Cetacea is a widely distributed and diverse clade of aquatic mammals that today consists of whales, dolphins, and porpoises. Cetaceans are carnivorous and finned. Most species live in the sea, some in rivers. The name is derived from the Latin cetus whale, itself from the Greek κῆτος kētos huge fish.There are around 89 extant species, which are divided into two groups or parvorders, the Odontoceti or toothed whales, a group of more than 70 species that includes the dolphins, porpoises, belugas, narwhals, sperm and beaked whales, and the Mysticeti or baleen whales, of which there are now 15 species. The extinct ancestors of modern whales are the Archaeoceti. While cetaceans were historically thought to have descended from mesonychids, molecular evidence supports them as a relative of Artiodactyls . Cetaceans belong to the order Cetartiodactyla and their closest living relatives are hippopotamuses and other hoofed mammals , having diverged about 50 million years ago. Cetaceans range in size from the 1 m and 50 kg Maui's dolphin to the 29.9 m and 173 t blue whale, which is also the largest animal ever known to have existed. Several species exhibit sexual dimorphism. They have streamlined bodies and two limbs that are modified into flippers. Though not as flexible or agile as seals, cetaceans can swim very quickly, with the killer whale able to travel at 56 kilometres per hour in short bursts and the fin whale able to cruise at 48 kilometres per hour . Dolphins are able to make very tight turns while swimming at high speeds. The hindlimbs of cetaceans are internal, and are thought to be vestigial. Baleen whales have short hairs on their mouth, unlike the toothed whales. Cetaceans have well-developed senses—their eyesight and hearing are adapted for both air and water, and baleen whales have a tactile system in their vibrissae. They have a layer of fat, or blubber, under the skin to maintain body heat in cold water. Some species are well adapted for diving to great depths. Although cetaceans are widespread, most species prefer the colder waters of the Northern and Southern Hemispheres. They spend their lives in the water, having to mate, give birth, molt or escape from predators, like killer whales, underwater. This has drastically affected their anatomy to be able to do so. They feed largely on fish and marine invertebrates; but a few, like the killer whale, feed on large mammals and birds, such as penguins and seals. Some baleen whales are specialised for feeding on benthic creatures. Male cetaceans typically mate with more than one female , although the degree of polygyny varies with the species. Cetaceans are not known to have pair bonds. Male cetacean strategies for reproductive success vary between herding females, defending potential mates from other males, or whale song which attracts mates. Calves are typically born in the fall and winter months, and females bear almost all the responsibility for raising them. Mothers of some species fast and nurse their young for a relatively short period of time, which is more typical of baleen whales as their main food source aren't found in their breeding and calving grounds . Cetaceans produce a number of vocalizations, notably the clicks and whistles of dolphins and the moaning songs of the humpback whale. The meat, blubber and oil of cetaceans have traditionally been used by indigenous peoples of the Arctic. Cetaceans have been depicted in various cultures worldwide. Dolphins are commonly kept in captivity and are even sometimes trained to perform tricks and tasks, other cetaceans aren't as often kept in captivity . Cetaceans have been relentlessly hunted by commercial industries for their products, although this is now forbidden by international law. The baiji has become Possibly Extinct in the past century, while the vaquita and Yangtze finless porpoise are ranked Critically Endangered by the International Union for Conservation of Nature. Besides hunting, cetaceans also face threats from accidental trapping, marine pollution, and ongoing climate change.
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