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Crocodilians
Temporal range: Late Cretaceous—Present, 83.5–0 Ma
Saltwater crocodile (Crocodylus porosus)
Scientific classification Edit this classification
Domain: Eukaryota
Kingdom: Animalia
Phylum: Chordata
Class: Reptilia
Clade: Archosauria
Clade: Pseudosuchia
Clade: Crocodylomorpha
Clade: Crocodyliformes
Clade: Eusuchia
Order: Crocodilia
Owen, 1842
Subgroups
Crocodylia distribution

The Crocodilia (or Crocodylia) are an order of large, predatory, semi-aquatic reptiles. They appeared 83.5 million years ago in the Late Cretaceous period (Campanian stage) and are the closest living relatives of birds, as the two groups are the only known survivors of the Archosauria. Members of the crocodilian total group, the clade Pseudosuchia, appeared about 250 million years ago in the Early Triassic period, 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, 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 airflow around the lungs, but like other reptiles they are ectotherms.

Crocodilians are found mainly in lowlands in the tropics, but alligators also live 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, molluscs, birds, and mammals; some species like the Indian gharial are specialised feeders, while others like the saltwater crocodile have generalised diets.

Eight species of crocodilians have attacked humans. The largest number of attacks come from the Nile crocodile. Humans are the largest threat to crocodilian populations through activities that include hunting and habitat destruction, but farming has greatly reduced unlawful trading in wild skins. Crocodilians appear in folklore and literature from around the world since 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). 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'.[1] The Greek name in turn is assumed[2] 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.[3]

Morphology and physiology

Mounted skeleton and model of Nile crocodile
Crocodilians, like this spectacled caiman, can hide in water with only their nostrils, eyes and ears at the surface.

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 weighs up to 2,000 kg (4,400 lb). They tend to be sexually dimorphic, with males much larger than females.[4] Though there is diversity in snout and tooth shape, crocodilian species have essentially the same body morphology.[5] They have solidly built, lizard-like bodies with elongated, flattened snouts and laterally compressed tails.[4] Their limbs are reduced in size; the front feet have five digits with little or no webbing, and the hind feet have four webbed digits and a rudimentary fifth.[6] The skeleton is somewhat typical of tetrapods, although the skull, pelvis and ribs are specialised;[4] in particular, the ribs to allow the animal to collapse its thorax during diving and expand it to accommodate large masses of food.[7] 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 the single penis in males and the clitoris in females.[4] The testes or ovaries are located near the kidneys.[8]

The eyes, ears and nostrils of crocodilians are at the top of the head. This allows them to stalk their prey with most of their body underwater.[9] 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.[6] While eyesight is fairly good in air, it is significantly weakened underwater.[10] The eardrums are protected by flaps that can be opened or closed by muscles.[4]

The ears are adapted for hearing both in air and underwater.[4] Crocodilians have a wide hearing range, with sensitivity comparable to most birds and many mammals.[11] They appear to have a well-developed olfactory system,[4] while the well-developed trigeminal nerve allows them to detect vibrations in the water (such as those made by potential prey).[12] 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 crocodilian leaves the water and dries off, this substance is visible as "tears".[6] The tongue cannot move freely but is held in place by a folded membrane.[7] While the brain of a crocodilian is fairly small, it is capable of greater learning than most reptiles.[13]

Locomotion

Crocodilians are excellent swimmers. During aquatic locomotion, the muscular tail undulates from side to side to drive the animal through the water while the limbs are held close to the body to reduce drag.[9][14] When the animal needs to stop, steer, or manoeuvre in a different direction, the limbs are splayed out.[9] Crocodilians generally cruise slowly on the surface or underwater with gentle sinuous movements of the tail, but when pursued or when chasing prey they can move rapidly, and can lunge out of the water in a manner reminiscent of dolphins.[15] Crocodilians are less well-adapted for moving on land, and are unusual among vertebrates in having two different means of terrestrial locomotion: the "high walk" and the "low walk".[6] Their ankle joints flex in a different way from those of other reptiles, a feature they share with some early archosaurs. 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.[16]

Crocodilians, like this American alligator, can "high walk" with the limbs held almost vertically, unlike other reptiles.

