Temporal range: late Eocene–Recent
|Humpback whale breaching|
The baleen whales (Mysticeti), also called whalebone whales, comprise one of two parvorders of the artiodactyl infraorder Cetacea (whales, dolphins, and porpoises). They are the edentulous whales, characterized by having baleen plates for filtering food from water, rather than teeth like in the toothed whales (Odontoceti). Living species of Mysticeti have teeth only during the embryonic phase, while fossil Mysticeti had teeth before baleen evolved.
The taxonomic name "Mysticeti" (Latin, plural) apparently derives from a translation error in early copies of Aristotle's Historia Animalium, in which "ὁ μῦς τὸ κῆτος" (ho mus to kētos, "the mouse, the whale so called") was mistakenly run together as "ὁ μυστικῆτος" (ο mustikētos, "the Mysticetus"), which Rice 1998 assumed was an ironic reference to the animals' great size. An alternate name for the suborder is "Mystacoceti" (from Greek μύσταξ "moustache" + κῆτος "whale"), which, although obviously more appropriate and occasionally used in the past, has been superseded by "Mysticeti".
Baleen whales are generally larger than toothed whales, and females are bigger than males. This group includes the largest known animal species, the blue whale.
The members of the four recognized families of baleen whales can be distinguished by several external and internal features:
- The right whales (Balaenidae, the "black" right whales and the bowhead whale) are robust, have an arched upper jaw, and long and narrow baleen plates. Their heads are remarkably large — one-third of the body length — and are equipped with a long, thin rostrum and huge bowed lower lips, but lack ventral grooves. The coronoid process is missing in the lower jaw and the cervical vertebrae are fused.
- The rorquals (Balaenopteridae) have short heads — less than a quarter of the body length — with short and wide baleen plates. They have a mostly small dorsal fin and numerous ventral grooves. The upper jaw is long and unarched and a coronoid process is present in the lower jaw, which is bowed outwards. The cervical vertebrae are unfused.
The two remaining families are intermediate in appearance between right whales and balaenopterids:
- The pygmy right whale (Neobalenidae) have short heads — a quarter of the body length — with arched upper jaw and bowed lower lips. Their relatively long baleen plates are yellowish white with a dark outer border. Many of the ribs are broadened and flattened.
- The gray whales (Eschrichtiidae, of which only Eschrichtus robustus is extant) are robust, have short and narrow heads, with a slightly arched rostrum, and relatively small baleen plates. Ventrally, the head has two to five deep creases.
In baleen whales, enlarged oral cavities adapted for suction feeding evolved before specializations for bulk filter feeding. In the early Eocene basilosaurid Saghacetus, the mandibular symphysis is long and rigid, the rostrum is narrow, and the edges of the maxillae are thickened, indicating an adaptation for raptorial feeding. In the toothed Oligocene mammalodontid Janjucetus, the symphysis is short and the oral cavity enlarged, the rostrum is wide, and the edges of the maxillae are thin, indicating an adaptation for suction feeding. The aetiocetid 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.
Baleen whales have two blowholes, causing a V-shaped blow. These paired blowholes are longitudinal slits that converge anteriorly and widen posteriorly. They are surrounded by a fleshy ridge that keeps water away while the whale breathes. The septum that separates the blowholes has two plugs attached to it, making the blowholes water-tight while the whale dives.
Mysticeti possess an incredibly unique form of dentition, known as "baleen plates" or simply "baleen". The evolution of this novel adaptation has had much study surrounding it, from its gradual appearance in ancestors, to other adaptations which relaxed evolutionary constraints on the baleen itself, and finally the genetic basis for the appearance of baleen. The evolutionary processes involved with this adaptation operated very slowly. These processes most likely include a mutation which caused the trait to arise, natural selection which then enforced the prevalence of the new trait and finally fixation of the trait within baleen populations. This is evidenced by the fossil record which shows ancestral forms of mysticeti who possess both baleen and regular teeth. Traits which unfortunately cannot be supported by the fossil record include behavioral traits. Researchers suggest that filter feeding as a behavior arose prior to the evolution of modern baleen. Instead, individuals who invested in this behavior possessed a primitive form of baleen along with common teeth. Due to the success of filter feeding as a mode of prey acquisition, it is assumed that the selectional pressures on common teeth were lessened and thus baleen was positively selected for because it increased the fitness of individuals possessing it. Having baleen also allowed for the size of these whales to increase, as they were able to occupy a new niche and expand their food source. Another aspect of studying dental evolution in baleen whales is the genetic mutation which arose to create this novel trait. Enamelysin is coded for in baleen whales in the gene known as MMP20. When studying this gene within several species of mysticete, researchers found a very interesting SINE insertion shared by 8 mysticete species, which are representative of all extant mysticete species. This insertion is what caused an end to enamel production in the common ancestor of mysticeti. Researchers argue that this is evidence that there was selection against the deleterious effects typical of a SINE insertion. It is clear that this mutation in the genome which brought out new diversity within extant mysticeti species is the true beginning of the evolution of baleen. 
