Referat engleza - BALENA-Whale referat







Whale, any of the marine mammals in the order Cetacea. They are unique among all mammals in that they carry out their complete life history, from birth to death, in water. The term cetacean is used to embrace all known species (about 79) of whales, dolphins, and porpoises




Evidence indicates that whales descended from a four-legged land animal, perhaps a primitive ungulate (hoofed mammal) that may also have given rise to modern ungulates. The earliest known whale fossils are 52 million years old, but many scientists estimate that whales date from 60 million years ago. Fossilized cetacean skeletons dating back to the Eocene epoch (56.5 million to 35.4 million years ago) were recently discovered in Pakistan. These fossils indicate that early whales swam by undulating their vertebral column, thus forcing their feet up and down in a way similar to modern otters.

Whales are divided into two suborders: toothed whales and baleen whales. Most smaller whales, and all the dolphins and porpoises, belong to the toothed whale suborder. Those more than 4 to 5 m (13 to 16 ft) long are generally referred to as whales, whereas smaller species are known as dolphins or porpoises. Toothed whales have teeth that are uniform in size and shape although they vary considerably in number, and they feed on fish and invertebrates such as squid and crustaceans; one species, the killer whale, has a more varied diet that includes seabirds and marine mammals. A few species are commercially valuable as exhibits in aquariums and oceanariums, and some of the smaller whales are hunted to a limited extent. One toothed whale, the sperm whale, sometimes known as a cachalot, is quite large: the male grows to a length of 18.3 m (60 ft), and the female grows to a length of 12.2 m (40 ft). It was heavily hunted in the past.

The rest of the larger whales belong to the baleen whale suborder. In this group of ten species—all of which have been or are currently being hunted—teeth have been replaced with large structures, known as baleen plates, that hang like vertical Venetian blinds from the upper jaw. The plates number 160 to 395 on each side, are frayed into bristles on their inner edges, and are used to capture the plankton or krill on which the animals subsist. When feeding, a baleen whale swims with its mouth open in order to engulf plankton and seawater by the ton. Then, shutting its cavernous mouth and pressing its tongue against the back of the baleen bristles, the whale forces the water out of its mouth, trapping the plankton on a mat of overlapping baleen plates.

Probably the largest animal ever to have lived is a baleen whale, the blue whale, which has been measured up to 30.5 m (100 ft) in length, with a weight of about 190 tonnes. Baleen whales tend to spend the summer in polar seas, where plankton blooms provide abundant food. After months of heavy feeding they migrate to temperate or tropical zones, often fasting there over the winter.

Characteristics






The dramatic streamlining of whales in the course of their evolution resulted in an animal that appears remarkably fish-like. Thus, the front limbs became modified as paddle-shaped flippers, the bones of which are still reminiscent of jointed limbs and digits, but the hind limbs were lost. The broad horizontal tail flukes that provide the main propulsive thrust bear no anatomical connection to the lost hind limbs, but are a separate and distinct development. They contain no bone, and owe their firm and yet flexible shape to underlying fibrous and elastic tissue. The body is enveloped in a thick layer of blubber that aids in buoyancy, helps to preserve body heat, and is a source of stored energy. A whale's skin is free from sweat glands, oil glands, or hair, and feels much like smooth, wet rubber to the touch.

Whales, like other mammals, have lungs. They breathe air through a single nostril, or pair of nostrils, situated on the top of the head (the blowhole); but contrary to a popular image, they do not spout water when they exhale. The visible “spout”, the size and shape of which is unique to many species, is simply water vapour in the lungs and a small amount of water present in the depression around the blowhole, which is blown into the air as the whale comes to the surface of water (breaches) and exhales.



A number of physiological adaptations enable whales to perform deep dives. First, they have a larger blood volume than land mammals of comparable size and weight, and they also have a greatly increased capacity to store oxygen in their blood and muscle tissue. Second, each breath provides an 80 to 90 per cent renewal of air in a whale's lung, compared with only 10 to 20 per cent in most land mammals. Third, whales have a resistance to the metabolic by-product carbon dioxide, the build up of which in the tissues, rather than a lack of oxygen, triggers the involuntary breathing response of most mammals. Baleen whales can hold their breath for up to 50 minutes when diving, and sperm whales for up to 60 minutes; sperm whales dive to depths of 1,000 m (3,300 ft) in search of their prey, the giant squid. Finally, whales are able to restrict blood flow to vital organs during a deep dive, so that essential organs such as the heart and the brain do not suffer injurious oxygen deprivation.

Life Cycle

Reproduction in whales is essentially the same as in other mammals. Sexually mature animals go through a courtship period followed by copulation, which in the whale takes place belly to belly in the water. (Whether mated pairs remain together after this time is not known.) The pregnant female carries her unborn young for 9 to 16 months, depending on the species, and usually a single large, well-developed calf is born underwater.

