Crocodilian

Crocodilia (or Crocodylia, both ) is an order of mostly large, predatory, semiaquatic reptiles, known as crocodilians. They first appeared 95 million years ago in the Late Cretaceous period (Cenomanian stage) and are the closest living relatives of birds, as the two groups are the only known survivors of the Archosauria. Members of the order's 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 gharial and false gharial (family Gavialidae). Although the term 'crocodiles' is sometimes used to refer to all of these, crocodilians is a less ambiguous vernacular term for members of this group.

Large, solidly built, lizard-like reptiles, crocodilians have 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", while smaller species are even capable of galloping. Their skin is thick and covered in non-overlapping scales. They have conical, peg-like teeth and a powerful bite. They have a four-chambered heart and, somewhat like birds, a unidirectional looping system of airflow within the lungs, but like other living 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. Crocodilians are typically solitary and territorial, though cooperative feeding does occur. During breeding, dominant males try to monopolise available females. Females lay eggs in holes or in mounds and, unlike most other reptiles, care for their hatched young.

Some species of crocodilians are known to have attacked humans. The largest number of attacks comes from the Nile crocodile. Humans are the greatest threat to crocodilian populations through activities that include hunting, poaching, and habitat destruction, but farming of crocodilians has greatly reduced unlawful trading in wild skins. Artistic and literary representations of crocodilians have appeared in human cultures around the world since Ancient Egypt. The earliest known mention of the story that crocodiles weep for their victims was in the 9th century; it was later spread by Sir John Mandeville in 1400 and then by William Shakespeare in the late 16th century and early 17th century.

Spelling and etymology

Crocodilia and Crocodylia have been used interchangeably for decades starting with Schmidt's redescription of the group from the formerly defunct term Loricata.Schmidt, K.P. 1953. A Checklist of North American Amphibians and Reptiles. Sixth edition. Amer. Soc. Ichthy. Herp. Chicago, University of Chicago Press. Schmidt used the older term Crocodilia, based on Owen's original name for the group.Owen, R. 1842. Report on British Fossil Reptiles. Part II. Report British Association Adv. Sci. Plymouth Meeting. 1841:60–240. Shortly after, Wermuth opted for Crocodylia as the proper name for this redescribed group,Wermuth, H. 1953. Systematik der Rezenten Krokodile. Mitt. Mus. Berlin. Vol. 29(2):275–514. basing it on the type genus '' Crocodylus'' ( Laurenti, 1768).Laurenti, J.N. 1768. Specimen Medicum, Exhibens Synopsin Reptilium Emendatum cum Experimentis Circa Venena et Antidota Reptilium Austriacorum. Joan. Thom. Nob. de Trattern, Vienna. Dundee—in a revision of many reptilian and amphibian names—argued strongly for Crocodylia to be the spelling for the group.Dundee, H.A. 1989. Higher Category Name Usage for Amphibians and Reptiles. Syst. Zool. Vol. 38(4):398–406, DOI 10.2307/2992405. However, it was not until the advent of cladistics and phylogenetic nomenclature that a more solid justification for assuming one spelling over the other was proposed.

Prior to 1988, Crocodilia/Crocodylia was a group that encompassed the modern-day animals as well as their more distant relatives now in the larger groups called Crocodylomorpha and Pseudosuchia. Under its current definition as a crown group (as opposed to a stem-based group), Crocodylia is now restricted to only the last common ancestor of today's modern-day crocodilians ( alligators, crocodiles, and gharials) and all of its descendants (living or extinct). This distinction is more important for paleontologists studying crocodilian evolution. As such, the alternate spellings Crocodilia and Crocodylia are still used interchangeably in the neontological literature.

Crocodilia appears to be a Latinizing of the Greek κροκόδειλος (''crocodeilos''), which means both lizard and Nile crocodile. Crocodylia, as coined by Wermuth, in regards to the genus ''Crocodylus'' appears to be derived from the ancient Greek κρόκη (''kroke'')—meaning shingle or pebble—and δρîλος or δρεîλος (''dr(e)ilos'') for "worm". The name may refer to the animal's habit of basking on the pebbled shores of the Nile.

