Whale Species
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Taxonomy
See also: List of whale species.
Cetaceans are divided into two suborders:
The largest suborder, Mysticeti (baleen whales) are characterized by baleen, a sieve-like structure in the upper jaw made of keratin, which it uses to filter plankton from the water.
Odontoceti (toothed whales) bear sharp teeth for hunting. Odontoceti also include dolphins and porpoises.
Both cetaceans and artiodactyl are now classified under the super-order Cetartiodactyla which includes both whales and hippopotamuses. Whales are the hippopotamuss closest living relatives.[5]
Evolution
Ambulocetus natans – a primitive whale
See also: Evolution of cetaceans .
All cetaceans, including whales, dolphins and porpoises, are descendants of land-living mammals of the Artiodactyl order (even-toed ungulates). Both are related to the Indohyus (an extinct semi-aquatic deer-like ungulate) from which they split around 54 million years ago.[6][7] Primitive whales probably first took to the sea about 50 million years ago and became fully aquatic about 5-10 million years later.[8]
Anatomy
Like all mammals, whales breathe air, are warm-blooded, nurse their young with milk from mammary glands, and have body hair.[citation needed]
Beneath the skin lies a layer of fat called blubber, which stores energy and insulates the body. Whales have a spinal column, a vestigial pelvic bone, and a four-chambered heart. The neck vertebrae are typically fused, trading flexibility for stability during swimming.[citation needed]
Blowhole(s)
Features of a blue whale
Whales breathe via blowholes; baleen whales have two and toothed whales have one. These are located on the top of the head, allowing the animal to remain mostly submerged whilst breathing. Breathing involves expelling excess water from the blowhole, forming an upward spout, followed by inhaling air into the lungs. Spout shapes differ among species and can help with identification.
Appendages
The body shape is fusiform and the modified forelimbs, or fins, are paddle-shaped. The end of the tail is composed of two flukes, which propel the animal by vertical movement, as opposed to the horizontal movement of a fish tail. Although whales do not possess fully developed hind limbs, some (such as sperm whales and baleen whales) possess discrete rudimentary appendages, which may even have feet and digits. Most species have a dorsal fin.[citation needed]
Dentition
Toothed whales, 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 has been worn away on the tip of the tooth, does enamel show.[9]
Instead of teeth, Baleen whales have a row of plates on the upper side of their jaws that resemble the “teeth” of a comb.
Ears
The whale ear has specific adaptations to the marine environment. In humans, the middle ear works as an impedance matcher between the outside airs low impedance and the cochlear fluids high impedance. In aquatic mammals such as whales, however, there is no great difference between the outer and inner environments. Instead of sound passing through the outer ear to the middle ear, whales receive sound through the throat, from which it passes through a low-impedance fat-filled cavity