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Crocodilians

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Crocodilians

Crocodilians
Temporal range: Late Cretaceous—Present, 83.5–0Ma
Saltwater crocodile (Crocodylus porosus)
Subgroups
Crocodylia Distribution

The Crocodilia (or Crocodylia) are an order of large reptiles that appeared 83.5 million years ago in the Late Cretaceous period (Campanian stage). They are the closest living relatives of birds, as the two groups are the only known survivors of the Archosauria.[1] Members of the crocodilian total group, the clade Pseudosuchia, appeared about 250 million years ago in the Early Triassic period,[2][3] and diversified during the Mesozoic era. The order Crocodilia includes the true crocodiles (family Crocodylidae), the alligators and caimans (family Alligatoridae) and the gharials (family Gavialidae). Although the term 'crocodiles' is sometimes used to refer to all of these, a less ambiguous vernacular term for this group is 'crocodilians'.

Crocodilians are large, solidly built lizard-like reptiles with long flattened snouts and laterally compressed tails, and eyes, ears and nostrils at the top of the head. They swim well and can move on land in a "high walk" and a "low walk". Their skin is thick and covered in non-overlapping scales. They have conical, peg-like teeth and a powerful bite. Like birds, they have a four-chambered heart and a unidirectional system of air flow around the lungs, but like other reptiles they are ectotherms.

Crocodilians are found mainly in lowlands in the tropics, but alligators also occur in the southeastern United States and the Yangtze River in China. They are largely carnivorous, the various species feeding on animals such as fish, crustaceans, mollusks, birds and mammals; some species like the Indian gharial are specialized feeders, while others like the saltwater crocodile have generalized diets.

Eight species of crocodilians have attacked humans. The largest number of attacks come from the Nile crocodile. Humans are the largest threat to crocodile populations through activities including hunting and habitat destruction, but crocodile farming has greatly reduced unlawful trading in wild crocodile skins. Crocodilians appear in folklore and literature from around the world from the time of Herodotus and Pliny the Elder. The story that crocodiles weep for their victims has been current since the 9th century and was spread by Sir John Mandeville in 1400 and then by William Shakespeare.

Spelling and etymology

The group is often spelled 'Crocodylia' for consistency with the genus Crocodylus (Laurenti, 1768). However, the original name as published by Richard Owen in 1842 had the -i- spelling, more accurately Latinizing the Greek κροκόδειλος (crocodeilos), which means both 'lizard' and 'Nile crocodile'.[4] The Greek name in turn is assumed[5] to derive from κρόκε (kroke), shingle or pebble, and δρîλος or δρεîλος (dr(e)ilos), worm. The name may refer to the animal's habit of basking on the pebbled shores of the Nile.[6]

Morphology and physiology


Crocodilians range in size from the Paleosuchus and Osteolaemus species which reach 1–1.5 m (3 ft 3 in–4 ft 11 in) to the saltwater crocodile which reaches 7 m (23 ft) and weigh up to 2,000 kg (4,400 lb). They have solidly built, lizard-like bodies with elongated, flattened snouts and laterally compressed tails.[7] Their limbs are reduced in size; the front feet have five digits with little or no webbing while the hind feet have four webbed digits and a rudimentary fifth.[8] The skeleton is somewhat typical of tetrapods, although the skull and pelvis are specialized,[7] as are the ribs to allow the animal to collapse its thorax during diving and expand it to accommodate large masses of food.[9] Both sexes have a cloaca, a single chamber and outlet at the base of the tail into which the intestinal, urinary, and genital tracts open. It houses which houses the single penis in males and the clitoris in females.[7] The testes or ovaries are located near the kidneys.[10]

The eyes, ears and nostrils of crocodilians are placed at the top of the head. This allows them to stalk their prey while most of their body is underwater.[11] When in bright light, the pupils of a crocodilian contract into narrow slits, whereas in darkness they become fully circular. This is typical for animals that hunt at night. Crocodilians also possess a tapetum lucidum which enhances vision in low light.[8] While eyesight is fairly good in air, it is significantly weakened underwater.[12] The eardrums are protected by flaps that can be opened or closed by muscles. The ears are adapted for hearing both in air and underwater.[7] Crocodilians have a wide hearing range, with sensitivity comparable to most birds and many mammals.[13] They appear to have a well-developed olfactory system,[7] while the trigeminal nerve in their snouts allows them to detect vibrations in the water.[14] When the animal completely submerges, the nictitating membranes cover its eyes. In addition, glands on the nictitating membrane secrete a salty lubricant that keeps the eye clean. When a crocodile leaves the water and dries off, this substance is visible as "tears".[8] The tongue can't move freely but is held in place by a folded membrane.[9]

Locomotion

Crocodilians are excellent swimmers. During aquatic locomotion, the muscular tail is undulated from side to side to drive the animal through the water while the limbs are held close to the body to reduce drag.[11][15] When the animal needs to stop, steer or maneuver in a different direction, the limbs are splayed out.[11] Crocodilians generally cruise slowly on the surface or underwater with gentle sinuous movements of the tail, however when being pursued or when chasing prey they can move rapidly, and can lunge out of the water in a manner reminiscent of dolphins.[16]

Crocodilians are less well adapted for moving on land, and are unusual among vertebrates in having two completely different means of terrestrial locomotion, the "high walk" and the "low walk".[8] They share a common feature with some early archosaurian reptiles in that their ankle joints flex in a different way from those of other reptiles. One of the upper row of ankle bones, the astragalus, moves with the tibia and fibula. The other, the calcaneum, is functionally part of the foot, and has a socket into which a peg from the astragalus fits. The result is that the legs can be held almost vertically beneath the body when on land, and the foot can swivel during locomotion with a twisting movement at the ankle.[17]