Morphology and physiology

Crocodilians range in size from the ''Paleosuchus'' and ''Osteolaemus'' species, which reach , to the saltwater crocodile, which reaches and weighs up to , though some prehistoric species such as the late Cretaceous '' Deinosuchus'' were even larger at up to about and . They tend to be sexually dimorphic, with males much larger than females.Grigg and Gans, pp. 326–327. Though there is diversity in snout and tooth shape, all crocodilian species have essentially the same body morphology. They have solidly built, lizard-like bodies with elongated, flattened snouts and laterally compressed tails. 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.Kelly, pp. 70–75. The skeleton is somewhat typical of tetrapods, although the skull, pelvis and ribs are specialised; in particular, the cartilaginous processes of the ribs allow the thorax to collapse during diving and the structure of the pelvis can accommodate large masses of food, or more air in the lungs. 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 penis in males and the clitoris in females. The crocodilian penis is permanently erect and relies on cloacal muscles for eversion and elastic ligaments and a tendon for recoil. The gonads are located near the kidneys.

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 bodies underwater. Crocodilians possess a '' tapetum lucidum'' which enhances vision in low light. While eyesight is fairly good in air, it is significantly weakened underwater. The fovea in other vertebrates is usually circular, but in crocodiles it is a horizontal bar of tightly packed receptors across the middle of the retina. 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".

The ears are adapted for hearing both in air and underwater, and the eardrums are protected by flaps that can be opened or closed by muscles.Grigg and Gans, p. 335. Crocodilians have a wide hearing range, with sensitivity comparable to most birds and many mammals. They have only one olfactory chamber and the vomeronasal organ is absent in the adults indicating all olfactory perception is limited to the olfactory system. Behavioural and olfactometer experiments indicate that crocodiles detect both air-borne and water-soluble chemicals and use their olfactory system for hunting. When above water, crocodiles enhance their ability to detect volatile odorants by gular pumping, a rhythmic movement of the floor of the pharynx. The well-developed trigeminal nerve allows them to detect vibrations in the water (such as those made by potential prey). The tongue cannot move freely but is held in place by a folded membrane. They appear to have lost their pineal organ, but still show signs of melatonin rhythms. Vocal folds (vocal cords) was found in ''Alligator mississipiensis'', that have a complex morphology consisting of epithelium, lamina propria and muscle, and according to Riede et al. (2015), "it is reasonable to expect species-specific morphologies in vocal folds/analogues as far back as basal reptiles". Crocodilians can produce vocalisations by vibrating three flaps in the larynx. The crocodilian larynx is capable of complex motor control similar to birds and mammals. While the brain of a crocodilian is fairly small, it is capable of greater learning than most reptiles.

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. When the animal needs to stop, steer, or manoeuvre in a different direction, the limbs are splayed out. 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.Mazzotti, pp. 43–46. 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". 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.Sues, p. 21.

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. 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 usual 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.

Though typically slow on land, crocodilians can produce brief bursts of speed, and some can run at for short distances. A fast entry into water from a muddy bank can be effected by plunging to the ground, twisting the body from side to side and splaying out the limbs. 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. During terrestrial locomotion, a crocodilian can keep its back and tail straight, since the scales are attached to the vertebrae by muscles. 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.

Jaws and teeth

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, and a crocodilian's jaws can be held shut by a person fairly easily. Conversely, the jaws are extremely difficult to pry open. The powerful closing muscles attach at the median portion of the lower jaw and the jaw hinge attaches to the atlanto-occipital joint, allowing the animal to open its mouth fairly wide.Huchzermeyer, pp. 7–10.

Crocodilians have some 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 . In a 2012 study, a saltwater crocodile's bite force was measured even higher, at . This study also found no correlation between bite force and snout shape. Nevertheless, the gharial's extremely slender jaws are relatively weak and built more for quick jaw closure. The bite force of ''Deinosuchus'' may have measured , even greater than that of theropod dinosaurs like '' Tyrannosaurus''.

Crocodilian teeth vary from blunt and dull to sharp and needle-like. 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 than those of alligators and caimans when the jaws are closed.Grigg and Gans, pp. 227–228. The easiest way to distinguish crocodiles from alligators is by looking at their jaw line. The teeth on the lower jaw of an alligator fit into sockets in the upper jaw, so only the upper teeth are visible when the mouth is closed. The teeth on the lower jaw of a crocodile fit into grooves on the outside of the top jaw making both the upper and lower teeth visible when the mouth is closed.

