"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".
