Crocodilia

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Crocodilia
Temporal range:
Ma
Clockwise from top-left: saltwater crocodile (Crocodylus porosus), American alligator (Alligator mississippiensis), and gharial (Gavialis gangeticus)
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 on land (green) and at sea (blue)

Crocodilia (or Crocodylia, both

Crocodylidae), the alligators and caimans (family Alligatoridae), and the gharial and false gharial (family Gavialidae
). Although the term crocodiles (or crocs) 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

ectotherms
.

Crocodilians are found mainly in lowlands in the

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

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.[1] Schmidt used the older term Crocodilia, based on Owen's original name for the group.[2] Shortly after, Wermuth opted for Crocodylia as the proper name for this redescribed group,[3] basing it on the type genus Crocodylus (Laurenti, 1768).[4] Dundee—in a revision of many reptilian and amphibian names—argued strongly for Crocodylia to be the spelling for the group.[5] However, it was not until the advent of cladistics and phylogenetic nomenclature that a more solid justification (priority) for one spelling over the other was proposed.[6]

Prior to 1988, Crocodilia (also called Crocodylia) was a group that encompassed the modern-day animals, as well as their more distant relatives now in the larger groups called

extinct).[6]

Crocodilia[2] appears to be a Latinizing of the Greek κροκόδειλος (crocodeilos), which means both lizard and Nile crocodile.[7] Crocodylia, as coined by Wermuth,[3] in regards to the genus Crocodylus appears to be derived from the ancient Greek[8] 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.[9]

Taxonomy and classification

Evolution

The main distinguishing characteristic of diapsid tetrapods is the presence of two openings (temporal fenestrae) on either side of the skull behind the eye. Living diapsids include modern reptiles and birds.[10] The feature that distinguishes archosaurs from other diapsids is an extra pair of openings in the skull (antorbital fenestrae) in front of the eye sockets. Archosauria is the crown group containing the most recent common ancestor of crocodilians and birds and all its descendants. It comprises the Pseudosuchia, the "false crocodiles", and the Avemetatarsalia, which in turn comprises the dinosaurs (including birds) and pterosaurs.[11] Pseudosuchia is defined as living crocodilians and all archosaurs more closely related to crocodilians than to birds. The Pseudosuchia–bird split is assumed to have occurred close to the Permian–Triassic mass extinction event.[12] In modern crocodilians, the antorbital fenestrae are walled off externally and exist merely as sinuses.[13] They were present in most of their fossil ancestors as small openings.[14]

Restoration of early crocodylomorph Protosuchus

The ancestors of today's

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.[14]

Skeletal mount of the giant crocodylian Deinosuchus from the Late Cretaceous of North America

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

Palaeogene.[18] Spanning the Cretaceous and Palaeogene periods is the genus Borealosuchus of North America, with six species, though its phylogenetic position is not settled.[19]

The three primary branches of Crocodilia had diverged by the end of the Mesozoic. The possible earliest-known members of the group may be Portugalosuchus from the Cenomanian (95 million years ago),[20] though the classification of this taxon has been disputed.[21][22] After are alligatoroids and gavialoids that lived in North America and Europe during the Campanian (around 83.6–72.1 million years ago). 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.[6]

Scientists conclude that environmental factors played a major role in the evolution of crocodilians and their ancestors, with warmer climate being associated with high evolutionary rates and large body sizes.[23]

Relationships

Further information: List of crocodilians

Crocodylia is

phylogenetic relationships of crocodilians has been the subject of debate and conflicting results. Many studies and their resulting cladograms, or "family trees" of crocodilians, have found the "short-snouted" families of Crocodylidae and Alligatoridae to be close relatives, with the long-snouted Gavialidae as a divergent branch of the tree. The resulting group of short-snouted species, named Brevirostres, was supported mainly by morphological studies which analyzed skeletal features alone.[25]

Range of skull shape in crocodilians, from narrow to broad-snouted

However, recent molecular studies using DNA sequencing of living crocodilians have rejected this distinct group Brevirostres, with the long-snouted gavialids more closely related to crocodiles than to alligators, with the new grouping of gavialids and crocodiles named Longirostres.[26][27][28][29][30]

Below is a cladogram showing the relationships of the major

extant crocodilian groups based on molecular studies, excluding separate extinct taxa:[30]

