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Crocodilia (or Crocodylia, both ) is an
order Order, ORDER or Orders may refer to: * Categorization, the process in which ideas and objects are recognized, differentiated, and understood * Heterarchy, a system of organization wherein the elements have the potential to be ranked a number of ...
of mostly large, predatory,
semiaquatic In biology, semiaquatic can refer to various types of animals that spend part of their time in water, or plants that naturally grow partially submerged in water. Examples are given below. Semiaquatic animals Semi aquatic animals include: * Ve ...
reptile Reptiles, as most commonly defined are the animals in the class Reptilia ( ), a paraphyletic grouping comprising all sauropsids except birds. Living reptiles comprise turtles, crocodilians, squamates ( lizards and snakes) and rhynchocephalia ...
s, known as crocodilians. They first appeared 95 million years ago in the Late
Cretaceous The Cretaceous ( ) is a geological period that lasted from about 145 to 66 million years ago (Mya). It is the third and final period of the Mesozoic Era, as well as the longest. At around 79 million years, it is the longest geological period of ...
period (
Cenomanian The Cenomanian is, in the ICS' geological timescale, the oldest or earliest age of the Late Cretaceous Epoch or the lowest stage of the Upper Cretaceous Series. An age is a unit of geochronology; it is a unit of time; the stage is a unit in ...
stage) and are the closest living relatives of
birds Birds are a group of warm-blooded vertebrates constituting the class Aves (), characterised by feathers, toothless beaked jaws, the laying of hard-shelled eggs, a high metabolic rate, a four-chambered heart, and a strong yet lightweigh ...
, as the two groups are the only known survivors of the
Archosaur Archosauria () is a clade of diapsids, with birds and crocodilians as the only living representatives. Archosaurs are broadly classified as reptiles, in the cladistic sense of the term which includes birds. Extinct archosaurs include non-avia ...
ia. Members of the order's
total group In phylogenetics, the crown group or crown assemblage is a collection of species composed of the living representatives of the collection, the most recent common ancestor of the collection, and all descendants of the most recent common ancestor. ...
, the
clade A clade (), also known as a monophyletic group or natural group, is a group of organisms that are monophyletic – that is, composed of a common ancestor and all its lineal descendants – on a phylogenetic tree. Rather than the English ter ...
Pseudosuchia Pseudosuchia is one of two major divisions of Archosauria, including living crocodilians and all archosaurs more closely related to crocodilians than to birds. Pseudosuchians are also informally known as "crocodilian-line archosaurs". Prior to ...
, appeared about 250 million years ago in the
Early Triassic The Early Triassic is the first of three epochs of the Triassic Period of the geologic timescale. It spans the time between Ma and Ma (million years ago). Rocks from this epoch are collectively known as the Lower Triassic Series, which is ...
period, and diversified during the
Mesozoic The Mesozoic Era ( ), also called the Age of Reptiles, the Age of Conifers, and colloquially as the Age of the Dinosaurs is the second-to-last era of Earth's geological history, lasting from about , comprising the Triassic, Jurassic and Cretace ...
era. The order Crocodilia includes the true crocodiles (family
Crocodylidae Crocodiles (family Crocodylidae) or true crocodiles are large semiaquatic reptiles that live throughout the tropics in Africa, Asia, the Americas and Australia. The term crocodile is sometimes used even more loosely to include all extant me ...
), the
alligator An alligator is a large reptile in the Crocodilia order in the genus ''Alligator'' of the family Alligatoridae. The two extant species are the American alligator (''A. mississippiensis'') and the Chinese alligator (''A. sinensis''). Additional ...
s and
caiman A caiman (also cayman as a variant spelling) is an alligatorid belonging to the subfamily Caimaninae, one of two primary lineages within the Alligatoridae family, the other being alligators. Caimans inhabit Mexico, Central and South America f ...
s (family
Alligatoridae The family Alligatoridae of crocodylians includes alligators, caimans and their extinct relatives. Phylogeny The superfamily Alligatoroidea includes all crocodilians (fossil and extant) that are more closely related to the American alligator ...
), and the
gharial The gharial (''Gavialis gangeticus''), also known as gavial or fish-eating crocodile, is a crocodilian in the family Gavialidae and among the longest of all living crocodilians. Mature females are long, and males . Adult males have a distinct ...
and
false gharial The false gharial (''Tomistoma schlegelii''), also known by the names Malayan gharial, Sunda gharial and tomistoma is a freshwater crocodilian of the family Gavialidae native to Peninsular Malaysia, Borneo, Sumatra and Java. It is listed as ...
(family
Gavialidae Gavialidae is a family of large semiaquatic crocodilians with elongated, narrow snouts. Gavialidae consists of two living species, the gharial (''Gavialis gangeticus'') and the false gharial (''Tomistoma schlegelii''), both occurring in Asia. Ma ...
). 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 The tropics are the regions of Earth surrounding the Equator. They are defined in latitude by the Tropic of Cancer in the Northern Hemisphere at N and the Tropic of Capricorn in the Southern Hemisphere at S. The tropics are also refer ...
, but alligators also live in the southeastern United States and the
Yangtze River The Yangtze or Yangzi ( or ; ) is the longest river in Asia, the third-longest in the world, and the longest in the world to flow entirely within one country. It rises at Jari Hill in the Tanggula Mountains (Tibetan Plateau) and flows ...
in China. They are largely
carnivorous A carnivore , or meat-eater (Latin, ''caro'', genitive ''carnis'', meaning meat or "flesh" and ''vorare'' meaning "to devour"), is an animal or plant whose food and energy requirements derive from animal tissues (mainly muscle, fat and other ...
, the various species feeding on animals such as fish, crustaceans, molluscs, birds, and mammals; some species like the Indian
gharial The gharial (''Gavialis gangeticus''), also known as gavial or fish-eating crocodile, is a crocodilian in the family Gavialidae and among the longest of all living crocodilians. Mature females are long, and males . Adult males have a distinct ...
are specialised feeders, while others like the
saltwater crocodile The saltwater crocodile (''Crocodylus porosus'') is a crocodilian native to saltwater habitats and brackish wetlands from India's east coast across Southeast Asia and the Sundaic region to northern Australia and Micronesia. It has been l ...
have generalised diets. Crocodilians are typically solitary and
territorial A territory is an area of land, sea, or space, particularly belonging or connected to a country, person, or animal. In international politics, a territory is usually either the total area from which a state may extract power resources or an ...
, 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 The Nile crocodile (''Crocodylus niloticus'') is a large crocodilian native to freshwater habitats in Africa, where it is present in 26 countries. It is widely distributed throughout sub-Saharan Africa, occurring mostly in the central, eastern, ...
. 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 Sir John Mandeville is the supposed author of ''The Travels of Sir John Mandeville'', a travel memoir which first circulated between 1357 and 1371. The earliest-surviving text is in French. By aid of translations into many other languages, the ...
in 1400 and then by
William Shakespeare William Shakespeare ( 26 April 1564 – 23 April 1616) was an English playwright, poet and actor. He is widely regarded as the greatest writer in the English language and the world's pre-eminent dramatist. He is often called England's nation ...
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 ''Crocodylus'' is a genus of true crocodiles in the family Crocodylidae. Taxonomy The generic name, ''Crocodylus'', was proposed by Josephus Nicolaus Laurenti in 1768. ''Crocodylus'' contains 13–14 extant (living) species and 5 extinct speci ...
'' ( 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 Cladistics (; ) is an approach to biological classification in which organisms are categorized in groups (" clades") based on hypotheses of most recent common ancestry. The evidence for hypothesized relationships is typically shared derived cha ...
and
phylogenetic nomenclature Phylogenetic nomenclature is a method of nomenclature for taxa in biology that uses phylogenetic definitions for taxon names as explained below. This contrasts with the traditional approach, in which taxon names are defined by a '' type'', which ...
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 Crocodylomorpha is a group of pseudosuchian archosaurs that includes the crocodilians and their extinct relatives. They were the only members of Pseudosuchia to survive the end-Triassic extinction. During Mesozoic and early Cenozoic times, cro ...
and
Pseudosuchia Pseudosuchia is one of two major divisions of Archosauria, including living crocodilians and all archosaurs more closely related to crocodilians than to birds. Pseudosuchians are also informally known as "crocodilian-line archosaurs". Prior to ...
. Under its current definition as a
crown group In phylogenetics, the crown group or crown assemblage is a collection of species composed of the living representatives of the collection, the most recent common ancestor of the collection, and all descendants of the most recent common ancestor ...
(as opposed to a stem-based group), Crocodylia is now restricted to only the last common ancestor of today's modern-day crocodilians (
alligators An alligator is a large reptile in the Crocodilia order in the genus ''Alligator'' of the family Alligatoridae. The two extant species are the American alligator (''A. mississippiensis'') and the Chinese alligator (''A. sinensis''). Additionall ...
, crocodiles, and
gharials Gavialidae is a family of large semiaquatic crocodilians with elongated, narrow snouts. Gavialidae consists of two living species, the gharial (''Gavialis gangeticus'') and the false gharial (''Tomistoma schlegelii''), both occurring in Asia. Ma ...
) and all of its descendants (living or
extinct Extinction is the termination of a kind of organism or of a group of kinds (taxon), usually a species. The moment of extinction is generally considered to be the death of the last individual of the species, although the capacity to breed and ...
). 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
Latin Latin (, or , ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally a dialect spoken in the lower Tiber area (then known as Latium) around present-day Rome, but through ...
izing of the
Greek Greek may refer to: Greece Anything of, from, or related to Greece, a country in Southern Europe: *Greeks, an ethnic group. *Greek language, a branch of the Indo-European language family. **Proto-Greek language, the assumed last common ancestor ...
κροκόδειλος (''crocodeilos''), which means both
lizard Lizards are a widespread group of squamate reptiles, with over 7,000 species, ranging across all continents except Antarctica, as well as most oceanic island chains. The group is paraphyletic since it excludes the snakes and Amphisbaenia altho ...
and
Nile crocodile The Nile crocodile (''Crocodylus niloticus'') is a large crocodilian native to freshwater habitats in Africa, where it is present in 26 countries. It is widely distributed throughout sub-Saharan Africa, occurring mostly in the central, eastern, ...
. 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 The Nile, , Bohairic , lg, Kiira , Nobiin: Áman Dawū is a major north-flowing river in northeastern Africa. It flows into the Mediterranean Sea. The Nile is the longest river in Africa and has historically been considered the longest riv ...
.


