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Baleen whales (), also known as whalebone whales, are of the parvorder Mysticeti in the infraorder Cetacea (, and ), which use baleen plates (or "whalebone") in their mouths to sieve plankton from the water. Mysticeti comprises the families Balaenidae (right whale and Bowhead whale whales), Balaenopteridae (), Eschrichtiidae (the gray whale) and Cetotheriidae (the pygmy right whale). There are currently 16 species of baleen whales. While cetaceans were historically thought to have descended from Mesonychia, molecular evidence instead supports them as a clade of even-toed ungulates (Artiodactyla). Baleen whales split from (Odontoceti) around 34 million years ago.
Baleen whales range in size from the and pygmy right whale to the and blue whale, the largest known animal to have ever existed.
They are sexually dimorphic. Baleen whales can have streamlined or large bodies, depending on the feeding behavior, and two limbs that are modified into flippers. The fin whale is the fastest baleen whale, recorded swimming at . Baleen whales use their baleen to filter out food from the water by either lunge-feeding or skim-feeding. Baleen whales have fused neck vertebrae, and are unable to turn their heads at all. Baleen whales have two blowholes. Some species are well adapted for diving to great depths. They have a layer of fat, or blubber, under the skin to keep warm in the cold water.Although baleen whales are widespread, most species prefer the colder waters of the Arctic and Antarctic. Gray whales are specialized for feeding on bottom-dwelling . Rorquals are specialized at lunge-feeding, and have a streamlined body to reduce drag while accelerating. Right whales skim-feed, meaning they use their enlarged head to effectively take in a large amount of water and sieve the slow-moving prey. Males typically mate with more than one female (polygyny), although the degree of polygyny varies with the species. Male strategies for reproductive success vary between performing ritual displays (whale song) or lek mating. Calves are typically born in the winter and spring months and females bear all the responsibility for raising them. Mothers fasting for a relatively long period of time over the period of migration, which varies between species. Baleen whales produce a number of infrasound whale sound, notably the songs of the humpback whale.
The meat, blubber, baleen, and oil of baleen whales have traditionally been used by the indigenous peoples of the Arctic. Once relentlessly whaling by commercial industries for these products, cetaceans are now protected by international law. These protections have allowed their numbers to recover. However, the North Atlantic right whale is ranked critically endangered by the International Union for Conservation of Nature. Besides hunting, baleen whales also face threats from marine pollution and ocean acidification. It has been speculated that man-made sonar results in strandings. They have rarely been kept in captivity, and this has only been attempted with juveniles or members of one of the smallest species.
Cetotheriidae consists of only one living member: the pygmy right whale ( Caperea marginata). The first descriptions date back to the 1840s of bones and resembling a smaller version of the right whale, and was named Balaena marginata. In 1864, it was moved into the genus Caperea after a skull of another specimen was discovered. Six years later, the pygmy right whale was classified under the family Neobalaenidae. Despite its name, the pygmy right whale is more genetics similar to rorquals and gray whales than to right whales. A study published in 2012, based on bone structure, moved the pygmy right whale from the family Neobalaenidae to the family Cetotheriidae, making it a living fossil; Neobalaenidae was demoted to subfamily level as Neobalaeninae.
Rorquals consist of three genera ( Balaenoptera, Megaptera, and Eschrichtius) and 11 species: the fin whale ( B. physalus), the Sei whale ( B. borealis), Bryde's whale ( B. brydei), Eden's whale ( B. edeni), Rice's whale ( B. ricei), the blue whale ( B. musculus), the common minke whale ( B. acutorostrata), the Antarctic minke whale ( B. bonaerensis), Omura's whale ( B. omurai), the humpback whale ( M. novaeangliae), and the gray whale ( E. robustus). In a 2012 review of cetacean taxonomy, Alexandre Hassanin (of the Muséum National d'Histoire Naturelle) and colleagues suggested that, based on phylogenic criteria, there are four extant genera of rorquals. They recommend that the genus Balaenoptera be limited to the fin whale, have minke whales fall under the genus Pterobalaena, and have Rorqualus contain the Sei whale, Bryde's whale, Eden's whale (and by extension Rice's whale), the blue whale, and Omura's whale. The gray whale was formerly classified in its own family. The two populations, one in the Sea of Okhotsk and Sea of Japan and the other in Pacific Ocean are thought to be genetically and physiologically dissimilar. However, there is some discussion as to whether the gray whale should be classified into its own family, or as a rorqual, with recent studies favoring the latter.