Crocodilia

Anatomy and physiology

Mounted skeleton and taxidermy of Nile crocodile

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), though some prehistoric species such as the late Cretaceous Deinosuchus were even larger at up to about 11 m (36 ft)[31] and 3,450 kg (7,610 lb).[28] They tend to be sexually dimorphic, with males much larger than females.[32] Though there is diversity in snout and tooth shape, all crocodilian species have essentially the same body morphology.[28] They have solidly built, lizard-like bodies with elongated, flattened snouts and laterally compressed tails.[32] 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.[33]

The skeleton is somewhat typical of

intestinal, urinary and genital tracts open.[32] It houses the penis in males and the clitoris in females.[36] The crocodilian penis is permanently erect and relies on cloacal muscles for eversion and elastic ligaments and a tendon for recoil.[37] The gonads are located near the kidneys.[38]

Locomotion

Nile crocodile swimming. Sequence runs from right to left.

Crocodilians are excellent swimmers. During

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.[42]

Crocodilians, like this American alligator, can "high walk" with the lower limb portions 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.[41] 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.[43]

Though typically slow on land, crocodilians can produce brief bursts of speed, and some can run at 12 to 14 km/h (7.5 to 8.7 mph) for short distances.

dorso-ventrally, and this sequence of movements is repeated.[45] During terrestrial locomotion, a crocodilian can keep its back and tail straight, since the scales are attached to the vertebrae by muscles.[34] 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.[39][46]

Jaws and teeth

Skull of American alligator
American alligator with mouth open, showing inside

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,[32] and a crocodilian's jaws can be held shut by a person fairly easily. Conversely, the jaws are extremely difficult to pry open.[47] 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. The tongue cannot move freely but is held in place by a folded membrane.[34]

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 2,125 lbf (9.45 kN).

theropod dinosaurs like Tyrannosaurus.[48]

Crocodilian teeth vary from blunt and dull to sharp and needle-like.[28] 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.[49] 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.[50]

Crocodilians are

tooth sockets.[52] 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.[53] Tooth replacement slows significantly and eventually stops as the animal grows old.[49]

Sense organs

Overhead view of broad-snouted caiman with eyes, ears and nostrils above water

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.

sclerotic rings, an annular pad of the lens and colored cone oil droplets. Since them, some crocodilians appears to have re-evolved a trichromatic color vision again.[54][55][56] While eyesight is fairly good in air, it is significantly weakened underwater.[57] 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".[33]

The ears are adapted for hearing both in air and underwater, and the

pineal organ, but still show signs of melatonin rhythms.[64]

Skin and scales

Main article: Crocodilian armor
Skin of a juvenile Nile crocodile

The skin of crocodilians is thick and

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.[65]

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.[32] The head and jaws lack actual scales and are instead covered in tight keratinised skin that cracks due to stress.[66] 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.[32][67] The scutes contain blood vessels and may act to absorb or radiate heat during thermoregulation.[32] 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.[68]

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

conspecifics, and the immune system is effective enough to heal wounds within a few days.[70]

In the genus Crocodylus the skin contains chromatophores, allowing them to change color from dark to light and vice versa.[71]

Circulation

Diagram of crocodilian heart and 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,[72] 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.[73] This system may allow the animals to remain submerged for a longer period,[74] but this explanation has been questioned.[75] 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.[76]

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.

haemoglobin in the blood to release oxygen.[78]

Respiration

X-ray fluoroscopy videos of a female American alligator showing contraction of the lungs while breathing (dorsoventral view above and lateral view below)

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

air sacs.[79][80]

The lungs of crocodilians are attached to the liver and the pelvis by the diaphragmaticus muscle (

rectus abdominis pulls the hips and liver forwards and the belly inward.[35][72][81][82] 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.[81]

Swimming and diving crocodilians appear to rely on

trachea.[32][33] This enables them to open their mouths underwater without drowning.[33] 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.[84] The maximum diving depth is unknown, but crocodiles can dive to at least 20 m (66 ft).[85]

Vocalizing is produced by vibrating

vocal folds in the larynx.[86][87] The folds of the American alligator 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".[88] Crocodilian vocal folds lack the elasticity of mammalian ones; but the larynx is still capable of complex motor control similar to birds and mammals and can adequately control its fundamental frequency.[88][89]

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.[90] Indigestible items are regurgitated as pellets.[91] 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.[39] When digesting a meal, CO2-rich blood towards the lungs is redirected to the stomach where glands make use of the CO2 to form bicarbonate and gastric acid secretions approximately 10 times the highest rates measured in mammals.[92][93] Alligators have a higher ability to digest carbohydrates relative to protein compared to crocodiles.[94] 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.[95] 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.[95]

