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Baleen whale

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Baleen whales(/bəˈliːn/), also known aswhalebone whales,aremarine mammalsof theparvorderMysticetiin the infraorderCetacea(whales,dolphinsandporpoises), which usekeratinaceousbaleenplates (or "whalebone" ) in their mouths to sieveplanktoniccreatures from the water. Mysticeti comprises thefamiliesBalaenidae(rightandbowheadwhales),Balaenopteridae(rorquals),Eschrichtiidae(thegray whale) andCetotheriidae(thepygmy right whale). There are currently 16speciesof baleen whales. While cetaceans were historically thought to have descended frommesonychians,molecularevidence instead supports them as acladeofeven-toed ungulates(Artiodactyla). Baleen whales split fromtoothed whales(Odontoceti) around 34million years ago.

Baleen whales
Temporal range:late Eocene–Present
A humpback whale breaching
Clockwise from top left:humpback whale,gray whale,North Atlantic right whale,andcommon minke whale
Scientific classificationEdit this classification
Domain: Eukaryota
Kingdom: Animalia
Phylum: Chordata
Class: Mammalia
Order: Artiodactyla
Infraorder: Cetacea
Parvorder: Mysticeti
Cope 1891
Subgroups
Diversity
16 species
Synonyms
  • Mystacoceti

Baleen whales range in size from the 6 m (20 ft) and 3,000 kg (6,600 lb)pygmy right whaleto the 31 m (102 ft) and 190 t (210 short tons)blue whale,thelargest known animalto have ever existed.[1][2]They aresexually dimorphic.Baleen whales can have streamlined or large bodies, depending on the feeding behavior, and two limbs that are modified intoflippers.Thefin whaleis the fastest baleen whale, recorded swimming at 10 m/s (36 km/h; 22 mph). Baleen whales use theirbaleen platesto filter out food from the water by eitherlunge-feedingor skim-feeding. Baleen whales have fusedneck vertebrae,and are unable to turn their heads at all. Baleen whales have twoblowholes.Some species are well adapted for diving to great depths. They have a layer of fat, orblubber,under the skin to keep warm in the cold water.

Although baleen whales are widespread, most species prefer the colder waters of theArcticandAntarctic.Gray whales are specialized for feeding on bottom-dwellingcrustaceans.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 forreproductive successvary between performingritual displays(whale song) orlek mating.Calves are typically born in the winter and spring months and females bear all the responsibility for raising them. Mothersfastfor a relatively long period of time over the period of migration, which varies between species. Baleen whales produce a number ofinfrasonicvocalizations,notably the songs of thehumpback whale.

The meat, blubber, baleen, and oil of baleen whales have traditionally been used by theindigenous peoples of the Arctic.Once relentlesslyhuntedby commercial industries for these products, cetaceans are now protected by international law. These protections have allowed their numbers to recover. However, theNorth Atlantic right whaleis ranked critically endangered by theInternational Union for Conservation of Nature.Besides hunting, baleen whales also face threats frommarine pollutionandocean acidification.It has been speculated thatman-made sonarresults instrandings.They have rarely been kept in captivity, and this has only been attempted with juveniles or members of one of the smallest species.

Taxonomy

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See also:List of cetaceansandList of extinct cetaceans

Baleen whales arecetaceansclassified under theparvorderMysticeti, and consist of fourextantfamilies:Balaenidae(right whales),Balaenopteridae(rorquals),Eschrichtiidae(gray whale) andCetotheriidae(pygmy right whale). Balaenids are distinguished by their enlarged head and thickblubber,[3][4]while rorquals and gray whales generally have a flat head, long throat pleats, and are more streamlined than Balaenids. Rorquals also tend to be longer than the latter.[5]Cetaceans (whales,dolphins,andporpoises) andartiodactylsare now classified under the orderCetartiodactyla,often still referred to as Artiodactyla (given that the cetaceans are deeply nested with the artiodactyls). The closest living relatives to baleen whales are toothed whales both from the infraorderCetacea.[6]

Classification

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Mysticeti
Phylogeny of baleen whales according to a 2019 analysis by McGowen and colleagues,[8]with the Bryde's whalecomplexexpanded following Rosel et al (2021).[7]

Balaenidae consists of two genera:Eubalaena(right whales) andBalaena(thebowhead whale,B. mysticetus). Balaenidae was thought to have consisted of only one genus until studies done through the early 2000s reported that bowhead whales and right whales aremorphologically(different skull shape) andphylogenicallydifferent. According to a study done by H. C. Rosenbaum (of theAmerican Museum of Natural History) and colleagues, the North Pacific (E. japonica) and Southern right (E. australis) whales are more closely related to each other than to the North Atlantic right whale (E. glacialis).[9]

