splavl-tree/Online-Algorithms/08/data/toi65.txt

Whales, dolphins and porpoises make up the classification order Cetacea,
which contains two suborders, Mysticeti and Odontoceti. The baleen whales
are members of the Mysticeti suborder, while the toothed whales, dolphins
and porpoises make up the suborder Odontoceti. 
Altogether, the two suborders contain eighty-one known species, separated
into thirteen different families. In each family are a number of species, each
classified further into 'sub-families', or genera, of which there are 40. 
What Are Cetaceans?
There are many misconceptions about cetaceans (whales, dolphins and
porpoises), the most common of which is the idea that cetaceans are fish.
They're not - they are mammals, like you and me. Millions of years ago,
they lived on land; their bodies were covered in hair, they had external
ears, they walked on four legs, they beared live young. 
As mammals, cetaceans have these characteristics that are common to all
mammals:
* They are warm-blooded animals.
* They breathe in air through their lungs.
* They bear their young alive and suckle them on their own milk.
* They have hair - though generally only a few 'whiskers'.
Another way of discerning a cetacean from a fish is by the shape of the
tail. The tail of a fish is vertical and moves from side to side when the fish
swims. The tail of a cetacean is horizontal and moves up and down instead.
The Cetacean's Adaptations for Sea Life
Over a period of millions of years, the cetacean returned to the sea - there
was more food there, and more space than on land. Because of this
increase in space, there was no natural limit to the cetacean's size (i.e. the
amount of weight its legs could hold) since the water provided buoyancy. It
had no longer any need for legs.
During this time, the cetacean lost the qualities that fitted it for land
existence and gained new qualities for life at sea. Its hind limbs
disappeared, its body became more tapered and streamlined - a form that
enabled it to move swiftly through the water. For the same reason, most of
its fur disappeared, reducing the resistance of the giant body to the water.
The cetacean's original tail was replaced by a pair of flukes that acted like a
propeller.
As part of this streamlining process, the bones in the cetacean's front limbs
fused together. In time, what had been the forelegs became a solid mass of
bone, blubber and tissue, making very effective flippers that balance the
cetacean's tremendous bulk.
After the cetacean's hair disappeared, it needed some way of preserving
their body heat. This came in the form of blubber, a thick layer of fat
between the skin and the flesh that also acts as an emergency source of
energy. In some cetaceans the layer of blubber can be more than a foot
thick.
Breathing, Seeing, Hearing and Echolocation
Since the cetacean is a mammal, it needs air to breathe. Because of this, it
needs to come to the water's surface to exhale its carbon dioxide and
inhale a fresh supply of air. Naturally it cannot breathe under water, so as it
dives a muscular action closes the blowholes (nostrils), which remain closed
until the cetacean next breaks the surface. When it does, the muscles open
the blowholes and warm air is exhaled.
To make this easier, the cetacean's blowholes have moved to the top of its
head, giving it a quicker chance to expel the stale air and inhale fresh air.
When the stale air, warmed from the lungs, is exhaled it condenses and
vapourises as it meets the cold air outside. This is rather like when you
breathe out on a cold day and a small cloud of warm air appears. This is
called the 'blow', or 'spout', and each cetacean's blow is different in terms
of shape, angle and height. This is how cetaceans can be identified at a
distance by experienced whalers or whale-watchers.
The cetacean's eyes are set well back and to either side of its huge head.
This means that cetaceans with pointed 'beaks' (such as dolphins) have
good binocular vision forward and downward, but others with blunt heads
(such as the Sperm Whale) can see either side but not directly ahead or
directly behind. The eyes shed greasy tears which protect them from the
salt in the water, and cetaceans have been found to have good vision both
in the water and out.
Akin to the eyes, the cetacean's ears are also small. Life in the sea
accounts for the cetacean's loss of its external ears, whose function is to
collect sound waves and focus them in order for them to become strong
enough to hear well. However, sound waves travel faster through the water
than in the air, and so the external ear was no longer needed, and is no
more than a tiny hole in the skin, just behind the eye. The inner ear,
however, has become so well developed that the cetacean can not only
hear sounds tens of miles away, but it can also discern from which direction
the sound comes.
Cetaceans use sound in the same way as bats - they emit a sound, which
then bounces off an object and returns to them. From this, cetaceans can
discern the size, shape, surface characteristics and movement of the
object, as well as how far away it is. This is called sonar, or echolocation,
and with it cetaceans can search for, chase and catch fast-swimming prey
in total darkness. It is so advanced that most cetaceans can discern
between prey and non-prey (such as humans or boats), and captive
cetaceans can be trained to distinguish between, for example, balls of
different colours, sizes or shapes.
Cetaceans also use sound to communicate, whether it be groans, moans,
whistles, clicks or the complex 'singing' of the Humpback Whale that is
becoming so popular on wildlife documentaries and relaxation tapes.
Feeding
When it comes to food and feeding, this is where cetaceans can be
separated into two distinct groups. The 'toothed whales' or Odontoceti
have lots of teeth that they use for catching fish, squid or other marine life.
They do not chew their food, but swallow it whole. The cetaceans in this
group include the Sperm Whale, dolphins and porpoises.
The 'baleen whales' or Mysticeti do not have teeth. Instead they have
plates made of keratin (the same substance as our fingernails) which hang
down from the upper jaw. These plates act like a giant filter, straining small
animals (such as plankton, krill and fish) from the seawater. Cetaceans
included in this group include the mighty Blue Whale, the Humpback Whale,
the Bowhead Whale and the Minke Whale.
<br><br><b>Bibliography</b><br><br>
NONE
www.catecea.com
<br><br>
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