The high walk of crocodilians is unique among living reptiles, with the belly and most of the tail being held off the ground. It somewhat resembles the walk of a mammal, with the same sequence of limb movements - left fore, right hind, right fore, left hind.[16] The low walk is similar to the high walk, but without the body being raised, and is quite different from the sprawling walk of salamanders and lizards. The animal can change from one walk to the other instantaneously, but the high walk is the normal means of locomotion on land. The animal may push its body up and use this form immediately, or may take one or two strides of low walk before raising the body higher. Unlike most other land vertebrates, when crocodilians increase their pace of travel they increase the speed at which the lower half of each limb (rather than the whole leg) swings forward; by this means, stride length increases while stride duration decreases.[18] Though typically slow on land, crocodilians can produce short bursts of speed and some can run at 12 to 14 km/h (7.5 to 8.7 mph) for short distances.[19] A fast entry into water from a muddy riverbank can be effected by plunging to the ground, twisting the body from side to side and splaying out the limbs.[16] In some small species such as the freshwater crocodile (Crocodylus johnsoni), a running gait can progress to a bounding gallop. This involves the hind limbs launching the body forward, with the fore limbs then taking the weight. Next, the hind limbs swing forward as the the spine flexes dorso-ventrally, and this sequence of movements is repeated.[20] During terrestrial locomotion, a crocodilian can keep its back and tail straight as the scales are attached to the vertebrae by muscles.[9] Whether on land or in water, crocodilians can jump or leap by pressing their tails and hind limbs against the substrate and then launching themselves into the air.[7][11]

Jaws and teeth

The snout shape of crocodilians vary between species. Crocodiles may have either broad or slender snouts while alligatorids have mostly broad ones. Gharials have snouts that are extremely elongated. The muscles that close the jaws are much more massive and powerful than the ones that open them,[7] and a crocodilian's jaws can be held shut by a person fairly easy. Conversely, the jaws are extremely difficult to pry open.[21] The powerful closing muscles are low-slung, allowing the top of the head to retain a flat profile. In addition, the jaw hinge attaches to the atlanto-occipital joint, allowing the animal to open its mouth fairly wide.[9] Crocodilians have among the strongest bite forces of any animal. In a study published in 2003, an American alligator's bite force was measured at up to 2,125 lbf (9,450 N).[22] In a 2012 published study, a saltwater crocodile's bite force was measured even higher, at 3,700 lbf (16,000 N). This study also found no correlation between bite force and snout shape, as is traditionally believed. Nevertheless, the gharial's extremely slender jaws are relatively weak.[23]

Crocodilian teeth are peg-like and conical. Broad snouted species have teeth that vary in size while those of slender-snouted species are more uniform. The teeth of crocodiles and gharials tend to be more visible then those of alligatorids when the jaws are closed.[7] Crocodilians are polyphyodonts and able to replace each of their 80 teeth up to 50 times in their 35 to 75-year lifespan.[24] Next to each full grown tooth there is a small replacement tooth and a odontogenic stem cell in the dental lamina in standby that can be activated when required.[25] Teeth are replaced continuously through much the animal's life but this process decreases significantly and eventually stops as the animal grows older.[7]

Skin and scales

Main article: Crocodilian armor


The skin of crocodilians is thick and cornified and is clad in non-overlapping scales known as scutes arranged in regular rows and patterns. These scales are continually being produced by cell division in the underlying layer of the epidermis, the stratum germinativum, and the surface of individual scutes sloughs off periodically. The outer surface of the scutes consists of the relatively rigid beta-keratin while the hinge region between the scutes contains only the more pliable alpha keratin.[26]

Many of the scutes are strengthened by bony plates known as osteoderms, which are the same size and shape as the superficial scales but grow beneath them. They are most numerous on the back and neck of the animal and may form a protective armor. They often have prominent, knobbly ridges and are covered in hard-wearing beta-keratin.[7] The skin on the head is fused to the skull.[9] The skin on the neck and flanks is loose while that on the abdomen and underside of the tail is sheathed in large, flat square-shaped scutes arranged in neat rows.[7][27] The scutes contain blood vessels and may act to absorb or radiate heat during thermoregulation.[7] Some scutes contain a single pore known as an integumentary sense organ. Crocodiles and gharials have these on large parts of their bodies while alligatorids only have then on the head. Their exact function is not fully understood but it has been suggested that they may be tactile organs,[28] or they may have a function similar to the lateral line organ of fish. Another possibility is that they may produce an oily secretion that prevents mud from adhering to the skin. There are prominent paired integumentary glands in skin folds on the throat and others in the side walls of the cloaca. Various functions for these have been suggested. They may play a part in communication between crocodilians and also seem to produce pheromones and play a part in courtship behavior.[7] The skins of crocodilians is tough and can withstand damage from conspecifics, and the immune system is effective enough to heal wounds within a few days.[29]

Respiration

File:X-ray video of a female American alligator (Alligator mississippiensis) while breathing - pone.0004497.s009.ogv As in birds, respiration in crocodilians is unidirectional, unlike mammals where airflow moves in and out tidally. When a crocodilian inhales, air flows through the trachea and into two primary bronchi or airways which branch off into narrower secondary passageways. The air continues to move though these and into even narrower tertiary airways and then into other secondary airways (which were bypassed the first time). The air then flows back into the primary airways and is exhaled back out, completing a one-directional loop around the lungs.[30][31]