The high walk of crocodilians, with the belly and most of the tail being held off the ground, is unique among living reptiles. It somewhat resembles the walk of a mammal, with the same sequence of limb movements: left fore, right hind, right fore, left hind.[15] 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.[17]

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.[18] 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.[15] In some small species such as the freshwater crocodile, a running gait can progress to a bounding gallop. This involves the hind limbs launching the body forward and the fore limbs subsequently taking the weight. Next, the hind limbs swing forward as the spine flexes dorso-ventrally, and this sequence of movements is repeated.[19] During terrestrial locomotion, a crocodilian can keep its back and tail straight, since the scales are attached to the vertebrae by muscles.[7] 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.[4][9]

Jaws and teeth

Skull of gharial

The snout shape of crocodilians varies between species. Crocodiles may have either broad or slender snouts, while alligators and caimans 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,[4] and a crocodilian's jaws can be held shut by a person fairly easily. Conversely, the jaws are extremely difficult to pry open.[20] The powerful closing muscles attach at the bottom of the skull, allowing the top of the head to retain a flat profile. The jaw hinge attaches to the atlanto-occipital joint, allowing the animal to open its mouth fairly wide.[7] Crocodilians have one of the strongest bite forces in the animal kingdom. In a study published in 2003, an American alligator's bite force was measured at up to 2,125 lbf (9,450 N).[21] In a 2012 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. Nevertheless, the gharial's extremely slender jaws are relatively weak.[5]

Crocodilian teeth vary from blunt and dull to sharp and needle-like.[5] 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 alligators and caimans when the jaws are closed.[4] Crocodilians are polyphyodonts and able to replace each of their 80 teeth up to 50 times in their 35 to 75-year lifespan.[22] They are the only non-mammalian vertebrates with tooth sockets.[23] Next to each full-grown tooth there is a small replacement tooth and a odontogenic stem cell in the dental lamina in standby, which can be activated when required.[24] The tooth replacement rate decreases significantly and eventually stops as the animal grows older.[4]

Skin and scales

Main article: Crocodilian armor
Skin of a juvenile Nile crocodile

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.[25]

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 armour. They often have prominent, lumpy ridges and are covered in hard-wearing beta-keratin.[4] Most of the skin on the head is fused to the skull.[7] 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 scutes arranged in neat rows.[4][26] The scutes contain blood vessels and may act to absorb or radiate heat during thermoregulation.[4] Some scutes contain a single pore known as an integumentary sense organ. Crocodiles and gharials have these on large parts of their bodies, while alligators and caimans only have then on the head. Their exact function is not fully understood, but it has been suggested that they may be tactile organs,[27] 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, as indirect evidence suggest that they secrete pheromones used in courtship or nesting.[4] The skin of crocodilians is tough and can withstand damage from conspecifics, and the immune system is effective enough to heal wounds within a few days.[28]

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,[4] and both a left and right aorta which are connected by a hole called the Foramen of Panizza. Like birds and mammals, crocodilians have heart valves that flip open when pressured by surges of blood and shut closely when the pressure subsides. They also have unique cog-teeth-like valves that, when interlocked, direct blood to the left aorta and away from the lungs, and then back around the body.[29] This system may allow the animals to remain submerged for a longer period,[30] but this explanation has been questioned;[31] other possible reasons for the peculiar circulatory system include that it assists with thermoregulatory needs, reduces pulmonary oedema, or speeds recovery from metabolic acidosis. Retaining carbon dioxide within the body permits 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.[32]

When submerged, a crocodilian's heart rate slows down to one or two beats a minute, and blood flow to the muscles is reduced. When it rises and takes a breath, its heart rate speeds up in seconds, and the muscles receive newly oxygenated blood.[33] Unlike many marine mammals, crocodilians have little myoglobin to store oxygen in their muscles. During diving, muscles are supplied oxygen when an increasing concentration of bicarbonate ions forces haemoglobin in the blood to release oxygen.[34]

Respiration

X-ray video of a female American alligator showing contraction of the lungs while breathing

Crocodilians were traditionally thought to breathe like mammals, with airflow moving in and out tidally, but studies published in 2010 and 2013 conclude that crocodilians breathe like birds. The studies found that 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, then 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 unidirectional loop around the lungs. It is not understood how they can breathe this way with a diaphragm instead of air sacs.[35][36]

