Mysticetes

The mysticetes comprise the baleen whales, which lack functional teeth and feed by filtering small prey. This suborder includes the largest animals ever to have lived on Earth, among them the blue whale. This page sets out the list of the 15 mysticete species recognised by the Society for Marine Mammalogy, arranged by family, with their IUCN conservation status and their evolutionary origin. To place this group within the cetacean tree, see also the odontocetes (toothed whales) and the difference between whale, sperm whale and dolphin.

The four families of living mysticetes

The reference classification used here is that of the Society for Marine Mammalogy (SMM, Committee on Taxonomy), which is updated regularly. Of the some 94 living cetacean species it recognises (about 15 mysticetes and 79 odontocetes), the living mysticetes fall into four families and 15 species:

Family Balaenopteridae (rorquals)

Rorquals are slender mysticetes with throat pleats (expandable ventral grooves, from a few dozen to around a hundred depending on the species) that feed by lunge filtering. The SMM recognises 9 species in the family: eight in the genus Balaenoptera and one in the genus Megaptera.

The humpback whale (Megaptera novaeangliae) is readily told apart from the other rorquals by its very long flippers (up to a third of the body length), its knobbly back and its complex songs. The rorquals of the genus Balaenoptera, more streamlined, resemble one another more closely and are distinguished by size, coloration and the number of throat pleats.

The Bryde’s whale complex: an open taxonomic debate

Bryde’s whales form a species complex whose taxonomy is not entirely resolved. The SMM recognises only a single species, Balaenoptera edeni, comprising two subspecies: B. e. edeni (the smaller coastal form) and B. e. brydei (the larger oceanic form). Some recent phylogenomic analyses suggest that these two forms could merit distinct specific rank, but no formal proposal has been adopted to date; “Balaenoptera brydei” is therefore not accepted as a valid species. One lineage long confused with this complex has, by contrast, genuinely been separated out: Rice’s whale (Balaenoptera ricei, Rosel, Wilcox, Yamada & Mullin, 2021), endemic to the Gulf of Mexico. It is the most recently described mysticete and one of the most threatened large cetaceans in the world: a population estimated at about 50 individuals (probably fewer than 50 mature animals, with some recent estimates putting the figure at around thirty), which earns it the Critically Endangered listing from the IUCN.

Family Balaenidae (right whales)

Right whales are large, stocky mysticetes with a very big head and a strongly arched jaw, and no throat pleats. They filter plankton at the surface, mouth open, swimming slowly (skim feeding). The family numbers 4 species.

The North Atlantic right whale is among the most threatened large cetaceans on the planet, with fewer than 400 individuals (recent estimates from the North Atlantic Right Whale Consortium stand at around 370 to 385 individuals in the early 2020s), a victim of ship strikes and of entanglement in fishing gear. The bowhead whale (Balaena mysticetus), an Arctic animal of exceptional longevity (more than 200 years), has the longest baleen of any cetacean. A caution on the French names: “baleine boréale” (Balaena mysticetus, the bowhead whale) and “rorqual boréal” (Balaenoptera borealis, the sei whale) are two distinct species belonging to two different families.

Family Eschrichtiidae (grey whale)

This family contains only one living species, specialised in benthic feeding: it scrapes soft bottoms and filters invertebrates out of the sediment, a feeding mode unique among cetaceans.

The global status of Least Concern conceals wide regional disparities. The western Pacific subpopulation (off Sakhalin) remains very small: long listed as “Critically Endangered”, it was reclassified as “Endangered” by the IUCN in 2018 on the strength of a slight improvement. The North Atlantic subpopulation, for its part, has been extinct for several centuries.

Family Neobalaenidae (pygmy right whale)

The pygmy right whale is the smallest of the mysticetes (6 to 6.5 m) and one of the most rarely seen. The SMM places it provisionally in the family Neobalaenidae, while noting an unsettled debate.

The exact position of Caperea marginata remains under discussion. On the basis of fossil data, Fordyce and Marx (2012) proposed attaching it to the family Cetotheriidae, long held to be entirely extinct, in which case Caperea would be its only living representative. This hypothesis, founded on morphology, is disputed by several molecular analyses that keep it within the Neobalaenidae. The SMM therefore retains the placement in Neobalaenidae provisionally, while mentioning the Cetotheriidae hypothesis as a line of enquiry without consensus.

Summary table of the mysticetes

FamilyGeneraNumber of speciesIUCN status (overview)
BalaenopteridaeBalaenoptera (8), Megaptera (1)9From Least Concern to Critically Endangered (B. ricei)
BalaenidaeEubalaena (3), Balaena (1)4From Least Concern to Critically Endangered (E. glacialis)
EschrichtiidaeEschrichtius (1)1Least Concern (global)
NeobalaenidaeCaperea (1)1Least Concern
Total–15 species–
Classification according to the Society for Marine Mammalogy (reference list updated regularly). IUCN Red List statuses. See the full overview on the conservation statuses of cetaceans.

Characteristics of the mysticetes

The mysticetes (Mysticeti), or baleen whales, form one of the two major groups of living cetaceans, distinct from the odontocetes (toothed whales). They are characterised by a unique filtering apparatus, the absence of functional teeth in the adult, and a diet based on filtering small prey. To recognise a mysticete in the field, see our guide to identifying a cetacean.

Baleen in place of teeth

Mysticetes have no functional teeth. In their place they carry horny baleen (keratin plates) suspended from the upper jaw, which filters seawater.