Cetotheriidae consists of only one living member: the pygmy right whale (Caperea marginata). The first descriptions date back to the 1840s of bones andbaleen platesresembling a smaller version of the right whale, and was namedBalaena marginata.In 1864, it was moved into the genusCapereaafter a skull of another specimen was discovered. Six years later, the pygmy right whale was classified under the family Neobalaenidae.[10]Despite its name, the pygmy right whale is moregeneticallysimilar to rorquals and gray whales than to right whales.[11]A study published in 2012, based on bone structure, moved the pygmy right whale from the family Neobalaenidae to the family Cetotheriidae, making it aliving fossil;Neobalaenidae was demoted to subfamily level as Neobalaeninae.[12]

Rorquals consist of three genera (Balaenoptera,Megaptera,andEschrichtius) and 11 species: thefin whale(B. physalus), theSei whale(B. borealis),Bryde's whale(B. brydei),Eden's whale(B. edeni),Rice's whale(B. ricei), theblue whale(B. musculus), thecommon minke whale(B. acutorostrata), theAntarctic minke whale(B. bonaerensis),Omura's whale(B. omurai), thehumpback whale(M. novaeangliae), and thegray whale(E. robustus). In a 2012 review of cetacean taxonomy, Alexandre Hassanin (of theMuséum National d'Histoire Naturelle) and colleagues suggested that, based on phylogenic criteria, there are four extant genera of rorquals. They recommend that the genusBalaenopterabe limited to the fin whale, have minke whales fall under the genusPterobalaena,and haveRorqualuscontain the Sei whale, Bryde's whale, Eden's whale (and by extension Rice's whale), the blue whale, and Omura's whale.[13]The gray whale was formerly classified in its own family. The two populations, one in theSea of OkhotskandSea of Japanand the other ineastern Pacificare thought to be genetically and physiologically dissimilar.[14]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.[15][16][17][18]

Etymology

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The taxonomic name "Mysticeti" (/ˌmɪstɪˈsiːtaɪ/) apparently derives from a translation error in early copies ofAristotle'sHistoria Animalium(inAncient Greek), in which "ὁ μῦς τὸ κῆτος"(ho mus to kētos,"the mouse, the whale so called" ) was mistakenly translated as "ὁ μυστικῆτος"(ho mustikētos,"the Mysticetus" ), which D. W. Rice (of theSociety for Marine Mammalogy) inRice 1998assumed was an ironic reference to the animals' great size.[19]An alternate name for the parvorder is "Mystacoceti" (from Greekμύσταξ"mustache" +κῆτος"whale" ), which, although obviously more appropriate and occasionally used in the past, has been superseded by "Mysticeti" (junior synonym).[19]

Mysticetes are also known as baleen whales for theirbaleen,which they use to sieveplanktonand other small organisms from the water. The term "baleen" (Middle Englishbaleyn, ballayne, ballien, bellane,etc.) is an archaic word for "whale", which came fromOld Frenchbaleine,derived from theLatinwordbalæna,derived itself from theAncient Greekφάλλαινα(phállaina).[20]

Right whales got their name because ofwhalerspreferring them over other species; they were essentially the "right whale" to catch.[21]

Differences between families

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Baleen whales vary considerably in size and shape, depending on their feeding behavior.

Rorquals use throat pleats to expand their mouths, which allow them to feed more effectively. However, rorquals need to build up water pressure in order to expand their mouths, leading to a lunge-feeding behavior. Lunge-feeding is where a whale rams abait ball(a swarm ofsmall fish) at high speed. Rorquals generally have streamlined physiques to reduce drag in the water while doing this.[22] Balaenids rely on their huge heads, as opposed to the rorquals' throat pleats, to feed effectively. This feeding behavior allows them to grow very big and bulky, without the necessity for a streamlined body. They havecallosities,unlike other whales, with the exception of the bowhead whale.[23]Rorquals have a higher proportion of muscle tissue and tend to be negatively buoyant, whereas right whales have a higher proportion of blubber and are positively buoyant.[24]Gray whales are easily distinguished from the other rorquals by their sleet-gray color, dorsal ridges (knuckles on the back), and their gray-white scars left from parasites. As with the other rorquals, their throat pleats increase the capacity of their throats, allowing them to filter larger volumes of water at once. Gray whales are bottom-feeders, meaning they sift through sand to get their food. They usually turn on their sides, scoop up sediment into their mouths and filter outbenthiccreatures likeamphipods,which leave noticeable marks on their heads.[25]The pygmy right whale is easily confused with minke whales because of their similar characteristics, such as their small size, dark gray tops, light gray bottoms, and light eye patches.[23]