The lungs of the crocodilian are attached to the liver and the pelvis by the diaphragm. During breathing, the lungs are stretched and compressed. 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, and the diaphragm pulls the liver back. When exhaling, the intercostal muscles push the ribs inward, while the rectus abdominis pulls the hips and liver forwards and the belly inward.[37][4][38] 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.[38] Swimming and diving crocodilians appear to rely on lung volume more for buoyancy than oxygen storage.[4] When submerging, the nostrils of a crocodilian shut tight.[6] 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.[4][6] This enables them to breathe even when the mouth is open underwater.[6] Crocodilians typically remain underwater for short periods of fifteen minutes or less, but some can hold their breaths for up to two hours under ideal conditions.[39]

Digestion

Crocodilian teeth are adapted for seizing and holding prey, and 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 that of any other vertebrate and contains ridges for gastroliths, which play a role in the mechanical breakdown of food. Digestion takes place more quickly at higher temperatures.[9] 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 are 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

Captive Indian gharial basking

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 for any crocodilian, while immersion in water may either raise its temperature by conduction, or cool the animal in hot weather. The main method for regulating its temperature is behavioural. 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 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).[4]

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 falls 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 and 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

Saltwater crocodile resting on beach

No species of crocodilian can be considered truly marine; 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. All crocodilians 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 through the skin, across the lining of the mouth, when water is drunk, incidentally while feeding, and when present in foods. Salts and water are lost from the body in the urine and faeces, 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 causes water loss by evaporation, but the skin is a largely effective barrier to both water and ions.[43] Large animals are better able to maintain homeostasis at times of osmotic stress than smaller ones.[4] 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.[4][43]

Distribution and habitat

Spectacled caimans on river bank

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 are the American and Chinese alligators, whose ranges extend as far north as the south-eastern 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. None of them permanently reside in the sea, though some can venture into it, and several 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 where 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. 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; in the rainy season, much of the savannah in the Venezuelan llanos is flooded, and they disperse widely across the plain.[47]

Gharial accidentally camouflaged with floating weed

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 forest, 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. Destruction of forest habitat is probably a greater threat to crocodilians than is hunting.[49]

Ecological role

In the Amazon basin, when caimans became scarce as a result of overhunting in the mid-20th century, the number of local fish, such as the important arapaima (Arapaima gigas), also decreased. These are nutrient-poor waters, and it was hypothesised that the urine and faeces 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. American alligator mounds are used by turtles and snakes, both for basking and for laying their own eggs. The Florida red-bellied turtle specialises in this, and alligator mounds may have several clutches of turtle eggs developing alongside the owner's eggs.[52] Alligators modify some wetland habitats in flat areas such as the Everglades by constructing small ponds known as "alligator holes". These create wetter or drier habitats for other organisms, such as plants, fish, invertebrates, amphibians, reptiles, and mammals. In the limestone depressions of cypress swamps, alligator holes tend to be large and deep. Those in marl prairies and rocky glades are usually small and shallow, and those in peat depressions of ridge and slough wetlands are more variable. Man-made holes do not appear to have as large an effect.[53]

Behaviour 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, when several individuals gather at remaining water sites. In addition, some species are also known to share basking sites at certain times of the day.[9]

Feeding

Nile crocodile ambushing migrating wildebeest crossing the Mara River

Crocodilians are largely carnivorous, and the diets of different species can vary with snout shape and tooth sharpness. Species with sharp teeth and long slender snouts, like the Indian gharial and Australian freshwater crocodile, are specialised for feeding on fish, insects, and crustaceans, while extremely broad-snouted species with blunt teeth, like the Chinese alligator and broad-snouted caiman, specialise in eating hard-shelled molluscs. Species whose snouts and teeth are intermediate between these two forms, such as the saltwater crocodile and American alligator, have generalised diets and opportunistically feed on invertebrates, fish, snakes, turtles, birds, and mammals.[5]

In general, crocodilians are stalk-and-ambush predators,[5] though hunting strategies vary depending on the individual species and the prey being hunted.[9] Terrestrial prey is stalked from the water's edge and then grabbed and drowned.[4][9] Gharials and other fish-eating species sweep their jaws sideways to snap up prey, and these animals can leap out of the water to catch birds, bats, and leaping fish.[54] Small animals can be killed by whiplash as the predator shakes its head.[4] Caimans use their tails and bodies to herd fish into shallow water.[9] They may also dig for bottom-dwelling invertebrates,[6] and the smooth-fronted caiman will even hunt on land.[5] Nile crocodiles are known to hunt cooperatively,[9] and several individuals may feed on the same carcass. Most species will eat anything suitable that comes within reach and are also opportunistic scavengers.[6]

