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Tiger Anatomy

Anatomy of a Supreme Hunter

Extracts from the outstanding book:
Tiger - The Ultimate Guide by Valmik Thapar
CDS Books, 2004, p 23 - 33

"The tiger, whose proper nutrition depends upon consuming one deer-size animal a week, evolved into a supreme hunter. Its sensory organs help it to locate elusive animals in the densest of undergrowth or on the blackest of nights, and its striped fur allows it to blend in with the surroundings. The tiger's padded paws permit a stealthy and silent approach when stalking a potential kill, and its skeleton and muscles provide the strength and speed necessary to catch prey as big as an elephant or as small as a grasshopper. A strong jaw and razor-sharp teeth can deliver a quick death, tearing tough hide and slicing through raw meat.

The Skeleton

Even experts have difficulty distinguishing between the skeleton of a tiger and that of a lion; the two cats are so similar in size and shape that without the telltale fur they are hard to tell apart.

An understanding of the tiger's hunting habits and daily behaviors illuminates the particularly striking features of its skeleton - one that enables it to endure the vastly different rigors of both speed and strength.

Tigers have longer hind legs than forelegs and can therefore spring forward 10 meters (32.5 feet), while the reduced size of their clavicle (collarbone) allows for greater stride length. Their solid forelimb bones, which can support large muscles, give their forelegs incredible power, enabling them to bring down large prey. In addition the bones of a tiger's feet are closely bound by ligaments to make them strong enough to survive the impact of landing, an important factor in the tiger's ability to sprint at high speeds.

The skull makes the hard-core engineering behind the business end of the tiger possible. By looking more closely at the shapes of the various skull pieces we can see how evolution has created a structure that is perfectly shaped for the job at hand. A short, rounded skull provides more power behind the jaws than can the kind of long skull found in herbivores such as sheep. The perfectly adapted teeth are thus strongly supported in the final act of dispatching prey. Because smell is less important than sight, the nose need not be long, and the brain-cavity space for the olfactory area can be compact.

Several other features of the skull contribute to its strength. The tiger's bony septum, which separates the cerebrum from the cerebellum, and which in humans and many other creatures consists of only a membrane, increase skull strength. The jaw joints do not allow side-to-side movement, making the bite stronger and firmer. The jaw muscles, which are attached to a special extra bony ridge on the top of the skull called the sagittal crest, react quickly and clamp down with crushing force.

Tigers have thirty teeth, fewer than other carnivores (dogs and bears have forty two) but no less dangerous, because of their specialization.

The tiger has the largest canines of all the big cats, at 6.4 to 7.6 centimeters (2.5 to 3 inches) long. Moreover, these canines are rich in pressure-sensitive nerves, enabling the tiger to make an accurate and deadly stab between a victim's neck bones to severe the spinal cord. The back teeth, called carnassials, act as shearing blades, with which tigers are able to slice the meat off their prey. The incisors (small front teeth) are positioned in a straight line, enabling them to efficiently pluck feathers and clean meat off the bone. Because of a huge gap between the canines and carnassials, the tiger can dig its teeth deep into its prey for the kill. One tiger in six has a broken canine as the result of an attack. Some old tigers, having lost as many as three canines, have become man-eaters because they are unable to kill their traditional prey.

Pads, Paws and Claws

Tigers walk on their toes, which have big soft pads to help them tread almost silently through the undergrowth. These sensitive pads can be a disadvantage: if a tiger treads on something sharp, the pads will bleed profusely; if it tries to cross hot ground its feet will burn and blister easily. These common injuries do heal quickly, with lost of licking and care.

A tiger's claws can take the face off a human in one swipe, and deer lucky enough to escape a confrontation with a tiger often bear the scars of the encounter on their hides. Claws play a critical role in the tiger's hunting abilities, helping it to grab and hold prey until its teeth can inflict the final blow. The claws can be a fearsome ten centimeters (four inches) long. A tiger has four of these deadly weapons on each paw, with an extra dewclaw on the front ones. Dewclaws are set back a little and do not touch the ground when the tiger is walking; they are used like thumbs in gripping prey and in climbing.

