Cetaceans - Dolphins, Porpoises and Whales

Cetaceans, members of the order Cetacea, are aquatic placental mammals (eutherians). They include whales, dolphins and porpoises.

The blue whale is the largest cetacean and the largest living animal in the world. A blue whale can grow to over 100 feet long and weigh more than 200 tons. As far as we know, the blue whale is the heaviest animal to have ever lived on Earth. (Some dinosaurs were longer but did not weigh as much.)

The smallest cetacean is the vaquita, a porpoise that lives in the Gulf of California. A vaquita may grow up to about 5 feet long and weigh up to about 110 pounds.

The vaquita is a critically endangered speciees.

Cetaceans are tetrapods. They descended from four-legged vertebrates that lived on land.

However, they have evolved so that their bodies are adapted to life underwater.

Common dolphinsThe first cetaceans appeared on Earth about 40 or 50 million years ago.

The closest relatives of the cetaceans are the Artiodactyla, or even-toed ungulates, which include pigs, peccaries, camels, llamas, hippopotamuses and ruminants, such as cows, sheep, goats, deer, and giraffes.

Certartiodactyla is the name that taxonomists give to the larger clade that includes both the cetaceans and the artiodactyls.

Like all placental mammals, cetaceans bear live young and nourish their young with milk that is produced by the mother's mammary glands.

Cetaceans can swim when they are born, but they are not strong swimmers. A calf will swim in its mother's slipstream - the current that she creates behind her when she swims.

There are two main groups of cetaceans - the Odontoceti, which includes toothed whales, porpoises and dolphins, and the Mysticeti, or baleen whales.

Physical Characteristics

Whales, dolphins and porpoises have streamlined bodies that allow them to move through water easily. Their bodies are large in the middle and taper at the ends. This shape is known as a fusiform shape.

Most fish are fusiform-shaped as well.

Fish and cetaceans resemble each other superficially because they have both evolved to live underwater. The process by which unrelated organisms develop similar characteristics because they must survive in similar environments is known as convergent evolution.

Cetaceans have very short necks.

A cetacean does not have any visible hind limbs. However, cetacean embryos do have hind limb buds during part of their development. Rarely, cetaceans are born with vestigial hind limbs. Cetaceans do retain fragments of a pelvic girdle.

The forelimbs of cetaceans are paddle-shaped and are known as flippers. The bones in the flippers are fused together, so that the elbow joints cannot rotate.

Cetacean's tails are horizontal. The tail of a cetacean has two muscular lobes that are known as flukes. A cetacean's tail moves up and down when the cetacean swims.

Fish have vertical tails that move side to side when they swim.

Whales, dolphins and porpoises have very little hair on their bodies.

To avoid heat loss while in the water, cetaceans have a thick layer of blubber (fat) that acts as insulation.


Like all mammals, cetaceans breathe air.

Cetaceans must come to the surface of the water at regular intervals in order breathe.

The nostrils of cetaceans have migrated to the tops of their heads. This allows them to breathe air while keeping their heads underwater.

Cetaceans' nostrils are known as blowholes. Some cetaceans have two blowholes and some have only one.

When a whale, dolphin or porpoise dives underwater, its blowhole (or blowholes) closes up until it comes to the surface to breathe. The blowhole (or blowholes) then opens so that it can inhale oxygen and exhale carbon dioxide.

Cetaceans can remain underwater for much longer than other mammals. Some whales can stay underwater for over an hour. The blood cells and muscles of cetaceans store oxygen more efficiently than the blood cells and muscles of non-aquatic mammals.

A cetacean's mouth is not connected to its respiratory system. This allows it to eat underwater without choking on water or food.

The spout of a dolphin, porpoise or whale is simply exhaled air that condensed and became visible when it came into contact with the cold air above the surface of the water.

Cetaceans are voluntary breathers, or conscious breathers. This means that, unlike land-dwelling mammals, such as human beings, cetaceans must think about breathing. They must know when they have to come up to the surface of the water and breathe.

If a cetacean must be anesthetized for medical reasons, it must first be attached to a machine that breathes for it. The cetacean will not breathe on its own if it is unconscious.

Studies of dolphin brain activity have shown that only one half of a dolphin's brain sleeps at a time. When the right side of a dolphin's brain is asleep, the left side is awake. When the left side is asleep, the right side is awake. This means that part of the brain is always awake and aware of the dolphin's need to breathe.

1 Whales 1520
2 Baleen Whales 1373
3 Toothed Whales 1238
4 Dolphins 2446
5 Porpoises 1953