The three baleen whale families
Eschrichtius,Balaenopteridae
Eubalaena,Balaenidae
Megaptera,Balaenopteridae
Balaena,Balaenidae
Cetotheriidae
Balaenoptera,Balaenopteridae
List of mysticetes

Evolutionary history

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Main article:Evolution of cetaceans
Restoration ofJanjucetus hunderi

Molecular phylogeny suggests MysticetisplitfromOdontoceti(toothed whales) between 26 and 17 million years ago between the lateOligoceneor middleMiocene,but the earliest Mysticeti fossils date to at least 34 million years ago.[30]Theirevolutionary linkto archaic toothed cetaceans (Archaeoceti) remained unknown until the extinctJanjucetus hunderiwas discovered in the early 1990s inVictoria, Australia.While, unlike a modern baleen whale,Janjucetuslacked baleen in its jaw, the anatomy shows sufficient similarity to baleen whales. It appears to have had very limited apparentbiosonarcapabilities. Its jaw contained teeth, withincisorsandcaninesbuilt for stabbing andmolarsandpremolarsbuilt for tearing. These early mysticetes were exceedingly small compared to modern baleen whales, with species likeMammalodonmeasuring no greater than 3 meters (10 ft). It is thought that their size increased with their dependence on baleen.[31]However, the discovery of a skull of the toothedLlanocetus,the second-oldest mysticete, yielded a total length of 8 meters (26 ft), indicating filter feeding was not a driving feature in mysticete evolution.[32]The discovery ofJanjucetusand others like it suggests that baleen evolution went through several transitional phases.[33]Species likeMammalodon colliverihad little to no baleen, while later species likeAetiocetus weltonihad both baleen and teeth, suggesting they had limited filter feeding capabilities; later genera likeCetotheriumhad no teeth in their mouth, meaning they were fully dependent on baleen and could only filter feed.[34]However, the 2018 discovery of the toothlessMaiabalaenaindicates some lineages evolved toothlessness before baleen.[35]

Archaeomysticetes, likeJanjucetus,had teeth.

Mystacodon selenensisis the earliest mysticete, dating back to 37 to 33 million years ago (mya) in theLate Eocene,and, like other early toothed mysticetes, or "archaeomysticetes",M. selenensishadheterodont dentitionused for suction feeding.[27]Archaeomysticetes from theOligoceneare theMammalodontidae(MammalodonandJanjucetus) fromAustralia.They were small with shortenedrostra,and a primitivedental formula(3.1.4.33.1.4.3).[36]In baleen whales, it is thought that enlarged mouths adapted for suction feeding evolved before specializations for bulkfilter feeding.In the toothed Oligocene mammalodontidJanjucetus,thesymphysisis short and the mouth enlarged, the rostrum is wide, and the edges of themaxillaeare thin, indicating an adaptation for suction feeding. TheaetiocetidChonecetusstill had teeth, but the presence of a groove on the interior side of eachmandibleindicates the symphysis was elastic, which would have enabled rotation of each mandible, an initial adaptation for bulk feeding like in modern mysticetes.[37]

The first toothless ancestors of baleen whales appeared before the firstradiationin the late Oligocene.[38]Eomysticetusand 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 thedorsal sideof the snout. Though thepalateis not well-preserved in these specimens, they are thought to have had baleen and been filter feeders.[36][39]Miocenebaleen whales were preyed upon by larger predators likekiller sperm whalesandmegalodon.[40]

Megalodonhunted Miocene baleen whales

Thelineagesof 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, orcopepoddiet like modern baleen whales. Baleen whales experienced their first radiation in themid-Miocene.It is thought this radiation was caused by global climate change and majortectonicactivity when Antarctica and Australia separated from each other, creating theAntarctic Circumpolar Current.[41]Balaenopterids grew bigger during this time, with species likeBalaenoptera sibbaldinaperhaps rivaling the blue whale in terms of size,[28]though other studies disagree that any baleen whale grew that large in the Miocene.[42]

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.[43]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 10 meters (33 ft) in length; the two largest Miocene species were less than 13 m (43 ft) in length.[44]The initial evolution of baleen and filter feeding long preceded the evolution of gigantic body size,[45]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.[44]Gigantism also was preceded by divergence of different mysticete lineages, meaning multiple lineages arrived at large size independently.[44]It is possible thePlio-Pleistoceneincrease in seasonally intenseupwellings,causing high-prey-density zones, led to gigantism.[44][46]

Anatomy

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A humpback whale skeleton. Notice how the jaw is split into two.

