Blue whale

The blue whale is a species of baleen whale and the largest marine mammal in the rorqual family Balaenopteridae. Reaching a maximum confirmed length of and weighing up to, it is the largest animal known to have ever existed. The blue whale's long and slender body can be of various shades of greyish-blue on its upper surface and somewhat lighter underneath. Four subspecies are recognized: B. m. musculus in the North Atlantic and North Pacific, B. m. intermedia in the Southern Ocean, B. m. brevicauda in the Indian Ocean and South Pacific Ocean, and B. m. indica in the Northern Indian Ocean. There is a population in the waters off Chile that may constitute a fifth subspecies.
In general, blue whale populations migrate between their summer feeding areas near the poles and their winter breeding grounds near the tropics. There is also evidence of year-round residencies, and partial or age- and sex-based migration. Blue whales are filter feeders; their diet consists almost exclusively of krill. They are generally solitary or gather in small groups, and have no well-defined social structure other than mother–calf bonds. Blue whales vocalize, with a fundamental frequency ranging from 8 to 25 Hz; their vocalizations may vary by region, season, behavior, and time of day. Orcas are their only natural predators.
The blue whale was abundant in nearly all the Earth's oceans until the end of the 19th century. It was hunted almost to the point of extinction by whalers until the International Whaling Commission banned all blue whale hunting in 1966. The International Union for Conservation of Nature has listed blue whales as Endangered as of 2018. Blue whales continue to face numerous man-made threats such as ship strikes, pollution, ocean noise, and climate change.

Taxonomy

Nomenclature

The genus name, Balaenoptera, means winged whale, while the species name, musculus, could mean "muscle" or a diminutive form of "mouse", possibly a pun by Carl Linnaeus when he named the species in Systema Naturae. One of the first published descriptions of a blue whale comes from Robert Sibbald's Phalainologia Nova, after Sibbald found a stranded whale in the estuary of the Firth of Forth, Scotland, in 1692. The name "blue whale" was derived from the Norwegian blåhval, coined by Svend Foyn shortly after he had perfected the harpoon gun. The Norwegian scientist G. O. Sars adopted it as the common name in 1874.
Blue whales were referred to as "Sibbald's rorqual", after Robert Sibbald, who first described the species. Whalers sometimes referred to them as "sulphur bottom" whales, as the bellies of some individuals are tinged with yellow. This tinge is due to a coating of huge numbers of diatoms.

Evolution

Blue whales are rorquals in the family Balaenopteridae. A 2018 analysis estimates that the Balaenopteridae family diverged from other families in between 10.48 and 4.98 million years ago during the late Miocene. The earliest discovered anatomically modern blue whale is a partial skull fossil from southern Italy identified as B. cf. musculus, dating to the Early Pleistocene, roughly 1.5–1.25 million years ago. The Australian pygmy blue whale diverged during the Last Glacial Maximum. Their more recent divergence has resulted in the subspecies having a relatively low genetic diversity, and New Zealand blue whales have an even lower genetic diversity.
Whole genome sequencing suggests that blue whales are most closely related to sei whales with gray whales as a sister group. This study also found significant gene flow between minke whales and the ancestors of the blue and sei whale. Blue whales also displayed high genetic diversity.

Hybridization

Blue whales are known to interbreed with fin whales. The earliest description of a possible hybrid between a blue whale and a fin whale was a anomalous female whale with the features of both the blue and the fin whales taken in the North Pacific. A whale captured off northwestern Spain in 1984, was found to have been the product of a blue whale mother and a fin whale father.
Two live blue-fin whale hybrids have since been documented in the Gulf of St. Lawrence, and in the Azores. DNA tests done in Iceland on a blue whale killed in July 2018 by the Icelandic whaling company Hvalur hf., found that the whale was the offspring of a male fin whale and female blue whale; however, the results are pending independent testing and verification of the samples. Because the International Whaling Commission classified blue whales as a "Protection Stock", trading their meat is illegal, and the kill is an infraction that must be reported. Blue-fin hybrids have been detected from genetic analysis of whale meat samples taken from Japanese markets. Blue-fin whale hybrids are capable of being fertile. Molecular tests on a pregnant female whale caught off Iceland in 1986 found that it had a blue whale mother and a fin whale father, while its fetus was sired by a blue whale.
In 2024, a genome analysis of North Atlantic blue whales found evidence that approximately 3.5% of the blue whales' genome was derived from hybridization with fin whales. Gene flow was found to be unidirectional from fin whales to blue whales. Comparison with Antarctic blue whales showed that this hybridization began after the separation of the northern and southern populations. Despite their smaller size, fin whales have similar cruising and sprinting speeds to blue whales, which would allow fin males to complete courtship chases with blue females.
There is a reference to a humpback–blue whale hybrid in the South Pacific, attributed to marine biologist Michael Poole.