Morphology and physiology

Crocodilians range in size from the ''
Paleosuchus ''Paleosuchus'' is a South American genus of reptiles in the subfamily Caimaninae of the family Alligatoridae. They are the smallest members of the order Crocodilia in the Americas The Americas, which are sometimes collectively called ...
'' and ''
Osteolaemus ''Osteolaemus'' is a genus of crocodiles. They are small, secretive crocodiles that occur in wetlands of West and Middle Africa. They are commonly known as the African dwarf crocodiles. Unlike other crocodiles, ''Osteolaemus'' are strictly noctur ...
'' species, which reach , to the saltwater crocodile, which reaches and weighs up to , though some prehistoric species such as the late
Cretaceous The Cretaceous ( ) is a geological period that lasted from about 145 to 66 million years ago (Mya). It is the third and final period of the Mesozoic Era, as well as the longest. At around 79 million years, it is the longest geological period of ...
''
Deinosuchus ''Deinosuchus'' () is an extinct genus of alligatoroid crocodilian, related to modern alligators and caimans, that lived 82 to 73 million years ago (Ma), during the late Cretaceous period. The name translates as "terrible crocodile" and i ...
'' were even larger at up to about and . They tend to be
sexually dimorphic Sexual dimorphism is the condition where the sexes of the same animal and/or plant species exhibit different morphological characteristics, particularly characteristics not directly involved in reproduction. The condition occurs in most ani ...
, 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 Lizards are a widespread group of squamate reptiles, with over 7,000 species, ranging across all continents except Antarctica, as well as most oceanic island chains. The group is paraphyletic since it excludes the snakes and Amphisbaenia altho ...
-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
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, and the hind feet have four webbed digits and a rudimentary fifth.Kelly, pp. 70–75. The skeleton is somewhat typical of
tetrapod Tetrapods (; ) are four-limbed vertebrate animals constituting the superclass Tetrapoda (). It includes extant and extinct amphibians, sauropsids ( reptiles, including dinosaurs and therefore birds) and synapsids ( pelycosaurs, extinct t ...
s, although the skull, pelvis and ribs are specialised; in particular, the cartilaginous processes of the ribs allow the
thorax The thorax or chest is a part of the anatomy of humans, mammals, and other tetrapod animals located between the neck and the abdomen. In insects, crustaceans, and the extinct trilobites, the thorax is one of the three main divisions of the c ...
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 In animal anatomy, a cloaca ( ), plural cloacae ( or ), is the posterior orifice that serves as the only opening for the digestive, reproductive, and urinary tracts (if present) of many vertebrate animals. All amphibians, reptiles and birds, ...
, a single chamber and outlet at the base of the tail into which the intestinal, urinary and
genital A sex organ (or reproductive organ) is any part of an animal or plant that is involved in sexual reproduction. The reproductive organs together constitute the reproductive system. In animals, the testis in the male, and the ovary in the female, a ...
tracts open. It houses the
penis A penis (plural ''penises'' or ''penes'' () is the primary sexual organ that male animals use to inseminate females (or hermaphrodites) during copulation. Such organs occur in many animals, both vertebrate and invertebrate, but males d ...
in males and the
clitoris The clitoris ( or ) is a female sex organ present in mammals, ostriches and a limited number of other animals. In humans, the visible portion – the glans – is at the front junction of the labia minora (inner lips), above the op ...
in females. The crocodilian penis is permanently erect and relies on cloacal muscles for eversion and elastic ligaments and a tendon for recoil. The
gonad A gonad, sex gland, or reproductive gland is a mixed gland that produces the gametes and sex hormones of an organism. Female reproductive cells are egg cells, and male reproductive cells are sperm. The male gonad, the testicle, produces sp ...
s are located near the
kidney The kidneys are two reddish-brown bean-shaped organs found in vertebrates. They are located on the left and right in the retroperitoneal space, and in adult humans are about in length. They receive blood from the paired renal arteries; blo ...
s. 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 The ''tapetum lucidum'' ( ; ; ) is a layer of tissue in the eye of many vertebrates and some other animals. Lying immediately behind the retina, it is a retroreflector. It reflects visible light back through the retina, increasing the light a ...
'' which enhances vision in low light. While eyesight is fairly good in air, it is significantly weakened underwater. The
fovea Fovea () (Latin for "pit"; plural foveae ) is a term in anatomy. It refers to a pit or depression in a structure. Human anatomy *Fovea centralis of the retina * Fovea buccalis or Dimple * Fovea of the femoral head * Trochlear fovea of the fr ...
in other vertebrates is usually circular, but in crocodiles it is a horizontal bar of tightly packed receptors across the middle of the
retina The retina (from la, rete "net") is the innermost, light-sensitive layer of tissue of the eye of most vertebrates and some molluscs. The optics of the eye create a focused two-dimensional image of the visual world on the retina, which the ...
. When the animal completely submerges, the
nictitating membrane The nictitating membrane (from Latin '' nictare'', to blink) is a transparent or translucent third eyelid present in some animals that can be drawn across the eye from the medial canthus to protect and moisten it while maintaining vision. All ...
s 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
eardrum In the anatomy of humans and various other tetrapods, the eardrum, also called the tympanic membrane or myringa, is a thin, cone-shaped membrane that separates the external ear from the middle ear. Its function is to transmit sound from the ...
s are protected by flaps that can be opened or closed by muscles.Grigg and Gans, p. 335. Crocodilians have a wide
hearing range Hearing range describes the range of frequencies that can be heard by humans or other animals, though it can also refer to the range of levels. The human range is commonly given as 20 to 20,000 Hz, although there is considerable variati ...
, with sensitivity comparable to most birds and many mammals. They have only one olfactory chamber and the
vomeronasal organ The vomeronasal organ (VNO), or Jacobson's organ, is the paired auxiliary olfactory (smell) sense organ located in the soft tissue of the nasal septum, in the nasal cavity just above the roof of the mouth (the hard palate) in various tetrapo ...
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 pharynx (plural: pharynges) is the part of the throat behind the mouth and nasal cavity, and above the oesophagus and trachea (the tubes going down to the stomach and the lungs). It is found in vertebrates and invertebrates, though its st ...
. The well-developed
trigeminal nerve In neuroanatomy, the trigeminal nerve ( lit. ''triplet'' nerve), also known as the fifth cranial nerve, cranial nerve V, or simply CN V, is a cranial nerve responsible for sensation in the face and motor functions such as biting and che ...
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 Melatonin is a natural product found in plants and animals. It is primarily known in animals as a hormone released by the pineal gland in the brain at night, and has long been associated with control of the sleep–wake cycle. In vertebrat ...
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 larynx (), commonly called the voice box, is an organ in the top of the neck involved in breathing, producing sound and protecting the trachea against food aspiration. The opening of larynx into pharynx known as the laryngeal inlet is about ...
. 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 Aquatic locomotion or swimming is biologically propelled motion through a liquid medium. The simplest propulsive systems are composed of cilia and flagella. Swimming has evolved a number of times in a range of organisms including arthropods, ...