Motion

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When swimming, baleen whales rely on their flippers for locomotion in a wing-like manner similar to penguins andsea turtles.Flipper movement is continuous. While doing this, baleen whales use their tail fluke to propel themselves forward through vertical motion while using their flippers for steering, much like anotter.[47]Some speciesleapout of the water, which may allow them to travel faster.[48]Because of their great size, right whales are not flexible or agile like dolphins, and none can move their neck because of the fusedcervical vertebrae;this sacrifices speed for stability in the water.[49]The hind legs are enclosed inside the body, and are thought to bevestigial organs.However, a 2014 study suggests that the pelvic bone serves as support forwhale genitalia.[50]

Rorquals, needing to build speed to feed, have several adaptions for reducingdrag,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 10 m/s (36 km/h; 22 mph), and sustaining a speed of 2.5 m/s (9.0 km/h; 5.6 mph) for an extended period.[51]While feeding, the rorqual jaw expands to a volume that can be bigger than the whale itself;[52]to do this, the mouth inflates. The inflation of the mouth causes the cavum ventrale, the throat pleats on the underside stretching to thenavel,to expand, increasing the amount of water that the mouth can store.[52]The mandible is connected to the skull by dense fibers and cartilage (fibrocartilage), allowing the jaw to swing open at almost a 90° angle. Themandibular symphysisis also fibrocartilaginous, allowing the jaw to bend which lets in more water.[53]To prevent stretching the mouth too far, rorquals have a sensory organ located in the middle of the jaw to regulate these functions.[54]

External anatomy

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Paired blowholes of a humpback and the V-shaped blow of a right whale

Baleen whales have two flippers on the front, near the head. Like all mammals, baleen whales breathe air and must surface periodically to do so. Their nostrils, orblowholes,are situated at the top of thecranium.Baleen whales have two blowholes, as opposed to toothed whales which have one. These paired blowholes are longitudinal slits that converge anteriorly and widen posteriorly, which causes a V-shaped blow. They are surrounded by a fleshy ridge that keeps water away while the whale breathes. Theseptumthat separates the blowholes has two plugs attached to it, making the blowholes water-tight while the whale dives.[55]

Like other mammals, the skin of baleen whales has anepidermis,adermis,ahypodermis,andconnective tissue.The epidermis, thepigmentedlayer, is 5 millimeters (0.2 in) thick, along with connective tissue. The epidermis itself is only 1 millimeter (0.04 in) 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.[56]Right whales have the thickest hypodermis of any cetacean, averaging 51 centimeters (20 in), 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 havesmall hairson the top of their head, stretching from the tip of the rostrum to the blowhole, and, in right whales, on the chin. Like othermarine mammals,they lacksebaceousandsweatglands.[57]

Accessorybaleenplates taper off into small hairs

The baleen of baleen whales arekeratinousplates. They are made of a calcified, hard α-keratin material, a fiber-reinforced structure made ofintermediate filaments(proteins). The degree of calcification varies between species, with the sei whale having 14.5%hydroxyapatite,amineralthat coats teeth and bones, whereas minke whales have 1–4% hydroxyapatite. In most mammals, keratin structures, such aswool,air-dry, but aquatic whales rely on calciumsaltsto form on the plates to stiffen them.[58]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.[59]

Unlike other whales (and most other mammals), the females are larger than the males.Sexual dimorphismis 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 throughwhale 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.[60]

Internal systems

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The unique lungs of baleen whales are built to collapse under the pressure instead of resisting the pressure which would damage the lungs,[61]enabling some, like the fin whale, to dive to a depth of −470 meters (−1,540 ft).[62]The whale lungs are very efficient at extracting oxygen from the air, usually 80%, whereas humans only extract 20% of oxygen from inhaled air.Lung volumeis relatively low compared to terrestrial mammals because of the inability of therespiratory tractto hold gas while diving. Doing so may cause serious complications such asembolism.Unlike other mammals, the lungs of baleen whales lack lobes and are more sacculated. Like in humans, the left lung is smaller than the right to make room for the heart.[61]To conserve oxygen, blood is rerouted from pressure-tolerant-tissue to internal organs,[63]and they have a high concentration ofmyoglobinwhich allows them to hold their breath longer.[64]

