Whale
Whales are a widely distributed and diverse group of fully aquatic placental marine mammals. As an informal and colloquial grouping, they correspond to large members of the infraorder Cetacea, i.e. all cetaceans apart from dolphins and porpoises. Dolphins and porpoises may be considered whales from a formal, cladistic perspective. Whales, dolphins and porpoises belong to the order Cetartiodactyla, which consists of even-toed ungulates. Their closest non-cetacean living relatives are the hippopotamuses, from which they and other cetaceans diverged about 54 million years ago. The two parvorders of whales, baleen whales and toothed whales, are thought to have had their last common ancestor around 34 million years ago. Mysticetes include four extant families: Balaenopteridae, Balaenidae, Cetotheriidae, and Eschrichtiidae. Odontocetes include the Monodontidae, Physeteridae, Kogiidae, and Ziphiidae, as well as the six families of dolphins and porpoises which are not considered whales in the informal sense.
Whales are fully aquatic, open-ocean animals: they can feed, mate, give birth, suckle and raise their young at sea. Whales range in size from the and dwarf sperm whale to the and blue whale, which is the largest known animal that has ever lived. The sperm whale is the largest toothed predator on Earth. Several whale species exhibit sexual dimorphism, in that the females are larger than males.
Baleen whales have no teeth; instead, they have plates of baleen, fringe-like structures that enable them to expel the huge mouthfuls of water they take in while retaining the krill and plankton they feed on. Because their heads are enormous—making up as much as 40% of their total body mass—and they have throat pleats that enable them to expand their mouths, they are able to take huge quantities of water into their mouth at a time. Baleen whales also have a well-developed sense of smell.
Toothed whales, in contrast, have conical teeth adapted to catching fish or squid. They also have such keen hearing—whether above or below the surface of the water—that some can survive even if they are blind. Some species, such as sperm whales, are particularly well adapted for diving to great depths to catch squid and other favoured prey.
Whales evolved from land-living mammals, and must regularly surface to breathe air, although they can remain underwater for long periods of time. Some species, such as the sperm whale, can stay underwater for up to 90 minutes. They have blowholes located on top of their heads, through which air is taken in and expelled. They are warm-blooded, and have a layer of fat, or blubber, under the skin. With streamlined fusiform bodies and two limbs that are modified into flippers, whales can travel at speeds of up to 20 knots, though they are not as flexible or agile as seals. Whales produce a great variety of vocalizations, notably the extended songs of the humpback whale. Although whales are widespread, most species prefer the colder waters of the Northern and Southern Hemispheres and migrate to the equator to give birth. Species such as humpbacks and blue whales are capable of travelling thousands of miles without feeding. Males typically mate with multiple females every year, but females only mate every two to three years. Calves are typically born in the spring and summer; females bear all the responsibility for raising them. Mothers in some species fast and nurse their young for one to two years.
Once relentlessly hunted for their products, whales are now protected by international law. The North Atlantic right whales nearly became extinct in the twentieth century, with a population low of 450, and the North Pacific grey whale population is ranked Critically Endangered by the IUCN. Besides the threat from whalers, they also face threats from bycatch and marine pollution. The meat, blubber and baleen of whales have traditionally been used by indigenous peoples of the Arctic. Whales have been depicted in various cultures worldwide, notably by the Inuit and the coastal peoples of Vietnam. Whales feature in literature and film, as with the great white whale in Herman Melville's novel Moby-Dick. Small whales, such as belugas, are sometimes kept in captivity, but breeding success has been poor and the animals often die within a few months of capture. Whale watching has become a form of tourism around the world.
Etymology and definitions
The word "whale" comes from the Old English hwæl, from Proto-Germanic *hwalaz, from Proto-Indo-European *kwal-o-, meaning "large sea fish". The etymology is unrelated to the country of Wales.The term "whale" is sometimes used to include dolphins and porpoises, acting as a synonym for Cetacea. Six species of oceanic dolphins have the word "whale" in their name, collectively known as blackfish: the orca or killer whale; the melon-headed whale; the pygmy killer whale; the false killer whale; and the two species of pilot whales.