The lungs of the crocodilian attach to the liver and the pelvis by a diaphragmaticus muscle.[32] During breathing, the lungs are stretched and pushed against like a piston.[7] When inhaling, the intercostal muscles expand the ribs, allowing the animal to take in more air, while the ischiopubis muscle causes the hips to swing downwards and push the belly outward. When exhaling, the intercostal muscles push the ribs inward, while the rectus abdominis pulls the hips forwards and the belly inward. Crocodilians can also move their lungs back and forth to control their buoyancy in the water. An animal floats when the lungs are pushed towards the head and sinks when they are pulled towards the tail. In addition, they can also spin and twist by moving their lungs laterally.[32] Swimming and diving crocodilians appear to rely on their lung volumes more for buoyancy than oxygen storage.[7]

When submerging, the nostrils of a crocodilian shut tight.[8] All species have a palatal valve, a membranous flap of skin at the back of the oral cavity, that prevents water from flowing into the throat, oesophagus and trachea.[7][8] This enables them to breathe even when the mouth is open underwater.[8] Crocodilians can hold their breath for four to fifteen minutes and perhaps as long as two hours.[33]

Circulation

The crocodilian has perhaps the most complex vertebrate circulatory system. It has a four-chambered heart and two ventricles, an unusual trait among extant reptiles,[7] and both a left and right aorta. Like birds and mammals, crocodilians have heart valves that flip open when pressured by surges of blood and shut closely when the pressure subsides. In addition, they also have unique cog-teeth-like valves that when interlocked, direct blood to the left aorta and away from the lungs and back around the body.[34] One possible advantage of this system is that it allows the animals to stay submerged underwater for a longer period.[35] However, this explanation has been questioned,[36] and other suggestions include assisting with thermoregulatory needs, reducing pulmonary edema or speeding recovery from metabolic acidosis. By retaining carbon dioxide within the body, there is an increase in the rate of gastric acid secretion and thus the efficiency of digestion, and other gastrointestinal organs such as the pancreas, spleen, small intestine and liver also function more efficiently.[37]

When submerged, a crocodilian's heart rate slows down to one or two beats a minute and blood flow to the muscles are reduced. When it rises and takes a breath, its heart rate speeds up in seconds and this muscles receives the full-energy of newly oxygenated rich blood.[38] Unlike many marine mammals, crocodilians have little myoglobin. Hence during diving, muscles are supplied oxygen when concentrations of bicarbonate ions removes oxygen from the hemoglobin, allowing it to flow into the tissues.[39]

Digestion

Crocodilian teeth are designed for seizing and holding prey and the food is swallowed unchewed. The digestive tract is relatively short as meat is a fairly simple substance to break down and digest. The stomach is divided into two parts, a muscular gizzard which grinds food and a digestive chamber where enzymes work on it.[40] The stomach is more acidic than in any other vertebrate and contains ridges that accommodate gastroliths, which play a role in mechanical breakdown of the food. Digestion takes place more quickly at higher temperatures.[11] Crocodilians have a very low metabolic rate and consequently, low energy requirements. This allows them to survive for many months on a single large meal, digesting the food slowly. They can withstand extended fasting, living on stored fat between meals. Even recently hatched crocodiles were able to survive 58 days without food, losing 23% of their bodyweight during this time.[41] An adult crocodile needs between a tenth and a fifth of the amount of food necessary for a lion of the same weight and can live for half a year without eating.[41]

Thermoregulation

Crocodilians are ectotherms, producing relatively little heat internally and relying on external sources to raise their body temperatures. Solar radiation is the main means of warming, while immersion in water may either raise their temperature by conduction or cool them in hot weather. The main method for regulating the animal's temperature is behavioral. For example an alligator in temperate regions may start the day by basking in the sun on land. Bulky animals warm up slowly, but at some time later in the day it moves into the water, still exposing its dorsal surface to the sun. At night it remains submerged and its temperature slowly falls. The basking period is extended in winter and reduced in summer. For crocodiles in the tropics, avoiding overheating is generally the main problem. They may bask briefly in the morning but then move into the shade, remaining there for the rest of the day, or submerge themselves in water to keep cool. Gaping with the mouth open can provide cooling by evaporation from the mouth lining.[42] By these means, the temperature range of crocodilians is maintained between about 25 and 35 °C (77 and 95 °F), and mainly stays in the range 30 to 33 °C (86 to 91 °F).[7]

The ranges of the American and Chinese alligator extend into regions that sometimes experience periods of frost in winter. Being ectothermic, the internal body temperature of crocodilians fall as the temperature drops and they become sluggish. They may become more active on warm days but do not usually feed at all during the winter. In cold weather, they remain submerged with their tails in deeper, less cold water with their nostrils just projecting through the surface. In very cold weather when ice forms on the water, they maintain ice-free breathing holes, and there have been occasions when their snouts have become frozen into the ice. Temperature sensing probes implanted in wild American alligators have found that their core body temperatures can descend to around 5 °C (41 °F) but as long as they remain able to breathe, they show no ill-effects when the weather warms up.[42]