Mysticetes are also known as baleen whales for their baleen, which they use to sieve plankton and other small organisms from the water. The term "baleen" (Middle English baleyn, ballayne, ballien, bellane, etc.) is an archaic word for "whale", which came from Old French , derived from the Latin word , derived itself from the Ancient Greek (phállaina).
Right whales got their name because of whaling preferring them over other species; they were essentially the "right whale" to catch.
Mystacodon selenensis is the earliest mysticete, dating back to 37 to 33 million years ago (mya) in the Late Eocene, and, like other early toothed mysticetes, or "archaeomysticetes", M. selenensis had heterodont used for suction feeding. Archaeomysticetes from the Oligocene are the Mammalodontidae ( Mammalodon and Janjucetus) from Australia. They were small with shortened rostra, and a primitive dental formula (). In baleen whales, it is thought that enlarged mouths adapted for suction feeding evolved before specializations for bulk filter feeding. In the toothed Oligocene mammalodontid Janjucetus, the symphysis is short and the mouth enlarged, the rostrum is wide, and the edges of the maxillae are thin, indicating an adaptation for suction feeding. The Aetiocetidae Chonecetus still had teeth, but the presence of a groove on the interior side of each mandible indicates the symphysis was elastic, which would have enabled rotation of each mandible, an initial adaptation for bulk feeding like in modern mysticetes.
The first toothless ancestors of baleen whales appeared before the first radiation in the late Oligocene. Eomysticetus and others like it showed no evidence in the skull of echolocation abilities, suggesting they mainly relied on their eyesight for navigation. The eomysticetes had long, flat rostra that lacked teeth and had blowholes located halfway up the dorsal side of the snout. Though the palate is not well-preserved in these specimens, they are thought to have had baleen and been filter feeders. Miocene baleen whales were preyed upon by larger predators like killer sperm whales and megalodon.
The lineages of rorquals and right whales split almost 20 mya. It is unknown where this occurred, but it is generally believed that they, like their descendants, followed plankton migrations. These primitive baleen whales had lost their dentition in favor of baleen, and are believed to have lived on a specialized benthic, plankton, or copepod diet like modern baleen whales. Baleen whales experienced their first radiation in the Middle Miocene. It is thought this radiation was caused by global climate change and major tectonic activity when Antarctica and Australia separated from each other, creating the Antarctic Circumpolar Current. Balaenopterids grew bigger during this time, with species like Balaenoptera sibbaldina perhaps rivaling the blue whale in terms of size, though other studies disagree that any baleen whale grew that large in the Miocene.
The increase in size is likely due to climate change which caused seasonally shifting accumulations of plankton in various parts of the world, necessitating travel over long distances, as well as the ability to feed on large baitballs to make such trips worthwhile. A 2017 analysis of body size based on data from the fossil record and modern baleen whales indicates that the evolution of gigantism in baleen whales occurred rather recently, within the last 3 million years. Before 4.5 million years ago, few baleen whales exceeded in length; the two largest Miocene species were less than in length. The initial evolution of baleen and filter feeding long preceded the evolution of gigantic body size, indicating the evolution of novel feeding mechanisms did not cause the evolution of gigantism. The formation of the Antarctic circumpolar current and its effects on global climate patterns is excluded as being causal for the same reason. Gigantism also was preceded by divergence of different mysticete lineages, meaning multiple lineages arrived at large size independently. It is possible the Plio-Pleistocene increase in seasonally intense , causing high-prey-density zones, led to gigantism.
