Porpoises, like other odontocetes, possess only one blowhole. Porpoises have a three-chambered stomach, including a fore-stomach and fundic and pyloric chambers. Unlike human teeth, which are composed mostly of enamel on the portion of the tooth outside of the gum, whale teeth have cementum outside the gum. Odontocetes possess teeth with cementum cells overlying dentine cells. Almost all species have female-biased sexual dimorphism, with the females being larger than the males, although those physical differences are generally small one exception is Dall's porpoise. Overall, they tend to be dwarfed by other cetaceans. Porpoises range in size from the 1.4 m (4 ft 7 in) and 54 kg (119 lb) Vaquita to the 2.3 m (7 ft 7 in) and 220 kg (490 lb) Dall's porpoise. Their skull has small eye orbits, small, blunt snouts, and eyes placed on the sides of the head. Porpoises have a bulbous head, no external ear flaps, a non-flexible neck, a torpedo shaped body, limbs modified into flippers, and a tail fin. Biology Anatomy Harbour porpoise skeleton Recently discovered hybrids between male harbour porpoises and female Dall's porpoises indicate the two species may actually be members of the same genus. asiaeorientalis – Yangtze finless porpoise phocaeniodes – Indo-Pacific finless porpoise The oldest fossils are known from the shallow seas around the North Pacific, with animals spreading to the European coasts and Southern Hemisphere only much later, during the Pliocene. The cetaceans diversified, and fossil evidence suggests porpoises and dolphins diverged from their last common ancestor around 15 Mya. During the Miocene (23 to 5 Mya), mammals were fairly modern, meaning they seldom changed physiologically from the time. Porpoises, along with whales and dolphins, are descendants of land-living ungulates (hoofed animals) that first entered the oceans around 50 million years ago (Mya). Some species of porpoises have been and are kept in captivity and trained for research, education and public display. Since the extinction of the baiji, the vaquita is considered the most endangered cetacean. The vaquita is nearly extinct due to bycatch in gill nets, with a predicted population of fewer than a dozen individuals. Larger threats to porpoises include extensive bycatch in gill nets, competition for food from fisheries, and marine pollution, in particular heavy metals and organochlorides. Porpoises were, and still are, hunted by some countries by means of drive hunting. In contrast to many dolphin species, porpoises do not form large social groups. Porpoises produce ultrasonic clicks, which are used for both navigation ( echolocation) and social communication. Calves are typically born in the spring and summer months and remain dependent on the female until the following spring. Females may have one calf every year under favourable conditions. Little is known about reproductive behaviour. Porpoises feed largely on fish and squid, much like the rest of the odontocetes. Porpoises are abundant and found in a multitude of environments, including rivers ( finless porpoise), coastal and shelf waters ( harbour porpoise, vaquita) and open ocean ( Dall's porpoise and spectacled porpoise), covering all water temperatures from tropical ( Sea of Cortez, vaquita) to polar ( Greenland, harbour porpoise). As all cetaceans, they have a layer of fat, or blubber, under the skin to keep them warm in cold water. Some species are well adapted for diving to great depths. Porpoises use echolocation as their primary sensory system. They have streamlined bodies and two limbs that are modified into flippers. Several species exhibit sexual dimorphism in that the females are larger than males. Porpoises range in size from the vaquita, at 1.4 metres (4 feet 7 inches) in length and 54 kilograms (119 pounds) in weight, to the Dall's porpoise, at 2.3 m (7 ft 7 in) and 220 kg (490 lb). Porpoises, and other cetaceans, belong to the clade Cetartiodactyla with even-toed ungulates. Hector's dolphin) also lack a pronounced beak. Porpoises are distinguished from dolphins by their flattened, spade-shaped teeth distinct from the conical teeth of dolphins, and lack of a pronounced beak, although some dolphins (e.g. There are eight extant species of porpoise, all among the smallest of the toothed whales. Although similar in appearance to dolphins, they are more closely related to narwhals and belugas than to the true dolphins. Porpoises ( / ˈ p ɔː r p ə s ɪ z/) are a group of fully aquatic marine mammals, all of which are classified under the family Phocoenidae, parvorder Odontoceti (toothed whales).
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