Osmoregulation

Although the saltwater crocodile and the American crocodile are able to swim out to sea, their normal habitat is river mouths, estuaries, mangrove swamps and hypersaline lakes and no species of crocodilian can be considered truly marine. They need to maintain the concentration of salt in body fluids at suitable levels. Osmoregulation is related to the quantity of salts and water exchanged with the environment. Intake of water and salts takes place when water is drunk, incidentally while feeding, when present in foods, through the skin and across the lining of the mouth. Salts and water are lost from the body in the urine and feces, during respiration, through the skin, and via salt excreting glands on the tongue, though these are only present in crocodiles and gharials.[43][44] Gaping the mouth causes loss of water by evaporation but the skin is a largely effective barrier to both water and ions.[43] Large animals are more able to maintain homeostasis at times of osmotic stress than are smaller ones.[7] Newly hatched crocodilians are much less tolerant of exposure to salt water than are older juveniles, presumably because they have a higher surface to volume ratio.[43]

The kidneys and excretory system are much the same as in other reptiles, but crocodilians do not have a bladder. In fresh water the concentration of ions in the plasma is much higher than it is in the surrounding water. The animals are well-hydrated and the urine in the cloaca is clear, copious, and dilute, with excess nitrogen being excreted as ammonium bicarbonate. Sodium loss is low and mainly takes place through the skin in freshwater conditions. In seawater, the opposite is true. The concentration of ions in the plasma is lower than the surrounding water and this is dehydrating for the animal. The cloacal urine is much more concentrated, white and opaque, with the nitrogenous waste being mostly excreted as insoluble uric acid.[7][43]

Distribution and habitat

Crocodilians are amphibious reptiles, spending part of their time in water and part on land. Typically they are creatures of the tropics, the main exceptions to this being the American and Chinese alligators whose ranges extend as far north as the southeastern United States and the Yangtze River respectively. Most crocodilians live in the lowlands and few are found above 1,000 metres (3,300 ft) where the temperatures are typically about 5°C (9°F) lower than at the coast. Although several can venture into the sea, none of them permanently reside there although a number of species can tolerate the brackish water of estuaries, mangrove swamps and hypersaline lakes.[45] The saltwater crocodile has the widest distribution of any crocodilian with a range extending from eastern India to New Guinea and northern Australia. Much of its success is due to its ability to swim out to sea and colonise new locations but it is not restricted to the marine environment and spends much time in estuaries, rivers and large lakes.[46]


Various types of aquatic habitats are used by different species of crocodilian. Some prefer swamps, ponds and the edges of lakes where they can bask in the sun and there is plenty of plant life supporting a diverse fauna. Others prefer the lower stretches of rivers, mangrove swamps and estuaries which also have a rich flora and plenty of food. The Asian gharials find the fish on which they feed in the pools and backwaters of swift rivers. The South American dwarf caimans inhabit cool, fast-flowing streams often near waterfalls, and other caimans live in warmer, turbid lakes and slow moving rivers. The crocodiles are mainly river dwellers and the Chinese alligator is found in slow-moving, turbid rivers flowing across China's floodplains. The American alligator is an adaptable species and inhabits swamps, rivers or lakes with clear or turbid water.[45] Climatic factors also affect crocodilians distribution locally. During the dry season, caimans can be restricted to deep pools in rivers for several months, while in the rainy season, much of the savanna in the Venezuelan llanos is flooded and they disperse widely across the plain.[47]

Dry land is also important to crocodilians as it provides opportunities for basking, nesting and escaping from temperature extremes. Several species make use of shallow burrows on land and muddy pools provide opportunity for wallowing.[48] The type of vegetation bordering the rivers and lakes inhabited by crocodilians is mostly humid tropical forests, with mangrove swamps in estuarine areas. These forests are of great importance to the crocodilians, creating suitable microhabitats where they can flourish. The roots of the trees absorb water when it rains, releasing it back slowly into the environment. When the forests are cleared to make way for agriculture, rivers tend to silt up, the water runs off rapidly, the water courses can dry up in the dry season and flooding can occur in the wet season. Flooding is a major cause of failure of crocodilians to breed successfully as nests are submerged, developing embryos are deprived of oxygen and juveniles get washed away. Destruction of forest habitat is probably a greater threat to crocodilians than is hunting.[49]

Ecological role

In the Amazon basin, it was noticed that when caimans became scarce as a result of overhunting in the mid 20th century, the number of fish present, such as the important arapaima (Arapaima gigas), also decreased. These are nutrient-poor waters, and it was hypothesized that the urine and feces of the caimans were an important source of nutrients which increased primary production. Thus the presence of the reptiles could have benefited the fish stock.[50] Whether or not this theory was correct, the number of crocodilians in a stretch of water appears to be correlated with the fish population.[51] The nest mounds built by some species of crocodilian are used by other creatures for their own purposes. The American alligator builds a nest of rotting vegetation as much as a meter above the surrounding lakes and marshes of the Everglades. These mounds, heated internally by fermentation, are used by turtles and snakes both for basking and for laying their own eggs.[52] The Florida red-bellied turtle specialises in this, and some alligator mounds may have several clutches of turtle eggs developing alongside the owner's eggs.

