Whales are descendants of land-dwelling mammals of the artiodactyl order (even-toed ungulates). They are relevant to the Indohyus, an wiped out chevrotain-like ungulate, from which they will split approximately 48 mil years ago.|19||20| Primitive cetaceans, or archaeocetes, first took to the sea about 49 million years ago to become fully aquatic 5-10 mil years later. What defines an archaeocete is the occurrence of anatomical features exceptional to cetaceans, alongside other primitive features not found in modern cetaceans, such as visible legs or asymmetrical teeth.|21||22||23||9| Their features became adapted for living in the marine environment. Major physiological changes included their hearing set-up that channeled shocks from the jaw to the earbone (Ambulocetus 49 mya), a streamlined body and the growth of flukes on the tail (Protocetus 43 mya), the immigration of the nostrils toward the most notable of the cranium (blowholes), as well as the modification of the forelimbs in flippers (Basilosaurus 35 mya), and the shrinking and inevitable disappearance of the hind hands or legs (the first odontocetes and mysticetes 34 mya).|24||25||26|
Whale morphology shows a number of examples of convergent evolution, the most obvious being the streamlined fish-like body shape.|27| Other examples include the utilization of echolocation for hunting in low light conditions - which is the same hearing adaptation used by bats - and, in the rorqual whales, jaw modifications, similar to those found in pelicans, that enable engulfment feeding.|28|
Today, the best living relatives of cetaceans are the hippopotamuses; these talk about a semi-aquatic ancestor that branched off from other artiodactyls some 60 mya.|9| Around 40 mya, a common ancestor between the two branched off into cetacea and anthracotheres; nearly all anthracotheres became extinct at the end from the Pleistocene 2 . 5 mya, eventually leaving only one making it through lineage - the hippopotamus.|29|
Whales split into two separate parvorders around thirty four mya - the baleen whales (Mysticetes) and the toothed whales (Odontocetes).
Whales have torpedo shaped physiques with non-flexible necks, arms and legs modified into flippers, nonexistent external ear flaps, a big tail fin, and flat heads (with the exception to this rule of monodontids and ziphiids). Whale skulls have little eye orbits, long snouts (with the exception of monodontids and ziphiids) and eyes placed on the edges of its head. Whales range in size from the 2 . 6-metre (8. 5 ft) and 135-kilogram (298 lb) dwarf sperm whale for the 34-metre (112 ft) and 190-metric-ton (210-short-ton) blue whale. Overall, they tend to dwarf other cetartiodactyls; the black whale is the largest person on earth. Several species have female-biased sexual dimorphism, while using females being larger than the males. One exception is by using the sperm whale, which has males larger than the females.|33||34|
Odontocetes, like the sperm whale, possess pearly whites with cementum cells overlying dentine cells. Unlike human teeth, which are composed mainly of enamel on the component of the tooth outside of the gum, whale teeth own cementum outside the gum. Only in larger whales, the place that the cementum is worn aside on the tip of the tooth, does enamel show. Mysticetes have large whalebone, compared to teeth, made of keratin. Mysticetes have two blowholes, although Odontocetes contain only one.|35|
Breathing involves expelling dull air from the blowhole, developing an upward, steamy spout, followed by inhaling fresh air in the lungs; a humpback whale's lungs can hold about a few, 000 litres of air. Spout shapes differ amongst species, which facilitates identity.|36||37|
The cardiovascular system of a whale weighs about 180-200 kg. It is 640 times bigger than a the heart. The heart of the unknown whale is the largest of any animal,|38| and the walls of the arterial blood vessels in the heart have been described as being "as thick since an iPhone 6 Plus is long".|39|
All whales have a thick layer of blubber. In species that live near the poles, the blubber can be as thick seeing that 11 inches. This blubber can help with buoyancy (which is useful for a 100-ton whale), safety to some extent as predators could have a hard time getting through a dense layer of fat, and energy for fasting when migrating to the equator; the principal usage for blubber is usually insulation from the harsh climate. It can constitute as much as 50% of a whale's body weight. Lower legs are born with just a thin layer of blubber, but some species compensate for this with thick lanugos.|40||41|
Whales have a two- to three-chambered stomach that may be similar in structure to terrestrial carnivores. Mysticetes include a proventriculus as an extension with the oesophagus; this contains pebbles that grind up meals. They also have fundic and pyloric chambers.