, 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 ''Astragalus'' is a large genus of over 3,000 species of herbs and small shrubs, belonging to the legume family Fabaceae and the subfamily Faboideae. It is the largest genus of plants in terms of described species. The genus is native to tempe ...
, moves with the
tibia The tibia (; ), also known as the shinbone or shankbone, is the larger, stronger, and anterior (frontal) of the two bones in the leg below the knee in vertebrates (the other being the fibula, behind and to the outside of the tibia); it conn ...
and
fibula The fibula or calf bone is a leg bone on the lateral side of the tibia, to which it is connected above and below. It is the smaller of the two bones and, in proportion to its length, the most slender of all the long bones. Its upper extremity ...
. The other, the
calcaneum In humans and many other primates, the calcaneus (; from the Latin ''calcaneus'' or ''calcaneum'', meaning heel) or heel bone is a bone of the tarsus of the foot which constitutes the heel. In some other animals, it is the point of the hock. S ...
, 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
salamander Salamanders are a group of amphibians typically characterized by their lizard-like appearance, with slender bodies, blunt snouts, short limbs projecting at right angles to the body, and the presence of a tail in both larvae and adults. All t ...
s 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 Theropoda (; ), whose members are known as theropods, is a dinosaur clade that is characterized by hollow bones and three toes and claws on each limb. Theropods are generally classed as a group of saurischian dinosaurs. They were ancestrally c ...
dinosaurs like ''
Tyrannosaurus ''Tyrannosaurus'' is a genus of large theropoda, theropod dinosaur. The species ''Tyrannosaurus rex'' (''rex'' meaning "king" in Latin), often called ''T. rex'' or colloquially ''T-Rex'', is one of the best represented theropods. ''Tyrannosa ...
''. 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
homodont In anatomy, a heterodont (from Greek, meaning 'different teeth') is an animal which possesses more than a single tooth morphology. In vertebrates, heterodont pertains to animals where teeth are differentiated into different forms. For example, ...
s, meaning each of their teeth are all of the same type (they do not possess different tooth types, such as canines and molars) and
polyphyodont A polyphyodont is any animal whose teeth are continually replaced. In contrast, diphyodonts are characterized by having only two successive sets of teeth. Polyphyodonts include most toothed fishes, many reptiles such as crocodiles and geckos, ...
s 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 multicellular organisms, stem cells are undifferentiated or partially differentiated cells that can differentiate into various types of cells and proliferate indefinitely to produce more of the same stem cell. They are the earliest type of ...
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
scute A scute or scutum (Latin: ''scutum''; plural: ''scuta'' "shield") is a bony external plate or scale overlaid with horn, as on the shell of a turtle, the skin of crocodilians, and the feet of birds. The term is also used to describe the anterior po ...
s, arranged in regular rows and patterns. These scales are continually being produced by
cell division Cell division is the process by which a parent cell divides into two daughter cells. Cell division usually occurs as part of a larger cell cycle in which the cell grows and replicates its chromosome(s) before dividing. In eukaryotes, there ...
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 Beta-keratin (β-keratin), is a member of a structural protein family found in the epidermis of reptiles and birds. Beta-keratins were named so because they are components of epidermal stratum corneum rich in stacked beta sheets, in contrast to a ...
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
osteoderm Osteoderms are bony deposits forming scales, plates, or other structures based in the dermis. Osteoderms are found in many groups of extant and extinct reptiles and amphibians, including lizards, crocodilians, frogs, temnospondyls (extinct amp ...
s, 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 Thermoregulation is the ability of an organism to keep its body temperature within certain boundaries, even when the surrounding temperature is very different. A thermoconforming organism, by contrast, simply adopts the surrounding temperatur ...
. 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 Acidosis is a process causing increased acidity in the blood and other body tissues (i.e., an increase in hydrogen ion concentration). If not further qualified, it usually refers to acidity of the blood plasma. The term ''acidemia'' describe ...
. 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
pheromone A pheromone () is a secreted or excreted chemical factor that triggers a social response in members of the same species. Pheromones are chemicals capable of acting like hormones outside the body of the secreting individual, to affect the behavio ...
s 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 The blood circulatory system is a system of organs that includes the heart, blood vessels, and blood which is circulated throughout the entire body of a human or other vertebrate. It includes the cardiovascular system, or vascular system, tha ...
. It has a four-chambered
heart The heart is a muscular Organ (biology), organ in most animals. This organ pumps blood through the blood vessels of the circulatory system. The pumped blood carries oxygen and nutrients to the body, while carrying metabolic waste such as ca ...
and two ventricles, an unusual trait among extant reptiles,Grigg and Gans, pp. 331–332. and both a left and right
aorta The aorta ( ) is the main and largest artery in the human body, originating from the left ventricle of the heart and extending down to the abdomen, where it splits into two smaller arteries (the common iliac arteries). The aorta distributes o ...
which are connected by a hole called the
Foramen of Panizza The foramen of Panizza (named for anatomist Bartolomeo Panizza) is a hole that connects the left and right aorta as they leave the heart of all animals of the order Crocodilia. Crocodilians have a completely separated ventricle with deoxygenated bl ...
. Like birds and mammals, crocodilians have
heart valve A heart valve is a one-way valve that allows blood to flow in one direction through the chambers of the heart. Four valves are usually present in a mammalian heart and together they determine the pathway of blood flow through the heart. A heart ...
s 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 Pulmonary edema, also known as pulmonary congestion, is excessive liquid accumulation in the tissue and air spaces (usually alveoli) of the lungs. It leads to impaired gas exchange and may cause hypoxemia and respiratory failure. It is due to ...
, or faster recovery from
metabolic acidosis Metabolic acidosis is a serious electrolyte disorder characterized by an imbalance in the body's acid-base balance. Metabolic acidosis has three main root causes: increased acid production, loss of bicarbonate, and a reduced ability of the kidneys ...
. 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 The pancreas is an organ of the digestive system and endocrine system of vertebrates. In humans, it is located in the abdomen behind the stomach and functions as a gland. The pancreas is a mixed or heterocrine gland, i.e. it has both an ...
,
spleen The spleen is an organ found in almost all vertebrates. Similar in structure to a large lymph node, it acts primarily as a blood filter. The word spleen comes .
,
small intestine The small intestine or small bowel is an organ (anatomy), organ in the human gastrointestinal tract, gastrointestinal tract where most of the #Absorption, absorption of nutrients from food takes place. It lies between the stomach and large intes ...
, and
liver The liver is a major organ only found in vertebrates which performs many essential biological functions such as detoxification of the organism, and the synthesis of proteins and biochemicals necessary for digestion and growth. In humans, it i ...
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 mammal Marine mammals are aquatic mammals that rely on the ocean and other marine ecosystems for their existence. They include animals such as seals, whales, manatees, sea otters and polar bears. They are an informal group, unified only by their ...
s, crocodilians have little