Rorquals, needing to build speed to feed, have several adaptions for reducing drag, including a streamlined body; a small dorsal fin, relative to its size; and lack of external ears or long hair. The fin whale is the fastest among baleen whales, having been recorded travelling as fast as , and sustaining a speed of for an extended period. While feeding, the rorqual jaw expands to a volume that can be bigger than the whale itself; to do this, the mouth inflates. The inflation of the mouth causes the cavum ventrale, the throat pleats on the underside stretching to the bellybutton, to expand, increasing the amount of water that the mouth can store. The mandible is connected to the skull by dense fibers and cartilage (fibrocartilage), allowing the jaw to swing open at almost a 90° angle. The mandibular symphysis is also fibrocartilaginous, allowing the jaw to bend which lets in more water. To prevent stretching the mouth too far, rorquals have a sensory organ located in the middle of the jaw to regulate these functions.
Like other mammals, the skin of baleen whales has an epidermis, a dermis, a hypodermis, and connective tissue. The epidermis, the layer, is thick, along with connective tissue. The epidermis itself is only thick. The dermis, the layer underneath the epidermis, is also thin. The hypodermis, containing blubber, is the thickest part of the skin and functions as a means to conserve heat. Right whales have the thickest hypodermis of any cetacean, averaging , though, as in all whales, it is thinner around openings (such as the blowhole) and limbs. Blubber may also be used to store energy during times of fasting. The connective tissue between the hypodermis and muscles allows only limited movement to occur between them. Unlike toothed whales, baleen whales have vibrissae on the top of their head, stretching from the tip of the rostrum to the blowhole, and, in right whales, on the chin. Like other , they lack sebaceous gland and sweat glands glands.
The baleen of baleen whales are plates. They are made of a calcified, hard α-keratin material, a fiber-reinforced structure made of intermediate filaments (proteins). The degree of calcification varies between species, with the sei whale having 14.5% hydroxyapatite, a mineral that coats teeth and bones, whereas minke whales have 1–4% hydroxyapatite. In most mammals, keratin structures, such as wool, air-dry, but aquatic whales rely on calcium salts to form on the plates to stiffen them. Baleen plates are attached to the upper jaw and are absent in the mid-jaw, forming two separate combs of baleen. The plates decrease in size as they go further back into the jaw; the largest ones are called the "main baleen plates" and the smallest ones are called the "accessory plates". Accessory plates taper off into small hairs.
Unlike other whales (and most other mammals), the females are larger than the males. Sexual dimorphism is usually reversed, with the males being larger, but the females of all baleen whales are usually five percent larger than males. Sexual dimorphism is also displayed through whale song, notably in humpback whales where the males of the species sing elaborate songs. Male right whales have bigger callosities than female right whales. The males are generally more scarred than females which is thought to be because of aggression during mating season.
The heart of baleen whales functions similarly to other mammals, with the major difference being the size. The heart can reach , but is still proportional to the whale's size. The muscular wall of the ventricle, which is responsible for pumping blood out of the heart, can be thick. The aorta, an artery, can be thick. Their resting heart rate is 60 to 140 beats per minute (bpm), as opposed to the 60 to 100 bpm in humans.
When diving, their heart rate will drop to 4 to 15 bpm to conserve oxygen. Like toothed whales, they have a dense network of blood vessels (rete mirabile) which prevents heat-loss. Like in most mammals, heat is lost in their extremities, so, in baleen whales, warm blood in the arteries is surrounded by veins to prevent heat loss during transport. As well as this, heat inevitably given off by the arteries warms blood in the surrounding veins as it travels back into the core. This is otherwise known as countercurrent exchange. To counteract overheating while in warmer waters, baleen whales reroute blood to the skin to accelerate heat-loss. They have the largest (red and white ) of any mammal, measuring in diameter, as opposed to human's blood corpuscles.When sieved from the water, food is swallowed and travels through the esophagus where it enters a three-chambered stomach. The first compartment is known as the fore-stomach; this is where food gets ground up into an acidic liquid, which is then squirted into the main stomach. Like in humans, the food is mixed with hydrochloric acid and protein-digesting . Then, the partly digested food is moved into the third stomach, where it meets fat-digesting enzymes, and is then mixed with an alkaline liquid to neutralize the acid from the fore-stomach to prevent damage to the intestinal tract. Their intestinal tract is highly adapted to absorb the most nutrients from food; the walls are folded and contain copious blood vessels, allowing for a greater surface area over which digested food and water can be absorbed. Baleen whales get the water they need from their food; however, the salt content of most of their prey () is similar to that of seawater, whereas the salt content of a whale's blood is considerably lower (three times lower) than that of seawater. The whale kidney is adapted to excreting excess salt; however, while producing urine more concentrated than seawater, it wastes a lot of water which must be replaced.