Behavior and life history

Adult crocodilians are typically territorial and solitary. Individuals may defend basking spots, nesting sites, feeding areas, nurseries and overwintering sites. Male saltwater crocodiles establish year-round territories that encompass several female nesting sites. Some species are occasionally gregarious, particularly during droughts, where several individuals gather at remaining water sites. In addition, some species are also known to share basking sites at certain times of the day.[11]

Feeding

Crocodilians are largely carnivorous, but the diets of different species can vary with snout shape and tooth sharpness. Long-and-slender snouted species with sharp teeth, like the Indian gharial and Australian freshwater crocodile, are specialized for feeding on fish, insects and crustaceans while extremely broad snouted species with blunt teeth, like the Chinese alligator and broad-snouted caiman, specialize in eating hard-shelled mollusks. Species whose snouts and teeth are intermediate between these two forms, such as the saltwater crocodile and American alligator, have generalized diets and opportunistically feed on invertebrates, fish, snakes, turtles, birds and mammals.[23]

In general, crocodilians are stalk-and-ambush predators,[23] though hunting strategies vary depending on the individual species and the prey being hunted.[11] Terrestrial prey is stalked from the water's edge and then grabbed and drowned.[11][7] Small animals can be taken by a sideways head strike and killed by whiplash as the predator shakes its head.[7] Gharials and other fish-eating species sweep their jaws sideways to snap up prey, and these animals are able to leap our of the water to catch birds, bats and leaping fish.[53] Caimans use their tails and bodies to herd fish into shallow water.[11] They may also dig for bottom-dwelling invertebrates[8] and the smooth-fronted caiman will even hunt on land.[23] Nile crocodiles are known to hunt cooperatively,[11] and several individuals may feed on the same carcass. Most species will eat anything suitable that comes within reach and are also opportunistic scavengers.[8]

Crocodilians are unable to chew and need to swallow food whole, and prey that is too large to swallow is torn in pieces. They may be unable to deal with a large animal with a thick hide, and may wait until it becomes putrid and comes apart more easily.[53] To tear a chunk of tissue from a large carcase, a crocodilian spins its body continuously while holding on with its jaws, a maneuver known as the "death roll".[54] During, cooperative feeding, some individuals may hold on to the prey, while others perform the roll. The animals do not fight, and each retires with a piece of flesh and await its next feeding turn.[55] Food is typically consumed by crocodilians with their heads above water. The food item is first held with the tips of the jaws, tossed towards the back of the mouth by an upward jerk of the head and then gulped down.[7] Nile crocodiles may store carcasses underwater for later consumption.[8] Though mostly carnivorous, several species of crocodilian have been observed to consume fruit and may play a role in seed dispersal.[56]

Reproduction and parenting

Crocodilians are generally polygynous and individual males try to mate with as many females as they can. Dominant males patrol and defend territories which contain several females. During courtship, males and females may rub against each other, circle around and preform swimming displays. Males of some species, like the American alligator, have elaborate courtship displays. Male alligators try to attract females with loud bellows and vibrate along the length of their bodies. Copulation typically occurs in the water. When a female is ready to mate, she arches her back while her head and tail submerge. The male rubs across the female's neck and then grasps her with his hindlimbs, placing his tail underneath her so their cloacas align and his penis can be inserted. Mating can last up to 15 minutes, during which time the pair continuously submerges and surfaces.[57] While dominant males usually monopolize reproductive females, multiple paternity is known to exist in American alligators, where as many as three different males may sire offspring in a single clutch. Within a month of mating, the female begins to make a nest.[11]

Depending on the species, female crocodilians may construct either holes or mounds as nests. These are typically found near dens or caves. Nests made by different females are sometimes close to each other, particularly in hole-nesting species. The number of eggs laid in a single clutch ranges from ten to fifty. As in all amniotes, crocodilian eggs are protected by hard shells. The incubation period is two to three months.[11] The temperature at which the eggs incubate determines the sex of the hatchlings. Constant nest temperatures above 32 °C (90 °F) produce males, while those below 31 °C (88 °F) produce females. However, sex in crocodilians may be determined in a short interval, and nests are subject to changes in temperature. Most natural nests produce hatchlings of both sexes though single-sex clutches do occur.[7]

The young may all hatch in a single night.[58] Crocodilians are unusual among reptiles in the amount of parental care provided after the young hatch.[11] The mother helps excavate hatchlings from the nest and carries them to water in her mouth. Newly-hatched crocodilians gather together and stay close to their mother.[59] For spectacled caimans in the Venezuelan llanos, individual mothers are known to leave their young in the same nurseries or crèches and one of the mothers guards them.[60] Hatchlings of many species tend to bask in a group during the day and disperse at nightfall to feed.[58] The time it takes young crocodilians to reach independence can vary. For American alligators, groups of young associate with adults for 1–2 years while young saltwater and Nile crocodiles become independent in a few months.[11]

Communication

Alligator bellow
File:Alligatorbellowedit.ogg
Alligator bellow, ogg/Vorbis format.

Another alligator bellow
File:27alligator2bellow.ogg
Alligator bellow, ogg/Vorbis format.

Alligator hiss
File:Alligatorhiss.ogg
Alligator hiss ogg/Vorbis format.

Problems playing these files? See media help.