These whales are found solitary or in small groups called pods.
Particularly known for its acrobatics is the humpback whale, but other baleen whales also break through the water surface with their bodies or beat it loudly with their fins. Some believe the male baleen whales try to show off in the presence of females to increase their mating success. Scientists speculate baleen whales and other cetaceans may engage in breaching to dislodge parasites, or scratch irritated skin. Breaching, and other behaviors like lobtailing, are also used to stun or kill nearby fish or krill.
In the Southern Hemisphere, huge concentrations of krill are found during summers, the food preferred by baleen whales. In the Northern Hemisphere, the prey available is more variable, and, for example, humpbacks and fin whales can feed exclusively on krill around Antarctica, but prey on schooling fish in the Arctic. All baleen whales except the gray whale feed near the water surface, rarely diving deeper than 100 m (330 ft) or for extended periods. The gray whale feeds on bottom-living organisms such as amphipods in shallow waters.
Baleen whales are not known to echolocate, but bowheads swimming under ice fields probably use sound for navigation. All baleen whales, however, 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 (about 20 Hz) moans can last for half a minute, reach almost 190 decibels, and be heard hundreds of kilometres 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 neighbouring population over a few breeding seasons.
Baleen whales are carnivorous filter-feeders; they consume vast numbers of small organisms by vacuum-cleaning the ocean and not, like toothed whales, by catching prey individually. To achieve this, baleen whales typically seek out a concentration of zooplankton, swim through it, either open-mouthed or gulping, and filter the prey from the water using their baleen. The baleen is a row of a large number of keratin plates attached to the upper jaw. These plates have a composition similar to those in human hair or fingernails. They are triangular in section with the larger inward-facing side bearing fine hairs that form a filtering mat.
When filter feeding, baleen whales can dynamically expand their oral cavity to accommodate enormous volumes of sea water. This is made possible by its kinetic skull joints, especially the elastic mandibular symphysis (central lower joint), which permits both dentaries to be rotated independently in two planes. This flexible jaw, which made the titanic body sizes of baleen whales possible, is not present in early whales and probably evolved within Mysticeti.
Baleen whales are either continuous or intermittent filter feeders:
Balaenids, the bowhead and right whale, continuously filter water through their mouths and have several anatomical adaptations for skim feeding: a frontal cleft between the two rows of baleen plates (known as the subrostral gap) and a large depression inside the lower lip. These adaptations are unique to these mysticetes, as are the fused cervical vertebrae, the firm tongue, and the semicircular lips that can reach up to the narrow rostrum. Balaenids regularly clean their baleen of accumulated prey. Right whales are slow swimmers with large heads and mouths. Their baleen plates are narrow and very long — up to 4 m (13 ft) in bowheads — and accommodated inside the enlarged lower lip, which fits onto the bowed upper jaw. As the right whale swims, water and prey are guided in through the subrostral gap, while the baleen filters out the water. Balaenids feed chiefly on tiny copepods that are about 1 mm (0.039 in), and their baleen is finely fringed for this purpose.
Intermittently filter-feeding mysticetes include the gray whale and rorquals such as the blue whale, fin whale, and humpback. They engulf a mouthful of water from which they filter the small prey using their baleen. Rorquals have several anatomical adaptations for this lunge feeding, including a loose mandibular joint, a large throat pouch with ventral folds, and a soft and agile tongue. Rorquals are fast swimmers with small heads and have short and broad baleen plates. To catch prey, they widely open their large lower jaw — almost 90° — swim into a swarm, gulping water and prey. They expand the capacity of their mouth by expanding the ventral grooves by pressing the tongue down. Humpbacks and other balaenopterids feed on larger prey (5–20 mm (0.20–0.79 in)) and consequently have coarser fringes than have balaenids.
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)/day just before delivery, and by 80 kg (180 lb)/day during suckling. Before weaning, the calf increases its body weight by 17 t (17 long tons; 19 short tons) and grows from 7–8 m (23–26 ft) at birth to 13–16 m (42.5-52.5 ft) long. When it reaches sexual maturity after 5–10 years, it will be 20 to 24 m (65.6–79 ft) long and possibly live as long as 80–90 years.