Thermoregulation

Captive Indian gharial basking and gaping

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.[96] By these means, the temperature range of crocodilians is usually maintained between 25 and 35 °C (77 and 95 °F), and mainly stays in the range 30 to 33 °C (86 to 91 °F).[97]

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 5 °C (41 °F), but as long as they remain able to breathe they show no ill effects when the weather warms up.[96]

Osmoregulation

Saltwater crocodile resting on beach

No living species of crocodilian can be considered truly marine; although the

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.[98] 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.[99] 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.[100][101] 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.[100] Large animals are better able to maintain homeostasis at times of osmotic stress than smaller ones.[102] 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.[100]

The kidneys and excretory system are much the same as in other reptiles, but crocodilians do not have a

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.[102] 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.[100][102]

Distribution and habitat

Spectacled caiman immersed in vegetation covered water

Crocodilians are

Yangtze River, respectively. Florida, in the United States, is the only place that crocodiles and alligators live side by side.[104] 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 the extreme salinity of hypersaline lakes.[105] 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.[106]

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.[105] 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.[107] 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.[108]

American crocodiles basking

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.[109] Four species of crocodilians climb trees to bask in areas lacking a shoreline.[110] 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.[111]

Ecological roles

Gharial camouflaged with floating weed

Being highly efficient predators, crocodilians tend to be top of the

Florida red-bellied turtle specialises in this, and alligator mounds may have several clutches of turtle eggs developing alongside the owner's eggs.[113] 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.[114]

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;[115] the number of crocodilians in a stretch of water appears to be correlated with the fish population.[116]

Behavior and life history

Cognition

Crocodilians are among the most cognitively complex nonavian reptiles, though their behavioral repertoire is less well understood than other vertebrates due to the difficulty of monitoring solitary and often nocturnal predators in aquatic habitats.

Embryological studies of developing amniotes have shown similar brain structures in the

hunt cooperatively, herding and chasing prey.[119] Play, or the free, intrinsically motivated activity by young individuals, has been observed on numerous occasions in crocodilians in both captive and wild settings, with young alligators and crocodiles regularly engaging in object play and social play.[citation needed] As with their mammalian and avian counterparts, play likely performs a significant role in helping these predators to hone their hunting skills and develop their understanding of behavioral tradeoffs. It is important to note that not all higher social behaviors are endemic across these clades. A 2023 study of tinamou, a paleognath avian, and American alligator test subjects found that while the paleognaths were able to engage in visual perspective taking – a cornerstone of advanced social cognition – the alligators did not appear to do so.[120] Some researchers have proposed to increase the use of crocodilians as test animals in comparative cognition studies.[121]

Spacing

Adult crocodilians are typically

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.[39]

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, amphibians, other reptiles, birds, and mammals.[28][112] Though mostly carnivorous, several species of crocodilian have been observed to consume fruit, and this may play a role in seed dispersal.[122]

In general, crocodilians are stalk-and-ambush predators,[28] though hunting strategies vary depending on the individual species and the prey being hunted.[39] Terrestrial prey is stalked from the water's edge and then grabbed and drowned.[39][123] 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.[112] A small animal can be killed by whiplash as the predator shakes its head.[123] Caimans use their tails and bodies to herd fish into shallow water.[39] They may also dig for bottom-dwelling invertebrates,[33] and the smooth-fronted caiman will even hunt on land.[28] Most species will eat anything suitable that comes within reach and are also opportunistic scavengers.[33]

A gharial eating a fish

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.[112] To tear a chunk of tissue from a large carcass, a crocodilian spins its body continuously while holding on with its jaws, a maneuver known as the "death roll".[124] During cooperative feeding, some individuals may hold on to the prey, while others perform the roll. The animals do not fight, and each retires with a piece of flesh and awaits its next feeding turn.[125] After feeding together, individuals may go their separate ways.[119] 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.[123] Nile crocodiles may store carcasses underwater for later consumption.[33]

Reproduction and parenting

Nile crocodile eggs

Crocodilians are generally

Dominant male crocodilians patrol and defend territories which contain several females. Males of some species, like the American alligator, try to attract females with elaborate courtship displays. During courtship, crocodilian males and females may rub against each other, circle around, and perform swimming displays. Copulation typically occurs in the water. When a female is ready to mate, she arches her back while her head and tail submerge. The male rubs across the female's neck and then grasps her with his hindlimbs, placing his tail underneath hers 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.[126] 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.[39]

