Cetacea

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Cetaceans
Image:Humpbackwhale37.jpg
Humpback Whale breaching
Scientific classification
Kingdom:Animalia
Phylum:Chordata
Class:Mammalia
Order:Cetacea
Brisson, 1762
Suborders

Mysticeti
Odontoceti
(see text for families)

The order Cetacea 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.

The order Cetacea contains eighty-one known species, divided into two suborders, Mysticeti (baleen whales) and Odontoceti (toothed whales, dolphins and porpoises).

Contents

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.

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 - called a "fluke" - is horizontal and moves up and down instead.

Evolution

Main article: Evolution of cetaceans

Cetaceans evolved from land mammals (most likely from certain hoofed carnivores distantly related to hippopotamuses) that returned to the sea about 50 million years ago.

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 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 oxygen. 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 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 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 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, such as Killer Whales (Orcinus orca) catching a fur seal, they tear 'chunks' of it that in turn are swallowed 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 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.

It is a fallacy to believe that all Mysticeti feed on plankton, because for some species their prey animals are not plankton. The larger whales tend to eat small shoaling fish, such as herrings and sardine. These fish are called micronecton, and not plankton. One species of Mysticeti, the Gray Whale (Escrichtius robustus) is a benthic feeder.

Taxonomic listing

The classification here closely follows "Marine Mammals of the World: Systematics and Distribution" by Dale W. Rice (1998). The work has become the standard taxonomy reference in the field. Differences reflect usage of common names and further discoveries since the publication of that work.

References

  • Rice, Dale W. (1998). Marine mammals of the world: systematics and distribution. Society of Marine Mammalogy Special Publication Number 4. 231 pp. See the Society's website for further details.

External links

Wikibooks has a Dichotomous Key related to this taxon:
Mammals
Monotremata

Placentalia: Xenarthra | Dermoptera | Desmostylia | Scandentia | Primates | Rodentia | Lagomorpha | Insectivora | Chiroptera | Pholidota | Carnivora | Perissodactyla | Artiodactyla | Cetacea | Afrosoricida | Macroscelidea | Tubulidentata | Hyracoidea | Proboscidea | Sirenia

Marsupialia: Didelphimorphia | Paucituberculata | Microbiotheria | Dasyuromorphia | Peramelemorphia | Notoryctemorphia | Diprotodontia

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