The Blue Whale (Balaenoptera musculus) is a marine mammal belonging to the suborder of baleen whales (called Mysticeti). At up to 33.59 metres (110 ft 2.5 in) in length and 181 metric tonnes (200 short tons) or more in weight, it is believed to be the largest animal ever to have existed.

Long and slender, the Blue Whale's body can be various shades of bluish-grey dorsally and somewhat lighter underneath. There are at least three distinct subspecies: B. m. musculus of the north Atlantic and north Pacific, B. m. intermedia, of the Southern Ocean and B. m. brevicauda (also known as the Pygmy Blue Whale) found in the Indian Ocean and South Pacific Ocean. B. m. indica, found in the Indian Ocean, may be another subspecies. As with other baleen whales, its diet consists mainly of small crustaceans known as krill, as well as small fish and sometimes squid.

Blue Whales were abundant in nearly all oceans until the beginning of the twentieth century. For over 40 years they were hunted almost to extinction by whalers until protected by the international community in 1966. A 2002 report estimated there were 5,000 to 12,000 Blue Whales worldwide located in at least five groups. More recent research into the Pygmy subspecies suggests this may be an underestimate. Before whaling the largest population was in the Antarctic, numbering approximately 239,000 (range 202,000 to 311,000). There remain only much smaller (around 2,000) concentrations in each of the North-East Pacific, Antarctic, and Indian Ocean groups. There are two more groups in the North Atlantic and at least two in the Southern Hemisphere.

Blue Whales are rorquals (family Balaenopteridae), a family that includes the Humpback Whale, the Fin Whale, Bryde's Whale, the Sei Whale and the Minke Whale. The family Balaenopteridae is believed to have diverged from the other families of the suborder Mysticeti as long ago as the middle Oligocene. However, it is not known when the members of those families diverged from each other. The Blue Whale is usually classified as one of seven species of whale in the genus Balaenoptera; one authority placed it in a separate monotypic genus Sibbaldus, but this was not accepted elsewhere. DNA sequencing analysis indicates that Blue Whales are phylogenetically closer to the Humpback (Megaptera) and the Gray Whale (Eschrichtius) than to other Balaenoptera species. If further research confirms these relationships, it will be necessary to reclassify the rorquals.

There have been at least 11 documented cases of Blue/Fin Whale hybrid adults in the wild. Arnason and Gullberg describe the genetic distance between a Blue and a Fin as about the same as that between a human and gorilla. Blue Whale/Humpback Whale hybrids are also known.

The specific name musculus is Latin and could mean "muscular", but it can also be interpreted as "little mouse". Linnaeus, who named the species in his seminal Systema Naturae of 1758, would have known this and may have intended the ironic double meaning. The species was called Sulphur-bottom by Herman Melville in his novel Moby-Dick due to an orangish-brown or yellow tinge on the underparts from diatom films on the skin. Other common names for the Blue Whale have included the Sibbald's Rorqual (after Sir Robert Sibbald), the Great Blue Whale and the Great Northern Rorqual. These names have fallen into disuse in recent decades.

Authorities classify the species into three or four subspecies: B. m. musculus, the Northern Blue Whale consisting of the north Atlantic and north Pacific populations, B. m. intermedia, the Southern Blue Whale of the Southern Ocean, B. m. brevicauda, the Pygmy Blue Whale found in the Indian Ocean and South Pacific, and the more problematic B. m. indica, the Great Indian Rorqual, which is also found in the Indian Ocean and although described earlier may be the same subspecies as B. m. brevicauda.

