A crocodile is any species belonging to the family
Crocodylidae (sometimes classified instead as the subfamily Crocodylinae). The
term can also be used more loosely to include all extant members of the order
Crocodilia: i.e. the true crocodiles, the alligators and caimans (family
Alligatoridae) and the gharials (family Gavialidae), as well as the
Crocodylomorpha, which include prehistoric crocodile relatives and ancestors.
Member species of the family Crocodylidae are large aquatic
reptiles that live throughout the tropics in Africa, Asia, the Americas and
Australia. Crocodiles tend to congregate in freshwater habitats such as rivers,
lakes, wetlands and sometimes in brackish water. They feed mostly on
vertebrates - fish, reptiles, and mammals, and sometimes on invertebrates -
molluscs and crustaceans, depending on species. They first appeared during the
Eocene epoch, about 55 million years ago.
Description
Crocodiles are among the more biologically complex reptiles
despite their prehistoric look. Unlike other reptiles, a crocodile has a
cerebral cortex, a four-chambered heart,and the functional equivalent of a
diaphragm, by incorporating muscles used for aquatic locomotion into
respiration (e.g. m. diaphragmaticus);[6] Its external morphology, on the other
hand, is a sign of its aquatic and predatory lifestyle.
A crocodile’s physical traits allow it to be a successful
predator. Its streamlined body enables it to swim swiftly, it also tucks its
feet to the side while swimming, which makes it faster by decreasing water
resistance. Its webbed feet, though not used to propel the animal through the
water, allow it to make fast turns and sudden moves in the water or initiate
swimming. Webbed feet are an advantage in shallower water, where the animal
sometimes moves around by walking.
Crocodiles have a palatal flap, a rigid tissue at the back
of the mouth that blocks the entry of water. The palate has a special path from
the nostril to the glottis that bypasses the mouth. The nostrils are closed
during submergence. Like other archosaurs, crocodilians are diapsid, although
their post-temporal fenestrae are reduced. The walls of the braincase are bony,
but lack supratemporal and postfrontal bones.[7] Their tongues are not free,
but held in place by a membrane that limits movement; as a result, crocodiles
are unable to stick out their tongues.[8]
Crocodilian scales have pores believed to be sensory in
function, analogous to the lateral line in fishes. They are particularly seen
on their upper and lower jaws. Another possibility is that they are secretory,
as they produce an oily substance, which appears to flush mud off.[7]
Crocodiles are very fast over short distances, even out of
water. Since they feed by grabbing and holding onto their prey, they have
evolved sharp teeth for tearing and holding onto flesh, and powerful muscles to
close the jaws and hold them shut. These jaws can bite down with immense force,
by far the strongest bite of any animal. The pressure of the crocodile's bite
is more than 5,000 pounds per square inch (30,000 kPa),[9] compared to just 335
pounds per square inch (2,300 kPa) for a Rottweiler, 400 pounds per square inch
(2,800 kPa) for a large great white shark, 800 pounds per square inch (6,000
kPa) to 1,000 pounds per square inch (7,000 kPa) for a hyena, or 2,000 pounds
per square inch (10,000 kPa) for a large alligator[citation needed]. The jaws
are opened, however, by a very weak set of muscles. 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. They have limited
lateral (side-to-side) neck movement.
Behaviour
Crocodiles are ambush hunters, waiting for fish or land
animals to come close, then rushing out to attack. As cold-blooded predators,
they have a very slow metabolism, so they can survive long periods without
food. Despite their appearance of being slow, crocodiles are top predators in
their environment, and various species have been observed attacking and killing
sharks.[10]
Herodotus claimed that Nile crocodiles had a symbiotic
relationship with certain birds, such as the Egyptian plover, which enter the crocodile's
mouth and pick leeches feeding on the crocodile's blood; with no evidence of
this interaction actually occurring in any crocodile species, it is most likely
mythical or allegorical fiction.[11]
Many large crocodilians swallow stones (called gastroliths
or stomach stones), which may act as ballast to balance their bodies or assist
in crushing food,[7] similar to grit in birds.
Salt glands are present in the tongues of most crocodylids
and they have a pore opening on the surface of the tongue. They appear to be
similar to those in marine turtles; they seem to be absent in Alligatoridae.[7]
Crocodilians can produce sounds during distress and in
aggressive displays. They can also hear well, but their tympanic membranes are
concealed by flat flaps that may be raised or lowered by muscles.[7]
A crocodile farm in Mexico
Crocodiles eat fish, birds, mammals and occasionally smaller
crocodiles.
Crocodiles are protected in many parts of the world, but
they also are farmed commercially. Their hides are tanned and used to make
leather goods such as shoes and handbags; crocodile meat is also considered a
delicacy. The most commonly farmed species are the saltwater and Nile
crocodiles, while a hybrid of the saltwater and the rare Siamese crocodile is
also bred in Asian farms. Farming has resulted in an increase in the saltwater
crocodile population in Australia, as eggs are usually harvested from the wild,
so landowners have an incentive to conserve their habitat.