The tiger keeps its claws retracted when not in use, in order to stalk silently and ensure that they remain sharp and ready for action. This is in fact the relaxed position, as ligaments hold them in their protective skin sheath and no muscular effort is needed. When necessary, other ligament pull the claws and straighten the toes. Curved claws allow tigers to climb trees head first, but to come down they must crawl backward or jump. This disadvantage, coupled with their size and weight, makes tigers inferior climbers compared with other big cats.

Stomach and Digestion

Meat can be digested far more efficiently than grass. It requires a far less complicated gut to convert meat to protein than it does to convert grass to protein. Thus tigers have shortened guts (four times their body length, as compared with five times in an omnivorous fox) and comparatively small and light abdomens, a factor that contributes to the tiger's ability to accelerate quickly when in pursuit of prey.

Recently, scientists have been observing how many animals self-medicate - that is, treat themselves when they are ill. Tigers, like other cats, eat grass to help with digestion or bringing up fur balls. But when looking at the scats of emaciated tigers who were riddled with parasites, zoologist George Schaller noted the presence of both grass and tapeworms, suggesting that tigers ate grass in attempts to cleanse intestines of the parasites. Other studies of tiger scats show that their stomachs can cope with everything from porcupine quills to bear claws.

Hair, Coat or Pelage

Hair, coat or pelage - the magnificent striping, so characteristic of the tiger - provides a perfect camouflage, but it functions primarily to keep the tiger warm and protect the skin. Fur traps air, which is a poor conductor of heat, thus insulating the tiger's body to keep it at a temperature of 37°C (99°F). The longer and denser the fur, the better the heat retention. This fact explains why tigers that live in warmer southern latitudes have shorter fur (7 to 20 millimeters of the back and 15 to 35 millimeters of the stomach) than those that live in the colder northern habitats (40 to 60 millimeters on the back and 70 to 105 millimeters on the stomach, with as many as 3 000 to 3 300 hairs per square centimeter). Males also have prominent ruffs.

A tiger's fur is made up of two types of hair: the outer guard hairs, which are longer and stronger, for physical protection; and the under fur, which is shorter and denser and is the fluffy heat retainer.

Like many other mammals, tigers shed their hair once or twice per year, having a longer winter coat and a shorter summer coat. And, just like domestic cats, tigers spend time grooming. As part of this, they use their tongues to spread the oils from their sebaceous glands over their coats, keeping them in condition.

Tigers are very clean animals, using their rasping tongues like combs to remove loose hairs and dirt, just like domestic cats. And like domestic cats, they lick their paws and then wash their heads. (They also carefully lick any injuries, coating them with antiseptic saliva from the tongue. This helps prevent infection from entering a wound. Observers in the wild have noted that wounds that the tongue has been able to reach and lick have healed, whereas those that are inaccessible have not.)

While all tigers share basic similarities, there are of course several distinct types. The orange color ranges from a page yellow in the northern extreme of the tiger's range to a deep reddish-ocher in the south of Sumatra. However, the huge color range within each subspecies means that color differences cannot be regarded as definitive. In all subspecies, the tiger's stripes are also found on the skin underneath, while the stomach, cheeks, chin, and the area around the eyes are whitish.

Exceptions, such as white tigers, exist, albeit rarely in the wild because they would be at an obvious disadvantage when hunting and so would seldom survive to pass on their white genes. White tigers are not actually albinos, which would be totally white with red eyes. Instead, they are described as leucocystic, meaning that they have a recessive trait that causes a lack of dark pigments, which produces a white coat, gray to brown stripes, and stunning blue eyes. For some reason, they also seem to grow faster and bigger than their yellow relatives.