Crocodilians are unable to chew and need to swallow food whole, so prey that is too large to swallow is torn into 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.[54] To tear a chunk of tissue from a large carcase, a crocodilian spins its body continuously while holding on with its jaws, a manoeuvre known as the "death roll".[55] During cooperative feeding, some individuals may hold onto the prey, while others perform the roll. The animals do not fight, and each retires with a piece of flesh and awaits its next feeding turn.[56] Food is typically consumed by crocodilians with their heads above water. The food is 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.[4] Nile crocodiles may store carcasses underwater for later consumption.[6] Though mostly carnivorous, several species of crocodilian have been observed to consume fruit, and this may play a role in seed dispersal.[57]

Reproduction and parenting

Nile crocodile eggs

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 perform 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 for crocodilians 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 submerge and surface.[58] While dominant males usually monopolise 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 crocodilian begins to make a nest.[9]

Mother American alligator with nest and young

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.[9] 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.[4]

The young may all hatch in a single night.[59] Crocodilians are unusual among reptiles in the amount of parental care provided after the young hatch.[9] 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.[60] 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.[61] Hatchlings of many species tend to bask in a group during the day and disperse at nightfall to feed.[59] The time it takes young crocodilians to reach independence can vary. For American alligators, groups of young associate with adults for one to two years, while juvenile saltwater and Nile crocodiles become independent in a few months.[9]

Communication

Alligator bellow
Alligator bellow, ogg/Vorbis format.
Another alligator bellow
Alligator bellow, ogg/Vorbis format.
Alligator hiss
Alligator hiss ogg/Vorbis format.
Problems playing these files? See media help.

The social life of a crocodilian 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 and even unrelated adults respond quickly to juvenile distress calls.[59]

Vocalisations 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 or alerting the youngsters to the presence of food. The range and quantity of vocalisations vary 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.[59]

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 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.[59] Dominant individuals may also display their body size while swimming at the water surface, and a subordinate will summit by holding its head at an acute angle with the jaws open before retreating underwater.[9]

Growth and mortality

Young saltwater crocodiles in captivity

Mortality is high for eggs and hatchlings, and nests face threats from floods, overheating, and predators.[9] 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.[49] 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.[62] 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.[63]

Despite the maternal care they receive, hatchlings commonly fall prey to fish, birds, mammals, and other reptiles.[64] 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 for 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 in any clutch there may be malformed individuals that are unlikely to survive.[63] In northern Australia, the survival rate for saltwater crocodile hatchlings is only twenty-five percent, but with each succeeding year of life this improves, reaching 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.[64] The jaguar[65] and the giant otter[66] may prey on caimans in South America. In other parts of the world, elephants and hippopotamuses may kill crocodiles defensively.[9] 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.[63]

Growth in hatchlings and young crocodilians depends on the food supply, and sexual maturity is linked with length rather than 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 maturity at 1.4 m (4 ft 7 in). The common caiman matures earlier, reaching its mature length of 1.2 m (3 ft 11 in) in four to seven years.[58] Crocodilians continue to grow throughout most of their lives. Males in particular continue to gain in weight as they get older, but this is mostly in the form of extra girth rather than length.[67] Crocodilians can live 35–75 years,[22] and their age can be determined by growth rings in their bones.[58][67]

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.[68] 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 comprises the Pseudosuchia, the "false crocodiles", and the Ornithosuchia, which in turn comprises the dinosaurs and their relatives, the pterosaurs, and the birds.[69] 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.[70]

Restoration of early crocodylomorph Protosuchus

The crocodylomorphs are the only pseudosuchians to have survived the Triassic–Jurassic extinction event, 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 recently extinguished groups. Unfolding fossil evidence shows that Mesozoic crocodylomorphs had a much greater diversity of forms than modern crocodilians. Some became small fast-moving insectivores, others specialist fish-eaters, still others marine and terrestrial carnivores, and a few became herbivores.[71] 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 Cretaceous 80 million years ago and includes all the crocodilians living today.[70]

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 breathe 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 Wight in the United Kingdom.[72]

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.[73]

Phylogeny

Borealosuchus wilsoni

The phylogeny of the Crocodilia is undergoing revision. This cladogram after Holliday and Gardner, 2012, preserves the traditional 'Brevirostres':[74]

Eusuchia

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:[75]

Crocodylia

Extant taxonomy

The male gharial has a long narrow snout with a boss at the tip.