The heart of a blue whale with a person standing next to it

The heart of baleen whales functions similarly to other mammals, with the major difference being the size. The heart can reach 454 kilograms (1,000 lb), but is still proportional to the whale's size. Themuscular wallof theventricle,which is responsible for pumping blood out of the heart, can be 7.6 to 12.7 centimeters (3 to 5 in) thick. The aorta, anartery,can be 1.9 centimeters (.75 in) thick. Theirresting heart rateis 60 to 140beats per minute(bpm),[65]as opposed to the 60 to 100 bpm in humans.[66]When diving, theirheart ratewill 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 theirextremities,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 thecore.This is otherwise known ascountercurrent exchange.To counteract overheating while in warmer waters, baleen whales reroute blood to the skin to accelerate heat-loss.[67][65]They have the largestblood corpuscles(redandwhiteblood cells) of any mammal, measuring 10 micrometers (4.1×10−4in) in diameter,[68]as opposed to human's 7.1-micrometer (2.8×10−4in) blood corpuscles.[69]

When sieved from the water, food is swallowed and travels through theesophaguswhere it enters a three-chambered-stomach. The first compartment is known as the fore-stomach; this is where food gets ground up into anacidicliquid, which is then squirted into the main stomach. Like in humans, the food is mixed withhydrochloric acidand protein-digestingenzymes.Then, the partly digested food is moved into the third stomach, where it meets fat-digesting enzymes, and is then mixed with analkalineliquid toneutralizethe acid from the fore-stomach to prevent damage to theintestinal tract.Their intestinal tract is highly adapted to absorb the most nutrients from food; the walls are folded and contain copiousblood 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 (invertebrates) 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 producingurinemore concentrated than seawater, it wastes a lot of water which must be replaced.[70]

Baleen whales have a relatively small braincompared to their body mass.Like other mammals, their brain has a large, foldedcerebrum,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%. Thecerebellum,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.[71]Necropsieson the brains of gray whales revealediron oxideparticles, which may allow them to findmagnetic northlike acompass.[72]

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 exhibitunihemispheric 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.[73]

It is largely unknown how baleen whales produce sound because of the lack of amelonandvocal cords.In a 2007 study, it was discovered that thelarynxhad 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 thearytenoid cartilages.The muscles surrounding the larynx may expel air rapidly or maintain a constant volume while diving.[74]

Senses

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Their eyes are relatively small for their size.

The eyes of baleen whales are relatively small for their size and are positioned near the end of the mouth. This is probably because they feed on slow or immobile prey, combined with the fact that most sunlight does not pass 9.1 meters (30 ft), and hence they do not need acute vision. A whale's eye is adapted for seeing both in theeuphoticandaphoticzones by increasing or decreasing thepupil's size to prevent damage to the eye. As opposed to land mammals which have a flattenedlens,whales have a spherical lens. Theretinais surrounded by a reflective layer of cells (tapetum lucidum), which bounces light back at the retina, enhancing eyesight in dark areas. However, light is bent more near the surface of the eye when in air as opposed to water; consequently, they can see much better in the air than in the water. The eyeballs are protected by a thick outer layer to prevent abrasions and an oily fluid (instead of tears) on the surface of the eye. Baleen whales appear to have limited color vision, as they lackS-cones.[75]

The mysticete ear is adapted for hearing underwater, where it can hear sound frequencies as low as 7Hzand as high as 22kHz,[76]distinct fromodontoceteswhose hearing is optimized forultrasonicfrequencies.[77]It is largely unknown how sound is received by baleen whales. Unlike in toothed whales, sound does not pass through the lower jaw. Theauditory meatusis blocked by connective tissue and an ear plug, which connects to theeardrum.The inner-ear bones are contained in thetympanic bulla,a bony capsule. However, this is attached to the skull, suggesting that vibrations passing through the bone is important.Sinusesmay reflect vibrations towards thecochlea.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.[78][79]

Baleen whales have a small, yet functional,vomeronasal organ.This allows baleen whales to detect chemicals andpheromonesreleased 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 theolfactory bulb,but they do have anolfactory tract.[80]Baleen whales have few if any taste buds, suggesting they have lost their sense of taste. They do retainsalt-receptortaste-buds suggesting that they can taste saltiness.[81]

Behavior

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Migration

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Most species of baleen whale migrate long distances from high latitude waters during spring and summer months to more tropical waters during winter months. This migration cycle is repeated annually.[82]The gray whale has the longest recorded migration of any mammal, with one traveling 23,000 kilometers (14,000 mi) from the Sea of Okhotsk to theBaja Peninsula.[83]

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.[84]Migration to warmer waters may also reduce the risk of calves being predated on bykiller whales.[85]

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.[86]

Foraging

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Humpback whales lunge-feeding in the course ofbubble net fishing

All modern mysticetes are obligate filter feeders, using their baleen to strain small prey items (including small fish, krill, copepods, and zooplankton) from seawater.[43]Despite their carnivorous diet, a 2015 study revealed they housegut florasimilar to that of terrestrial herbivores.[87]Different kinds of prey are found in different abundances depending on location, and each type of whale is adapted to a specialized way of foraging.