White tigers were first observed in India around the turn of the nineteenth century, and may have surfaced due to inbreeding in overhunted and fragmented populations. They are common nowadays because of the large zoo population, most of which stems from a white Bengal tiger cub name Mohan, the 1st known wild white tiger, who was caught and set to breeding by the maharajah of Rewa in 1951.

A few black tigers have been reported, too, but the only evidence for their existence is a skin, which had a black head and back (but stripes at the bottom of the sides), and a photograph of tigers with only a few very broad stripes. Unlike black leopards and jaguars, this is obviously not a true melanistic form, just a darker version of the "normal" tiger.

We are even less certain of the existence of the blue tiger, supposed to have a bluish coat with dark-gray stripes. A handful of sightings, often from reliable sources, is the only proof. However, blue lynx and bobcats occur in the wild, so we cannot rule out the possibility of blue tigers. There are also reports of some white and black tigers having no stripes at all.

The stripes themselves differ greatly in color and thickness, depending on the subspecies, but are also quite distinctive to each individual as well as to each side of the body. If you look at the top of a tiger's head, the stripes make a pattern that looks like the Chinese character of "wang," meaning "king".

The Tiger's Tail

The tiger's tail is surprisingly important in catching prey. A meter of more long, it gathers a lot of momentum when swung from side to side, helping the tiger balance when it has to turn suddenly during a chase. It also plays a part in tiger communication.

A relaxed tiger has a droopy tail. A tiger meeting a friend waves a raised tail slowly. A tiger's tail swishing madly from side to side or held low with just the occasional twitch indicates aggression.

The Sense of Touch

Sensitive whiskers and padded feet give tigers the ability to feel their way silently - even in pitch darkness, through dense cover - to approach unwary prey.

Tigers have specialized hairs on their heads known as whiskers, or vibrissae, which serve a sensory function. About twice as thick as the outer guard hairs of the coat, they are rooted more deeply in the skin, in a capsule of blood. When the whiskers brushes against something, its root moves the blood, thus simplifying the movement. Nerves pick up on the movement and send signals to the brain.

Whiskers are grouped in four different positions. The most important and well-developed are the mystacial group on the muzzle. Typically these are extended, at right angles to the jaws, when the tiger is resting. When sniffing, tigers retract these whiskers against the side of the face. As they advance, they bring them forward in the direction of the mouth like a circular net. The net of whiskers senses a prey's attempts to escape and provides crucial information about shere to inflict the fatal bite. We now know that the whiskers and eyes function together, complementing each other.

The mystacial whiskers also help the tiger to find its way in the dark when the pupils, which are fully dilated, have difficulty focusing on nearby objects. IT is now believed that there is a link between the tiger's sensory and visual inputs and that a degree of parallel processing takes place within the brain. For example, touching the end of the whiskers on a domestic cat triggers an automatic blink response in the cat, which serves to protect the eyes from injury.

The entire area of the tiger's face is well-endowed for sensory input: the skin between the whiskers has separate receptors which are very sensitive to pressure. IT may not even be necessary for the whiskers to come into contact with an object, since they can detect the slight disturbance of air pressure when passing close by it.

Additional whiskers, known as the superciliary whiskers, are located above the eyes. This group may have a protective function, augmenting those on the side of the face. A third group, know as the genal or cheek whiskers, occurs farther back on the sides of the face than the mystacial whiskers and is probably less significant. There are other hairs on the body, notably at the back of the forelegs; these, sometimes called carpal hairs, are very responsive to touch. Body whiskers, or tylotrichs, are slightly longer single hairs spread apparently randomly across a cat's coat. These operate in the same way as facial whiskers to deliver sensory information.

a tiger's facial whiskers can be about fifteen centimeters (six inches) long, with those of males longer and heavier than those of females. Of all tigers, the Sunda Island subspecies Panthera tigris sondaica possesses the most generous number of whiskers.

