Whales, dolphins and porpoises (Cetacea), also referred to as cetaceans, are a group of marine mammals that includes baleen whales (such as gray whales, right whales, fin whales, blue whales and minke whales) and toothed whales (such as river dolphins, narwhals, porpoises and dolphins).
Cetaceans are highly specialized mammals with numerous adaptations that make them well-suited for living in water. To help them move efficiently through the water, they have developed a streamlined body, flipper-like front limbs and a tail that broadens into two horizontal flukes. To conserve body heat, cetaceans have a thick layer of blubber that acts as an efficient form of insulation. To breathe, they have blowholes on top of their head that are simply muscular nostrils that the cetaceans can open to inhale and close to block the inflow of water. Cetaceans no longer have visible hind limbs. Instead, their hind limbs are present as small vestigial bones burried within their body wall.
Although cetaceans in many ways resemble large fish, they are in fact mammals. Like all mammals, cetaceans breath air with lungs (most fish breath using gills), they have hair (although hair is very sparse on cetaceans) and females have mammary glands which they use to nourish their young with milk.
Cetaceans have evolved the ability to remain underwater for extended periods of time. When submurged, their heart rate slows to nearly one half of its normal rate, reducing the frequency at which they need to resurface to breathe. Additionally, as cetaceans dive, the water pressure forces blood from the vessels near the skin back into the internal organs where it can provide needed nourishment. Some whales dive to impressive depths where water pressures are immense. Sperm whales, for example, dive to depths of as much as 3,300 feet and remain submerged for as long as 90 minutes.
Cetaceans have very good hearing. Toothed whales use echolocation to navigate the waters they inhabit. The emit high-frequency clicks from a structure in their forehead called the melon. The clicks bounce of obstacles in their surroundings and are absorbed by the animal's jawbones, giving them a information about the distance and location of nearby objects.
Cetaceans are divided into two groups, the baleen whales and the toothed whales. Baleen whales (also known as filter feeding whales) are notable for their large size. There are about 12 species of baleen whales, including the northern right whale, gray whale, bowhead, fin whale, blue whale, humpback whale, minke whale, and others.
Toothed whales are the more diverse of the two groups of cetaceans, with 71 species including porpoises, dolphins, river dolphins, sperm whales, beaked whales, and white whales. Toothed whales are mostly medium-sized marine mammals that have teeth instead of baleen.
Although present day whales, dolphins and porpoises are highly specialized marine mammals, their ancestors were land mammals. Fossils of creodonts, creatures that lived between 60 to 37 million years ago, showed initial signs of whale-like evolution with skulls that resembled those of the first true whales, the Archaeocetes. Other early whales included creatures such as Pakicetus, Indohyus, Rodhocetus and Zygorhiza.Many species of cetaceans have suffered such intense hunting pressure that they have experienced drastic population declines. Most species are in danger of extinction. Among the baleen whales at most risk are blue whales, fin whales, humpback whales, northern right whales, southern right whales, bowhead whales, grey whales, sei whales and sperm whales. Some of the toothed whales at risk are Yangtze river dolphins, vaquitas, Hector's dolphins, Ganges river dolphins and baijis.