There are two types of feeding behaviors: skim-feeding and lunge-feeding,[43]but some species do both depending on the type and amount of food. Lunge-feeders feed primarily oneuphausiids(krill), though some lunge feeders also prey on schools of fish.[88]Skim-feeders, like bowhead whales, feed upon primarily smaller plankton such ascopepods.[89]They feed alone or in small groups.[90]Baleen whales get the water they need from their food, and their kidneys excrete excess salt.[70]

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.[52]Just before they ram the baitball, the jaw swings open at almost a 90° angle and bends which lets in more water.[53]To prevent stretching the mouth too far, rorquals have a sensory organ located in the middle of the jaw to regulate these functions.[54]Then they must decelerate. This process takes a lot of mechanical work and is only energy-effective when used against a large baitball.[91]Lunge feeding is more energy-intensive than skim-feeding due to the acceleration and deceleration required.[43]

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.[92]

Foraging efficiency for both lunge feeding and continuous ram filter feeding is highly dependent upon prey density.[91][93][94]The efficiency of a blue whale lunge is approximately 30 times higher at krill densities of 4.5 kg/m3(0.28 lb/cu ft) than at low krill densities of 0.15 kg/m3(0.0094 lb/cu ft).[91]Baleen whale have been observed seeking out highly specific areas within the local environment in order to forage at the highest density prey aggregations.[95][86]

Predation and parasitism

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Orangewhale liceon a right whale

Baleen whales, primarily juveniles and calves, are preyed on by killer whales. It is thought that annual whale migration occurs to protect the calves from the killer whales.[85]There have also been reports of a pod of killer whales attacking and killing an adult bowhead whale, by holding down its flippers, covering the blowhole, and ramming and biting until death.[96]Generally, a mother and calf pair, when faced with the threat of a killer whale pod, will either fight or flee. Fleeing only occurs in species that can swim away quickly, the rorquals. Slower whales must fight the pod alone or with a small family group.[97]There has been one report of a shark attacking and killing a whale calf. This occurred in 2014 during thesardine runwhen a shiver ofdusky sharksattacked a humpback whale calf.[98]Usually, the only shark that will attack a whale is thecookiecutter shark,which leaves a small, non-fatal bite mark.[99][100]

Many parasites and epibiotics latch onto whales, notablywhale liceandwhale barnacles.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.[101]Though not a parasite, whale barnacles latch onto the skin of a whale during theirlarval stage.However, in doing so it does not harm nor benefit the whale, so their relationship is often labeled as an example ofcommensalism.[102]Some baleen whales will deliberately rub themselves on substrate to dislodge parasites.[103]Some species of barnacle, such asConchoderma auritumand whale barnacles, attach to the baleen plates, though this seldom occurs.[104]A species of copepod,Balaenophilus unisetus,inhabits baleen plates of whales. A species of Antarcticdiatom,Cocconeis ceticola,forms afilmon the skin, which takes a month to develop; this film causes minor damage to the skin. They are also plagued by internal parasites such asstomach worms,cestodes,nematodes,liver flukes,andacanthocephalans.[100]

Reproduction and development

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See also:Whale penis
Female right whale with calf

Before reaching adulthood, baleen whales grow at an extraordinary rate. In the blue whale, the largest species, the fetus grows by some 100 kg (220 lb) per day just before delivery, and by 80 kg (180 lb) per day during suckling. Before weaning, the calf increases its body weight by 17 t (17 long tons; 19 short tons) and grows from 7 to 8 m (23 to 26 ft) at birth to 13 to 16 m (43 to 52 ft) long. When it reachessexual maturityafter 5–10 years, it will be 20 to 24 m (66 to 79 ft) long and possibly live as long as 80–90 years. Calves are bornprecocial,needing to be able to swim to the surface at the moment of their birth.[105]