The Blue Whale has a long tapering body that appears stretched in comparison with the stockier build of other whales. The head is flat and U-shaped and has a prominent ridge running from the blowhole to the top of the upper lip. The front part of the mouth is thick with baleen plates; around 300 plates (each around one metre (3.2 ft) long) hang from the upper jaw, running 0.5 m (1.6 ft) back into the mouth. Between 60 and 90 grooves (called ventral pleats) run along the throat parallel to the body. These pleats assist with evacuating water from the mouth after lunge feeding (see feeding below). The dorsal fin is small, visible only briefly during the dive sequence. Located around three-quarters of the way along the length of the body it varies in shape from one individual to another; some only have a barely perceptible lump, but others may have prominent and falcate dorsals. When surfacing to breathe, the Blue Whale raises its shoulder and blowhole out of the water to a greater extent than other large whales such as the Fin or Sei. This trait may be used by observers to differentiate between species at sea. Some Blue Whales in the North Atlantic and North Pacific raise their tail fluke when diving. When breathing, the whale emits a spectacular vertical single column blow (up to 12 m (40 ft), typically 9 m (30 ft)) that can be seen from a great distance on a calm day. Its lung capacity is 5,000 litres (1320 US gallons). Blue whales have twin blowholes, shielded by a large splashguard.

The flippers are three to four metres (10 to 13 ft) long. The upper sides are grey with a thin white border. The lower sides are white. The head and tail fluke are generally uniformly grey. The whale's upper parts, and sometimes the flippers, are usually mottled. The degree of mottling varies substantially from individual to individual. Some may have a uniform slate-grey colour all over, but others demonstrate a considerable variation of dark blues, greys and blacks, all tightly mottled.

Blue Whales can reach speeds of 50 km/h (30 mph) over short bursts, usually when interacting with other whales, but 20 km/h (12 mph) is a more typical travelling speed. When feeding they slow down to 5 km/h (3 mph).

Blue Whales most commonly live alone or with one other individual. It is not known whether those that travel in pairs stay together over long periods or form more loose relationships. In locations where there is a high concentration of food, as many as 50 Blue Whales have been seen scattered over a small area. However, they do not form the large close-knit groups seen in other baleen species.

Blue Whales are difficult to weigh because of their size. Most Blue Whales killed by whalers were not weighed whole, but cut up into manageable pieces first. This caused an underestimate of the total weight of the whale, due to the loss of blood and other fluids. Nevertheless, measurements between 150 and 170 tonnes (160 and 190 short tons) were recorded of animals up to 27 m (88 ft 6 inches) in length. The weight of a 30 m (98 ft) individual is believed by the American National Marine Mammal Laboratory (NMML) to be in excess of 180 tonnes (200 short tons). The largest Blue Whale accurately weighed by NMML scientists to date was a female that weighed 177 tonnes (196 short tons).

The Blue Whale is believed to be the largest animal ever to have lived. The largest known dinosaur of the Mesozoic era was the Argentinosaurus, which is estimated to have weighed up to 90 tonnes (100 short tons), though a controversial vertebra of Amphicoelias fragillimus may indicate an animal of up to 122 tonnes (135 short tons) and 40–60 meters (130–200 ft). The extinct fish Leedsichthys may have reached similar sizes. However, complete fossils are difficult to come by, making size comparisons difficult.

There is some uncertainty about the biggest Blue Whale ever found as most data comes from Blue Whales killed in Antarctic waters during the first half of the twentieth century and was collected by whalers not well-versed in standard zoological measurement techniques. The longest whales ever recorded were two females measuring 33.6 m and 33.3 m (110 ft 3 in and 109 ft 3 in) respectively. However, the reliability of these measurements is disputed. The longest whale measured by scientists at the NMML was 29.9 m (98 ft).

A Blue Whale's tongue weighs around 2.7 tonnes (3 short tons) and when fully expanded its mouth is large enough to hold up to 90 tonnes (100 short tons) of food and water. Despite the size of its mouth, the dimensions of its throat are such that a Blue Whale cannot swallow an object wider than a beach ball. Its heart weighs 600 kg (1,320 lb) and is the largest known in any animal. A Blue Whale's aorta is about 23 cm (9 in) in diameter. During the first 7 months of its life, a Blue Whale calf drinks approximately 400 litres (100 US gallons) of milk every day. Blue Whale calves gain weight quickly, as much as 90 kg (200 lb) every 24 hours. Even at birth, they weigh up to 2,700 kilograms (6,000 lb) – the same as a fully-grown hippopotamus.

