Crocodilians are more closely related to birds and dinosaurs than to most animals classified as reptiles, the three families being included in the group
Archosauria ('ruling reptiles'). Despite their prehistoric look, crocodiles are among the more biologically complex reptiles. Unlike other reptiles, a crocodile has a
cerebral cortex and a four-chambered
heart. Crocodilians also have the functional equivalent of a diaphragm by incorporating muscles used for aquatic locomotion into respiration.
Salt glands are present in the tongues of crocodiles and they have a pore opening on the surface of the tongue, a trait that separates them from alligators. Salt glands are dysfunctional in Alligatoridae. Four species of freshwater crocodile climb trees to bask in areas lacking a shoreline.
Senses Crocodiles have acute senses, an evolutionary advantage that makes them successful predators. The eyes, ears and nostrils are located on top of the head, allowing the crocodile to lie low in the water, almost totally submerged and hidden from prey.
Vision Crocodiles have very good night vision, and are mostly
nocturnal hunters. They use the disadvantage of most prey animals' poor nocturnal vision to their advantage. The light receptors in crocodilians' eyes include
cones and numerous
rods, so it is assumed all crocodilians can see colours. Crocodiles have vertical-slit shaped pupils, similar to those of domestic cats. One explanation for the evolution of slit pupils is that they exclude light more effectively than a circular pupil, helping to protect the eyes during daylight. On the rear wall of the eye is a
tapetum lucidum, which reflects incoming light back onto the retina, thus utilizing the small amount of light available at night to best advantage. In addition to the protection of the upper and lower eyelids, crocodiles have a
nictitating membrane (sometimes called a "third eyelid") that can be drawn over the eye from the inner corner while the lids are open. The eyeball surface is thus protected under the water while a certain degree of vision is still possible.
Olfaction Crocodilian
sense of smell is also very well developed, aiding them to detect prey or animal carcasses that are either on land or in water, from far away. It is possible that crocodiles use olfaction in the egg prior to hatching. 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. Crocodiles close their nostrils when submerged, so olfaction underwater is unlikely. Underwater food detection is presumably gustatory and tactile.
Hearing Crocodiles can hear well; their
tympanic membranes are concealed by flat flaps that may be raised or lowered by muscles. Crocodiles can feel touch on their skin.
Cranial: The upper and lower jaws are covered with sensory pits, visible as small, black speckles on the skin, the crocodilian version of the
lateral line organs seen in fish and many amphibians, though arising from a completely different origin. These pigmented nodules encase bundles of
nerve fibers innervated beneath by branches of the trigeminal nerve. They respond to the slightest disturbance in surface water, detecting vibrations and small pressure changes as small as a single drop. This makes it possible for crocodiles to detect prey, danger and intruders, even in total darkness. These sense organs are known as domed pressure receptors (DPRs).
Post-Cranial: While alligators and caimans have DPRs only on their jaws, crocodiles have similar organs on almost every scale on their bodies. The function of the DPRs on the jaws is clear; to catch prey, but it is still not clear what the function is of the organs on the rest of the body. The receptors flatten when exposed to increased osmotic pressure, such as that experienced when swimming in sea water
hyperosmotic to the body fluids. When contact between the integument and the surrounding sea water solution is blocked, crocodiles are found to lose their ability to discriminate salinities. It has been proposed that the flattening of the sensory organ in hyperosmotic sea water is sensed by the animal as "touch", but interpreted as chemical information about its surroundings.