The social life of a crocodile begins while it is still in the egg because the young start communicating with each other before they are hatched. It has been shown that a light tapping noise near the nest will be repeated by the young, one after another. Such early communication may help them to hatch simultaneously. Once it has broken out of the egg, a juvenile produces yelps and grunts, either spontaneously or as a result of external stimuli. Nearby adults respond quickly to juvenile distress calls as was demonstrated at a Papua New Guinea crocodile farm. When a juvenile squeaked on being handled injudiciously, nearby adults became highly active, splashing into the water and thrashing about. The dominant male flung himself against the fence separating the enclosures while the females swam agitatedly around making deep calls and engaging in headslapping.[58]

Vocalizations are frequent as the juveniles disperse, and again as they congregate in the morning. Nearby adults, presumably the parents, also give signals warning of predators and others alerting the youngsters to the presence of food. The range and quantity of vocalizations varies between species. Alligators are the noisiest, while some crocodiles species are almost completely silent. Adult female New Guinea crocodiles and Siamese crocodiles roar when approached by another adult, while Nile crocodiles grunt or bellow in a similar situation. The American alligator is exceptionally noisy. It emits a series of about seven throaty bellows each a couple of seconds long, at ten second intervals. It also makes various grunts and hisses.[58]

Another form of acoustic communication is the headslap. This typically starts with an animal in the water elevating its snout and remaining stationary. After some time the jaws are opened sharply then clamped shut with a biting motion that makes a loud slapping sound, and this is immediately followed by a loud splash after which the head may be submerged and copious numbers of bubbles produced. Some species then roar while others slap the water with their tails. Episodes of headslapping spread through the group. The purpose varies but it seems to be associated with maintaining social relationships, and is also used in courtship.[58]

Growth and mortality

Mortality is high for eggs and hatchlings and nests face threats from floods, overheating and predators.[11] One of the chief predators of alligators' eggs in Florida is the raccoon, which is attracted by olfactory clues after the nest has been disturbed by turtles. The Florida black bear also raids alligator eggs.[61] In Africa, mongooses, honey badgers, baboons, otters, warthogs, bushpigs and spotted hyenas are all fond of crocodile eggs but the monitor lizard is thought to be the most important nest raider. This lizard is also a major hazard in Asia where other egg predators include civets, mongooses, rats, sloth bears, jackals and dogs.[62]

Despite the maternal care they receive, hatchlings commonly fall prey to fish, birds, mammals and other reptiles.[63] While the female is transporting some to the nursery area, others are picked off by predators that lurk near the nest. The hatchlings are a source of food to most of the creatures that feed on eggs and are also subject to aquatic attacks by turtles, fish and snakes. Birds of prey take their toll and there are usually some malformed individuals that are unlikely to survive.[62] In northern Australia, the survival rate for saltwater crocodile hatchlings is only twenty-five percent but with each succeeding year this improves and reaches sixty percent by year five. Mortality rates are fairly low among subadult and adult crocodilians, though they are occasionally preyed on by large cats and snakes.[63] The jaguar is an important predator of some caimans in South America[64] and in other parts of the world, elephants and hippos may kill crocodiles defensively.[11] Authorities differs as to whether much cannibalism takes place among crocodilians. Adults do not normally eat their own offspring but there is some evidence of subadults feeding on juveniles and of adults attacking subadults. In Nile crocodiles, rival males sometimes kill each other during the breeding season, but the greatest cause of mortality among crocodilians of all ages is humans who kill them from fear, spite, through habitat destruction or for commercial gain.[62]

Growth in hatchlings and young crocodilians depends on the food supply, and sexual maturity is linked with length rather then age. Female saltwater crocodiles reach maturity at 2.2–2.5 m (7 ft 3 in–8 ft 2 in), while males mature at 3 m (9.8 ft). Australian freshwater crocodiles take ten years to reach 1.4 m (4 ft 7 in) and maturity. The common caiman matures earlier, reaching its mature length of 1.2 m (3 ft 11 in) in four to seven years.[57] Crocodilians continue to grow throughout most of their lives. Males in particularly continue to gain in weight as they get older but this is mostly in the form of extra girth rather than increase in length.[65] Crocodilians can live for more than forty years[11] and their age can be determined by growth rings in their bones.[57][65]

Evolution and classification

Evolution

Diapsids are tetrapods characterised by having two openings (temporal fenestrae) on either side of the skull behind the eye. Living diapsids include all crocodiles, lizards, snakes, tuataras and birds.[66] The feature that distinguishes archosaurs from other diapsid reptiles is an extra pair of openings in the skull (antorbital fenestrae) in front of the eye sockets. Archosauria is defined as the group that includes the common ancestor of crocodiles and birds and all of its descendants. It is comprises the Pseudosuchia, the "false crocodiles", and the Ornithosuchia, which itself comprises the dinosaurs and their relatives, the pterosaurs and the birds.[67] Pseudosuchia is defined as living crocodilians and all archosaurs more closely related to crocodilians than to birds. Modern crocodilians have lost the antorbital fenestrae but they were present in most of their fossil ancestors as small openings.[68]


The crocodylomorphs are the only pseudosuchians to have survived the Triassic–Jurassic extinction event some 201.3 million years ago. During the early Jurassic period, the dinosaurs became dominant on land and the crocodylomorphs underwent major adaptive diversications to fill ecological niches vacated by recenty extinguished groups. Unfolding fossil evidence shows that Mesozoic crocodylomorphs had a much greater diversity of forms than do modern crocodilians. Some became small fast-moving insectivores, others specialist fish-eaters, still others marine and terrestrial carnivores and a few became herbivores.[69] The earliest stage of crocodilian evolution was the protosuchians which evolved in the late Triassic and early Jurassic. They were followed by the mesosuchians which diversified widely during the Jurassic and the Tertiary. Another group, the eusuchians appeared in the late Cretacious some 80 million years ago and includes all the crocodilians living today.[68]

