Toothed whale

The toothed whales (also called odontocetes, systematic name Odontoceti) are a parvorder of cetaceans that includes dolphins, porpoises, and all other whales with teeth, such as beaked whales and the sperm whales. 73 species of toothed whales are described. They are one of two living groups of cetaceans, with the other being the baleen whales (Mysticeti), which have baleen instead of teeth. The two groups are thought to have diverged around 34 million years ago (mya).

Toothed whales range in size from the 1.4 m (4 ft 7 in) and 54 kg (119 lb) vaquita to the 20 m (66 ft) and 100 t (98 long tons; 110 short tons) sperm whale. Several species of odontocetes exhibit sexual dimorphism, in that there are size or other morphological differences between females and males. They have streamlined bodies and two limbs that are modified into flippers. Some can travel at up to 30 knots. Odontocetes have conical teeth designed for catching fish or squid. They have well-developed hearing that is adapted for both air and water, so much so that some can survive even if they are blind. Some species are well adapted for diving to great depths. Almost all have a layer of fat, or blubber, under the skin to keep warm in the cold water, with the exception of river dolphins.

Toothed whales consist of some of the most widespread mammals, but some, as with the vaquita, are restricted to certain areas. Odontocetes feed largely on fish and squid, but a few, like the orca, feed on mammals, such as pinnipeds. Males typically mate with multiple females every year, making them polygynous. Females mate every two to three years. Calves are typically born in the spring and summer, and females bear the responsibility for raising them, but more sociable species rely on the family group to care for calves. Many species, mainly dolphins, are highly sociable, with some pods reaching over a thousand individuals.

Once hunted for their products, cetaceans are now protected by international law. Some species are very intelligent. At the 2012 meeting of the American Association for the Advancement of Science, support was reiterated for a cetacean bill of rights, listing cetaceans as nonhuman persons. Besides whaling and drive hunting, they also face threats from bycatch and marine pollution. The baiji, for example, is considered functionally extinct by IUCN, with the last sighting in 2004, due to heavy pollution to the Yangtze River. Whales sometimes feature in literature and film, as in the great white sperm whale of Herman Melville's Moby-Dick. Small odontocetes, mainly dolphins, are kept in captivity and trained to perform tricks. Whale watching has become a form of tourism around the world.

The tube in the head, through which this kind fish takes its breath and spitting water, located in front of the brain and ends outwardly in a simple hole, but inside it is divided by a downward bony septum, as if it were two nostrils; but underneath it opens up again in the mouth in a void.

In Aristotle's time, the fourth century BC, whales were regarded as fish due to their superficial similarity. Aristotle, however, could already see many physiological and anatomical similarities with the terrestrial vertebrates, such as blood (circulation), lungs, uterus, and fin anatomy.^{[citation needed]} His detailed descriptions were assimilated by the Romans, but mixed with a more accurate knowledge of the dolphins, as mentioned by Pliny the Elder in his Natural history. In the art of this and subsequent periods, dolphins are portrayed with a high-arched head (typical of porpoises) and a long snout. The harbor porpoise is one of the most accessible species for early cetologists, because it could be seen very close to land, inhabiting shallow coastal areas of Europe. Many of the findings that apply to all cetaceans were therefore first discovered in the porpoises. One of the first anatomical descriptions of the airways of the whales on the basis of a harbor porpoise dates from 1671 by John Ray. It nevertheless referred to the porpoise as a fish.

Toothed whales, along with baleen whales, are descendants of land-dwelling mammals of the artiodactyl order (even-toed ungulates). They are closely related to the hippopotamus, sharing a common ancestor that lived around 54 million years ago (mya). The primitive cetaceans, or archaeocetes, first took to the sea approximately 49 mya and became fully aquatic within 5–10 million years. The ancestors of toothed whales and baleen whales diverged in the early Oligocene. This was due to a change in the climate of the southern oceans that affected the environment of the plankton that these whales ate.

The adaptation of echolocation and enhanced fat synthesis in blubber occurred when toothed whales split from baleen whales, and distinguishes modern toothed whales from fully aquatic archaeocetes. This happened around 34 mya. Unlike toothed whales, baleen whales do not have wax ester deposits nor branched fatty chain acids in their blubber. Thus, more recent evolution of these complex blubber traits occurred after baleen whales and toothed whales split, and only in the toothed whale lineage.