Hunting and diet Crocodiles are
ambush predators, waiting for fish or land animals to come close, then rushing out to attack. Crocodiles mostly eat
fish,
amphibians,
crustaceans,
molluscs,
birds,
reptiles, and
mammals, and they occasionally
cannibalize smaller crocodiles. What a crocodile eats varies greatly with species, size and age. From the mostly fish-eating species, like the
slender-snouted and
freshwater crocodiles, to the larger species like the
Nile crocodile and the
saltwater crocodile that prey on large mammals, such as
buffalo,
deer and
wild boar, diet shows great diversity. Diet is also greatly affected by the size and age of the individual within the same species. All young crocodiles hunt mostly
invertebrates and small
fish, gradually moving on to larger prey. Being
ectothermic (cold-blooded) predators, they have a very slow
metabolism, so they can survive long periods without food. Despite their appearance of being slow, crocodiles have a very fast strike and are top
predators in their environment, and various species have been observed attacking and killing other
predators such as
sharks and
big cats. Crocodiles are also known to be aggressive
scavengers who feed upon
carrion and steal from other predators. Evidence suggests that crocodiles also feed upon fruits, based on the discovery of seeds in stools and stomachs from many subjects as well as accounts of them feeding. Crocodiles have the most acidic stomach of any vertebrate. They can easily digest bones, hooves and horns. The
BBC TV reported that a
Nile crocodile that has lurked a long time underwater to catch prey builds up a large
oxygen debt. When it has caught and eaten that prey, it closes its right
aortic arch and uses its left aortic arch to flush blood loaded with
carbon dioxide from its muscles directly to its stomach; the resulting excess acidity in its blood supply makes it much easier for the stomach lining to secrete more
stomach acid to quickly dissolve bulks of swallowed prey flesh and bone. Many large crocodilians swallow stones (called gastroliths or stomach stones), which may act as ballast to balance their bodies or assist in crushing food,
Bite in
Kruger National Park, South Africa Since they feed by grabbing and holding onto their prey, they have
evolved sharp teeth for piercing and holding onto flesh, and powerful muscles to close the jaws and hold them shut. The teeth are not well-suited to tearing flesh off of large prey items as are the dentition and claws of many mammalian carnivores, the hooked bills and talons of
raptorial birds, or the serrated teeth of sharks. However, this is an advantage rather than a disadvantage to the crocodile since the properties of the teeth allow it to hold onto prey with the least possibility of the prey animal escaping. Cutting teeth, combined with the exceptionally high
bite force, would pass through flesh easily enough to leave an escape opportunity for prey. The jaws can bite down with immense force, by far the strongest bite of any animal. The force of a large crocodile's bite is more than , which was measured in a
Nile crocodile, in the field; comparing to for a
Rottweiler, for a
hyena, for an
American alligator, and for the largest confirmed
great white shark. A long saltwater crocodile has been confirmed as having the strongest
bite force ever recorded for an animal in a laboratory setting. It was able to apply a bite force value of , and thus surpassed the previous record of made by a long
American alligator. Based on regression of mean body mass versus mean bite force, the bite force of a saltwater crocodile with an estimated mass of was estimated at between and . current data on modern crocodilians can be used to estimate the bite force of extinct species. An
Deinosuchus would apply a force of , nearly twice that of the latest, higher bite force estimations of
Tyrannosaurus (). The extraordinary bite of crocodilians is a result of their
anatomy. The space for the jaw muscle in the
skull is very large, which is easily visible from the outside as a bulge at each side. The
muscle is so stiff, it is almost as hard as bone to touch, as if it were the continuum of the skull. Another trait is that most of the muscle in a crocodile's jaw is arranged for clamping down. Despite the strong muscles to close the jaw, crocodiles have extremely small and weak muscles to open the jaw. Crocodiles can thus be subdued for study or transport by
taping their jaws or holding their jaws shut with large
rubber bands cut from automobile
inner tubes.
Locomotion Crocodiles can move quickly over short distances, even out of water. The
land speed record for a crocodile is measured in a galloping
Australian freshwater crocodile. Maximum speed varies between species. Some species can gallop, including Cuban crocodiles, Johnston's crocodiles, New Guinea crocodiles,
African dwarf crocodiles, and even small
Nile crocodiles. The fastest means by which most species can move is a "belly run", in which the body moves in a snake-like (sinusoidal) fashion, limbs splayed out to either side paddling away frantically while the tail whips to and fro. Crocodiles can reach speeds of when they "belly run", and often faster if slipping down muddy riverbanks. When a crocodile walks quickly, it holds its legs in a straighter and more upright position under its body, which is called the "high walk". This walk allows a speed of up to 5 km/h. Crocodiles may possess a
homing instinct. In northern Australia, three rogue saltwater crocodiles were relocated by
helicopter, but returned to their original locations within three weeks, based on data obtained from tracking devices attached to them.
Longevity Measuring crocodile age is unreliable, although several techniques are used to derive a reasonable guess. The most common method is to measure lamellar growth rings in bones and teeth—each ring corresponds to a change in growth rate which typically occurs once a year between dry and wet seasons. Bearing these inaccuracies in mind, it can be safely said that all crocodile species have an average lifespan of at least 30–40 years, and in the case of larger species an average of 60–70 years. The oldest crocodiles appear to be the largest species.
C. porosus is estimated to live around 70 years on average, with limited evidence of some individuals exceeding 100 years. In captivity, some individuals are claimed to have lived for over a century. A male crocodile lived to an estimated age of 110–115 years in a Russian zoo in
Yekaterinburg. A male freshwater crocodile lived to an estimated age of 120–140 years at the
Australia Zoo. Known affectionately as "Mr. Freshie", he was rescued around 1970 by
Bob Irwin and
Steve Irwin, after being shot twice by hunters and losing an eye as a result, and lived until 2010.
