known that crocodiles will tolerate certain species of birds picking external parasites from their skin, but not necessarily inside of their mouth.

Male crocodilians are territorial during the breeding season, and chase other males away from places that have good nesting, basking, and feeding habitat. The male animals proclaim their territory by roaring loudly, and sometimes by snapping their jaws together. Intruders are aggressively chased away, but evenly matched animals may engage in vicious fights. Territory-holding males do not actively assemble females into a harem. Rather, they focus on chasing other males away from a territory. Females will enter the defended territory if they consider it to be of high quality.

All crocodilians are predators, and they have large, strong jaws with numerous sharp teeth for gripping their prey. Crocodiles do not have cutting teeth. If they capture prey that is larger than they can eat in a single gulp, it is dismembered by gripping strongly with the teeth and rolling their body to tear the carcass. Some crocodiles will opportunistically cooperate to subdue a large mammal, and then to tear it into bits small enough to be swallowed. However, extremely large animals with tough skin, such as a dead hippopotamus, will be left to rot for some time until the carcass softens and can be torn apart by the crocodiles.

All crocodilians are oviparous, laying hard, white-shelled eggs. The nest may be a pit dug into a beach above high water, or it may be made of heaps of aquatic vegetation, which help to incubate the eggs through heat produced during decomposition. Often, a number of females will nest close to each other, but each builds a separate nest. The nesting grounds are used by the same females, year after year. In many species the female carefully guards and tends her nest. Fairly open, sandy beaches are generally preferred as sites upon which to build the nest or nest mound.

Typically, 20-40 eggs are laid at a time, but this varies with species and the size of the female. Incubation time varies with species and temperature, but ranges, in the case of the Nile crocodile, from 11 to 14 weeks. Predators as diverse as monitor lizards, mongooses, dogs, raccoons, and even ants seek out crocodile nests to eat the eggs and newly hatched young.

An infant crocodilian has a small, so-called”egg tooth“at the end of its snout, which helps it to break out of the shell when ready to hatch. All of the baby crocodilians hatch within a short time of each other, synchronized in part by the faint peeping noises they make during the later stages of incubation. In some crocodilians, the mother assists her babies in hatching, by gently taking eggs into her mouth and cracking them with her teeth. The mother also may guard her offspring for some time after hatching, often allowing them to climb onto her body and head. Female crocodiles are very aggressive against intruders while their eggs are hatching and newborn babies are nearby, and under these circumstances they will even emerge from the water to chase potential predators away. Nevertheless, young crocodilians are vulnerable to being eaten by many predators, and this is a high-risk stage of the life cycle.

Potentially, crocodilians are quite long-lived animals. Individuals in zoos have lived for more than 50 years, and the potential longevity of some species may be as great as a century.

Species of crocodilians

The gavial or gharial (family Gavialidae) is a single species, Gavialus gangeticus, which lives in a number of sluggish, tropical rivers in India, Nepal, Bangladesh, and Indochina. Gavials have a long, slender snout, and are almost exclusively fish eaters, catching their prey with sideways sweeps of the open-mouthed head. Gavials can attain a length of about 20 ft (6 m). They are considered holy in the Hindu religion, and this has afforded these animals a measure of protection in India. Unfortunately, this is not sufficiently the case anymore, and gavials have become endangered as a result of overhunting for their hide.

The true crocodiles (family Crocodylidae) include about 14 species that live in tropical waters. Crocodiles are large, stout animals, with a much heavier snout than that of the gavial. The main food of crocodiles is fish, but some species can catch and subdue large mammals that venture close to their aquatic habitat, or attempt to cross rivers in which the crocodiles are living. Perhaps the most famous species is the Nile crocodile (Crocodylus niloticus ) of Africa, which can grow to a length of 23 ft (7 m). This crocodile can be a predator of unwary humans, although its reputation in this respect far exceeds the actual risks, except in certain places. This species used to be very abundant and widespread in Africa, but unregulated hunting, and to a lesser degree habitat loss, have greatly reduced its population.

The most dangerous crocodilian to humans and the largest crocodilian is the estuarine or saltwater crocodile (Crocodylus porosus ), which lives in salt and brackish waters from northern Australia and New Guinea, through most of Southeast Asia, to southern India. This species can achieve a length of more than 23 ft (7 m). Individuals of this crocodile species sometimes occur well out to sea.

Other species are the mugger crocodile (Crocodylus palustris ) of India, Bangladesh, and Ceylon; the Australian crocodile (C. johnsoni ) of northern Australia; and the New Guinea crocodile (C. novaeguineae )of New Guinea and parts of the Philippines.

The American crocodile (Crocodylus acutus ) is a rare and endangered species of brackish estuaries in southern Florida, occurring more widely in central and northwestern South America and the Caribbean. This species can achieve a length of 20 ft (6 m). The Orinoco crocodile (C. intermedius ) occurs in the Orinoco and Amazon Rivers of South America.

The false gavial (Tomistoma schlegelii ) is a slender-snouted species of Southeast Asia.

The alligators and caimans (family Alligatoridae) are eight species that occur in freshwater, with a broader head and more rounded snout than crocodiles. The American alligator (Alligator mississipiensis ) can achieve a length of 13 ft (4 m), and occurs in the southeastern United States as far north as South Carolina and Alabama. This species was endangered by unregulated hunting for its hide. However, strict conservation measures have allowed for a substantial recovery of the species, and it is now the subject of a regulated hunt.

