Pakicetus, extinctgenus of early cetaceanmammals known from fossils discovered in 48.5-million-year-old river delta deposits in present-day Pakistan. Pakicetus is one of the earliest whales and the first cetacean discovered with functional legs. In addition, it still retained many other features of terrestrial mammals, including an auditory system that was better for hearing in air than in water, a dentition not unlike that of its closest terrestrial relatives, such as the mesonychids, and functional feet capable of locomotion on land.
The body mass of Pakicetus was estimated at 45 kg (100 pounds), roughly the size of a wolf or large dog. The dentition of the animal indicates that it had a diet primarily of fish; however, its skeleton and skull suggest that it spent a considerable amount of time on land.
The climate of the early Eocene Epoch (56 million to 40 million years ago) was the warmest of the Cenozoic Era, nearly 10 °C (18 °F) warmer than the global average of the present day. Whales evolved during the Eocene in the warm, shallow tropical Tethys Sea, which lay sandwiched between the mainland of Asia and Europe to the north and Africa, Arabia, Madagascar, and the Indian subcontinent to the south. Most archaeocetes (first cetaceans) lived in the Tethys or along its margins. Pakicetus fossils, which include many broken teeth, skulls, and skeletons, were found in the Kuldana Formation in Pakistan, a site that was located near the northern edge of the Tethys Sea during the Eocene.
The later descendants of Pakicetus were fully aquatic. Modern whales are descended from the archaeocete basilosaurids, a group of toothed whales that had extremely long bodies and tails. The archaeocete basilosaurids appeared later in the Eocene and early Oligocene (34 million to 23 million years ago) and lived in the Tethys Sea and Atlantic Ocean.
whale, any of the larger species of aquatic mammals belonging to the order Cetacea. The term whale can be used in reference to any cetacean, including porpoises and dolphins, but in general it is applied to those more than 3 metres (10 feet) long. An exception is the 2.7-metre dwarf sperm whale (Kogia simus), so called for its otherwise striking resemblance to its larger namesake. Whales are the heaviest known animals, living or fossil, reaching a maximum size in the blue whale (Balaenoptera musculus) of perhaps more than 30 metres and 200 metric tons (220 short [U.S.] tons). Large predatory whales, including the killer whale, are often considered to be the apex predators (meaning they have no natural predators or enemies) of their marineenvironments.
Whales are distributed throughout the world’s oceans and seas, from the Equator to the polar ice, except for the landlocked Caspian and Aral seas. They are mammals, and they share the defining traits of that group: they breathe air, are warm-blooded, give live birth, suckle their young on milk, and have hair. All are entirely aquatic, with specialized adaptations such as flippers and tail flukes for living in water. Whales must surface regularly to breathe, evacuating their lungs more completely than most mammals in an almost explosive breath known as a blow. Blows are visible because water vapour in the whale’s hot breath condenses when the blow is released.
Despite living in a medium that has much greater thermal conduction characteristics than air, whales, like other mammals, must regulate their body temperature. Hair, however, is restricted to the head, appearing mainly as isolated whiskers (vibrissae) near the mouth and blowhole. Blubber serves as an insulating layer to protect small whales from hypothermia. Large whales have the opposite problem in that they can produce too much heat; they possess elaborate thermoregulation mechanisms to prevent overheating.
Because of the restricted utility of vision underwater, whales use sound to perceive and interpret their environment and to communicate, sometimes over vast distances. Biologists have computed that the 10-hertz sounds of fin whales (Balaenoptera physalus), for example, can travel over 1,800 km (1,100 miles). Toothed whales can produce sounds and interpret their reflections via active echolocation. The extent to which baleen whales have this ability is unknown.
Following internal fertilization, female whales are pregnant for about a year. The young are relatively large when born—one-third to one-half the length of the mother. They are nursed for about six months on extremely rich milk containing almost 50 percent fat. Whales have one pair of nipples located at the rear of the abdomen near the genital opening.
Humpback whales: Size, behavior, and threatsLearn about the humpback whale.
The first fossil whales are known from rocks approximately 50 million years old (Early Eocene Epoch). These members of the extinct suborder Archaeoceti are the primitive whales from which modern whales are derived. They show many similarities to terrestrial mammals, including a differentiated dentition (heterodonty) consisting of incisors, canines, premolars, and molars. Archaeocetes gave rise to the living suborders: the baleen whales (suborder Mysticeti) and the toothed whales (suborder Odontoceti).
Mysticetes have baleen plates and take small prey by the mouthful, mainly in the form of drifting (planktonic) crustaceans such as copepods and krill, but they also occasionally consume small schooling fish or squid. One interesting departure from this pattern is the gray whale (Eschrichtius robustus), which commonly eats shrimp and other bottom-dwelling creatures: it scoops up mud and strains it through the baleen plates, retaining its food. Odontocetes have simple teeth (homodonty) and pursue individual squid, fish, and other prey. The largest odontocete, the sperm whale (Physeter macrocephalus), occasionally feeds on giant squid.
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Explore the Whale Museum and the whales around the San Juan IslandsA discussion of whales around the San Juan Islands of Washington, from the documentary Science at Sea: The Whale Museum.
Humans have long utilized stranded whales as a food resource, and historically whales were hunted for whale oil and baleen. In the early 20th century, as demand increased and technology enabled meat to be frozen at sea, whales began to be taken in larger numbers for human consumption and for specialty products. Scientific concern over the increased catch of whales in the Southern Hemisphere in the 1930s led to the ratification of the International Whaling Convention and the founding of the International Whaling Commission in 1946. Through the years this agency has acted to moderate whaling, and it instituted a moratorium on commercial whaling in the late 1980s. Some whaling still occurs under special permit. Native peoples are also allowed to continue traditional whale hunting that has been a part of their culture.
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