myoglobin Myoglobin (symbol Mb or MB) is an iron- and oxygen-binding protein found in the cardiac and skeletal muscle tissue of vertebrates in general and in almost all mammals. Myoglobin is distantly related to hemoglobin. Compared to hemoglobin, myoglob ...
to store oxygen in their muscles. During diving, muscles are supplied with oxygen when an increasing concentration of
bicarbonate In inorganic chemistry, bicarbonate (IUPAC-recommended nomenclature: hydrogencarbonate) is an intermediate form in the deprotonation of carbonic acid. It is a polyatomic anion with the chemical formula . Bicarbonate serves a crucial biochemi ...
ions causes
haemoglobin Hemoglobin (haemoglobin BrE) (from the Greek word αἷμα, ''haîma'' 'blood' + Latin ''globus'' 'ball, sphere' + ''-in'') (), abbreviated Hb or Hgb, is the iron-containing oxygen-transport metalloprotein present in red blood cells (erythrocyte ...
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 Air sacs are spaces within an organism where there is the constant presence of air. Among modern animals, birds possess the most air sacs (9–11), with their extinct dinosaurian relatives showing a great increase in the pneumatization (presence o ...
. The lungs of crocodilians are attached to the liver and the pelvis by the diaphragmaticus muscle ( analogous of the
diaphragm Diaphragm may refer to: Anatomy * Thoracic diaphragm, a thin sheet of muscle between the thorax and the abdomen * Pelvic diaphragm or pelvic floor, a pelvic structure * Urogenital diaphragm or triangular ligament, a pelvic structure Other * Diap ...
in mammals). During inhalation, the
external intercostal muscles The external intercostal muscles, or external intercostals (Intercostales externi) are eleven in number on both sides. Structure The muscles extend from the tubercles of the ribs behind, to the cartilages of the ribs in front, where they en ...
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 The internal intercostal muscles (intercostales interni) are a group of skeletal muscles located between the ribs. They are eleven in number on either side. They commence anteriorly at the sternum, in the intercostal spaces between the cartila ...
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 The esophagus ( American English) or oesophagus (British English; both ), non-technically known also as the food pipe or gullet, is an organ in vertebrates through which food passes, aided by peristaltic contractions, from the pharynx to ...
, and
trachea The trachea, also known as the windpipe, is a cartilaginous tube that connects the larynx to the bronchi of the lungs, allowing the passage of air, and so is present in almost all air- breathing animals with lungs. The trachea extends from t ...
. 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 The gizzard, also referred to as the ventriculus, gastric mill, and gigerium, is an organ found in the digestive tract of some animals, including archosaurs (pterosaurs, crocodiles, alligators, dinosaurs, birds), earthworms, some gastropods, so ...
that grinds food, and a digestive chamber where
enzymes Enzymes () are proteins that act as biological catalysts by accelerating chemical reactions. The molecules upon which enzymes may act are called substrates, and the enzyme converts the substrates into different molecules known as products. ...
work on it.Mazzotti, p. 54. The stomach is more acidic than that of any other vertebrate and contains ridges for
gastrolith A gastrolith, also called a stomach stone or gizzard stone, is a rock held inside a gastrointestinal tract. Gastroliths in some species are retained in the muscular gizzard and used to grind food in animals lacking suitable grinding teeth. In oth ...
s, 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 The lion (''Panthera leo'') is a large cat of the genus '' Panthera'' native to Africa and India. It has a muscular, broad-chested body; short, rounded head; round ears; and a hairy tuft at the end of its tail. It is sexually dimorphic; adu ...
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
ectotherm An ectotherm (from the Greek () "outside" and () "heat") is an organism in which internal physiological sources of heat are of relatively small or of quite negligible importance in controlling body temperature.Davenport, John. Animal Life ...
s, 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 Evaporation is a type of vaporization that occurs on the surface of a liquid as it changes into the gas phase. High concentration of the evaporating substance in the surrounding gas significantly slows down evaporation, such as when h ...
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 American(s) may refer to: * American, something of, from, or related to the United States of America, commonly known as the "United States" or "America" ** Americans, citizens and nationals of the United States of America ** American ancestry, pe ...
and
Chinese alligator The Chinese alligator (; ), also known as the Yangtze alligator (), China alligator, or historically the muddy dragon, is a crocodilian endemic to China. It and the American alligator (''A. mississippiensis'') are the only living species in th ...
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 The saltwater crocodile (''Crocodylus porosus'') is a crocodilian native to saltwater habitats and brackish wetlands from India's east coast across Southeast Asia and the Sundaic region to northern Australia and Micronesia. It has been l ...
and the American crocodile are able to swim out to sea, their normal habitats are river mouths,
estuaries An estuary is a partially enclosed coastal body of brackish water with one or more rivers or streams flowing into it, and with a free connection to the open sea. Estuaries form a transition zone between river environments and maritime environme ...
,
mangrove swamp Mangrove forests, also called mangrove swamps, mangrove thickets or mangals, are productive wetlands that occur in coastal intertidal zones. Mangrove forests grow mainly at tropical and subtropical latitudes because mangroves cannot withstand fre ...
s, and
hypersaline lake A hypersaline lake is a landlocked body of water that contains significant concentrations of sodium chloride, brines, and other salts, with saline levels surpassing that of ocean water (3.5%, i.e. ). Specific microbial species can thrive in ...
s, 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 Solomon Islands is an island country consisting of six major islands and over 900 smaller islands in Oceania, to the east of Papua New Guinea and north-west of Vanuatu. It has a land area of , and a population of approx. 700,000. Its capit ...
coastlines. All crocodilians need to maintain the concentration of salt in body fluids at suitable levels.
Osmoregulation Osmoregulation is the active regulation of the osmotic pressure of an organism's body fluids, detected by osmoreceptors, to maintain the homeostasis of the organism's water content; that is, it maintains the fluid balance and the concentration o ...
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 In biology, homeostasis ( British also homoeostasis) (/hɒmɪə(ʊ)ˈsteɪsɪs/) is the state of steady internal, physical, and chemical conditions maintained by living systems. This is the condition of optimal functioning for the organism and ...
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 The urinary bladder, or simply bladder, is a hollow organ in humans and other vertebrates that stores urine from the kidneys before disposal by urination. In humans the bladder is a distensible organ that sits on the pelvic floor. Urine en ...
. In fresh water, the osmolality (the concentration of solutes that contribute to a solution's
osmotic pressure Osmotic pressure is the minimum pressure which needs to be applied to a solution to prevent the inward flow of its pure solvent across a semipermeable membrane. It is also defined as the measure of the tendency of a solution to take in a pure ...
) 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 Ammonium bicarbonate is an inorganic compound with formula (NH4)HCO3. The compound has many names, reflecting its long history. Chemically speaking, it is the bicarbonate salt of the ammonium ion. It is a colourless solid that degrades readily to ...
.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 Uric acid is a heterocyclic compound of carbon, nitrogen, oxygen, and hydrogen with the formula C5H4N4O3. It forms ions and salts known as urates and acid urates, such as ammonium acid urate. Uric acid is a product of the metabolic breakdown ...
.