Blue Whales feed almost exclusively on krill, though they also take small numbers of copepods. The species of this zooplankton eaten by Blue Whales varies from ocean to ocean. In the North Atlantic Meganyctiphanes norvegica, Thysanoessa raschii, Thysanoessa inermis and Thysanoessa longicaudata are the usual food. In the North Pacific Euphausia pacifica, Thysanoessa inermis, Thysanoessa longipes, Thysanoessa spinifera, Nyctiphanes symplex and Nematoscelis megalops; in the Antarctic Euphausia superba, Euphausia crystallorophias and Euphausia valentin.

An adult blue whale can eat up to 40 million krill in a day. The whales always feed in the areas with the highest concentration of krill, sometimes eating up to 3,600 kg (8,000 lb) of krill in a single day. This means that they typically feed at depths of more than 100 m (330 ft) during the day, and only surface feed at night. Dive times are typically 10 minutes when feeding, though dives of up to 20 minutes are common. The longest recorded dive is 36 minutes (Sears 1998). The whale feeds by lunging forward at groups of krill, taking the animals and a large quantity of water into its mouth. The water is then squeezed out through the baleen plates by pressure from the ventral pouch and tongue. Once the mouth is clear of water, the remaining krill, unable to pass through the plates, are swallowed. The Blue Whale also incidentally consumes small fish, crustaceans and squid caught up with krill.

Mating starts in late autumn, and continues to the end of winter. Little is known about mating behaviour or breeding grounds. Females typically give birth once every two to three years at the start of the winter after a gestation period of ten to twelve months. The calf weighs about two and a half tonnes (2.75 short tons) and is around 7 m (23 ft) in length. Blue Whale calves drink 380–570 litres (100–150 US gallons) of milk a day. Weaning takes place for about six months, by which time the calf has doubled in length. Sexual maturity is typically reached at eight to ten years by which time males are at least 20 m (66 ft) long (or more in the Southern Hemisphere). Females are larger still, reaching sexual maturity at around the age of five, by which they are about 21 m (69 ft) long.

Scientists estimate that Blue Whales can live for at least 80 years; however, since individual records do not date back into the whaling era, this will not be known with certainty for many years. The longest recorded study of a single individual is 34 years, in the north-east Pacific (reported in Sears, 1998). The whales' only natural predator is the Orca. Studies report that as many as 25% of mature Blue Whales have scars resulting from Orca attacks. The rate of mortality due to such attacks is unknown.

Blue Whale strandings are extremely uncommon, and, because of the species' social structure, mass strandings are unheard of. However when strandings do occur they can become the focus of public interest. In 1920, a Blue Whale washed up near Bragar on the Isle of Lewis in the Outer Hebrides of Scotland. It had been shot in the head by whalers, but the harpoon had failed to explode. As with other mammals, the fundamental instinct of the whale was to try to carry on breathing at all costs, even though this meant beaching to prevent itself from drowning. Two of the whale's bones were erected just off a main road on Lewis, and remain a tourist attraction.

Estimates made by Cummings and Thompson (1971) suggest that source level of sounds made by Blue Whales are between 155 and 188 decibels when measured relative to a reference pressure of one micropascal at one metre. All Blue Whale groups make calls at a fundamental frequency of between 10 and 40 Hz, and the lowest frequency sound a human can typically perceive is 20 Hz. Blue Whale calls last between ten and thirty seconds. Additionally Blue Whales off the coast of Sri Lanka have been recorded repeatedly making "songs" of four notes duration lasting about two minutes each, reminiscent of the well-known Humpback Whale songs. Researchers believe that as this phenomenon has not been seen in any other populations, it may be unique to the B. m. brevicauda (Pygmy) subspecies. The reason for vocalization is unknown. Richardson et al (1995) discuss six possible reasons:

Blue Whales are not easy to catch or kill. Their speed and power meant that they were rarely pursued by early whalers who instead targeted Sperm and Right Whales. In 1864 the Norwegian Svend Foyn equipped a steamboat with harpoons specifically designed for catching large whales. Although initially cumbersome and with a low success rate, Foyn perfected the harpoon gun and soon several whaling stations had been established on the coast of Finnmark in northern Norway. Because of disputes with the local fishermen, the last whaling station in Finnmark was closed down in 1904.