The Sense of Sight

The beautiful golden glint of a tiger's eye's suggests its complexity and precision. Tigers' eyes also have several special features to increase their ability to see in the dark: large lenses and pupils receive more light; specially sensitive cells absorb the light; and reflective layers at the back of the retina (the light-sensitive cells at the back of the eye) give light more than one chance at stimulating light-receptor cells. The retina contains two different types of light-receptor cell, which connect via the optic nerve to the tiger's brain. These cells are known as rods and cones; rods are more responsive in low levels of light, while cones are sensitive to high levels of light and are used in color vision. Tigers, like many other animals, traded in many cones and in return acquired more rods, so they have increased night vision, which is useful for hunting, at the expense of color vision. Finally, the eyes work together to create a 3-D image (a phenomenon known as binocular vision) - a quality that is of huge importance in calculating how far away the prey is and then striking with accuracy.

Cats have a broad horizontal streak (the visual streak) near the center of the eye, where a high concentration of nerve cells leads to the optic nerve. This increases their visial acuity horizontally, making them better than humans at spotting movement and fleeing prey moving across the plane. In comparison, our visual streak is symmetrically round and so allows us to spot movement in all fields but not in any specialized plane.

Tigers also have a tapetum lucidum ("bright carpet"), meaning a reflective retinal layer. This layer causes their eyes to glow at night and increases their ability to see in the dark. The tapetum lucidum is a highly evolved layered structure that reflects light back to the retina, stimulating he light receptors again and so creating a brighter image. Research on domestic cats has shown that they require only one sixth of the light that humans need to see effectively.

The upper and lower eyelids of cats, like those of humans, sheathe the eyeballs. For further protection, all cats have an opaque, white third eyelid, called the nictitating membrane, between the lower lid and inside corner of the eye. This layer helps moisten the eye and clear dust from the surface of the cornea. Many other animals, including dogs, horses, and birds, also have a nictitating membrane.

The Sense of Sound

All tiger have noticeble white spots on the backs of their ears. The evolutionary purpose of these spots has not yet been determined. It has been suggested that the spots look like false eyes or "predator spots" to anything approaching from behind, making the tiger seem bigger and watchful to the rear, so putting off an attacker. This seems a plausible explanation, as many other species, including some caterpillars and butterflies, use this technique for self-preservation.

A second possibility is that the spots are used in aggressive communication. When one tiger threatens another, it twists its ears so that the backs face forward, prominently displaying the white markings. It is probable that both ideas have an element of truth in them and are not mutually exclusive.

Whatever the purpose of these spots, a tiger's hearing is its most highly developed sense, far more important to its success as a hunter than either sight or smell. With large

pinnae - external ear flaps - rotating like radar dishes, a tiger can catch many sounds and with experience determine precisely where they originated. The ear picks up the high-frequency sounds made by prey rustling in the undergrowth and also low-frequency contact calls, neither of which humans can hear. The tiger's sensitive hearing alerts it not only to prey but also to the footsteps of people, making it one of the most elusive of animals. In the days when tiger hunting was a common pastime, tigers quickly learned to make their escapes when they heard the distant noise of a gun being loaded.

The tiger's ability to communicate by infrasound, a sound wave with a frequency below the range of normally audible sound (twenty hertz), has opened a new and exciting area of research. Many animals, such as elephant and whales, communicate at this low-pitched level. Infrasound is fantastically useful for communicating over long distances or through dense vegetation because it literally passes straight through objects ranging from leaves to trees to mountains. IT is the perfect tool with which a solitary animal like the tiger can communicate with rivals and mates who are distributed widely in dense jungles. Edward J. Walsh and his team in Omaha, Nebraska, are now working with captive tigers in an effort to learn more about tiger sounds and, in particular, to discover whether different tigers can be identified by their infrasound roars. If so, conservationists would be able to count tigers more easily and accurately in the future".





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