Distribution and habitat

Crocodilians are
amphibious Amphibious means able to use either land or water. In particular it may refer to: Animals * Amphibian, a vertebrate animal of the class Amphibia (many of which live on land and breed in water) * Amphibious caterpillar * Amphibious fish, a fish ...
reptiles, spending part of their time in water and part on land. The last surviving fully terrestrial genus, ''
Mekosuchus ''Mekosuchus'' is a genus of extinct Australasian crocodiles within the subfamily Mekosuchinae. They are believed to have been made extinct by the arrival of humans on the South Pacific islands where they lived. The species of this genus were s ...
'', became extinct about 3000 years ago after humans had arrived on its Pacific islands, making the extinction possibly
anthropogenic Anthropogenic ("human" + "generating") is an adjective that may refer to: * Anthropogeny, the study of the origins of humanity Counterintuitively, anthropogenic may also refer to things that have been generated by humans, as follows: * Human i ...
. Typically they are creatures of the
tropics The tropics are the regions of Earth surrounding the Equator. They are defined in latitude by the Tropic of Cancer in the Northern Hemisphere at N and the Tropic of Capricorn in the Southern Hemisphere at S. The tropics are also refer ...
; the main exceptions are the American and Chinese alligators, whose ranges consist of the south-eastern United States and the
Yangtze River The Yangtze or Yangzi ( or ; ) is the longest river in Asia, the third-longest in the world, and the longest in the world to flow entirely within one country. It rises at Jari Hill in the Tanggula Mountains (Tibetan Plateau) and flows ...
, 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 Brackish water, sometimes termed brack water, is water occurring in a natural environment that has more salinity than freshwater, but not as much as seawater. It may result from mixing seawater (salt water) and fresh water together, as in estuari ...
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 caiman Cuvier's dwarf caiman (''Paleosuchus palpebrosus'') is a small crocodilian in the alligator family from northern and central South America. It is found in Brazil, Bolivia, Colombia, Ecuador, French Guiana, Guyana, Paraguay, Peru, Suriname, Trini ...
s 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
floodplain A floodplain or flood plain or bottomlands is an area of land adjacent to a river which stretches from the banks of its channel to the base of the enclosing valley walls, and which experiences flooding during periods of high discharge.Goudi ...
s. 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 Mauritania (; ar, موريتانيا, ', french: Mauritanie; Berber: ''Agawej'' or ''Cengit''; Pulaar: ''Moritani''; Wolof: ''Gànnaar''; Soninke:), officially the Islamic Republic of Mauritania ( ar, الجمهورية الإسلامية ...
have adapted to their arid environment by staying in caves or burrows in a state of
aestivation Aestivation ( la, aestas (summer); also spelled estivation in American English) is a state of animal dormancy, similar to hibernation, although taking place in the summer rather than the winter. Aestivation is characterized by inactivity and a ...
during the driest periods. When it rains, the reptiles gather at
guelta A guelta ( ar, قلتة, also transliterated qalta or galta; Berber: agelmam) is a pocket of water that forms in drainage canals or wadis in the Sahara. The size and duration will depend on the location and conditions. It may last year-round th ...
s. 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 A food chain is a linear network of links in a food web starting from producer organisms (such as grass or algae which produce their own food via photosynthesis) and ending at an apex predator species (like grizzly bears or killer whales), de ...
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 The Florida red-bellied cooter or Florida redbelly turtle (''Pseudemys nelsoni'') is a species of turtle in the family Emydidae. Etymology The specific name, ''nelsoni'', is in honor of American biologist George Nelson (born 1873). Geographic ...
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 Cypress is a common name for various coniferous trees or shrubs of northern temperate regions that belong to the family Cupressaceae. The word ''cypress'' is derived from Old French ''cipres'', which was imported from Latin ''cypressus'', the l ...
swamps, alligator holes tend to be large and deep. Those in
marl prairies Marl prairies are wet prairies that allow for a slow seepage of overland water and exist in the Everglades, usually bordering deeper sloughs, and contain low-growth vegetation. Description Marl is loose earthy deposits mixed with clay and calcium ...
and rocky glades are usually small and shallow, while those in
peat Peat (), also known as turf (), is an accumulation of partially Decomposition, decayed vegetation or organic matter. It is unique to natural areas called peatlands, bogs, mires, Moorland, moors, or muskegs. The peatland ecosystem covers and ...
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 In ecology, primary production is the synthesis of organic compounds from atmospheric or aqueous carbon dioxide. It principally occurs through the process of photosynthesis, which uses light as its source of energy, but it also occurs through ...
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 A territory is an area of land, sea, or space, particularly belonging or connected to a country, person, or animal. In international politics, a territory is usually either the total area from which a state may extract power resources or an ...
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 A carnivore , or meat-eater (Latin, ''caro'', genitive ''carnis'', meaning meat or "flesh" and ''vorare'' meaning "to devour"), is an animal or plant whose food and energy requirements derive from animal tissues (mainly muscle, fat and other ...
, 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 Spermatophyte plants, seed dispersal is the movement, spread or transport of seeds away from the parent plant. Plants have limited mobility and rely upon a variety of dispersal vectors to transport their seeds, including both abiotic vector ...
. 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 The smooth-fronted caiman (''Paleosuchus trigonatus''), also known as Schneider's dwarf caiman or Schneider's smooth-fronted caiman,scavenger Scavengers are animals that consume dead organisms that have died from causes other than predation or have been killed by other predators. While scavenging generally refers to carnivores feeding on carrion, it is also a herbivorous feedin ...
s. 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. To tear a chunk of tissue from a large carcass, a crocodilian spins its body continuously while holding on with its jaws, a manoeuvre known as the "death roll". 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.Pooley, pp. 88–91. 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. Nile crocodiles may store carcasses underwater for later consumption.


Reproduction and parenting

Crocodilians are generally
polygynous Polygyny (; from Neoclassical Greek πολυγυνία (); ) is the most common and accepted form of polygamy around the world, entailing the marriage of a man with several women. Incidence Polygyny is more widespread in Africa than in any ...
, and individual males try to mate with as many females as they can. Monogamous pairings have been recorded in American alligators. 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 display A courtship display is a set of display behaviors in which an animal, usually a male, attempts to attract a mate; the mate exercises choice, so sexual selection acts on the display. These behaviors often include ritualized movement ("dances"), ...
s. 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.Kelly, pp. 86–88. 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 A clutch is a mechanical device that engages and disengages power transmission, especially from a drive shaft to a driven shaft. In the simplest application, clutches connect and disconnect two rotating shafts (drive shafts or line shafts). ...
. Within a month of mating, the female crocodilian begins to make a
nest A nest is a structure built for certain animals to hold eggs or young. Although nests are most closely associated with birds, members of all classes of vertebrates and some invertebrates construct nests. They may be composed of organic materi ...
. Depending on the species, female crocodilians may construct either holes or
mound A mound is a heaped pile of earth, gravel, sand, rocks, or debris. Most commonly, mounds are earthen formations such as hills and mountains, particularly if they appear artificial. A mound may be any rounded area of topographically highe ...
s as nests, the latter made from vegetation, litter, sand, or soil. 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 Incubation period (also known as the latent period or latency period) is the time elapsed between exposure to a pathogenic organism, a chemical, or radiation, and when symptoms and signs are first apparent. In a typical infectious disease, the in ...
is two to three months. The temperature at which the eggs incubate determines the sex of the hatchlings. Constant nest temperatures above produce more males, while those below 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. The young may all hatch in a single night. Crocodilians are unusual among reptiles in the amount of
parental care Parental care is a behavioural and evolutionary strategy adopted by some animals, involving a parental investment being made to the evolutionary fitness of offspring. Patterns of parental care are widespread and highly diverse across the animal ki ...
provided after the young hatch. 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. Both male and female adult crocodilians will respond to vocalizations by hatchlings. 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. Hatchlings of many species tend to bask in a group during the day and disperse at nightfall to feed. 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.


Communication

Crocodilians can communicate with various sounds, including bellows, roars, growls, grunts, barks, coughs, hisses, toots, moos, whines, and chirps. 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.Lang, pp. 104–109. 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 New Guinea crocodiles and
Siamese crocodile The Siamese crocodile (''Crocodylus siamensis'') is a medium-sized freshwater crocodile native to Indonesia (Borneo and possibly Java), Brunei, East Malaysia, Laos, Cambodia, Myanmar, Thailand and Vietnam. The species is critically endangered a ...
s 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. Males create vibrations in the water to send out infrasonic signals that serve to attract females and intimidate rivals. The enlarged boss of the male gharial may serve as a sound resonator. 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. 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.