Whales have two flippers on the front, and a end fin. These flippers have four digits. Although whales do not possess fully developed hind limbs, some, such as the semen whale and bowhead whale, possess discrete rudimentary appendages, which may contain feet and digits. Whales are quickly swimmers in comparison to seals, which in turn typically cruise at 5-15 kn, or 9-28 kms per hour (5. 6-17. 4 mph); the fin whale, in comparison, can travel for speeds up to 47 kilometres per hour (29 mph) as well as the sperm whale can reach speeds of 35 kms per hour (22 mph). The fusing of the neck backbone, while increasing stability when ever swimming at high rates of speed, decreases flexibility; whales cannot turn their heads. The moment swimming, whales rely on their particular tail fin propel these people through the water. Flipper movements is continuous. Whales go swimming by moving their end fin and lower body up and down, propelling themselves through vertical movement, while all their flippers are mainly used for steerage. Some species log out of your water, which may allow them to travelling faster. Their skeletal anatomy allows them to be quickly swimmers. Most species possess a dorsal fin.|43||44|
Whales are adapted for diving to great depths. In addition to their streamlined bodies, they can slow all their heart rate to conserve oxygen; blood vessels is rerouted from cells tolerant of water pressure to the heart and head among other organs; haemoglobin and myoglobin store air in body tissue; and in addition they have twice the concentration of myoglobin than haemoglobin. Before going on long dives, many whales exhibit a behaviour known as sounding; they will stay close to the surface for any series of short, shallow divine while building their o2 reserves, and then make a sound dive.
The whale ear has certain adaptations to the marine environment. In humans, the middle headsets works as an impedance equalizer between the outside air's low impedance and the cochlear fluid's high impedance. In whales, and other marine mammals, there is absolutely no great difference between the external and inner environments. Rather than sound passing through the outer hearing to the middle ear, whales receive sound through the throat, from which it passes by using a low-impedance fat-filled cavity to the inner ear.|46| The whale ear is acoustically isolated from the head by air-filled sinus pouches, which allow for greater directional hearing underwater.|47| Odontocetes send out high frequency clicks from an organ known as the melon. This melon contains fat, and the skull of any such creature containing a melon will have a large depression. The melon size may differ between species, the bigger the greater dependent they are of it. A beaked whale for example possesses a small bulge sitting on top of its skull, whereas a sperm whale's head is filled up mainly with the melon.|48||49||50||51|
The whale eye is relatively small for its size, however they do retain a good amount of eyesight. As well as this, the eyes of a whale are put on the sides of their head, so their eye-sight consists of two fields, rather than binocular view like human beings have. When belugas surface area, their lens and cornea correct the nearsightedness that results from the refraction of light; that they contain both rod and cone cells, meaning they can see in both poor and bright light, but they own far more rod cells than they do cone cells. Whales do, however , lack brief wavelength sensitive visual tones in their cone cells articulating a more limited capacity for colouring vision than most mammals.|52| Most whales have slightly flattened eyeballs, enlarged pupils (which get smaller as they surface to prevent damage), slightly flattened corneas and a tapetum lucidum; these kinds of adaptations allow for large amounts of sunshine to pass through the eye and, consequently , a very clear image of the surrounding area. They also have glands for the eyelids and outer corneal layer that act as security for the cornea.|53||54|
The olfactory flambeau are absent in toothed whales, suggesting that they have no sense of smell. Some whales, such as the bowhead whale, possess a vomeronasal organ, which does imply that they can "sniff out" krill.|55|
Whales are not considered to have a good sense of taste, as their taste buds will be atrophied or missing entirely. However , some toothed whales have preferences between different kinds of fish, indicating some sort of attachment to taste. The existence of the Jacobson's organ implies that whales can smell food once inside their mouth area, which might be similar to the sensation of taste.
2019-02-11 12:41:28 * 2019-02-11 03:42:30
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