Subspecies and stocks

At least four subspecies of blue whale are traditionally recognized, some of which are divided into population stocks or "management units". Like many large rorquals, the blue whale is a cosmopolitan species. They have a worldwide distribution, but are mostly absent from the Arctic Ocean and the Mediterranean, Okhotsk, and Bering Sea.

Northern subspecies
*North Atlantic population – This population is mainly documented from New England along eastern Canada to Greenland, particularly in the Gulf of St. Lawrence, during summer, though some individuals may remain there all year. They also aggregate near Iceland and have increased their presence in the Norwegian Sea. They are reported to migrate south to the West Indies, the Azores and northwest Africa.
*Eastern North Pacific population – Whales in this region mostly feed off California's coast from summer to fall and then Oregon, Washington State, the Alaska Gyre and Aleutian Islands later in the fall. During winter and spring, blue whales migrate south to the waters of Mexico, mostly the Gulf of California, and the Costa Rica Dome, where they both feed and breed.
*Central/Western Pacific population – This stock is documented around the Kamchatka Peninsula during the summer; some individuals may remain there year-round. They have been recorded wintering in Hawaiian waters, though some can be found in the Gulf of Alaska during fall and early winter.
Northern Indian Ocean subspecies – This subspecies can be found year-round in the northwestern Indian Ocean, though some individuals have been recorded travelling to the Crozet Islands during between summer and fall.
Pygmy blue whale
*Madagascar population – This population migrates between the Seychelles and Amirante Islands in the north and the Crozet Islands and Prince Edward Islands in the south where they feed, passing through the Mozambique Channel.
*Australia/Indonesia population – Whales in this region appear to winter off Indonesia and migrate to their summer feeding grounds off the coast of Western Australia, with major concentrations at Perth Canyon and an area stretching from the Great Australian Bight and Bass Strait.
*Eastern Australia/New Zealand population – This stock may reside in the Tasman Sea and the Lau Basin in winter and feed mostly in the South Taranaki Bight and off the coast of eastern North Island. Blue whales have been detected around New Zealand throughout the year.
Antarctic subspecies – This subspecies includes all populations found around the Antarctic. They have been recorded to travel as far north as the Eastern Tropical Pacific, the central Indian Ocean, and the waters of southwestern Australia and northern New Zealand.

Blue whales off the Chilean coast might be a separate subspecies based on their geographic separation, genetics, and unique song types. Chilean blue whales might overlap in the Eastern Tropical Pacific with Antarctica blue whales and Eastern North Pacific blue whales. Chilean blue whales are genetically differentiated from Antarctica blue whales such that interbreeding is unlikely. However, the genetic distinction is less between them and the Eastern North Pacific blue whale, hence there might be gene flow between the Southern and Northern Hemispheres. A 2019 study by Luis Pastene, Jorge Acevedo and Trevor Branch provided new morphometric data from a survey of 60 Chilean blue whales, hoping to address the debate about the possible distinction of this population from others in the Southern Hemisphere. Data from this study, based on whales collected in the 1965/1966 whaling season, shows that both the maximum and mean body length of Chilean blue whales lies between these values in pygmy and Antarctic blue whales. Data also indicates a potential difference in snout-eye measurements between the three, and a significant difference in fluke-anus length between the Chilean population and pygmy blue whales. This further confirms Chilean blue whales as a separate population, and implies that they do not fall under the same subspecies as the pygmy blue whale.
A 2024 genomic study of the global blue whale population found support for the subspecific status of Antarctic and Indo-western Pacific blue whales but not eastern Pacific blue whales. The study found "...divergence between the eastern North and eastern South Pacific, and among the eastern Indian Ocean, the western South Pacific and the northern Indian Ocean." and "no divergence within the Antarctic".