Baleen whales have a relatively small brain compared to their body mass. Like other mammals, their brain has a large, folded cerebrum, the part of the brain responsible for memory and processing sensory information. Their cerebrum only makes up about 68% of their brain's weight, as opposed to human's 83%. The cerebellum, the part of the brain responsible for balance and coordination, makes up 18% of their brain's weight, compared to 10% in humans, which is probably due to the great degree of control necessary for constantly swimming. Necropsy on the brains of gray whales revealed iron oxide particles, which may allow them to find magnetic north like a compass.
Unlike most animals, whales are conscious breathers. All mammals sleep, but whales cannot afford to become unconscious for long because they may drown. They are believed to exhibit unihemispheric slow-wave sleep, in which they sleep with half of the brain while the other half remains active. This behavior was only documented in toothed whales until footage of a humpback whale sleeping (vertically) was shot in 2014.
It is largely unknown how baleen whales produce sound because of the lack of a melon and . Research has found that the larynx had U-shaped folds which are thought to be similar to vocal cords. They are positioned parallel to air flow, as opposed to the perpendicular vocal cords of terrestrial mammals. These may control air flow and cause vibrations. The walls of the larynx are able to contract which may generate sound with support from the arytenoid cartilages. The muscles surrounding the larynx may expel air rapidly or maintain a constant volume while diving.
The mysticete ear is adapted for hearing underwater, where it can hear sound frequencies as low as 7 hertz and as high as 22 kilohertz, distinct from odontocetes whose hearing is optimized for ultrasonic frequencies. It is largely unknown how sound is received by baleen whales. Unlike in toothed whales, sound does not pass through the lower jaw. The auditory meatus is blocked by connective tissue and an ear plug, which connects to the eardrum. The inner-ear bones are contained in the tympanic bulla, a bony capsule. However, this is attached to the skull, suggesting that vibrations passing through the bone is important. Sinuses may reflect vibrations towards the cochlea. It is known that when the fluid inside the cochlea is disturbed by vibrations, it triggers sensory hairs which send electric current to the brain, where vibrations are processed into sound.
Baleen whales have a small, yet functional, vomeronasal organ. This allows baleen whales to detect chemicals and released by their prey. It is thought that 'tasting' the water is important for finding prey and tracking down other whales. They are believed to have an impaired sense of smell due to the lack of the olfactory bulb, but they do have an olfactory tract. Baleen whales have few if any taste buds, suggesting they have lost their sense of taste. They do retain saltiness taste-buds suggesting that they can taste saltiness.
It is thought that plankton blooms dictate where whales migrate. Many baleen whales feed on the massive plankton blooms that occur in the cold, nutrient-rich waters of polar regions during the sunny spring and summer months. Baleen whales generally then migrate to calving grounds in tropical waters during the winter months when plankton populations are low. Migration is hypothesized to benefit calves in a number of ways. Newborns, born with underdeveloped blubber, would likely otherwise be killed by the cold polar temperatures. Migration to warmer waters may also reduce the risk of calves being predated on by .
Migratory movements may also reflect seasonally shifting patterns of productivity. California blue whales are hypothesized to migrate between dense patches of prey, moving from central California in the summer and fall, to the Gulf of California in the winter, to the central Baja California Pacific coast in spring.