Unlike most reptiles, crocodilians care for their young even after they have hatched.[129]
Mother American alligator with nest and young

Depending on the species, female crocodilians may construct either holes or mounds as nests,[39] the latter made from vegetation, litter, sand, or soil.[102] Nests are typically found near dens or caves. Those 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. Crocodilian eggs are protected by hard shells made of calcium carbonate. The incubation period is two to three months.[39] The temperature at which the eggs incubate determines the sex of the hatchlings. Constant nest temperatures above 32 °C (90 °F) produce more males, while those below 31 °C (88 °F) produce more 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.[102]

The young may all hatch in a single night.[130] Crocodilians are unusual among reptiles in the amount of parental care provided after the young hatch.[129][39] 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.[131] Both male and female adult crocodilians will respond to vocalizations by hatchlings.[129] 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.[132] Hatchlings of many species tend to bask in a group during the day and disperse at nightfall to feed.[130] 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.[39]

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.

Crocodilians can communicate with various sounds, including bellows, roars, growls, grunts, barks, coughs, hisses, toots, moos, whines, and chirps.[86] 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.[130]

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 crocodile species are almost completely silent. Adult female

infrasonic signals that serve to attract females and intimidate rivals.[133] When individuals bellow at the water's surface, the infrasound disturbs the water in nonrandom but visually appealing patterns, leading some ecotourists to describe it as 'water dance'. The enlarged boss of the male gharial may serve as a sound resonator.[134]

Another form of acoustic communication is the head slap. 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 head slapping spread through the group. The purpose varies, but it seems to be associated with maintaining social relationships, and is also used in courtship.[130] Dominant individuals may also display their body size while swimming at the water surface, and a subordinate will submit by holding its head at an acute angle with the jaws open before retreating underwater.[39]

Growth and mortality

Young saltwater crocodiles in captivity

Mortality is high for eggs and hatchlings, and nests face threats from floods, overheating, and predators.[39] Flooding is a major cause of failure of crocodilians to breed successfully: nests are submerged, developing embryos are deprived of oxygen, and juveniles get washed away.[111] Numerous predators, both mammalian and reptilian, may raid nests and eat crocodilian eggs.[135][136] Despite the maternal care they receive, hatchlings commonly fall to predation.[137] While the female is transporting some to the nursery area, others are picked off by predators that lurk near the nest. In addition to terrestrial predators, the hatchlings are also subject to aquatic attacks by fish. Birds take their toll, and in any clutch there may be malformed individuals that are unlikely to survive.[135] 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.[137]

Mortality rates are fairly low among subadults and adults, though they are occasionally preyed on by large cats and snakes.

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. Rival male Nile crocodiles sometimes kill each other during the breeding season.[135]

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 (7–8 ft), while males mature at 3 m (10 ft). Australian freshwater crocodiles take ten years to reach maturity at 1.4 m (4 ft 7 in). The spectacled caiman matures earlier, reaching its mature length of 1.2 m (4 ft) in four to seven years.[126] Crocodilians continue to grow throughout 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.[140] Crocodilians can live 35–75 years,[51] and their age can be determined by growth rings in their bones.[126][140] The oldest known crocodile, which is also the largest known individual, is an Australian crocodile that has lived in captivity since 1984 and is estimated to be 120 years old.[141]

Interactions with humans

Farming and ranching

Main article: Crocodile farm
Aerial view of a crocodile farm in Cambodia
Crocodile farm in Mamba Village, Mombasa, Kenya

Alligators and crocodiles were first farmed in the early 20th century, but the facilities involved were

Convention on International Trade in Endangered Species (CITES) by demonstrating that, in the area concerned, they do not adversely impact the wild population.[142]

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 gallbladders are valued in East Asia, and the heads are sometimes made into ornaments.[143] 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.[144]

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. Several 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.[145]

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.[146] Although many attacks go unreported, there are estimated to be over 300 per year, 63% of which are fatal.[145] 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.[147]

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.[145] Alligators are considered to be less aggressive than either the Nile or saltwater crocodile,[148] but the increase in density of the human population in the Everglades has brought people and alligators into proximity and increased the risk of alligator attacks.[145][148] Conversely in Mauritania, where the crocodiles' growth is severely stunted by the arid conditions, the local people swim with them without being attacked.[108]

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.[149]

In medicine

The blood of alligators and crocodiles contains peptides with antibiotic properties. According to National Geographic, these may contribute to future antibacterial drugs.[150] Cartilage from crocodiles raised in crocodile farms is also used in research to 3D-print new cartilage for humans by mixing human stem cells with liquified crocodile cartilage after proteins that may trigger the human immune system has been removed.[151]

Conservation

Osteolaemus tetraspis
) at the Natural History Museum, London.