The term "Great Whales" covers those species currently regulated by the International Whaling Commission: the Odontoceti family Physeteridae ; and the Mysticeti families Balaenidae, Eschrichtiidae, and some of the Balaenopteridae.
Taxonomy and evolution
Phylogeny
The whales are part of the largely terrestrial mammalian clade Laurasiatheria. The immediate clade is Cetacea, but if the dolphins and porpoises are excluded, the whales are paraphyletic. If on the other hand the dolphins and porpoises are accepted as whales, then the sensu lato group is a clade, the Cetacea. The phylogenetic tree shows the relationships of whales and other mammals, with whale groups marked in green boldface.Cetaceans are divided into two parvorders. The larger parvorder, Mysticeti, is characterized by the presence of baleen, a sieve-like structure in the upper jaw made of keratin, which it uses to filter plankton, among others, from the water. Odontocetes are characterized by bearing sharp teeth for hunting, as opposed to their counterparts' baleen.
Evolution
Whales are descendants of land-dwelling mammals of the artiodactyl order. They are related to the Indohyus, an extinct chevrotain-like ungulate, from which they split approximately 48 million years ago. Primitive cetaceans, or archaeocetes, first took to the sea approximately 49 million years ago and became fully aquatic 5–10 million years later. What defines an archaeocete is the presence of anatomical features exclusive to cetaceans, alongside other primitive features not found in modern cetaceans, such as visible legs or asymmetrical teeth. Their features became adapted for living in the marine environment. Major anatomical changes included their hearing set-up that channeled vibrations from the jaw to the earbone, a streamlined body and the growth of flukes on the tail, the migration of the nostrils toward the top of the cranium, and the modification of the forelimbs into flippers, and the shrinking and eventual disappearance of the hind limbs.Whale morphology shows several examples of convergent evolution, the most obvious being the streamlined fish-like body shape. Other examples include the use of echolocation for hunting in low light conditions — which is the same hearing adaptation used by bats — and, in the rorqual whales, jaw adaptations, similar to those found in pelicans, that enable engulfment feeding.
Today, the closest living relatives of cetaceans are the hippopotamuses; these share a semi-aquatic ancestor that branched off from other artiodactyls some 60 mya. Around 40 mya, a common ancestor between the two branched off into cetacea and anthracotheres; nearly all anthracotheres became extinct at the end of the Pleistocene 2.5 mya, eventually leaving only one surviving lineage – the hippopotamus.
Whales split into two separate parvorders around 34 mya – the baleen whales and the toothed whales.
Biology
Anatomy
Whales have torpedo-shaped bodies with non-flexible necks, limbs modified into flippers, non-existent external ear flaps, a large tail fin, and flat heads. Whale skulls have small eye orbits, long snouts and eyes placed on the sides of its head. Whales range in size from the and dwarf sperm whale to the and blue whale. Overall, they tend to dwarf other cetartiodactyls; the blue whale is the largest creature on Earth. Several species have female-biased sexual dimorphism, with the females being larger than the males. One exception is with the sperm whale, which has males larger than the females.Odontocetes, such as the sperm whale, possess teeth with cementum cells overlying dentine cells. 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. Only in larger whales, where the cementum is worn away on the tip of the tooth, does enamel show. Mysticetes have large whalebone, as opposed to teeth, made of keratin. Mysticetes have two blowholes, whereas Odontocetes contain only one.
Breathing involves expelling stale air from the blowhole, forming an upward, steamy spout, followed by inhaling fresh air into the lungs; a humpback whale's lungs can hold about of air. Spout shapes differ among species, which facilitates identification.
All whales have a thick layer of blubber. In species that live near the poles, the blubber can be as thick as. This blubber can help with buoyancy, protection to some extent as predators would have a hard time getting through a thick layer of fat, and energy for fasting when migrating to the equator; the primary usage for blubber is insulation from the harsh climate. It can constitute as much as 50% of a whale's body weight. Calves are born with only a thin layer of blubber, but some species compensate for this with thick lanugos.
Whales have a two- to three-chambered stomach that is similar in structure to those of terrestrial carnivores. Mysticetes contain a proventriculus as an extension of the oesophagus; this contains stones that grind up food. They also have fundic and pyloric chambers.