There are two types of feeding behaviors: skim-feeding and lunge-feeding, but some species do both depending on the type and amount of food. Lunge-feeders feed primarily on Krill (krill), though some lunge feeders also prey on schools of fish. Skim-feeders, like bowhead whales, feed upon primarily smaller plankton such as . They feed alone or in small groups.
Baleen whales get the water they need from their food, and their kidneys excrete excess salt.The lunge-feeders are the rorquals. To feed, lunge-feeders expand the volume of their jaw to a volume bigger than the original volume of the whale itself. To do this, the mouth inflates, which causes the throat pleats to expand, increasing the amount of water that the mouth can store. Just before they ram the baitball, the jaw swings open at almost a 90° angle and bends which lets in more water. To prevent stretching the mouth too far, rorquals have a sensory organ located in the middle of the jaw to regulate these functions. Then they must decelerate. This process takes a lot of mechanical work and is only energy-effective when used against a large baitball. Lunge feeding is more energy-intensive than skim-feeding due to the acceleration and deceleration required.
The skim-feeders are right whales, gray whales, pygmy right whales, and sei whales (which also lunge feed). To feed, skim-feeders swim with an open mouth, filling it with water and prey. Prey must occur in sufficient numbers to trigger the whale's interest, be within a certain size range so that the baleen plates can filter it, and be slow enough so that it cannot escape. The "skimming" may take place on the surface, underwater, or even at the ocean's bottom, indicated by mud occasionally observed on right whales' bodies. Gray whales feed primarily on the ocean's bottom, feeding on benthic creatures.
Foraging efficiency for both lunge feeding and continuous ram filter feeding is highly dependent upon prey density. The efficiency of a blue whale lunge is approximately 30 times higher at krill densities of than at low krill densities of . Baleen whale have been observed seeking out highly specific areas within the local environment in order to forage at the highest density prey aggregations.
Many parasites and epibiotics latch onto whales, notably whale lice and . Almost all species of whale lice are specialized towards a certain species of whale, and there can be more than one species per whale. Whale lice eat dead skin, resulting in minor wounds in the skin. Whale louse infestations are especially evident in right whales, where colonies propagate on their callosities. Though not a parasite, whale barnacles latch onto the skin of a whale during their larval stage. However, in doing so it does not harm nor benefit the whale, so their relationship is often labeled as an example of commensalism. Some baleen whales will deliberately rub themselves on substrate to dislodge parasites.
Some species of barnacle, such as Conchoderma and whale barnacles, attach to the baleen plates, though this seldom occurs. A species of copepod, Balaenophilus unisetus, inhabits baleen plates of whales. A species of Antarctic diatom, Cocconeis ceticola, forms a biofilm on the skin, which takes a month to develop; this film causes minor damage to the skin. They are also plagued by internal parasites such as Anisakis, , , , and .
Most rorquals mate in warm waters in winter to give birth almost a year later.
Baleen whales have fibroelastic (connective tissue) penises, similar to those of artiodactyls. The tip of the penis, which tapers toward the end, is called the pars intrapraeputialis or terminal cone. The blue whale has the largest penis of any organism on the planet, typically measuring .
Accurate measurements of the blue whale are difficult to take because the whale's erect length can only be observed during mating. The penis on a right whale can be up to – the testes, at up to in length, in diameter, and weighing up to , are also the largest of any animal on Earth.
Commercial whaling was historically important as an industry well throughout the 19th and 20th centuries. Whaling was at that time a sizable European industry with ships from Britain, France, Spain, Denmark, the Netherlands, and Germany, sometimes collaborating to hunt whales in the Arctic. By the early 1790s, whalers, namely the British (Australian) and Americans, started to focus efforts in the South Pacific; in the mid-1900s, over 50,000 humpback whale were taken from the South Pacific.