Growth and mortality

Mortality is high for eggs and hatchlings, and nests face threats from floods, overheating, and predators. 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. Numerous predators, both mammalian and reptilian, may raid nests and eat crocodilian eggs.Pooley and Ross, pp. 94–97. Despite the maternal care they receive, hatchlings commonly fall to predation.Kelly, p. 91. 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. 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 subadults and adults, though they are occasionally preyed on by large cats and snakes. The
jaguar The jaguar (''Panthera onca'') is a large cat species and the only living member of the genus ''Panthera'' native to the Americas. With a body length of up to and a weight of up to , it is the largest cat species in the Americas and the th ...
and the giant otter may prey on caimans in South America. In other parts of the world,
elephant Elephants are the largest existing land animals. Three living species are currently recognised: the African bush elephant, the African forest elephant, and the Asian elephant. They are the only surviving members of the family Elephantida ...
s and
hippopotamus The hippopotamus ( ; : hippopotamuses or hippopotami; ''Hippopotamus amphibius''), also called the hippo, common hippopotamus, or river hippopotamus, is a large semiaquatic mammal native to sub-Saharan Africa. It is one of only two exta ...
es may kill crocodiles defensively. Authorities differ as to whether much
cannibalism Cannibalism is the act of consuming another individual of the same species as food. Cannibalism is a common ecological interaction in the animal kingdom and has been recorded in more than 1,500 species. Human cannibalism is well documented, b ...
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. 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 , while males mature at . Australian freshwater crocodiles take ten years to reach maturity at . The spectacled caiman matures earlier, reaching its mature length of in four to seven years. 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.Huchzermeyer, p. 31. Crocodilians can live 35–75 years, and their age can be determined by growth rings in their bones.


Taxonomy and classification


Evolution

The main distinguishing characteristic of
diapsid Diapsids ("two arches") are a clade of sauropsids, distinguished from more primitive eureptiles by the presence of two holes, known as temporal fenestrae, in each side of their skulls. The group first appeared about three hundred million years a ...
tetrapods is the presence of two openings ( temporal fenestrae) on either side of the
skull The skull is a bone protective cavity for the brain. The skull is composed of four types of bone i.e., cranial bones, facial bones, ear ossicles and hyoid bone. However two parts are more prominent: the cranium and the mandible. In humans, th ...
behind the eye. Living diapsids include modern reptiles and
bird Birds are a group of warm-blooded vertebrates constituting the class Aves (), characterised by feathers, toothless beaked jaws, the laying of hard-shelled eggs, a high metabolic rate, a four-chambered heart, and a strong yet lightweig ...
s. The feature that distinguishes
archosaur Archosauria () is a clade of diapsids, with birds and crocodilians as the only living representatives. Archosaurs are broadly classified as reptiles, in the cladistic sense of the term which includes birds. Extinct archosaurs include non-avia ...
s 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 In phylogenetics, the crown group or crown assemblage is a collection of species composed of the living representatives of the collection, the most recent common ancestor of the collection, and all descendants of the most recent common ancestor ...
containing the
most recent common ancestor In biology and genetic genealogy, the most recent common ancestor (MRCA), also known as the last common ancestor (LCA) or concestor, of a set of organisms is the most recent individual from which all the organisms of the set are descended. The ...
of crocodilians and birds and all its descendants. It comprises the
Pseudosuchia Pseudosuchia is one of two major divisions of Archosauria, including living crocodilians and all archosaurs more closely related to crocodilians than to birds. Pseudosuchians are also informally known as "crocodilian-line archosaurs". Prior to ...
, the "false crocodiles", and the
Avemetatarsalia Avemetatarsalia (meaning "bird metatarsals") is a clade of diapsid reptiles containing all archosaurs more closely related to birds than to crocodilians. The two most successful groups of avemetatarsalians were the dinosaurs and pterosaurs. ...
, which in turn comprises the
dinosaur Dinosaurs are a diverse group of reptiles of the clade Dinosauria. They first appeared during the Triassic period, between 243 and 233.23 million years ago (mya), although the exact origin and timing of the evolution of dinosaurs is t ...
s (including birds) and
pterosaur Pterosaurs (; from Greek ''pteron'' and ''sauros'', meaning "wing lizard") is an extinct clade of flying reptiles in the order, Pterosauria. They existed during most of the Mesozoic: from the Late Triassic to the end of the Cretaceous (228 ...
s. 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. In modern crocodilians, the antorbital fenestrae are walled off externally and exist merely as sinuses. They were present in most of their fossil ancestors as small openings.Buffetaut, pp. 26–37. The
crocodylomorph Crocodylomorpha is a group of pseudosuchian archosaurs that includes the crocodilians and their extinct relatives. They were the only members of Pseudosuchia to survive the end-Triassic extinction. During Mesozoic and early Cenozoic times, cro ...
s are the only pseudosuchians to have survived the Triassic–Jurassic extinction event, 201.3 million years ago. During the early
Jurassic The Jurassic ( ) is a geologic period and stratigraphic system that spanned from the end of the Triassic Period million years ago (Mya) to the beginning of the Cretaceous Period, approximately Mya. The Jurassic constitutes the middle period of ...
period, the dinosaurs became dominant on land, and the crocodylomorphs underwent major adaptive diversifications to fill
ecological niche In ecology, a niche is the match of a species to a specific environmental condition. Three variants of ecological niche are described by It describes how an organism or population responds to the distribution of resources and competitors (for ...
s vacated by recently extinguished groups. Unfolding fossil evidence shows that
Mesozoic The Mesozoic Era ( ), also called the Age of Reptiles, the Age of Conifers, and colloquially as the Age of the Dinosaurs is the second-to-last era of Earth's geological history, lasting from about , comprising the Triassic, Jurassic and Cretace ...
crocodylomorphs had a much greater diversity of forms than modern crocodilians. Some became small fast-moving
insectivore A robber fly eating a hoverfly An insectivore is a carnivorous animal or plant that eats insects. An alternative term is entomophage, which can also refer to the human practice of eating insects. The first vertebrate insectivores were ...
s, others specialist fish-eaters, still others marine and terrestrial
carnivore A carnivore , or meat-eater (Latin, ''caro'', genitive ''carnis'', meaning meat or "flesh" and ''vorare'' meaning "to devour"), is an animal or plant whose food and energy requirements derive from animal tissues (mainly muscle, fat and other s ...
s, and a few became
herbivore A herbivore is an animal anatomically and physiologically adapted to eating plant material, for example foliage or marine algae, for the main component of its diet. As a result of their plant diet, herbivorous animals typically have mouthpar ...
s. The earliest stage of crocodilian evolution was the
protosuchia Protosuchia is a group of extinct Mesozoic crocodyliforms. They were small in size (~1 meter in length) and terrestrial. In phylogenetic terms, Protosuchia is considered an informal group because it is a grade of basal crocodyliforms, not a tru ...
ns, 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 The Cretaceous ( ) is a geological period that lasted from about 145 to 66 million years ago (Mya). It is the third and final period of the Mesozoic Era, as well as the longest. At around 79 million years, it is the longest geological period of ...
80 million years ago and includes all the crocodilians living today. 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 bone In anatomy, the palatine bones () are two irregular bones of the facial skeleton in many animal species, located above the uvula in the throat. Together with the maxillae, they comprise the hard palate. (''Palate'' is derived from the Latin ...
s to form a secondary bony palate and a great extension of the nasal passages to near the
pterygoid bone The pterygoid is a paired bone forming part of the palate of many vertebrates Vertebrates () comprise all animal taxa within the subphylum Vertebrata () ( chordates with backbones), including all mammals, birds, reptiles, amphibians, and ...
s. 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. The oldest known eusuchian is ''
Hylaeochampsa vectiana ''Hylaeochampsa'' is an extinct genus of eusuchian crocodylomorphs. It is known only from a partial skull recovered from Barremian-age rocks of the Lower Cretaceous Vectis Formation (Wealden Group) of the Isle of Wight. This skull, BMNH R 177, ...
'' from the lower Cretaceous of the
Isle of Wight The Isle of Wight ( ) is a county in the English Channel, off the coast of Hampshire, from which it is separated by the Solent. It is the largest and second-most populous island of England. Referred to as 'The Island' by residents, the Is ...
in the United Kingdom. It was followed by crocodilians such as the Planocraniidae, the so-called 'hoofed crocodiles', in the
Palaeogene The Paleogene ( ; also spelled Palaeogene or Palæogene; informally Lower Tertiary or Early Tertiary) is a geologic period and system that spans 43 million years from the end of the Cretaceous Period million years ago ( Mya) to the beginning ...
. Spanning the Cretaceous and Palaeogene periods is the genus '' Borealosuchus'' of North America, with six species, though its phylogenetic position is not settled. The three primary branches of Crocodilia had diverged by the end of the Mesozoic. The earliest-known members of the group is '' Portugalosuchus'' from the
Cenomanian The Cenomanian is, in the ICS' geological timescale, the oldest or earliest age of the Late Cretaceous Epoch or the lowest stage of the Upper Cretaceous Series. An age is a unit of geochronology; it is a unit of time; the stage is a unit in ...
(95 million years ago) and after are alligatoroids and gavialoids that lived in North America and Europe during the
Campanian The Campanian is the fifth of six ages of the Late Cretaceous Epoch on the geologic timescale of the International Commission on Stratigraphy (ICS). In chronostratigraphy, it is the fifth of six stages in the Upper Cretaceous Series. Campani ...
(around 83.6–72.1 million years ago). The first known crocodyloids appeared in the
Maastrichtian The Maastrichtian () is, in the ICS geologic timescale, the latest age (uppermost stage) of the Late Cretaceous Epoch or Upper Cretaceous Series, the Cretaceous Period or System, and of the Mesozoic Era or Erathem. It spanned the inte ...
(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. 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.