Most rorquals mate in warm waters in winter to give birth almost a year later.[82]A 7-to-11 month lactation period is normally followed by a year of rest before mating starts again. Adults normally start reproducing when 5–10 years old and reach their full length after 20–30 years.[106][107][108]In the smallest rorqual, the minke whale, 3 m (10 ft) calves are born after a 10-month pregnancy and weaning lasts until it has reached about 5 to 5.5 m (16 to 18 ft) after 6–7 months.[109]Unusual for a baleen whale, female minkes (and humpbacks) can become pregnant immediately after giving birth; in most species, there is a two-to-three-year calving period. In right whales, the calving interval is usually three years. They grow very rapidly during their first year, after which they hardly increase in size for several years.[110][111]They reach sexual maturity when 13 to 14 m (43 to 46 ft) long. Baleen whales areK-strategists,meaning they raise one calf at a time, have a long life-expectancy, and a low infant mortality rate.[112]Some 19th century harpoons found in harvested bowheads indicate this species can live more than 100 years.[113]Baleen whales arepromiscuous,with none showingpair bonds.[114]They arepolygynous,in that a male may mate with more than one female. The scars on male whales suggest they fight for the right to mate with females during breeding season, somewhat similar tolek mating.[115]

Baleen whales have fibroelastic (connective tissue) penises, similar to those of artiodactyls. The tip of the penis, which tapers toward the end, is called thepars intrapraeputialisorterminal cone.[116]The blue whale has thelargest penis of any organismon the planet, typically measuring 2.4–3.0 metres (8–10 ft).[117]Accurate measurements of the blue whale are difficult to take because the whale's erect length can only be observed during mating.[118]The penis on a right whale can be up to 2.7 m (8.9 ft) – thetestes,at up to 2 m (6.6 ft) in length, 78 cm (2.56 ft) in diameter, and weighing up to 238 kg (525 lb), are also the largest of any animal on Earth.[119]

Whale song

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Main article:Whale vocalization
Spectrogramof humpback whale vocalizations: detail is shown for the first 24 seconds of the 37-second recording "Singing Humpbacks". The whale songs are heard before and after a set of echolocation clicks in the middle.
Singing Humpbacks
Recording of Humpback Whales singing and clicking
Problems playing this file? Seemedia help.

All baleen whales use sound for communication and are known to "sing", especially during the breeding season. Blue whales produce the loudest sustained sounds of any animals: their low-frequency (infrasonic,under 20 Hz) moans can last for half a minute, reach almost 190 decibels, and be heard hundreds of kilometers away. Adult male humpbacks produce the longest and most complex songs; sequences of moans, groans, roars, sighs, and chirps sometimes lasting more than ten minutes are repeated for hours. Typically, all humpback males in a population sing the same song over a breeding season, but the songs change slightly between seasons, and males in one population have been observed adapting the song from males of a neighboring population over a few breeding seasons.[120]

Intelligence

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Unlike their toothed whale counterparts, baleen whales are hard to study because of their immense size. Intelligence tests such as themirror testcannot be done because their bulk and lack of body language make a reaction impossible to be definitive. However, studies on the brains of humpback whales revealedspindle cells,which, in humans, controltheory of mind.Because of this, it is thought that baleen whales, or at least humpback whales, haveconsciousness.[121]

Relationship with humans

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History of whaling

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Main article:History of whaling
World population graph ofblue whales

Whaling by humans has existed since theStone Age.Ancient whalers usedharpoonsto spear the bigger animals from boats out at sea.[122]People from Norway started hunting whales around 4,000 years ago, and people from Japan began hunting whales in the Pacific at least as early as that.[123]Whales are typically hunted for theirmeatand blubber by aboriginal groups; they used baleen for baskets or roofing, and made tools and masks out of bones.[123]TheInuithunt whales in the Arctic Ocean.[123]TheBasquesstarted whaling as early as the 11th century, sailing as far asNewfoundlandin the 16th century in search of right whales.[124][125]18th and 19th century whalers hunted down whales mainly for theiroil,which was used as lamp fuel and a lubricant, and baleen (or whalebone), which was used for items such ascorsetsandskirt hoops.[123]The most successful whaling nations at this time were the Netherlands, Japan, and the United States.[126]

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.[127]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.[128]At its height in the 1880s, U.S. profits turned toUSD10,000,000, equivalent to US$225,000,000 in 2000. Commonly exploited species included arctic whales such as the gray whale, right whale, and bowhead whale because they were close to the main whaling ports, likeNew Bedford.After those stocks were depleted, rorquals in the South Pacific were targeted by nearly all whaling organizations; however, they often out-swam whaling vessels. Whaling rorquals was not effective until theharpoon cannonwas invented in the late 1860s.[129]Whaling basically stopped when stocks of all species were depleted to a point that they could not be harvested on a commercial scale.[130]Whaling was controlled in 1982 when theInternational Whaling Commission(IWC) placed a moratorium setting catch limits to protect species from dying out from over-exploitation, and eventually banned it:[131]

Notwithstanding the other provisions of paragraph 10, catch limits for the killing for commercial purposes of whales from all stocks for the 1986 coastal and the 1985/86 pelagic seasons and thereafter shall be zero. This provision will be kept under review, based upon the best scientific advice, and by 1990 at the latest the Commission will undertake a comprehensive assessment of the effects of this decision on whale stocks and consider modification of this provision and the establishment of other catch limits.

