http://www.photonette.net/

 

Cetacea

From Wikipedia, the free encyclopedia

 

Cetaceans

Fossil range: Early Eocene - Recent

Humpback Whale breaching

Humpback Whale breaching

Scientific classification

Kingdom:

Animalia

Phylum:

Chordata

Subphylum:

Vertebrata

Class:

Mammalia

Order:

Cetacea
Brisson, 1762

Suborders

Mysticeti
Odontoceti
Archaeoceti (extinct)
(see text for families)

 

The order Cetacea (IPA: [siːteɪʃə], L. cetus, whale) includes whales, dolphins and porpoises. Cetus is Latin and is used in biological names to mean "whale"; its original meaning, "large sea animal," was more general. It comes from Greek ketos ("sea monster"). Cetology is the branch of marine science associated with the study of cetaceans.

Cetaceans are the mammals most fully adapted to aquatic life. Their body is fusiform (spindle-shaped). The forelimbs are modified into flippers. The tiny hindlimbs are vestigial; they do not attach to the backbone and are hidden within the body. The tail has horizontal flukes. Cetaceans are nearly hairless, and are insulated by a thick layer of blubber. While the specifics are debatable, cetaceans as a group are noted for their considerable intelligence among the animal kingdom.

The order Cetacea contains ninety species, all marine except for five species of freshwater dolphins. The order is divided into two suborders, Mysticeti (baleen whales) and Odontoceti (toothed whales, which includes dolphins and porpoises).

 

Evolution

Cetaceans evolved from land mammals (most likely from certain hoofed carnivores which also gave rise to the artiodactyls – the even-hoofed mammals, including pigs and the hippopotamus) that adapted to marine life about 50 million years ago.

 

Adaptations for sea life

Over a period of a few million years during the Eocene period, the cetaceans returned to the sea, where there was a niche for large, surface-dwelling predators that had been empty since the demise of the mosasaurs and plesiosaurs. Because of the increase in available living space, there was no natural limit to the cetaceans' 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 forelimbs disappeared, and then its hind limbs; its body became more tapered and streamlined – a form that enabled it to move swiftly through the water. The cetacean's original tail was replaced by a pair of flukes that sculled with a vertical motion.

As part of this streamlining process, the bones in the cetaceans' 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 cetaceans' tremendous bulk.

To preserve body heat in cold oceanic waters, the cetacean developed 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. No longer needed for warmth, the cetacean's fur coat disappeared, further reducing the resistance of the giant body to the water.

The ear bone called the hammer (malleus) is fused to the walls of the bone cavity where the ear bones are, making hearing in air nearly impossible. Instead sound is transmitted through their jaws and skull bones.

 

[Respiration, vision, 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 oxygen. 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.

Cetaceans' blowholes have evolved to a position on top of the head, allowing more time to expel stale air and inhale fresh air. When the stale air, warmed from the lungs, is exhaled, it condenses as it meets the cold air outside. As with a terrestrial mammal breathing out on a cold day, a small cloud of 'steam' appears. This is called the 'blow' or 'spout' and is different in terms of shape, angle and height, for each cetacean species. Cetaceans can be identified at a distance, using this characteristic, 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. Tear glands secrete greasy tears, which protect the eyes from the salt in the water. Cetaceans also have an almost spherical lens in their eyes, which is most efficient at focusing what little light there is in the deep waters. Cetaceans make up for their generally quite poor vision (with the exception of the dolphin) with excellent hearing.

As with 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 airborne sound waves and focus them in order for them to become strong enough to hear well. However, water is a better conductor of sound than air, so the external ear was no longer needed: It 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.

Some cetaceans are capable of echolocation. Many toothed whales emit clicks similar to those in echolocation, but it has not been demonstrated that they echolocate. Mysticeti have little need of echolocation, as they prey upon small fish that would be impractical to locate with echolocation. Some members of Odontoceti, such as dolphins and porpoises, do perform echolocation. These cetaceans use sound in the same way as bats - they emit a sound (called a click), 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. With this ability cetaceans can search for, chase and catch fast-swimming prey in total darkness. Echolocation is so advanced in most Odontoceti that they can distinguish between prey and non-prey (such as humans or boats); captive cetaceans can be trained to distinguish between, for example, balls of different sizes or shapes.

Cetaceans also use sound to communicate, whether it be groans, moans, whistles, clicks or the complex 'singing' of the Humpback Whale.

 

Feeding

 

Various Cetacea: 1- Bowhead Whale; 2- Orca; 3- Right Whale; 4- Sperm Whale; 5- Narwhal; 6- Blue Whale; 7- Rorqual; 8- Beluga

When it comes to food and feeding, cetaceans can be separated into two distinct groups. The 'toothed whales', Odontoceti like sperm whales, beluga whales, dolphins and porpoises, usually 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. In the rare cases that they catch large prey, as when Orca (Orcinus orca) catch a seal, they tear 'chunks' off it that in turn are swallowed whole.

The 'baleen whales' or Mysticeti do not have teeth. Instead they have plates made of keratin (the same substance as human fingernails) which hang down from the upper jaw. These plates act like a giant filter, straining small animals (such as krill and fish) from the seawater. Cetaceans included in this group include the Blue Whale, the Humpback Whale, the Bowhead Whale and the Minke Whale.

Not all Mysticeti feed on plankton: the larger whales tend to eat small shoaling fish, such as herrings and sardine, called micronecton. One species of Mysticeti, the Gray Whale (Eschrichtius robustus), is a benthic feeder, primarily eating sea floor crustaceans.

 

Mammalian nature

Cetaceans are mammals, that is, members of the class mammalia. The closest living relative of cetaceans is the hippopotamus.

As mammals, cetaceans have characteristics that are common to all mammals: They are warm-blooded, breathe in air through their lungs, bear their young alive and suckle them on their own milk, and have hair, although very little of it.

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 – called a "fluke" – is horizontal and moves up and down, as cetaceans' spines bend in the same manner as a human spine.

 

Taxonomic listing

The classification here closely follows Dale W. Rice, Marine Mammals of the World: Systematics and Distribution (1998), which has become the standard taxonomy reference in the field. There is very close agreement between this classification and that of Mammal Species of the World: 3rd Edition (Wilson and Reeder eds., 2005). Any differences are noted using the abbreviations "Rice" and "MSW3" respectively. Further differences due to recent discoveries are also noted.

Discussion of synonyms and subspecies are relegated to the relevant genus and species articles.

 

 

Wikipedia

 

http://en.wikipedia.org/w/index.php?title=Cetacea&action=history

 

http://www.gnu.org/copyleft/fdl.html