WHALES, DOLPHINS, AND PORPOISES: THE ORDER “CETACEA”

What are Cetacean's?

There are over 70 species of whales, dolphins and porpoises known to exist in the world today.  All whales, dolphins and porpoises are mammals. This means that they all share characteristics which are found in terrestrial (land) based mammals. For example, they are warm blooded, they breath air and give birth to living young. These young are then suckled on milk secreted by mammary glands of the mother (hence they are given the zoological classification “Mammalia” –the same classification that is given to humans).

All whales, dolphins and porpoises belong to the zoological order “Cetacea” and are commonly referred to as cetaceans. Cetaceans are assigned by scientists into two main groups or “sub-orders” – the “Mysticeti” or baleen whales, and the “Odontoceti” or toothed whales. These sub-orders, and the smaller sub-groups or families into which each is divided, are based upon a logical, well-defined system of classifying animals by commonly held characteristics.

Mysticetes or Baleen Whales

Baleen whales are sometimes referred to as the “great whales”. The largest of the baleen whales, the blue whale (Balaenoptera musculus), is the largest living animal ever known to have existed. They can reach 30 meters in length and weigh up to 190 tonnes. Their tongue alone can weigh four tonnes!

Baleen whales differ from toothed whales in a numbers of ways. Besides being much larger, the main difference is in the baleen apparatus which takes the place of teeth. This baleen appears as two rows of fringed plates hanging from the upper roof of the whale’s mouth. Despite their commercial name “whalebone”, these plates are not made of bone, but are actually a material related to keratin and, therefore, they are similar in structure to human finger nails. The ends of these baleen plates look like the bristles on a broom and the whales feed by straining large quantities of water containing plankton and other small organisms through these “bristles”.

Since dense swarms of suitable plankton occur in the upper layers of the sea, baleen whales tend to travel in the top 100 meters of the water and to be more shallow in their cruising and feeding movements than some species of toothed whales. Because they are so large, baleen whales tend to be slower moving and less agile than the toothed whales. Another prominent difference is that baleen whales have a blowhole which is comprised of two external holes which appear as parallel slits, whereas toothed whales have only a single blowhole.

Odontocetes or Toothed Whale

Toothed whales comprise a much larger group of the order Cetacea. These include dolphins, porpoise and the smaller whales that have teeth. They demonstrates an incredible range of forms, behaviour and lifestyle that reflect their evolutionary history and the variety of environments in which they live. There are toothed whales that are exclusively marine, some that are exclusively freshwater and some that can move between the two. Toothed whales feed mainly on fish and squid, which they pursue and capture. Because they are hunters, they must have speed and agility in the water. Toothed whales are very social animals. They are sometimes found in pods (a group of cetaceans is called a pod) that contain hundreds of animals. Because they often hunt collectively, this form of social structure may be an important survival technique.

This group also has the ability to locate their prey using “echolocation”. They emit intense, short pulses of sound in the form of a rapid series of clicks. These clicks, and other sounds, bounce off objects in their path producing echoes from which the whale or dolphin is able to build up a “sound picture” of its surroundings. Communication also appears to be an important aspect of socialisation and is accomplished through a variety of vocalisations, movements, body posture and contact, and sexual behaviour.

Porpoises are another type of toothed whale and are frequently mistaken for dolphins. Porpoises belong to the family Phocoenidae, whereas most dolphins belong to the family Delphinidae. Porpoises differ from dolphins in a numbers of ways. For example, they have a less distinct rostrum or beak and usually have a dorsal fin that is less pronounced than that of a dolphin. They tend to have rounded heads and are usually smaller than dolphins. The teeth of a porpoise are spade-shaped, not cone shaped like dolphin’s teeth. There are less species of porpoises than dolphins and they tend to be restricted to specific areas. We do not usually see any porpoises in Australian waters.

Most people assign the names whale, dolphin or porpoise according to size, whales being bigger, dolphins and porpoises smaller. However, scientists prefer to use the terms baleen (mysticete) and toothed (odontocete) to refer to the different groups of cetaceans. Other types of marine mammals are found in oceans around the world. These include the sea otter (in the order Carnivora), seals and sea lions which belongs to the order Pinnepedia, and dugongs and other sea cows which belong to the order Sirenia.