Soon blue whales were being hunted in Iceland (1883), the Faroe Islands (1894), Newfoundland (1898), and Spitsbergen (1903). In 1904-05 the first blue whales were taken off South Georgia. By 1925, with the advent of the stern slipway in factory ships, and the use of steam-driven whale catchers, the catch of blue whales, and baleen whales as a whole, in the Antarctic and sub-Antarctic began to increase dramatically. Between 1930 and 1931, these ships killed 29,400 Blue Whales in the Antarctic alone. By the end of World War II populations had been significantly depleted, and in 1946 the first quotas restricting international trade in whales were introduced, but they were ineffective because of the lack of differentiation between species. Rare species could be hunted on an equal footing with those found in relative abundance. Blue Whale hunting was banned in the 1960s by the International Whaling Commission, and illegal whaling by the USSR finally halted in the 1970s, by which time 330,000 Blue Whales had been killed in the Antarctic, 33,000 in the rest of the Southern Hemisphere, 8,200 in the North Pacific, and 7,000 in the North Atlantic. The largest original population, in the Antarctic, had been reduced to 0.15% of their initial numbers.

Whalers had clearly driven the Blue Whale to near-extinction, but rather than taking smaller harvests over a longer period, whalers continued to deplete the population. In hindsight, had the whaling industry taken into account monitoring and regulation by marine biologists, more whales might have been commercially available, albeit over a longer time span. The population dynamics involved in harvesting long-lived mammals are quite different from those involved in harvesting shorter-lived fish. Due to longer rates of reproduction (gestation of more than a year) and smaller litter size (one or two calves), whale populations recover much more slowly than the populations of smaller animals, which tend to invest less time and resources in individual young.

Since the introduction of the whaling ban, studies have failed to ascertain whether the global Blue Whale population is increasing or remaining stable. In the Antarctic, best estimates show a significant increase at 7.3% per year since the end of illegal Soviet Union whaling, but numbers remain at under 1% of their original levels. It has also been suggested that Icelandic and Californian populations are increasing but these increases are not statistically significant. The total world population was estimated to be between 5,000 and 12,000 in 2002, although there are high levels of uncertainty in available estimates for many areas. The Blue Whale remains listed as "endangered" on the IUCN Red List of threatened species as it has been since the list's inception. The largest known concentration, consisting of about 2,000 individuals, is the North-East Pacific population of the Northern Blue Whale (B. m. musculus) subspecies that ranges from Alaska to Costa Rica, but is most commonly seen from California in summer. Sometimes this population strays over to the North-West Pacific; infrequent sightings between Kamchatka and the northern tip of Japan have been recorded.

In the North Atlantic, two stocks of B. m. musculus are recognized. The first is found off Greenland, Newfoundland, Nova Scotia and the Gulf of Saint Lawrence. This group is estimated to total about 500. The second, more easterly group is spotted from the Açores in Spring to Iceland in July and August; it is presumed that the whales follow the Mid-Atlantic Ridge between the two volcanic islands. Beyond Iceland, Blue Whales have been spotted as far north as Spitsbergen and Jan Mayen though such sightings are rare. Scientists do not know where these whales spend their winters. The total North Atlantic population is estimated to be between 600 and 1500.