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.[152]

Young gharial in Kukrail Reserve Forest

The gharial has undergone a chronic long-term decline, combined with a rapid short-term decline, leading the

gill nets and the loss of riverine habitat.[153] The gharial population continues to be threatened by environmental hazards such as heavy metals and protozoan parasites,[154] but as of 2013 numbers are rising, due to the protection of nests against egg predators.[155] The Chinese alligator was historically widespread throughout the eastern Yangtze River system but is currently restricted to some areas in southeastern Anhui province thanks to habitat fragmentation and degradation. The wild population is believed to exist only in small fragmented ponds. In 1972, the species was declared a Class I endangered species by the Chinese government and received the maximum amount of legal protection. Since 1979, captive breeding programs were established in China and North America, creating a healthy captive population.[156] In 2008, alligators bred in the Bronx Zoo were successfully reintroduced to Chongming Island.[157] The Philippine crocodile is perhaps the most threatened crocodilian and is considered by the IUCN to be critically endangered. Hunting and destructive fishing habits have reduced its population to around 100 individuals by 2009. In the same year, 50 captive bred crocodiles were released into the wild to help boost the population. Support from local people is crucial for the species' survival.[158]

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.[159] Much research into alligator ranching has been undertaken at the Rockefeller Wildlife Refuge, a large area of marshland in the state of Louisiana. Income from the alligators kept at Rockefeller Wildlife Refuge contributes to conservation of the marshland.[160] 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.[161]

Cultural depictions

See also:
List of fictional crocodiles and alligators

In mythology, religion, 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

Aztecs had a crocodilian god of fertility named Cipactli who protected crops. In Aztec mythology, the earth deity Tlaltecuhtli is sometimes represented as a crocodile-like monster.[167] The Maya also associated crocodilians with fertility and death.[168]

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 but friendship ends after the gharial confess they tried to lure him into this house to eat him.

Dreamtime story tells of a crocodile ancestor who had fire all to himself, until a "rainbow bird" stole fire-sticks for man; hence the crocodile lives in water.[173]

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 (c. 440 BC) 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 an Egyptian plover) to remove leeches.[174] The crocodile was described in the late-13th century Rochester Bestiary, based on classical sources, including Pliny's Historia naturalis (c. 79 AD)[175] and Isidore of Seville's Etymologies.[176][177] Isidore asserts that the crocodile is named for its saffron colour (Latin croceus, 'saffron') and may be killed by fish with serrated crests sawing into its soft underbelly.[178]

Crocodiles have been

John Mandeville wrote his description of "cockodrills":[180]

"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."[180]
The Crocodile stretching the nose of the Elephant's Child in one of Rudyard Kipling's Just So Stories. Illustration by Kipling, 1902

Crocodilians, especially the crocodile, have been recurring characters in stories for children. Lewis Carroll's Alice's Adventures in Wonderland (1865) contains the poem How Doth the Little Crocodile,[181] a parody of a moralising poem by Isaac Watts, Against Idleness and Mischief.[182] In J. M. Barrie's novel Peter and Wendy (1911), Captain Hook lost his hand to the crocodile.[183] 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.[184] Roald Dahl's The Enormous Crocodile (1978) tells how a crocodile wanders the jungle looking for children to eat.[185]

In media

Crocodilians are often represented as dangerous obstacles[186] in action/adventure movies like Live and Let Die (1973) and Indiana Jones and the Temple of Doom (1984) or as monstrous man-eaters in horror films like Eaten Alive (1977), Alligator (1980), Lake Placid (1999), Crocodile (2000), Rogue (2007), Primeval (2007), Black Water (2007), and Crawl (2019).[187] In the film Crocodile Dundee, the title character's nickname comes from the animal that bit off his leg.[188] 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.[186]

See also

  • Phytosaurs

References

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Bibliography

External links
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