There are two extant species of Gavialidae: the gharial and the false gharial. Gharials can be recognised by the long narrow snout, with an enlarged boss at the tip. They are rare and found only in South Asia.[9]

The lower teeth of alligators, like this Chinese alligator, are not normally visible when the mouth is closed.

The extant Alligatoridae are two species in the genus Alligator, and six species of caimans grouped into three genera. They can be recognised by the broad snout, in which the fourth tooth of the lower jaw cannot be seen when the mouth is closed.[9]

The fourth lower tooth of a crocodile is exposed when the mouth is closed.

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 recognised because the fourth tooth of the lower jaw is visible when the mouth is closed.[9]

Interactions with humans

Farming and ranching

Main article: Alligator farm
Aerial view of a crocodile farm in Cambodia

Alligators and crocodiles were first farmed in the early 20th century, but the facilities involved were zoo-like and their main source of income was from tourism. By the early 1960s, the feasibility of farming these reptiles on a commercial scale was investigated in response to the decline of many crocodilian species around the world. Farming involves breeding and rearing captive stock on a self-contained basis, whereas ranching means the use of eggs, juveniles, or adults taken each year from the wild. Commercial organisations must satisfy the criteria of the Convention on International Trade in Endangered Species (CITES) by demonstrating that, in the area concerned, they do not adversely impact the wild population.[76]

Alligator and crocodile farming began because of demand for their hides, but now nearly all parts of the animal are put to use. The side and belly skin make the best leather, the meat is eaten, the gall bladders are valued in the Far East, and the heads are sometimes made into ornaments.[77] In traditional Chinese medicine, alligator meat is said to cure the common cold and prevent cancer, while various internal organs are believed to have medicinal properties.[78]

Attacks

Main article: Crocodile attack
See also: Alligator wrestling

Crocodilians are opportunistic predators that are at their most dangerous in water and at the edge of water. Eight species are known to attack humans and may do so to defend their territories, nests, or young; by mistake, while attacking domestic animals such as dogs; or 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, the Morelet's crocodile, the mugger crocodile, the American crocodile, the gharial, and the freshwater crocodile.[79]

Sign in Florida warning of alligators

The Nile crocodile has a reputation as the biggest killer of large animals, including humans, on the African continent. It is widely distributed, found in many habitats and cryptically coloured. From a waiting position with only its eyes and nostrils above the water, it can lunge at drinking animals, fishermen, bathers, or people collecting water or washing clothes. Once seized and dragged into the water, there is little chance for the victim to escape. Analysis of attacks show that most take place during the breeding season or when crocodiles are guarding nests or newly hatched young.[80] Although many attacks go unreported, there are estimated to be over 300 per year, 63% of which are fatal.[79] Wild saltwater crocodiles in Australia carried out 62 confirmed and unprovoked attacks causing injury or death between 1971 and 2004. These animals have also caused fatalities in Malaysia, New Guinea, and elsewhere. They are highly territorial and resent intrusion into their territories by other crocodiles, humans, or boats such as canoes. Attacks may come from animals of various sizes, but the larger males are generally responsible for fatalities. As their size increases, so does their need for larger mammalian prey; pigs, cattle, horses, and humans are all within the size range they seek. Most of the people attacked were either swimming or wading, but in two instances they were asleep in tents.[81]

American alligators are recorded as making 242 unprovoked attacks between 1948 and mid-2004, causing sixteen human fatalities. Ten of these were in the water and two were on land; the circumstances of the other four are not known. Most attacks were in the warmer months of the year, though in Florida, with its warmer climate, attacks can happen at any time of year.[79] Alligators are considered to be less aggressive than either the Nile or saltwater crocodile,[82] but the increase in density of the human population in the Everglades has brought people and alligators into closer proximity and increased the risk of alligator attacks.[79][82]

As pets

Several species of crocodilian are traded as exotic pets. They are appealing when young, and pet-store owners can easily sell them, but crocodilians do not make good pets; they grow large and are both dangerous and expensive to keep. As they grow older, pet crocodilians are often abandoned by their owners, and feral populations of spectacled caimans exist in the United States and Cuba. Most countries have strict regulations for keeping these reptiles.[83]

Conservation

Crocodile skins can be used to make items such as these shoes on display at a conservation exhibit at Bristol Zoo, England.