Protosuchians were small, mostly terrestrial animals with short snouts and long limbs. They had bony armor in the form of two rows of plates extending from head to tail and this armor is retained by most modern crocodilians. Their vertebrae were convex on the two main articulating surfaces and their bony palates were little developed. The mesosuchians saw a fusion of the palatine bones to form a secondary bony palate and a great extension of the nasal passages to near the pterygoid bones. This allowed the animal to breath through its nostrils while its mouth was open under the water. The eusuchians continued this process with the interior nostrils now opening through an aperture in the pterygoid bones. The vertebrae of eusuchians had one convex and one concave articulating surface allowing for a ball and socket type joint between the vertebrae bringing greater flexibility and strength.[70] The oldest known eusuchian is Hylaeochampsa vectiana from the lower Cretaceous of the Isle of White in the United Kingdom.[71]

The three primary branches of Crocodylia had diverged by the end of the Mesozoic. The earliest-known members of the group are alligatoroids and gavialoids that lived in North America and Europe during the Campanian (around 83.6–72.1 million years ago). Although the first known crocodyloids appeared in the Maastrichtian (around 72.1–66.0 million years ago), that lineage must have been present during the Campanian, and the earliest alligatoroids and gavialoids include highly derived forms which indicates that the time of the actual divergence between the three lineages must have been a pre-Campanian event.[72]

Phylogeny

The phylogeny of the Crocodilia is (2012) undergoing revision. A cladogram after Holliday and Gardner, 2012, preserving the traditional 'Brevirostres':[73]

Eusuchia

Isisfordia




Hylaeochampsa




Aegyptosuchidae


Crocodylia

Borealosuchus



Gavialoidea



Brevirostres (alligators, crocodiles and relatives)






The success of the crocodilians has been due to their complete dominance of the predatory, water-edge niche, a position they have occupied for more than 85 million years. The musculo-skeletal adaptations they developed can generate a higher bite-force and tooth pressure than that of any other living animal. Since the bony structure and musculature was modified in this way it has remained essentially unchanged, and modern crocodilians vary little from their ancestors in this respect. In 2012, Erickson et al. measured the biomechanical properties of their jaws and combined the findings with a phylogeny formed from DNA sequencing to give a maximum likelihood cladogram of the extant crocodilians (excluding the Yacare caiman for which no DNA evidence was available). In this, the position of Brevirostres is superseded:[74]

Crocodylia
Alligatoridae



Caiman



Melanosuchus




Paleosuchus




Alligator




Crocodylidae

Crocodylus




Mecistops



Osteolaemus




Gavialidae

Gavialis



Tomistoma





Extant taxonomy

There are two extant species of Gavialidae, the gharial and the false gharial. Gharials can be recognized by the long narrow snout, with a somewhat enlarged boss at the tip. They are rare and found only in South Asia.[75]


The extant Alligatoridae are two species in the genus Alligator, and six species of caimans grouped into three genera. They can be recognized by the broad snout, in which the fourth tooth of the lower jaw cannot be seen when the mouth is closed.[75]


The extant Crocodylidae are twelve species in the genus Crocodylus, and two species in other genera. They have a variety of snout shapes, but can be recognized because the fourth tooth of the lower jaw is visible when the mouth is closed.[75]

Human relations

Attacks on humans

Main article: Crocodile attack

Eight species of crocodilians are known to attack humans.[78] Crocodilians may attack humans to defend their territory, nests, or young; by mistake, while attacking domestic animals such as dogs; or simply for food, as larger crocodilians can take prey as big as or bigger than humans. The species on which there is most data are the saltwater crocodile, the Nile crocodile and the American alligator. Other species which have sometimes attacked humans are the black caiman, Morelet’s crocodile, mugger crocodile, American crocodile, gharial and Australian freshwater crocodile. The Nile crocodile is the species that has most often attacked humans, and while many attacks go unreported, there are probably over 300 per year.[78] American alligators carried out some hundreds of attacks between 1973 and 2005 in the Florida Everglades, killing sixteen people; before then the Everglades had been sparsely populated.[79] Wild saltwater crocodiles in Australia carried out 62 confirmed and unprovoked attacks causing injury or death between 1971 and 2004.[80]

Conservation

The main threat to crocodilians around the world is human activity, including hunting and habitat destruction. The 1973 Convention on Trade in Endangered Species (CITES) attempted to prevent trade in body parts of endangered animals, such as the skins of crocodiles. This proved in the 1980s to be problematic, as crocodiles were in some parts of Africa both abundant and dangerous to humans, and it was legal to hunt them. At the Conference of the Parties in Botswana in 1983, it was argued on behalf of aggrieved local people that it was reasonable to sell the lawfully hunted skins. However, early in the 1970s, more than 2 million wild crocodilian skins of a variety of species but mainly Caiman crocodilus had been traded, driving down the majority of crocodilian populations, in some cases almost to extinction. Fortunately, in the late 1970s, crocodiles began to be farmed in different countries, starting with eggs taken from the wild. In the 1980s, farmed crocodile skins started to be produced in sufficient numbers to destroy the unlawful trade in wild crocodilians. By 2000, crocodilian skins from twelve species, whether harvested lawfully in the wild or farmed, were traded by thirty countries, and the unlawful trade in the products had virtually vanished.[81]