Conservation and management issues

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See also:Whaling controversy
Japanese scientific whaling on a pair ofAntarctic minke whales

As of 2021, theInternational Union for Conservation of Nature(IUCN) recognizes 15 mysticete species (while not yet officially recognizingRice's whaleas a species, it still gives it a conservation status as adistinct population segment). Two species—the North Atlantic right whale (with only around 366 individuals left) and Rice's whale (with less than 100 individuals left)—are consideredcritically endangered.Three more are classified asendangered(the North Pacific right whale, the blue whale, and the sei whale), one asvulnerable(the fin whale), one asnear-threatened(Antarctic minke whale), and one asdata deficient(Omura's whale).[132]Species that live in polar habitats are vulnerable to the effects of ongoingclimate change,particularly declines in sea ice, as well asocean acidification.[133]

Thewhale-watchingindustry andanti-whalingadvocates 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.[134]Peru, Uruguay, Australia, and New Zealand also support proposals to permanently forbid whaling south of the Equator, asSolor(an island of Indonesia) is the only place of theSouthern Hemispherethat takes whales.[135]Anti-whaling groups, such as theInternational Fund for Animal Welfare(IFAW), claim that countries which support a pro-whaling stance are damaging their economies by driving away anti-whaling tourists.[136]

A protest against Japan's scientific whaling

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.[130]: 327–333 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 theendangered species list.[137]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.[138]Conversely, theeastern stock of North Atlantic right whalewas extirpated from much of its former range, which stretched from the coast of North Africa to theNorth Seaand Iceland; it is thought that the entire stock consists of only ten individuals, making the eastern stockfunctionally extinct.[130][139]

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.[140]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.[141]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.[142]Animal-rights activist groups, such as theGreenpeace,[143]object to Japan's scientific whaling, with some calling it a substitute for commercial whaling.[144]In 2014, theInternational Court of Justice(theUNjudicial branch) banned the taking of whales for any purpose in theSouthern Ocean Whale Sanctuary;[145]however, Japan refuses to stop whaling and has only promised to cut their annual catches by a third (around 300 whales per year).[146]

The remains of aNorth Atlantic right whaleafter it collided with a shippropeller

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. TheLloyd's mirroreffect results in low frequency propeller sounds not being discernible near the surface, where most accidents occur. Combined with spreading andacoustic shadowing effects,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.[147]A 2014 study noted that a lower vessel speed correlated with lower collision rates.[148]The ever-increasing amount of ocean noise, includingsonar,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.[149][150]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).[149]

Poisoning from toxic substances such aspolychlorinated biphenyl(PCB) is generally low because of their lowtrophic level.[151]However,oil spillscan be a significant threat, especially to small populations; the already endangered Rice's whale was likely devastated by theDeepwater Horizon oil spill,with some estimates indicating a decline of up to 22% in the species.[152]

Some baleen whales can become victims ofbycatch,which is especially serious for North Atlantic right whales considering their small number.[153]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.[154]

In captivity

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A gray whale in captivity

Baleen whales have rarely been kept in captivity. Their large size and appetite make them expensive creatures to maintain. Pools of proper size would also be very expensive to build. For example, a single gray whale calf would need to eat 215 kilograms (475 lb) of fish per day, and the pool would have to accommodate the 4-meter (13 ft) calf, along with ample room to swim.[155]Only gray whales have survived being kept in captivity for over a year. The first gray whale, which was captured inScammon's Lagoon,Baja California Sur,in 1965, was named Gigi and died two months later from an infection.[156]The second gray whale, which was captured in 1971 from the same lagoon, was named Gigi II and was released a year later after becoming too big.[157]The last gray whale, J.J., beached itself inMarina del Rey, California,where it was rushed toSeaWorld San Diegoand, after 14 months, was released because it got too big to take care of. Reaching 8,700 kilograms (19,200 lb) and 9.4 meters (31 ft), J.J. was the largest creature to be kept in captivity.[158]

The Mito Aquarium inNumazu, 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.[159]

References

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