The main threat to crocodilians around the world is human activity, including hunting and habitat destruction. Early in the 1970s, more than 2 million wild crocodilian skins of a variety of species had been traded, driving down the majority of crocodilian populations, in some cases almost to extinction. Starting in 1973, CITES attempted to prevent trade in body parts of endangered animals, such as the skins of crocodiles. This proved to be problematic in the 1980s, as crocodiles were abundant and dangerous to humans in some parts of Africa, 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. In the late 1970s, crocodiles began to be farmed in different countries, started from eggs taken from the wild. By the 1980s, farmed crocodile skins were produced in sufficient numbers to destroy the unlawful trade in wild crocodilians. By 2000, skins from twelve crocodilian species, whether harvested lawfully in the wild or farmed, were traded by thirty countries, and the unlawful trade in the products had almost vanished.[84]

The gharial has undergone a chronic long-term decline, combined with a rapid short-term decline, leading the IUCN to list the species as critically endangered. In 1946, the gharial population had been widespread, numbering around 5,000 to 10,000; by 2006, however, it had declined 96–98%, reduced to a small number of widely spaced subpopulations of fewer than 235 individuals. This long-term decline had a number of causes, including egg collection and hunting, such as for indigenous medicine. The rapid decline of about 58% between 1997 and 2006 was caused by increasing use of gill nets and the loss of riverine habitat.[85] The gharial population continues to be threatened by environmental hazards such as heavy metals and protozoan parasites.[86]

The American Alligator has also suffered serious declines from hunting and habitat loss throughout its range, threatening it with extinction. In 1967 it was listed as an endangered species, but the United States Fish and Wildlife Service and state wildlife agencies in the Southern United States stepped in and worked towards its recovery. Protection allowed the species to recuperate, and in 1987 it was removed from the endangered species list.[87] Much research into alligator ranching has been undertaken at the Rockefeller Wildlife Refuge, a large area of marshland in the state of Louisiana. The resulting data has increased understanding of penning, stocking rates, egg incubation, hatching, rearing, and diet, and this information has been used at other establishments around the world. Income from the alligators kept at Rockefeller Wildlife Refuge contributes to conservation of the marshland.[88] A study examining alligator farms in the United States showed that they have generated significant conservation gains, and poaching of wild alligators has greatly diminished.[89]

Cultural depictions

Main article: List of fictional crocodiles and alligators

In mythology and folklore

Relief of Egyptian god Sobek

Crocodilians have had prominent roles in the myths and legends of various cultures around the world and may even have inspired stories of dragons.[90] In Ancient Egyptian religion, Ammit, the demoniac devourer of unworthy souls, and Sobek, the god of power, protection, and fertility, are both represented in the form of the crocodile. This reflects the Egyptians' view of the crocodile both as an important part of the Nile ecosystem and as a terrifying predator. The crocodile was one of several animals that the Egyptian mummified.[91] Crocodiles were also associated with various water deities by tribes of West Africa.[92] The Leviathan described in the Book of Job may have been based on a crocodile.[93] In Mesoamerica, the Aztecs had a crocodilian god of fertility named Cipactli who protected 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.[94]

The gharial is featured in the folk tales of India. In one story, a gharial and a 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.[95] Similar stories exist in Native American legends, and in the African American folktale of an alligator and Br'er Rabbit.[96]

In a Malay folk tale, the mouse deer Sang Kancil wants to cross a river to reach the fruit trees on the far side, but Sang Buaya, a crocodile, is waiting in the river to eat him. Sang Kancil asks all the crocodiles to line up across the river so he can count them for the king, and makes them promise not to eat him as he counts. He then steps on their heads one by one, calling out "One! Two! Three!" as he goes. When he reaches the far side, he thanks them for helping him cross the river, and feasts on fruit. Sang Buaya does not fare as well, since the other crocodiles are angry with him for letting Sang Kancil trick them.[97] A legend from East Timor tells how a boy rescues a gigantic crocodile that becomes stranded. In return, the crocodile protects him for the rest of its life, and when it dies, its scaly ridged back becomes the hills of Timor.[98] One Australian Dreamtime story tells of a crocodile ancestor who had fire all to himself. One day, a "rainbow bird" stole fire-sticks from the crocodile and gave it to man. Hence the crocodile lives in water.[99]