In the Southern Hemisphere, there appear to be two distinct subspecies, B. m. intermedia, the Southern Blue Whale and the little studied Pygmy Blue Whale, B. m. brevicauda, found in Indian Ocean waters. Recent abundance estimates for the Antarctic subspecies range from 1100 to 1700. Research into the number of Pygmy Blues is on-going. Estimates from a 1996 survey were that 424 Pygmy Blue Whales were in a small area south of Madagascar alone, thus it is likely that numbers in the entire Indian Ocean are in the thousands. If this is true, the global numbers would be much higher than estimates predict.

A fourth subspecies, B. m. indica, was identified by Blyth in 1859 in the northern Indian Ocean, but difficulties in identifying distinguishing features for this subspecies lead to it being used a synonym for B. m. brevicauda, the Pygmy Blue Whale. Records for Soviet catches seem to indicate that the female adult size is closer to that of the Pygmy Blue than B. m. musculus, although the populations of B. m. indica and B. m. brevicauda appear to be discrete and the breeding seasons differ by almost six months.

Migratory patterns of these subspecies are not well known. For example, Pygmy Blue Whales have been recorded in the northern Indian Ocean (Oman, Maldives, Sri Lanka) where they may form a distinct resident population. In addition, the population of Blue Whales occurring off Chile and Peru may also be a distinct population. Some Antarctic blue whales approach the eastern South Atlantic coast in winter, and occasionally their vocalizations are heard off Peru, Western Australia, and in the northern Indian Ocean. In Chile, the Cetacean Conservation Center, with support from the Chilean Navy, is undertaking extensive research and conservation work on a recently discovered feeding aggregation of the species off the coast of Chiloe Island in an area named "Golfo del Corcovado" where actually, sometimes, can be seen close to the coast with 326 animals spotted in 2007 summer season.

Efforts to calculate the Blue Whale population more accurately are supported by marine mammologists at Duke University who maintain the OBIS-SEAMAP (Ocean Biogeographic Information System - Spatial Ecological Analysis of Megavertebrate Populations), a collation of marine mammal sighting data from around 130 sources.

Due to their enormous size, power and speed, adult Blue Whales have virtually no natural predators. There is however one documented case in National Geographic Magazine of a Blue Whale being attacked by Orcas, although the Orcas were unable to kill the animal outright during their attack, the Blue Whale sustained massive wounds and probably died as a cause of them shortly after the attack.[citation needed]

Blue Whales may be wounded, sometimes fatally, after colliding with ocean vessels as well as becoming trapped or entangled in fishing gear. The ever-increasing amount of ocean noise drowns out the vocalizations produced by whales, which may make it harder for whales to communicate. Human threats to the potential recovery of Blue Whale populations also include accumulation of polychlorinated biphenyl (PCB) chemicals within the whale's body.

With global warming causing glaciers and permafrost to rapidly melt and allowing a large amount of fresh water to flow into the oceans, there are concerns that if the amount of fresh water in the oceans reaches a critical point there will be a disruption in the thermohaline circulation. Considering the Blue Whale's migratory patterns are based on ocean temperature, a disruption in this circulation which moves warm and cold water around the world would be likely to have an effect on their migration. The whales summer in the cool, high latitudes, where they feed in krill-abundant waters; they winter in warmer, low latitudes, where they mate and give birth.

The change in ocean temperature would also affect the Blue Whale’s food supply. The warming trend and decreased salinity levels would cause a significant shift in krill location and abundance.

The Natural History Museum in London contains a famous mounted skeleton and life-size model of a Blue Whale, which were both the first of their kind in the world, but has since been replicated at the University of California, Santa Cruz. Similarly, the American Museum of Natural History in New York City has a full-size model in its Milstein Family Hall of Ocean Life. Living Blue Whales may be encountered on whale-watching cruises in the Gulf of Maine, and are the main attractions along the north shore of the Gulf of Saint Lawrence and in the Saint Lawrence estuary. The Blue Whale has been represented in children's popular culture in the 1967 movie Doctor Dolittle, where it appears as a symbol of size and strength when it is employed to move an island.