Crocodilians are homodonts, meaning each of their teeth are all of the same type (they do not possess different tooth types, such as canines and molars) and polyphyodonts are able to replace each of their approximately 80 teeth up to 50 times in their 35 to 75-year lifespan. They are the only non-mammalian vertebrates with tooth sockets. Next to each full-grown tooth there is a small replacement tooth and an odontogenic stem cell in the dental lamina in standby, which can be activated when required. Tooth replacement slows significantly and eventually stops as the animal grows old.

Skin and scales

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.

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. The head and jaws lack actual scales and are instead covered in tight keratinised skin that cracks due to stress. 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. The scutes contain blood vessels and may act to absorb or radiate heat during thermoregulation. Research also suggests that alkaline ions released into the blood from the calcium and magnesium in these dermal bones act as a buffer during prolonged submersion when increasing levels of carbon dioxide would otherwise cause acidosis.

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 them on the head. Their exact function is not fully understood, but it has been suggested that they may be mechanosensory organs. 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. 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.

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,Grigg and Gans, pp. 331–332. 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 direct blood flow in a single direction through the heart chambers. 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. This system may allow the animals to remain submerged for a longer period, but this explanation has been questioned. Other possible reasons for the peculiar circulatory system include assistance with thermoregulatory needs, prevention of pulmonary oedema, or faster 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.

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. Unlike many marine mammals, crocodilians have little myoglobin to store oxygen in their muscles. During diving, muscles are supplied with oxygen when an increasing concentration of bicarbonate ions causes haemoglobin in the blood to release oxygen.

Respiration

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 more like birds, with airflow moving in a unidirectional loop within the lungs. 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 through 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. These aerodynamic valves within the bronchial tree have been hypothesised to explain how crocodilians can have unidirectional airflow without the aid of avian-like air sacs.

The lungs of crocodilians are attached to the liver and the pelvis by the diaphragmaticus muscle ( analogous of the diaphragm in mammals). During inhalation, the external 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 diaphragmaticus pulls the liver back. When exhaling, the internal intercostal muscles push the ribs inward, while the rectus abdominis pulls the hips and liver forwards and the belly inward. Because the lungs expand into the space formerly occupied by the liver and are compressed when it moves back into position, this motion is sometimes referred to as a "hepatic piston". Crocodilians can also use these muscles to adjust the position of their lungs; thereby controlling their buoyancy in the water. An animal sinks when the lungs are pulled towards the tail and floats when they move back towards the head. This allows them to move through the water without creating disturbances that could alert potential prey. They can also spin and twist by moving their lungs laterally.

Swimming and diving crocodilians appear to rely on lung volume more for buoyancy than oxygen storage. Just before diving, the animal exhales to reduce its lung volume and achieve negative buoyancy. When submerging, the nostrils of a crocodilian shut tight. 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. This enables them to open their mouths underwater without drowning. Crocodilians typically remain underwater for fifteen minutes or less at a time, but some can hold their breath for up to two hours under ideal conditions. The maximum diving depth is unknown, but crocodiles can dive to at least .

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 digest. The stomach is divided into two parts: a muscular gizzard that grinds food, and a digestive chamber where enzymes work on it.Mazzotti, p. 54. 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. 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. 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.

''Amona glabra'' (Pond-apple tree) seeds recovered from the stomach of an ''Alligator mississippiensis'' (American alligator) captured in the Florida Coastal Everglades were nonviable under ideal germination conditions and were likely destroyed by stomach acids. Alligators are unlikely to be dispersers of Pond-apple seeds and may instead act as seed predators.

Thermoregulation

Crocodilians are ectotherms, producing relatively little heat internally and relying on external sources to raise their body temperatures. The sun's heat 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. A bulky animal, it warms 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.Mazzotti, pp. 48–51. By these means, the temperature range of crocodilians is usually maintained between , and mainly stays in the range .

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. If 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 , but as long as they remain able to breathe they show no ill effects when the weather warms up.