In literature

Crocodile in the mediaeval Rochester Bestiary, late 13th century

Ancient historians have described crocodilians from the earliest historical records, though often their descriptions contain as much legend as fact. The Ancient Greek historian Herodotus described the crocodile in detail, though much of his description is fanciful; he claimed that it would lie with its mouth open to permit a "trochilus" bird (possibly a sandpiper) to enter and remove any leeches it found.[100] The crocodile was one of the beasts described in the late-13th century Rochester Bestiary, based on classical sources, including Pliny's Historia naturalis[101] and Isidore of Seville's Etymologies.[102][103] Isidore asserts that the crocodile is named for its saffron colour (Latin croceus, 'saffron'), and that it is often twenty cubits (10 m (33 ft)) long. He further claimed that the crocodile may be killed by fish with serrated crests sawing into its soft underbelly, and that the male and female take turns guarding the eggs.[104]

Crocodiles have been reputed to weep for their victims since the 9th century Bibliotheca by Photios I of Constantinople.[105] The story was repeated in later accounts such as that of Bartholomaeus Anglicus in the 13th century.[106] It became widely known in 1400 when the English traveller Sir John Mandeville wrote his description of "cockodrills":[107]

"In that country [of Prester John] and by all Ind [India] 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."[107]

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

Florida Gators mascots Albert and Alberta

Crocodilians, especially the crocodile, have been recurring characters in stories for children throughout the modern era. Lewis Carroll's Alice's Adventures in Wonderland (1865) contains the poem How Doth the Little Crocodile,[108] a parody of a moralising poem by Isaac Watts, Against Idleness and Mischief.[109] In J. M. Barrie's novel Peter and Wendy (1911), the character Captain Hook has lost his arm to the crocodile. Hook fears the crocodile, but is warned of its approach by the ticking of a clock which it has swallowed.[110] 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.[111] Roald Dahl's The Enormous Crocodile (1978), illustrated by Quentin Blake, tells how a crocodile wanders the jungle looking for children to eat, trying one trick after another.[112]

In sports and media

Crocodilians are sometimes used mascots for sports teams. The Canton Crocodiles were a baseball team in the Frontier League,[113] for example, and the Southern Districts football team in the Australian Northern Territory Football League uses the crocodile as its emblem.[114] The University of Florida sport teams are known as the Florida Gators, in reference to the American alligator, and their mascots are Albert and Alberta Gator.[115] In film and television, crocodilians are represented as dangerous obstacles in lakes and rivers, as in the 1986 Australian comedy film Crocodile Dundee,[116] or as monstrous man-eaters in horror films like Eaten Alive (1977), Alligator (1980), Lake Placid (1999), Primeval (2007), and Black Water (2007).[117] Some media have attempted to portray these reptiles in more positive or educational light, such as Steve Irwin's wildlife documentary series The Crocodile Hunter.[118] Crocodilians in animation include the Hanna-Barbera character Wally Gator, and Ben Ali Gator from the Dance of the Hours segment of Disney's 1940 film Fantasia.

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Bibliography

--- Sues, Hans-Dieter. "The Place of Crocodilians in the Living World". pp. 14–25. {{cite book}}: Missing or empty |title= (help)
--- Buffetaut, Eric. "Evolution". pp. 26–41. {{cite book}}: Missing or empty |title= (help)
--- Mazzotti, Frank J. "Structure and Function". pp. 42–57. {{cite book}}: Missing or empty |title= (help)
--- Ross, Charles A.; Magnusson, William Ernest. "Living Crocodilians". pp. 58–73. {{cite book}}: Missing or empty |title= (help)CS1 maint: multiple names: authors list (link)
--- Pooley, A. C. "Food and Feeding Habits". pp. 76–91. {{cite book}}: Missing or empty |title= (help)
--- Pooley, A. C.; Ross, Charles A. "Mortality and Predators". pp. 92–101. {{cite book}}: Missing or empty |title= (help)CS1 maint: multiple names: authors list (link)
--- Lang, Jeffrey W. "Social Behaviour". pp. 102–117. {{cite book}}: Missing or empty |title= (help)
--- Magnusson, William Ernest; Vliet, Kent A.; Pooley, A. C.; Whitaker, Romulus. "Reproduction". pp. 118–135. {{cite book}}: Missing or empty |title= (help)CS1 maint: multiple names: authors list (link)
--- Alcala, Angel C.; Dy-Liacco, Maria Teresa S. "Habitats". pp. 136–153. {{cite book}}: Missing or empty |title= (help)CS1 maint: multiple names: authors list (link)
--- Pooley, A. C.; Hines, Tommy C.; Shield, John. "Attacks on Humans". pp. 172–187. {{cite book}}: Missing or empty |title= (help)CS1 maint: multiple names: authors list (link)

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