Relationships

Crocodylia is
cladistically Cladistics (; ) is an approach to biological classification in which organisms are categorized in groups ("clades") based on hypotheses of most recent common ancestry. The evidence for hypothesized relationships is typically shared derived char ...
defined as the last common ancestor of ''Gavialis gangeticus'' (the
gharial The gharial (''Gavialis gangeticus''), also known as gavial or fish-eating crocodile, is a crocodilian in the family Gavialidae and among the longest of all living crocodilians. Mature females are long, and males . Adult males have a distinct ...
), ''Alligator mississippiensis'' (
American alligator The American alligator (''Alligator mississippiensis''), sometimes referred to colloquially as a gator or common alligator, is a large crocodilian reptile native to the Southeastern United States. It is one of the two extant species in the gen ...
), and ''Crocodylus rhombifer'' (the Cuban crocodile) and all of its descendents. The
phylogenetic In biology, phylogenetics (; from Greek φυλή/ φῦλον [] "tribe, clan, race", and wikt:γενετικός, γενετικός [] "origin, source, birth") is the study of the evolutionary history and relationships among or within groups ...
relationships of crocodilians has been the subject of debate and conflicting results. Many studies and their resulting
cladogram A cladogram (from Greek ''clados'' "branch" and ''gramma'' "character") is a diagram used in cladistics to show relations among organisms. A cladogram is not, however, an evolutionary tree because it does not show how ancestors are related to ...
s, 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 Brevirostres is a paraphyletic group of crocodilians that included alligatoroids and crocodyloids. Brevirostres are crocodilians with small snouts, and are distinguished from the long-snouted gharials. It is defined phylogenetically as the last c ...
, was supported mainly by morphological studies which analyzed skeletal features alone. However, recent molecular studies using
DNA sequencing DNA sequencing is the process of determining the nucleic acid sequence – the order of nucleotides in DNA. It includes any method or technology that is used to determine the order of the four bases: adenine, guanine, cytosine, and thymine. T ...
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 Longirostres is a clade of crocodilians that includes the crocodiles and the gavialids, to the exclusion of the alligatoroids. Named in 2003 by Harshman ''et al.'', Longirostres is a crown group defined phylogenetically as including the last c ...
. Below is a cladogram showing the relationships of the major
extant Extant is the opposite of the word extinct. It may refer to: * Extant hereditary titles * Extant literature, surviving literature, such as ''Beowulf'', the oldest extant manuscript written in English * Extant taxon, a taxon which is not extinct, ...
crocodilian groups based on molecular studies, excluding separate extinct taxa:


Interactions with humans


Farming and ranching

Alligators and crocodiles were first farmed in the early 20th century, but the facilities involved were
zoo A zoo (short for zoological garden; also called an animal park or menagerie) is a facility in which animals are kept within enclosures for public exhibition and often bred for conservation purposes. The term ''zoological garden'' refers to z ...
-like and their main source of income was from
tourism Tourism is travel for pleasure or business; also the theory and practice of touring (disambiguation), touring, the business of attracting, accommodating, and entertaining tourists, and the business of operating tour (disambiguation), tours. Th ...
. 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 (shorter name for the Convention on International Trade in Endangered Species of Wild Fauna and Flora, also known as the Washington Convention) is a multilateral treaty to protect endangered plants and animals from the threats of intern ...
(CITES) by demonstrating that, in the area concerned, they do not adversely impact the wild population. 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. In
traditional Chinese medicine Traditional Chinese medicine (TCM) is an alternative medical practice drawn from traditional medicine in China. It has been described as "fraught with pseudoscience", with the majority of its treatments having no logical mechanism of acti ...
, alligator meat is said to cure the common cold and prevent cancer, while various internal organs are believed to have medicinal properties.


Attacks

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 saltwater crocodile (''Crocodylus porosus'') is a crocodilian native to saltwater habitats and brackish wetlands from India's east coast across Southeast Asia and the Sundaic region to northern Australia and Micronesia. It has been l ...
, the
Nile crocodile The Nile crocodile (''Crocodylus niloticus'') is a large crocodilian native to freshwater habitats in Africa, where it is present in 26 countries. It is widely distributed throughout sub-Saharan Africa, occurring mostly in the central, eastern, ...
, and the
American alligator The American alligator (''Alligator mississippiensis''), sometimes referred to colloquially as a gator or common alligator, is a large crocodilian reptile native to the Southeastern United States. It is one of the two extant species in the gen ...
. Other species which have sometimes attacked humans are the
black caiman The black caiman (''Melanosuchus niger'') is a species of large crocodilian and is the largest species of the family Alligatoridae. It is a carnivorous reptile that lives along slow-moving rivers, lakes, seasonally flooded savannas of the Amazon ...
, the Morelet's crocodile, the
mugger crocodile The mugger crocodile (''Crocodylus palustris'') is a medium-sized broad-snouted crocodile, also known as mugger and marsh crocodile. It is native to freshwater habitats from southern Iran to the Indian subcontinent, where it inhabits marshe ...
, the American crocodile, the
gharial The gharial (''Gavialis gangeticus''), also known as gavial or fish-eating crocodile, is a crocodilian in the family Gavialidae and among the longest of all living crocodilians. Mature females are long, and males . Adult males have a distinct ...
, and the freshwater crocodile. 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.Pooley, Hines and Shield, pp. 174–177. Although many attacks go unreported, there are estimated to be over 300 per year, 63% of which are fatal. 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. 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. Alligators are considered to be less aggressive than either the Nile or saltwater crocodile,Kelly, pp. 61–62. but the increase in density of the human population in the
Everglades The Everglades is a natural region of tropical wetlands in the southern portion of the U.S. state of Florida, comprising the southern half of a large drainage basin within the Neotropical realm. The system begins near Orlando with the Kissim ...
has brought people and alligators into proximity and increased the risk of alligator attacks. Conversely in Mauritania, where the crocodiles' growth is severely stunted by the arid conditions, the local people swim with them without being attacked.


As pets

Several species of crocodilian are traded as
exotic pet An exotic pet is a pet which is relatively rare or unusual to keep, or is generally thought of as a wild species rather than as a domesticated pet. The definition varies by culture, location, and over time—as animals become firmly enough est ...
s. 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.


In medicine

The blood of alligators and crocodiles contains
peptide Peptides (, ) are short chains of amino acids linked by peptide bonds. Long chains of amino acids are called proteins. Chains of fewer than twenty amino acids are called oligopeptides, and include dipeptides, tripeptides, and tetrapeptides. ...
s with
antibiotic An antibiotic is a type of antimicrobial substance active against bacteria. It is the most important type of antibacterial agent for fighting bacterial infections, and antibiotic medications are widely used in the treatment and prevention ...
properties. According to
National Geographic ''National Geographic'' (formerly the ''National Geographic Magazine'', sometimes branded as NAT GEO) is a popular American monthly magazine published by National Geographic Partners. Known for its photojournalism, it is one of the most widel ...
, these may contribute to future antibacterial drugs.