The Chinese alligator (Alligator sinensis ) occurs in the lower reaches of the Yangtze and Kiang rivers in southern China, and it is the only member of its family to occur outside of the Americas. The Chinese alligator can grow as long as 6.5 ft (2 m).

Caimans are animals of freshwater habitat in South and Central America. The black caiman (Melanosuchus niger ) can achieve a length of 16 ft (5 m). The spectacled or common caiman (Caiman crocodilus ) and the broad-nosed caiman (C. latisrostris ) of eastern Brazil can both grow somewhat longer than 6.5 ft (2 m). The dwarf caiman (Paleosuchus palpebrosus ) and the smooth-fronted caiman (P. trigonatus ) live in more swiftly flowing streams and rivers and are relatively small species that do not exceed about 5 ft (1.5 m) in length. The Yacaré (Caiman yacare ) has the southernmost distribution of any caiman and is found in Brazil, Argentina, Bolivia, and Paraguay. This species reaches lengths of (2.5–3 m) and eats a variety of aquatic animals, especially snails and fish.

Crocodilians and people

The larger species of crocodilians are fierce predators. In particular, crocodiles have posed a longstanding risk to domestic livestock that try to drink from their aquatic habitat as well as to unwary humans. For this reason, crocodiles are commonly regarded as dangerous pests, and they are sometimes killed to reduce the risks associated with their presence.

Because some species of crocodilians are dangerous, they are greatly feared in many places. This fear is justified in some cases, at least in places where large human-eaters are abundant. In some cultures, the deep fear and revulsion that people have for dangerous crocodilians has transformed into an attitude of reverence. For example, a pool near Karachi, Pakistan, contains a number of large mugger crocodiles, which are venerated as priest-like entities and worshipped by pilgrims. In other places, human sacrifices have been made to crocodiles to pacify animist spirits.

The skins of crocodilians can be used to make a very tough and beautiful leather. This valuable product is widely sought for use in making expensive shoes, handbags, wallets, belts, suitcases, and other items. Crocodilians are readily hunted at night, when they can be found using searchlights that reflect brightly off

KEY TERMS

Endangered— Refers to species or populations of organisms that are so small that there is a likelihood of imminent local extirpation, or even global extinction over its entire range.

Extirpated— The condition in which a species is eliminated from a specific geographic area of its habitat.

Nictitating membrane— An inner eyelid.

Overhunting— Hunting of an animal at a rate that exceeds its productivity, so that the population size decreases, often to the point of endangerment.

Oviparous— This refers to an animal that lays eggs, from which the young hatch after a period of incubation.

Poikilotherm— Refers to animals that have no physiological mechanism for the regulation of their internal body temperature. These animals are also known, less accurately, as”cold-blooded.“In many cases, these animals bask in the sun or engage in other behaviors to regulate their body temperature.

their eyes. In almost all parts of the range of crocodilians, they have been hunted to endangerment or extirpation. Most species in the crocodile family are endangered to some degree.

In a few places, however, strict conservation regulations have allowed the populations of crocodilians to increase from historically depleted lows. This has been particularly true of the American alligator, which was considered to be an endangered species only a few decades ago, but has now recovered sufficiently to allow for a carefully regulated sport and market hunt.

Some species of crocodilians are also ranched, usually by capturing young animals in the wild and feeding them in confinement until they reach a large enough size to slaughter for their hide. The meat of crocodilians is also a saleable product, but it is secondary in importance to the hide.

Crocodilians are sometimes used to entertain people, and some species are kept as pets. The Romans, for example, sometimes displayed Nile crocodiles in their circuses. To amuse the masses of assembled people, the crocodiles would be killed by humans, or alternatively, humans would be killed by the crocodiles.

In more modern times, alligator wrestling has been popular in some places, for example, in parts of Florida. The key to successful alligator wrestling is to hold the jaws of the animal shut, which can be accomplished using only the hands (but watch out for the lashing tail!). Crocodilians have very powerful muscles for closing their jaws, but the muscles to open their mouth are quite weak.

Resources

BOOKS

Alderton, D. Crocodiles and Alligators of the World. New ed. New York: Facts on File, 2004.

Dudley, K. Alligators and Crocodiles. New ed. London: A & C Black, 2000.

Grenard, S. Handbook of Alligators and Crocodiles. New York: Krieger, 1999.

Halliday, T.R., and K. Adler. The New Encyclopedia of Reptiles and Amphibians. New York: Oxford University Press, 2002.

Messel, H., F.W. King, and J.P. Ross, eds. Crocodiles: An Action Plan for Their Conservation. Gland, Switzerland: IUCN, 1992.

Webb, G., S. Manolis, and P. Whitehead, eds. Crocodiles and Alligators. Australia: Surrey Beatty, 1988.

Zug, George R., Laurie J. Vitt, and Janalee P. Caldwell. Herpetology: An Introductory Biology of Amphibians and Reptiles. 2nd ed. New York: Academic Press, 2001.

Zweifel, R.G., H.G. Cogger, and D. Kirshner, eds. Encyclopedia of Reptiles and Amphibians. 2nd ed. Academic Press, 1998.

Bill Freedman