Cetaceans

Cetaceans are the group of aquatic mammals that includes whales, dolphins and porpoises. They are adapted to spending their entire lives in water and are recognised by a combination of features, especially blowholes, paddle-like forelimbs, absence of external hindlimbs and a tail ending in horizontal flukes.

Mammals adapted to an entirely aquatic life

Despite their fish-like appearance, cetaceans are mammals. Their basic mammalian characteristics are shared with other marine mammals and do not, by themselves, distinguish cetaceans.

• They are warm-blooded, breathe air through lungs, give birth to live young and nourish their young with milk.
• They must surface to breathe because they lack gills; their nostrils open through blowholes on the upper surface of the head.
• Their bodies are streamlined, external hair is greatly reduced, and an insulating layer of blubber beneath the skin helps limit heat loss.

Body structure and swimming

The cetacean body combines steering flippers with a powerful tail-based propulsion system. These features should be considered together rather than treated individually as unique to cetaceans.

• The forelimbs form flippers, retaining the underlying bones of mammalian forelimbs.
• There are no external hindlimbs, although reduced pelvic bones remain within the body.
• The tail ends in paired horizontal flukes, which move up and down to propel the animal; the flippers mainly help with steering and stability.
• Unlike seals and sea lions, cetaceans cannot support their bodies for movement on land and remain aquatic throughout their life cycle.

Two groups with different feeding adaptations

Cetaceans are divided into baleen whales and toothed whales. Their feeding structures and blowhole arrangement provide useful identifying features.

• Baleen whales have plates of baleen hanging from the upper jaw instead of teeth in adults; these plates strain food from water. They have two blowhole openings.
• Toothed whales, including dolphins and porpoises, possess teeth and have a single blowhole opening.
• Toothed whales use echolocation, producing sounds and interpreting returning echoes to obtain information about their surroundings and locate prey.