The whale-watching industry and anti-whaling advocates argue that whaling catches "friendly" whales that are curious about boats, as these whales are the easiest to catch. This analysis claims that once the economic benefits of hotels, restaurants and other tourist amenities are considered, hunting whales is a net economic loss. This argument is particularly contentious in Iceland, as it has among the most-developed whale-watching operations in the world and the hunting of minke whales resumed in August 2003. Brazil, Argentina and South Africa argue that whale watching is a growing billion-dollar industry that provides more revenue than commercial whaling would provide. Peru, Uruguay, Australia, and New Zealand also support proposals to permanently forbid whaling south of the Equator, as Solor (an island of Indonesia) is the only place of the Southern Hemisphere that takes whales. Anti-whaling groups, such as the International Fund for Animal Welfare (IFAW), claim that countries which support a pro-whaling stance are damaging their economies by driving away anti-whaling tourists.
Commercial whaling was historically important for the world economy. All species were exploited, and as one type's stock depleted, another type was targeted. The scale of whale harvesting decreased substantially through the 1960s as all whale stocks had been depleted, and practically stopped in 1988 after the International Whaling Commission placed a moratorium which banned whaling for commercial use. Several species that were commercially exploited have rebounded in numbers; for example, gray whales may be as numerous as they were prior to whaling, making it the first marine mammal to be taken off the endangered species list. The Southern right whale was hunted to near extinction in the mid-to-late 20th century, with only a small (unknown) population around Antarctica. Because of international protection, the Southern right whale's population has been growing 7% annually since 1970.
Conversely, the eastern stock of North Atlantic right whale was extirpated from much of its former range, which stretched from the coast of North Africa to the North Sea and Iceland; it is thought that the entire stock consists of only ten individuals, making the eastern stock functionally extinct.Baleen whales continue to be harvested. Only three nations take whales: Iceland, Norway, and Japan. All these nations are part of the IWC, with Norway and Iceland rejecting the moratorium and continuing commercial whaling. Japan, being part of the IWC, whales under the Scientific Permit stated in Article VIII in the Convention for the Regulation of Whaling, which allows the taking of whales for scientific research. Japan has had two main research programs: the Joint Aquatic Resources Permit Application (JARPA) and the Japanese Research Program in the North (JARPN). JARPN is focused in the North Pacific and JARPA around the Antarctic. JARPA mainly caught Antarctic minke whales, catching nearly 7,000; to a far lesser extent, they also caught fin whales.
Baleen whales can also be affected by humans in more indirect ways. For species like the North Atlantic right whale, which migrates through some of the world's busiest shipping lanes, the biggest threat is from being struck by ships. The Lloyd's mirror effect results in low frequency propeller sounds not being discernible near the surface, where most accidents occur. Combined with spreading and acoustic shadow, the result is that the whale is unable to hear an approaching vessel before it has been run over or entrapped by the hydrodynamic forces of the vessel's passage. A 2014 study noted that a lower vessel speed correlated with lower collision rates. The ever-increasing amount of ocean noise, including sonar, drowns out the vocalizations produced by whales, notably in the blue whale which produces the loudest vocalization, which makes it harder for them to communicate. Blue whales stop producing foraging D calls once a mid-frequency sonar is activated, even though the sonar frequency range (1–8 kHz) far exceeds their sound production range (25–100 Hz).
Poisoning from toxic substances such as polychlorinated biphenyl (PCB) is generally low because of their low trophic level. However, can be a significant threat, especially to small populations; the already endangered Rice's whale was likely devastated by the Deepwater Horizon oil spill, with some estimates indicating a decline of up to 22% in the species.
Some baleen whales can become victims of bycatch, which is especially serious for North Atlantic right whales considering their small number. Right whales feed with a wide-open mouth, risking entanglement in any rope or net fixed in the water column. The rope wraps around their upper jaw, flippers and tail. Some are able to escape, but others remain entangled. If observers notice, they can be successfully disentangled, but others die over a period of months. Other whales, such as humpback whales, can also be entangled.
The Mito Aquarium in Numazu, Shizuoka, Japan, housed three minke whales in the nearby bay enclosed by nets. One survived for three months, another (a calf) survived for two weeks, and another was kept for over a month before breaking through the nets.
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