The same life pattern can be seen in other balaenopterids; they mate in warm waters in winter to give birth almost a year later. 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. In the smallest balaenopterid, the minke whale, 3 m (9.8 ft) calves are born after a 10-month pregnancy and weaning lasts until it has reached about 5–5.5 m (16.5–18 ft) after 6–7 months. Unusual for a baleen whale, female minkes (and humpbacks) can become pregnant immediately after giving birth; in most species, a 2– to 3-year calving period is the norm. In right whales, the calving interval is usually 3 years. Bowheads grow very rapidly during their first year, after which they hardly increase in size for several years. They reach sexual maturity when 13–14 m (43–46 ft) long. Some 19th-century harpoons found in harvested bowheads indicate this species can live more than 100 years.
Importance to humans
From the 11th to the late 20th centuries, baleen whales were hunted commercially for their oil and baleen. Their oil was used for cooking and making products such as margarine and lamp oils, while their baleen was used to stiffen corsets, as parasol ribs, and for the manufacture of various implements today more commonly made of plastic, such as combs and paper-creasing tools.
The evolution of Mysticeti as evidenced by the fossil record, was a gradual one that involved the transition from toothed, to teeth along with baleen, and finally to strictly baleen.Llanocetus, known from the late Eocene of Seymour Island, West Antarctica, is the earliest mysticete. It is a large animal with a basilosaurid-like skull with several mysticete-like features, including a wide, flat, and dorsoventrally flattened rostrum. Its jaw had heterodont teeth separated by wide diastemata, the cheek teeth are two-rooted and palmate with accessory denticles. Additionally, fine grooves around the alveoli indicate that the palate had a rich blood supply. Llanocetus has been interpreted as a filter feeder. Other early toothed mysticeti or "archaeomysticetes" from the Oligocene are the Mammalodontidae (Mammalodon and Janjucetus) from Australia. They are small with shortened rostra, and a primitive dental formula (220.127.116.11).
The most derived group of toothed mysticetes is the aetiocetids: Aetiocetus, Ashorocetus, Morawanocetus, Chonecetus, and Willungacetus. They are known from the late Oligocene of the North Pacific, except Willungacetus, which is known from the early Oligocene of Australia. Specimens in which the teeth are preserved, are polydont to some degree and have 11 cheek teeth, one more than in basilosaurids. In Aeotiocetus, palatal foramina and sulci suggest their blood vessels supplied some kind of "protobaleen", although the earliest preserved baleen is from the Miocene.
In the early 1990s, the species Janjucetus hunderi was discovered in Victoria by a surfer, and was described by Fitzgerald 2006. Janjucetus was a baleen whale with sharp teeth that hunted fish and squid, as well as larger prey, potentially including sharks and dolphin-like cetaceans. These fossils hint the early baleen whales were predatory and eventually evolved into the gentler, toothless whales known today. Deméré et al. 2008 identified palatal foramina and bony impressions of the blood vessels that "feed" the baleen racks in the toothed mysticete Aetiocetus weltoni. Deméré et al. concluded this discovery implies baleen whale previously possessed both teeth and baleen, and Aetiocetus serves as an intermediate adaptive role between primitive, toothed mysticetes and more derived, toothless mysticetes.
The first baleen-bearing, toothless baleen whales (such as Eomysticetus and Micromysticetus) appeared in the late Oligocene. The Eomysticetidae had long, flat rostra that lacked teeth and had external nares located halfway up the dorsal side of the snout. Though the palate is not well-preserved in these specimens, they are thought to have been baleen filter feeders. Early baleen whales probably could not echolocate; no anatomical evidence preserved in the skulls and ear regions of any fossil baleen whales show any of the adaptations associated with echolocation as in toothed whales.
The taxonomic classification of baleen whales was considerably re-evaluated by Bisconti, Lambert & Bosselaers 2013. They introduced a new superfamily, Thalassotherii, to avoid paraphyly within Mysticeti.
The "†" signs denote extinct families and genera.
Parvorder Mysticeti: baleen whales
- Family †Aetiocetidae
- †Family Llanocetidae
- †Family Mammalodontidae
- Clade Chaeomysticeti
- Superfamily Eomysticetoidea
- Clade Balaenomorpha
- Superfamily Balaenoidea
- Clade Thalassotherii
- Family †Aglaocetidae (invalid subgroup)
- Family Cetotheriidae
- †Family Diorocetidae (invalid subgroup)
- †Family Pelocetidae (invalid subgroup)
- Superfamily Balaenopteroidea
- Family Balaenopteridae: rorquals
- Family Eschrichtiidae
- Family incertae sedis
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