Social behaviour and vocalisation Crocodiles are the most social of reptiles. Even though they do not form social groups, many species congregate in certain sections of
rivers, tolerating each other at times of
feeding and
basking. Most species are not highly territorial, with the exception of the saltwater crocodile, which is a highly
territorial and aggressive species: a mature, male saltwater crocodile will not tolerate any other males at any time of the year, but most other species are more flexible. There is a certain form of
hierarchy in crocodiles: the largest and heaviest males are at the top, having access to the best basking site, while females are priority during a group feeding of a big kill or carcass. A good example of the hierarchy in crocodiles would be the case of the
Nile crocodile. This species clearly displays all of these behaviours. Studies in this area are not thorough, however, and many species are yet to be studied in greater detail.
Mugger crocodiles are also known to show toleration in group feedings and tend to congregate in certain areas. However, males of all species are aggressive towards each other during mating season, to gain access to females. Crocodilians are the most vocal of all reptiles, producing a wide variety of sounds depending on species, age, size and sex. Depending on the context, some species can communicate over 20 different messages through
vocalisations alone. Some of these vocalisations are made during social communication, especially during
territorial displays towards the same sex and
courtship with the opposite sex; the common concern being
reproduction. Therefore most
conspecific vocalisation is made during the
breeding season, with the exception being year-round
territorial behaviour in some species and quarrels during feeding. Crocodiles also produce different distress calls and in aggressive displays to their own kind and other animals; notably other predators during
interspecific predatory confrontations over carcasses and terrestrial kills. Specific vocalisations include — •
Chirp: When about to hatch, the young make a "peeping" noise, which encourages the female to excavate the nest. The female then gathers the hatchlings in her mouth and transports them to the water, where they remain in a group for several months, protected by the female •
Distress call: A high-pitched call used mostly by younger animals to alert other crocodiles to imminent danger or an animal being attacked. •
Threat call: A hissing sound that has also been described as a coughing noise. •
Hatching call: Emitted by a female when breeding to alert other crocodiles that she has laid eggs in her nest. •
Bellowing: Male crocodiles are especially vociferous. Bellowing choruses occur most often in the spring when breeding groups congregate, but can occur at any time of year. To bellow, males noticeably inflate as they raise the tail and head out of water, slowly waving the tail back and forth. They then puff out the throat and with a closed mouth, begin to vibrate air. Just before bellowing, males project an
infrasonic signal at about 10 Hz through the water, which vibrates the ground and nearby objects. These low-frequency vibrations travel great distances through both air and water to advertise the male's presence and are so powerful they result in the water's appearing to "dance".
Reproduction Mating Courtship takes place in a series of behavioural interactions that include a variety of snout rubbing and submissive display that can take a long time. Mating always takes place in water, where the pair can be observed mating several times. Females are highly protective of their nests and young. Crocodile
embryos do not have sex chromosomes, and unlike humans, sex is not determined genetically.
Sex is determined by temperature, where at or less most hatchlings are females and at , offspring are of both sexes. A temperature of gives mostly males whereas above in some species continues to give males, but in other species resulting in females, which are sometimes called high-temperature females. Temperature also affects growth and survival rate of the young, which may explain the
sexual dimorphism in crocodiles. The average
incubation period is around 80 days, and also is dependent on temperature and species that usually ranges from 65 to 95 days. The eggshell structure is very conservative through evolution but there are enough changes to tell different species apart by their eggshell microstructure. At the time of hatching, the young start calling within the eggs. Hearing the calls, the female usually excavates the nest and sometimes takes the unhatched eggs in her mouth, slowly rolling the eggs to help the process. The young have an egg-tooth at the tip of their snouts, which is developed from the skin, and that helps them pierce out of the shell. Once hatched, the young are usually carried to the water in the female's mouth. She will then introduce her hatchlings to the water and even feed them. The mother takes care of her young for over a year before the next mating season. In the absence of the mother crocodile, the father will act in her place to take care of the young. However, even with sophisticated
parental nurturing, young crocodiles have a very high mortality rate due to their vulnerability to predation. A group of
hatchlings is called a pod or
crèche. Crocodiles
cooperatively hunt. Large numbers of crocodiles swim in circles to trap fish and take turns snatching them. In hunting larger prey, crocodiles swarm in, with one holding the prey down as the others rip it apart.
Vladimir Dinets of the
University of Tennessee, observing crocodile's use of
twigs as bait was inconclusive. ==Relationship with humans==