Different species of crocodilians such as the gharial and the American alligator have widely differing conservation status. The gharial has undergone both chronic long term and a rapid short-term declines, so the IUCN has listed the species as Critically Endangered. The gharial population has declined by 96–98% over a three-generation period since 1946, and the once widespread species with a population of around 5,000 to 10,000 was reduced to a small number of widely spaced subpopulations of fewer than 235 individuals by 2006. The drastic decline had causes including hunting, egg collection, killing for indigenous medicine and killing by fishermen. The decline of about 58% between 1997 and 2006 was caused by increasing use of gill nets and loss of riverine habitat.[82] In December 2007, several gharials were found dead in the Chambal River. Their bodies contained high levels of lead and cadmium, which together with stomach ulcers and protozoan parasites probably caused their death.[83] The American Alligator similarly declined seriously through hunting and habitat loss throughout its range, threatening it with extinction, and in 1967 it was listed as an endangered species, but then the United States Fish and Wildlife Service and state wildlife agencies in the Southern United States contributed to its recovery. Protection allowed the species to recuperate, and in 1987 it was removed from the endangered species list.[84]

Cultural depictions

In mythology and folklore

Crocodilians have had prominent roles in myths and legends of various cultures around the world. In Ancient Egyptian religion, the crocodile represents Sobek, god of power, protection and fertility, and Ammit, the demoniac devourer of unworthy souls. This reflects the fact that the Egyptians saw the crocodile both as an important part of the Nile river ecosystem and a terrifying predator. Crocodiles were one of several animals that the Egyptian mummified.[85] The Aztecs also had a crocodilian god of fertility named Cipactli, who protects crops. In Aztec mythology, the sea monster Tlaltecuhtli is sometimes described as merging with a "great caiman". The Mayans also associated crocodilians with fertility and death.[86]

The gharial is featured in the folktales of India. In one story, a gharial and monkey become friends when the monkey gives the gharial fruit. The gharial's wife demands that her husband bring home the monkey to eat, believing that the fruit made the monkey's heart sweet. The gharial complies at first and attempts to lure the monkey to his home, but soon comes clean about the plan. Their friendship ends after that.[87] Similar tales exist in Native American and African American folklore, with an alligator and Br'er Rabbit.[88]

In a Malay folktale, the mouse deer Sang Kancil wanted to cross the river to reach the fruit trees on the far side, but Sang Buaya, the big bad crocodile was waiting in the river to eat him. Sang Kancil called to Sang Buaya and told him the king was inviting everyone to a feast, for which he needed to know how many crocodiles would be coming. Sang Kancil asked all the crocodiles to line up across the river, so he could count them for the king, and made them promise not to eat him as he counted. He then stepped on their heads, one by one, calling out "One! Two! Three!" as he went. When he reached the far side he thanked them for helping him cross the river, and feasted on the delicious fruit, but Sang Buaya did not do so well, as all the other crocodiles were angry with him for letting Sang Kancil trick them.[89] A legend from East Timor tells how a boy rescued a gigantic crocodile that had become stranded. In return, the crocodile protected him for the rest of its life, and when it died, its scaly ridged back became the hills of Timor for the boy to live in.[90]

In literature

The historian Herodotus described the crocodile as lying with its mouth open to permit a bird, the trochilus, to enter its mouth and remove any leeches, so it lived at peace with that bird.[91] The crocodile was one of the beasts described in the late 13th century Rochester Bestiary, based on classical sources including Pliny's Historia naturalis,[92] and Isidore of Seville's Etymologies.[93][94] Isidore asserts that the crocodile is named for its saffron color (Latin croceus, 'saffron'), may be killed by fish with serrated crests by sawing into its soft underbelly, and is often twenty cubits (10 m (33 ft)) long, while the male and female take it in turns to guard the eggs.[95]

Crocodiles have been supposed to weep for their victims since the 9th century Bibliotheca by Photios I of Constantinople.[96] The story was repeated in later accounts such as that of Bartholomaeus Anglicus in the 13th century.[97] It became widely known in 1400 when the English traveller Sir John Mandeville wrote his description of "cockodrills":[98]

In that country and by all Ind be great plenty of cockodrills, that is a manner of a long serpent, as I have said before. And in the night they dwell in the water, and on the day upon the land, in rocks and in caves. And they eat no meat in all the winter, but they lie as in a dream, as do the serpents. These serpents slay men, and they eat them weeping; and when they eat they move the over jaw, and not the nether jaw, and they have no tongue.[98]

William Shakespeare refers to crocodile tears in his plays Othello ( Act IV, Scene i), Henry VI, part 2 (Act III, Scene i), and Antony and Cleopatra (Act II, Scene vii).


In children's books

In Rudyard Kipling's Just So Stories (1902), the Elephant's Child acquires his trunk by having his nose pulled very hard by the Crocodile "on the banks of the great grey-green, greasy Limpopo river". The newly elongated nose allows him to pick fruit instead of waiting for it to fall, and to do many other useful things.[99]

Roald Dahl's The Enormous Crocodile (1978), illustrated as a picture book by the cartoonist Quentin Blake, tells how the crocodile wanders the jungle promising to eat children. All the animals including Muggle-Wump the monkey and the Roly-Poly Bird tell him to stop, to no avail, and the crocodile attempts one trick after another, such as camouflaging himself as a bench, to catch the children. Finally Trunky the elephant kills the crocodile.[100]

As symbols

Crocodiles and alligators have been used as symbols, for example in the name of the Canton Crocodiles, a baseball team in the Frontier league,[101] and as the emblem of the Southern Districts football team in the Australian Northern Territory Football League.[102] The alligator is the mascot of the University of Florida sport teams.[103] The army of East Timor adopted two saltwater crocodiles as its mascots after independence.[90]

References

Bibliography

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External links

  • Mikko's Phylogeny Archive Crocodyliformes
  • Florida's Museum of Natural History: Crocodilians

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