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Don’t just splash into the river after a long hot day on your African safari. Our can rivers are flush with man-eating crocodiles just waiting for some imported meat.
A crocodile’s construction allows it to be a successful predator. Its streamlined body enables it to swim really fast – up to 30km per hour: over 3 times as fast as a human. Crocodiles tuck their feet to the side while swimming, which decreases water resistance. They have webbed feet which, though not used to propel the animal through the water, allow them 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.
The tongues of crocodiles are not free, but held in place by a membrane that limits movement; as a result, crocodiles are unable to stick out their tongues. Crocodiles have smooth skin on their bellies and sides, while their dorsal surfaces are armored with large bony scales. 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 allows blood through the scales to absorb heat. 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.
Crocodile skin has been used for making armour.
Size greatly varies between species, from the dwarf crocodile to the saltwater crocodile. Species of Osteolaemus grow to an adult size of just 1.5 to 1.9 m (4.9 to 6.2 ft), whereas the saltwater crocodile can grow to sizes over 7 m (23 ft) and weigh 1,000 kg (2,200 lb). Several other large species can reach over 5.2 m (17 ft) long and weigh over 900 kg (2,000 lb). Crocodilians show pronounced sexual dimorphism, with males growing much larger and more rapidly than females. Despite their large adult sizes, crocodiles start their lives at around 20 cm (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.
The largest crocodile ever held in captivity is an estuarine–Siamese hybrid named Yai (born 10 June 1972) at the Samutprakarn Crocodile Farm and Zoo, Thailand. This animal measures 6 m (20 ft) in length and weighs 1,114.27 kg (2,456.5 lb).
The longest crocodile captured alive is Lolong, which was measured at 6.17 m (20.2 ft) and weighed at 1,075 kg (2,370 lb) by a National Geographic team in Agusan del Sur Province, Philippines.
Crocodiles are able to replace each of their 80 teeth up to 50 times in their 35 to 75-year lifespan. Next to the full grown tooth there is a small replacement tooth
Biology and behavior
Despite their prehistoric look, crocodiles are among the more biologically complex reptiles. 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. Salt glands are present in the tongues of crocodiles and they have a pore opening on the surface of the tongue, which is a trait that separates them from alligators. Salt glands are dysfunctional in Alligators. Their function appears to be similar to that of salt glands in marine turtles. Crocodiles do not have sweat glands and release heat through their mouths. They often sleep with their mouths open and may pant like a dog.
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.
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 both cones and numerous rods, so it is assumed all crocodilians can see colors. Crocodiles have vertical-slit shaped pupils, similar to domestic cats. In addition to the protection of the upper and lower eyelids, crocodiles have a nictitating membrane which 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.
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.
Chemoreception in crocodiles is especially interesting because they hunt both in terrestrial and in aquatic surroundings. Crocodiles detect both air-borne and water-soluble chemicals and use their sense of smell for hunting. When above water, crocodiles enhance their ability to detect volatile odorants through a rhythmic movement of the floor of the throat.
Unlike turtles, crocodiles close their nostrils when submerged, so the use of sense of smell underwater is unlikely.
Old boer advice is to cut the valves which close the nostrils on the tip of a crocodiles nose when caught, resulting in the inability to stop water from flooding the lungs.
Crocodiles can hear well; their eardrums concealed by flat flaps that may be raised or lowered by muscles.
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).
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 is the function of the organs on the rest of the body.
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, mammals and occasionally cannibalize on 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, buck and warthogs. Diet is also greatly affected by size and age of the individual within the same species. All young crocodiles hunt mostly invertebrates and small fish, gradually moving onto larger prey.
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 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 have the most acidic stomach of any vertebrate. They can easily digest bones, hooves and horns. 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, similar to grit ingested by birds.
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. 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 to escape. Otherwise combined with the exceptionally high bite force, the flesh would easily cut through; thus creating an escape opportunity for the prey item.
The jaws can bite down with immense force, by far the strongest bite of any animal.