Osmoregulation

No living 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 habitats are river mouths, estuaries, mangrove swamps, and hypersaline lakes, though several extinct species have had marine habitats, including the recently extinct '' Ikanogavialis papuensis'', which occurred in a fully marine habitat in the Solomon Islands coastlines. 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 across the lining of the mouth, when water is drunk, incidentally while feeding, and when present in foods. Water is lost from the body during breathing, and both salts and water are lost in the urine and faeces, through the skin, and via salt-excreting glands on the tongue, though these are only present in crocodiles and gharials.Mazzotti, pp. 52–55. The skin is a largely effective barrier to both water and ions. Gaping causes water loss by evaporation from the lining of the mouth, and on land, water is also lost through the skin. Large animals are better able to maintain homeostasis at times of osmotic stress than smaller ones. Newly hatched crocodilians are much less tolerant of exposure to salt water than are older juveniles, presumably because they have a higher surface-area-to-volume ratio.

The kidneys and excretory system are much the same as in other reptiles, but crocodilians do not have a bladder. In fresh water, the osmolality (the concentration of solutes that contribute to a solution's osmotic pressure) 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 abundant and dilute, nitrogen being excreted as ammonium bicarbonate.Grigg and Gans, pp. 333–334. Sodium loss is low and mainly takes place through the skin in freshwater conditions. In seawater, the opposite is true. The osmolality in the plasma is lower than the surrounding water, which 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.

Distribution and habitat

Crocodilians are amphibious reptiles, spending part of their time in water and part on land. The last surviving fully terrestrial genus, '' Mekosuchus'', became extinct about 3000 years ago after humans had arrived on its Pacific islands, making the extinction possibly anthropogenic. Typically they are creatures of the tropics; the main exceptions are the American and Chinese alligators, whose ranges consist of the south-eastern United States and the Yangtze River, respectively. Florida, in the United States, is the only place that crocodiles and alligators live side by side. Most crocodilians live in the lowlands, and few are found above , 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 the extreme salinity of hypersaline lakes.Alcala and Dy-Liacco, pp. 136–139. 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.Ross, p. 68.

Various types of aquatic habitats are used by different crocodilians. Some species are relatively more terrestrial and 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 spend more time in the water and inhabit the lower stretches of rivers, mangrove swamps, and estuaries. These habitats also have a rich flora and provide 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. 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 savanna in the Orinoco Llanos is flooded, and they disperse widely across the plain.Alcala and Dy-Liacco, p. 141. Desert crocodiles in Mauritania have adapted to their arid environment by staying in caves or burrows in a state of aestivation during the driest periods. When it rains, the reptiles gather at gueltas.

Dry land is also important as it provides opportunities for basking, nesting, and escaping from temperature extremes. Gaping allows evaporation of moisture from the mouth lining and has a cooling effect, and several species make use of shallow burrows on land to keep cool. Wallowing in mud can also help prevent them from overheating.Alcala and Dy-Liacco, pp. 144–146. Four species of crocodilians climb trees to bask in areas lacking a shoreline. 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 hunting.Alcala and Dy-Liacco, pp. 148–152.

Ecological roles

Being highly efficient predators, crocodilians tend to be top of the food chain in their watery environments.Pooley, pp. 76–80. The nest mounds built by some species of crocodilian are used by other animals 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.Alcala and Dy-Liacco, p. 145. 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, while those in peat depressions of ridge and slough wetlands are more variable. Man-made holes do not appear to have as large an effect.

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 the urine and faeces of the caimans may have increased primary production by contributing plant nutrients. Thus the presence of the reptiles could have benefited the fish stock; the number of crocodilians in a stretch of water appears to be correlated with the fish population.Alcala and Dy-Liacco, pp. 146–148.

Behaviour and life history

Spacing

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. Individuals of some species may share basking sites at certain times of the day.

Feeding

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, amphibians, other reptiles, birds, and mammals. Though mostly carnivorous, several species of crocodilian have been observed to consume fruit, and this may play a role in seed dispersal.

In general, crocodilians are stalk-and-ambush predators, though hunting strategies vary depending on the individual species and the prey being hunted. Terrestrial prey is stalked from the water's edge and then grabbed and drowned.Grigg and Gans, pp. 229–330. 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. A small animal can be killed by whiplash as the predator shakes its head. Caimans use their tails and bodies to herd fish into shallow water. They may also dig for bottom-dwelling invertebrates, and the smooth-fronted caiman

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