Distribution of crocodiles
Crocodiles are more closely related to birds and dinosaurs
than to most animals classified as reptiles, the three being included in the
group Archosauria ('ruling reptiles'). See Crocodilia for more information.
Crocodile embryos do not have sex chromosomes, and unlike
humans, sex is not determined genetically. Sex is determined by temperature,
with males produced at around 31.6 °C (89 °F), and females produced at slightly
lower and higher temperatures. The average incubation period is around 80 days,
and also is dependent on temperature.[12]
Crocodiles may possess a form of homing instinct. In
northern Australia, three rogue saltwater crocodiles were relocated 400
kilometres by helicopter, but had returned to their original locations within
three weeks, based on data obtained from tracking devices attached to the
reptiles.[13]
The land speed record for a crocodile is 17 km/h (11 mph)
measured in a galloping Australian freshwater crocodile.[14] Maximum speed
varies from species to species. Certain species can indeed gallop, including
Cuban crocodiles, New Guinea crocodiles, African dwarf crocodiles, and even
small Nile crocodiles. The fastest means by which most species can move is a
kind of "belly run", where the body moves in a snake-like fashion,
limbs splayed out to either side paddling away frantically while the tail whips
to and fro. Crocodiles can reach speeds of 10 or 11 km/h (around 7 mph) when
they "belly run", and often faster if slipping down muddy riverbanks.
Another form of locomotion is the "high walk", where the body is
raised clear of the ground.
Siamese crocodile sleeping with its mouth open to pant
Crocodiles do not have sweat glands and release heat through
their mouths. They often sleep with their mouths open and may even pant like a
dog.[15]
The BBC TV[16] 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.
Size
A large saltwater crocodile in captivity in Australia
Size greatly varies between species, from the dwarf
crocodile to the saltwater crocodile. Species of Palaeosuchus and Osteolaemus
grew to an adult size of just 1 metre (3.3 ft) to 1.5 metres (4.9 ft). Larger
species can reach over 4.85 metres (15.9 ft) long and weigh well over 1,200
kilograms (2,600 lb). Crocodilians show pronounced sexual dimorphism, with
males growing much larger and more rapidly than females.[7] Despite their large
adult sizes, crocodiles start their lives at around 20 centimetres (7.9 in)
long. The largest species of crocodile is the saltwater crocodile, found in
eastern India, northern Australia, throughout South-east Asia, and in the
surrounding waters.
Two larger certifiable records are both of 6.2 metres (20
ft) crocodiles. The first was shot in the Mary River in the Northern Territory
of Australia in 1974 by poachers, and measured by wildlife rangers.[citation
needed] The second crocodile was killed in 1983 in the Fly River, Papua New
Guinea. In the case of the second crocodile, it was actually the skin that was
measured by zoologist Jerome Montague, and as skins are known to underestimate
the size of the actual animal, it is possible this crocodile was at least
another 10 cm longer.[citation needed]
The largest crocodile ever held in captivity is an
estuarine–Siamese hybrid named Yai (Thai: ใหญ่,
meaning big) (born 10 June 1972) at the famous Samutprakarn Crocodile Farm and
Zoo, Thailand. This animal measures 6 m (19.69 ft) (19 ft 8 in) in length and weighs
1114.27 kg.[17]
Meanwhile, the longest crocodile captured alive is Lolong,
which was measured at 6.17 metres (20.24 ft) and weighed at 1,075 kilos by a
National Geographic team in Agusan del Sur Province, Philippines.[18] [19] [20]
Wildlife experts, however, argue the largest crocodile so
far found in the Bhitarkanika was almost 23 feet (7.0 m) long, which could be
traced from the skull preserved by the Kanika royal family. The crocodile was
shot near Dhamara in 1926 and later its skull was preserved by the then Kanika
king. Crocodile experts estimated the animal was between 20 feet (6.1 m) and 23
feet (7.0 m) long, as the size of the skull was measured one-ninth of the total
length of the body.[citation needed]
Age
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.[21]
Bearing these inaccuracies in mind, the oldest crocodilians 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. One of the
oldest crocodiles recorded died in a zoo in Russia. A male freshwater crocodile
at the Australia Zoo is estimated to be 130 years old. He was rescued from the
wild by Bob Irwin and Steve Irwin after being shot twice by hunters. As a
result of the shootings, this crocodile (known affectionately as "Mr.
Freshy") has lost his right eye.[22]
Skin
Crocodiles have smooth skin on their bellies and sides,
while their dorsal surfaces are armoured with large osteoderms. The armoured
skin has scales and is thick and rugged, providing some protection. They are
still able to absorb heat through this armour, as a network of small
capillaries allow blood through the scales to absorb heat.