The bite of a large crocodile’s bite is more than 5,000 lbf (22,000 N), which was measured in a 5.5 m (18 ft) Nile crocodile, on the field, compared to just 335 lbf (1,490 N) for a Rottweiler, 670 lbf (3,000 N) for a great white shark, 800 lbf (3,600 N) for a hyena, or 2,200 lbf (9,800 N) for an American alligator. A 5.2 m (17 ft) 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 3,700 lbf (16,000 N), and thus surpassed the previous record of 2,125 lbf (9,450 N) made by a 3.9 m (13 ft) long American alligator.
Taking the measurements of several 5.2 m (17 ft) crocodiles as reference, the bite forces of 6-m individuals were estimated at 7,700 lbf (34,000 N). The study, lead by Dr. Gregory M. Erickson, also shed light to the larger, extinct species of crocodilians. Since crocodile anatomy has changed only slightly for the last 80 million years, current data on modern crocodilians can be used to estimate the bite force of extinct species. An 11 to 12 metres (36–39 ft) long Deinosuchus would apply a force of 23,100 lbf (103,000 N), 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 nature of 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.
Crocodiles can gallop with their front and back legs working together and they are very fast over short distances.
The land speed record for a crocodile is 17 km/h (11 mph) measured in a galloping Australian freshwater crocodile. 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 10–11 km/h (6–7 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. Crocodiles may possess a form of homing instinct. In northern Australia, three rogue saltwater crocodiles were relocated 400 km (249 mi) by helicopter, but had returned to their original locations within three weeks, based on data obtained from tracking devices attached to the reptiles.
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.
A male crocodile lived to an estimated age of 110–115 years in a Russian zoo in Yekaterinburg. Named Kolya, he joined the zoo around 1913 to 1915, fully grown, after touring in an animal show, and lived until 1995.
Crocworld in Kwazulu-Natal is the home of Henry, the oldest known crocodile in captivity. He was captured by an elephant hunter known as sir Henry in the Okavango Delta in Botswana in 1903. The crocodile is estimated to have been born around 1900. The croc was a man-eater and ate several children. Sir Henry was asked by the tribe to kill the crocodile, but after consultation they decided to let the crocodile live as punishment.
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 behavior and vocalization
Crocodiles are the most social of reptiles. Even though they do not form social groups, many species congregate in certain section of a 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 will not tolerate any other males at any time of the year. Most of the species however, are more flexible. There is a certain form of hierarchy in crocodiles, where the largest and heaviest males are at the top; having access to the best basking site, females and priority during a group feeding of a big kill or carcass. A good example to the hierarchy in crocodiles would be the case of the Nile crocodile. This species clearly displays all of these behaviors. Studies in this area are not thorough, 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 to certain areas. However, males of all species are aggressive towards each other during mating season, to gain access to females.
Crocodiles are also the most vocal of all reptiles, producing a wide variety of sounds during various situations and conditions, depending on species, age, size and sex. Depending on the context, some species can communicate over 20 different messages through vocalizations alone. Some of these vocalizations 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 vocalization is made during the breeding season, with the exception being year-round territorial behavior 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 mostly used by younger animals that alerts 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 females 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 appearing to ‘dance’.
Crocodiles reproduce by laying eggs, which are either laid in hole or mound nests, depending on species. A hole nest is usually excavated in sand and a mound nest is usually constructed out of vegetation. Nesting period ranges from a few weeks up to six months. Courtship takes place in a series of behavioral 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 can build or dig several trial nests which appear incomplete and abandoned later.
Egg laying usually takes place at night and about 30–40 minutes. Females are highly protective of their nests and young. The eggs are hard shelled but translucent at the time of egg-laying. Depending on the species crocodile, a number of 7-95 eggs are laid. Crocodile embryos do not have sex chromosomes, and unlike humans, sex is not determined genetically. Sex is determined by temperature, where at 30 °C (86 °F) or less most hatchlings are females and at 31 °C (88 °F), offspring are of both sexes. A temperature of 32 to 33 °C (90 to 91 °F) gives mostly males whereas above 33 °C (91 °F) in some species continues to give males but in other species resulting in females, which are sometimes called as 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. At the time of hatching, the young start calling within the eggs. They have an egg-tooth at the tip of their snouts, which is developed from the skin, helps them pierce out of the shell. 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 is usually carried to the water in the mouth. A group of hatchlings is called a pod or crèche and may be protected for months.
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