A healthy calf can swim from the instant it is born, rising to the surface for its first breath. In some species it is helped to the surface by its mother. Soon after, it begins to suckle from either of two mammary teats enclosed in slits located on each side of its mother's genital opening. Whale milk is very rich, and calves grow rapidly. For example, a blue whale calf that is 7 m (23 ft) long and weighs 1.8 tonnes at birth doubles its weight in the first week; at seven months the infant is 17 m (56 ft) long and weighs 22 tonnes. Juvenile whales are probably weaned off their mothers' milk somewhere between eight months and two years after birth; the age at which a young whale leaves its mother, however, is not known for many species. In some species, such as the killer whale, young animals appear to remain permanently in the family pod, which usually numbers 5 to 12 animals.

A whale reaches sexual maturity at 6 to 13 years of age. The lifespan varies: for small-toothed whales such as the beluga it is thought to be about 30 years; for larger-toothed cetaceans such as the sperm whale it is up to 50 years or more; and baleen whales probably live for as long as 80 years. As with all other animals, not all whales survive to old age. Disease, injury, and predation by killer whales, sharks, and whalers all take their toll.

Senses and Intelligence



Sound and hearing are to whales what vision and smell are to most land mammals. The wide range of sounds that they can hear include low- and high-pitched sounds undetectable to humans. The loud low moans of the blue whale can be heard 160 km (100 mi) away. At least two kinds of sound are produced: echolocation clicks and vocalizations. Both are probably produced as air moves in and out of nasal sacs. Echolocation sounds function as a kind of biological sonar, whereas vocalization sounds—the most famous of which are the “songs” of the humpback whale—seem to function as a means of communication between members of the same species.

Porpoises and probably many whales can explore their environment and the objects in it through the use of echolocation. By directing sounds, produced in the head region, towards an object and receiving back the sound waves that bounce off the object, the animals can make fine discriminations as to size, density, distance, and so on. Because the sound waves are waterborne, cetaceans have been able to do without the external ears that land mammals developed to gather airborne sounds. This system of sensing the environment is obviously of enormous advantage in orienting, navigating, and capturing prey in dark or turbid waters. In essence, it is a means of scanning by sound for the same information humans and most other land mammals perceive by vision. On the other hand, cetaceans do not necessarily have poor eyesight; the visual acuity of the killer whale underwater, for example, is equal to that of a cat on land. Echolocation research has mainly concentrated on the bottlenose dolphin; similar sounds emitted by other species of cetaceans have been assumed to be echolocation sounds.



These unusual sensory abilities of whales have given rise to considerable speculation as to their intelligence. Cetaceans, in fact, are the only animals (other than the elephant) to possess a brain larger than a human does. For example, the brain of an average adult sperm whale weighs 9 kg (20 lb); of an elephant, 4.5 kg (10 lb); of a bottle-nosed dolphin, 1.7 kg (3.75 lb); and of a human being, 1.35 kg (3 lb). The significance of this brain size in terms of intelligence, however, is not known. In captivity, whales and dolphins show considerable capacity for learning and a well-developed sense of play and caring for others; but because it is extremely difficult to observe cetaceans in their natural state, little is known of their social interaction and communication in the wild. Some species exist for the most part as solitary animals, whereas others occur in family groups or in pods or schools that range from six or seven to hundreds of individuals. Killer whales exhibit a high degree of cooperative hunting, an activity that would indicate active communication among pod members. Despite much popular writing about the cetacean “language” and attempts at human-cetacean communication, such claims remain unproved.

Whales and Humans

Half of all whale species can be considered rare as opposed to endangered; that is, individuals of such species are not (and perhaps never were) very numerous. Many of the unusual beaked whales fall into this category. Most of the commercially valuable whale species, however, are indeed endangered. These include the blue, sei, Bryde's, fin, bowhead, humpback, and right whales and many stocks of sperm whales. They are endangered as a result of overhunting, to the point that the animals killed outnumber the animals being born. If unregulated whaling continues, these species will be wiped out entirely; the numbers of some of them are perhaps already so low that they can never recover.

With the possible exception of the eastern Pacific grey whale, numbers of whales of any existing species are at best only estimated. Therefore, because of the drastic decline in commercial species, and because scientists do not yet fully understand the intricacies of whale behaviour and biology, conservationists worldwide are actively seeking a total ban on commercial whaling. The primary agency for regulation of commercial whaling is the International Whaling Commission (IWC), an organization of whaling and non-whaling nations that has been involved in whaling regulation since 1946. The stated purpose of the IWC is not primarily to conserve and preserve whales, but to manage whales for the orderly continuance of the whaling industry. Thus, the IWC has banned the hunting of certain species and has set quotas on others, and signatory nations are expected to abide by these quotas. Non-member nations, however, can and do operate outside such regulations—a serious threat to any conservation attempts by the IWC or individual countries.

Whales can endanger themselves when they become stranded on beaches as they cannot live long out of water. They may overheat or drown when the tide comes in and covers their blowholes. It is not fully understood why whales swim on to beaches but it has been suggested that a failure in their echolocation signalling can be caused by disease or parasites in the whales' ears.

Scientific classification: Whales belong to the order Cetacea. Toothed whales belong to the suborder Odontoceti, and baleen whales make up the suborder Mysticeti.




'Whale,' Microsoft® Encarta® 97 Encyclopedia. © 1993-1996 Microsoft Corporation. All rights reserved.












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