Conservation

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 CITES (shorter name for the Convention on International Trade in Endangered Species of Wild Fauna and Flora, also known as the Washington Convention) is a multilateral treaty to protect endangered plants and animals from the threats of intern ...
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. The gharial has undergone a chronic long-term decline, combined with a rapid short-term decline, leading the
IUCN The International Union for Conservation of Nature (IUCN; officially International Union for Conservation of Nature and Natural Resources) is an international organization working in the field of nature conservation and sustainable use of natu ...
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 net Gillnetting is a fishing method that uses gillnets: vertical panels of netting that hang from a line with regularly spaced floaters that hold the line on the surface of the water. The floats are sometimes called "corks" and the line with corks is ...
s and the loss of riverine habitat. The gharial population continues to be threatened by environmental hazards such as heavy metals and protozoan parasites, but as of 2013 numbers are rising, due to the protection of nests against egg predators. The Chinese alligator was historically widespread throughout the eastern Yangtze River system but is currently restricted to some areas in southeastern
Anhui Anhui , (; formerly romanized as Anhwei) is a landlocked province of the People's Republic of China, part of the East China region. Its provincial capital and largest city is Hefei. The province is located across the basins of the Yangtze Riv ...
province thanks to
habitat fragmentation Habitat fragmentation describes the emergence of discontinuities (fragmentation) in an organism's preferred environment (habitat), causing population fragmentation and ecosystem decay. Causes of habitat fragmentation include geological process ...
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. In 2008, alligators bred in the
Bronx Zoo The Bronx Zoo (also historically the Bronx Zoological Park and the Bronx Zoological Gardens) is a zoo within Bronx Park in the Bronx, New York. It is one of the largest zoos in the United States by area and is the largest metropolitan zoo in ...
were successfully reintroduced to
Chongming Island Chongming, formerly known as Chungming, is an alluvial island at the mouth of the Yangtze River in eastern China covering as of 2010. Together with the islands Changxing and Hengsha, it forms Chongming District, the northernmost area of the ...
. 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. 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 The United States Fish and Wildlife Service (USFWS or FWS) is an agency within the United States Department of the Interior dedicated to the management of fish, wildlife, and natural habitats. The mission of the agency is "working with othe ...
and state wildlife agencies in the
Southern United States The Southern United States (sometimes Dixie, also referred to as the Southern States, the American South, the Southland, or simply the South) is a geographic and cultural region of the United States of America. It is between the Atlantic Ocean ...
stepped in and worked towards its recovery. Protection allowed the species to recuperate, and in 1987 it was removed from the endangered species list. 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. 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.


Cultural depictions


In mythology, religion, and folklore

Crocodilians have had prominent roles in the myths and legends of various cultures around the world and may even have inspired stories of dragons. In Ancient Egyptian religion, Ammit, the demoniac devourer of unworthy souls, and Sobek, the god of power, protection, and fertility, are both represented as having crocodile heads. This reflects the Ancient Egypt, Egyptians' view of the crocodile both as a terrifying predator and an important part of the Nile ecosystem. The crocodile was one of several Animal mummy, animals that the Egyptians mummified. Crocodiles were also associated with various water deities by peoples of West Africa. During the Benin Empire, crocodiles were considered the "policemen of the waters" and symbolised the power of the king or ''Oba (ruler), oba'' to punish wrongdoers. The Leviathan described in the Book of Job may have been based on a crocodile. In Mesoamerica, the 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. The Maya civilization, Maya also associated crocodilians with fertility and death. 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. Similar stories exist in Native Americans in the United States, Native American legends, and in the African American folktale of an alligator and Br'er Rabbit. In a popular Malaysia, Malay folk tale, a Lesser mouse-deer, mouse deer tricks a group of crocodiles into becoming a bridge for him to cross a river without eating him. 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. 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.


In literature

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; Cleaning symbiosis#History, he claimed that it would lie with its mouth open to permit a "trochilus" bird (possibly an Egyptian plover) to enter and remove any leeches it found. The crocodile was one of the beasts described in the late-13th century Rochester Bestiary, based on classical sources, including Pliny the Elder, Pliny's Natural History (Pliny), ''Historia naturalis'' (c. 79 AD) and Isidore of Seville's Etymologiae, ''Etymologies''. Isidore asserts that the crocodile is named for its Saffron (color), saffron colour (Latin croceus, 'saffron'), and that it is often twenty cubits () 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. Crocodiles have been crocodile tears, reputed to weep for their victims since the 9th century ''Bibliotheca (Photius), Bibliotheca'' by Photios I of Constantinople. The story was repeated in later accounts such as that of Bartholomeus Anglicus in the 13th century. It became widely known in 1400 when the English traveller Sir
John Mandeville Sir John Mandeville is the supposed author of ''The Travels of Sir John Mandeville'', a travel memoir which first circulated between 1357 and 1371. The earliest-surviving text is in French. By aid of translations into many other languages, the ...
wrote his description of "cockodrills": :"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." 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'', a parody of a moralising poem by Isaac Watts, ''wikisource:Against Idleness and Mischief, Against Idleness and Mischief''. In J. M. Barrie's novel ''Peter and Wendy'' (1911), the character Captain Hook has lost his hand to the crocodile. Hook fears the crocodile, but is warned of its approach by the ticking of a clock which it has swallowed. In Rudyard Kipling's ''Just So Stories'' (1902), the Elephant's Child acquires his trunk by having his (short) 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. 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. In Andrew Fusek Peters' story book, Monkey's Clever Tale, a crocodile is tricked by a monkey. The monkey asks the crocodile to carry it across a river, promising to give its tail to eat in return, but escaped with the tail intact.


In sports and media

Crocodilians are sometimes used as mascots for sports teams. The Canton Crocodiles were a baseball team in the Frontier League, while 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. In film and television, crocodilians are represented as dangerous obstacles or as monstrous man-eaters in horror films like ''Eaten Alive'' (1977), ''Alligator (film), Alligator'' (1980), ''Lake Placid (film), Lake Placid'' (1999), ''Crocodile (2000 film), Crocodile'' (2000), ''Primeval (film), Primeval'' (2007) and ''Black Water (2007 film), Black Water'' (2007). In the film ''Crocodile Dundee'', the title character's nickname comes from the animal that bit off his leg.Kelly. pp. 234–235. Some media have attempted to portray these reptiles in more positive or educational light, such as Steve Irwin's Nature documentary, wildlife documentary series ''The Crocodile Hunter''.Kelly. p. 228.


See also

* Phytosaurs


References


Citations


Bibliography

* * * * ** Sues, Hans-Dieter. "The Place of Crocodilians in the Living World". pp. 14–25. ** Buffetaut, Eric. "Evolution". pp. 26–41. ** Mazzotti, Frank J. "Structure and Function". pp. 42–57. ** Ross, Charles A.; Magnusson, William Ernest. "Living Crocodilians". pp. 58–73. ** Pooley, A. C. "Food and Feeding Habits". pp. 76–91. ** Pooley, A. C.; Ross, Charles A. "Mortality and Predators". pp. 92–101. ** Lang, Jeffrey W. "Social Behaviour". pp. 102–117. ** Magnusson, William Ernest; Vliet, Kent A.; Pooley, A. C.; Whitaker, Romulus. "Reproduction". pp. 118–135. ** Alcala, Angel C.; Dy-Liacco, Maria Teresa S. "Habitats". pp. 136–153. ** Pooley, A. C.; Hines, Tommy C.; Shield, John. "Attacks on Humans. pp. 172–187. *


External links


Florida's Museum of Natural History: Crocodilians

Crocodylians
(photos with information), Flickr {{Taxonbar, from=Q25363 Crocodilians, Articles containing video clips Extant Campanian first appearances Taxa named by Richard Owen