  • Baleen is made of keratin. Its length varies a great deal between species: from a few tens of centimetres in the minke whales up to about 4 metres (even 4 to 5 m) in the bowhead whale, which holds the record.
  • It retains small prey: krill, copepods, small schooling fish.
  • The lower jaw and the throat, both highly mobile, allow the mouth to expand when huge volumes of water are taken in.

For more on the anatomy and growth of these structures, see our page devoted to baleen in mysticete cetaceans.

Three main filtering strategies

Mysticetes exploit distinct filtering modes according to family. These techniques are set out in detail on our page on cetacean diets.

  • Balaenopteridae: filtering by engulfment (lunge / gulp feeding), the rorqual charges into a shoal of prey, mouth open, and engulfs a considerable volume of water that its throat pleats let expand.
  • Balaenidae: filtering by skimming (skim feeding), the whale swims slowly with its mouth open, the water passing continuously through the curtain of baleen (e.g. Eubalaena, Balaena mysticetus).
  • Eschrichtiidae: benthic filtering by suction of the sediment (grey whale).

Two blowholes

All mysticetes have two symmetrical blowholes, unlike the odontocetes, which have only one. The exhaled blow is tall and vertical; it is distinctly bifid (V-shaped) in the right whales, but forms instead a single column in the large rorquals, a useful criterion for identifying a cetacean from its blow.

A body size beyond the ordinary

The mysticetes include the largest vertebrates in the whole of evolutionary history. The blue whale (Balaenoptera musculus) reaches about 30 metres according to reliable modern measurements (rostrum to tail notch), for a mass that can approach 150 to 190 tonnes. The lengths of “33 metres” sometimes quoted come from old whaling registers and are regarded as unverified. These extreme dimensions are tied to a bulk feeding strategy in ocean areas very rich in plankton. Compare the sizes of the various species with our scale comparator and our page on cetacean morphometrics.

Migration cycle and breeding

Most mysticetes are long-distance migrants:

  • They alternate between feeding areas at high latitudes (summer) and breeding areas in warm water (winter). See our pages on annual migrations and sighting hotspots.
  • Females give birth on average every 2 to 3 years, to a single calf after 10 to 13 months of gestation.
  • Newborns suckle for several months on a milk extremely rich in lipids (up to ~40%), which supports rapid growth. More detail on cetacean life cycles.

Origin and evolution: from teeth to baleen

Like all cetaceans, the mysticetes descend from terrestrial artiodactyls that returned to the marine environment during the Eocene. Molecular estimates place the divergence between the cetaceans and their closest living relatives, the hippopotamuses (the Whippomorpha clade), about 53 to 55 million years ago. This transition is illustrated by a series of emblematic fossils: Pakicetus (~50 Ma), a semi-aquatic mammal tied to fresh water rather than strictly terrestrial, whose closest known relative is Indohyus (~48 Ma, a raoellid), then Ambulocetus, Rodhocetus, and finally the fully aquatic cetaceans Basilosaurus and Dorudon. The full account is developed on our pages devoted to cetacean evolution, to the terrestrial ancestor of cetaceans and to the fossils of cetacean ancestors.

The split between mysticetes and odontocetes (the two lineages forming the Neoceti) goes back about 34 to 39 million years (around 34 Ma according to the palaeontological record). The most striking characteristic of the mysticetes, the replacement of teeth by baleen, then came into place gradually during the Oligocene (about 33 to 23 million years ago). The oldest known mysticetes still had teeth and no baleen:

  • Mystacodon selenensis (Late Eocene, Peru, ~36 million years ago): one of the oldest mysticetes, fully toothed, probably adapted to suction predation.
  • Coronodon and Janjucetus (Oligocene): toothed mysticetes without baleen; their teeth could have served for a form of filtering or for active predation.
  • Aetiocetus (Oligocene, ~30 million years ago): a transitional form showing both teeth and indications of baleen, evidence of an intermediate stage in the shift to filter feeding.

These fossils show that the loss of teeth and the appearance of baleen were not simultaneous: several lineages tried out intermediate solutions before baleen filtering became established in the modern mysticetes. To explore the relationships, see our interactive cetacean phylogeny.

Frequently asked questions about the mysticetes

How many mysticete species are there?

The Society for Marine Mammalogy recognises about 15 living mysticete species, spread across four families: Balaenopteridae (9), Balaenidae (4), Eschrichtiidae (1) and Neobalaenidae (1). They make up one of the two cetacean suborders, alongside the some 79 odontocetes, giving about 94 living cetacean species in all.

What is the difference between a mysticete and an odontocete?

Mysticetes carry baleen, have two blowholes and filter small prey; the odontocetes (dolphins, sperm whales, killer whales) have teeth, a single blowhole and use echolocation. See also the difference between whale, sperm whale and dolphin.

Is Rice’s whale a distinct species?

Yes. Balaenoptera ricei, described in 2021, is a valid rorqual species endemic to the Gulf of Mexico, previously confused with the Bryde’s whale complex. With a population estimated at about 50 individuals (probably fewer than 50 mature animals), it is listed as Critically Endangered by the IUCN.

Why is the classification of the pygmy right whale debated?

The Society for Marine Mammalogy places Caperea marginata in the Neobalaenidae, but a hypothesis founded on fossils (Fordyce & Marx, 2012) attaches it to the Cetotheriidae, a family long reputed extinct. The molecular data do not confirm that attachment, which remains a proposal without consensus.

Which is the largest of the mysticetes?

The blue whale (Balaenoptera musculus) is the largest living animal, and probably the largest ever to have existed, with a reliable length of about 30 metres and a mass that can approach 190 tonnes.

Further reading

Round out this page with the cetacean glossary, the general cetacean FAQ and the cetacean demography data.