This group, which includes all the animals commonly known as whales, dolphins and porpoises, has two main branches, the suborder Mysticeti of baleen whales, and the suborder Odontoceti of toothed whales. There are some 11 species of baleen whales and six times that many toothed whales. Representatives of both branches occur in the waters around India.
Cetaceans are medium sized to very large animals. Their forelimbs are modified into flippers with no digits or claws. Their hind limbs have been dispensed with by evolution. The tail fin is horizontal, unlike fishes’ tails. Overall, however, cetaceans do have a fishlike appearance.
Cetaceans are exclusively aquatic animals, but being mammals, they must come to the surface to breathe. During the course of evolution, their nostrils have moved from the front to the top of the head, where they open as blowholes. Odontocetes have only one nostril in the blowhole. They differ in this from baleen whales, which have a paired opening in the blowhole. Another difference between the two groups of cetaceans is that odontocetes have teeth whilst teeth are absent in baleen whales. All cetaceans are streamlined and are practically hairless.
In place of hair, cetaceans have for insulation a layer of fat under their skin, called blubber. Blubber may be 30 cm thick in Sperm Whales Physeter macrocephalus, and as much as 70 cm thick in the Bowhead Whale Balaena mysticetus.
The hind portion of the cetacean body is known as the tail or tail stock. The two horizontal flukes are appendages. Unlike fishes’ tails, which move from side to side, the flukes of a cetacean move up and down. The flippers are not used for propulsion. Even the largest whales can work up sufficient momentum to leap out of the water.
Dolphins, whales and porpoises feed their newly born youngsters with milk, as all mammals do. In all instances where toothed whale births have been observed, the young have emerged tail first. This is significant, as the young one’s tail may project for hours. A young cetacean with its head protruding in the water would stand the risk of being drowned. The final steps of birth are very quick. Once the calf emerges, it can swim immediately, but its mother nudges it to the surface for its first breath, usually within 10 seconds. Little is known about how baleen whale calves are born.
The water-world in which cetaceans live is rather different from ours. Water conducts sound much farther than air, and at a much higher speed. Light does not penetrate more than a couple of hundred metres. Often, coastal waters are murky, such as due to river discharge. In this world, cetaceans use sound more than sight to survive. Their sense of touch is also very highly developed.