Blue whale

The blue whale (Balaenoptera musculus) is a baleen whale and the largest living animal on Earth. Its streamlined, elongated body typically displays a bluish-grey dorsal surface and a paler underside. Although modern measurements have confirmed lengths exceeding 30 metres and masses approaching 150 tonnes, recent analyses suggest that several extinct dinosaur species may have attained comparable body masses. Blue whales were once abundant across the world’s oceans but were driven to the brink of extinction by commercial whaling before being protected internationally in the mid-twentieth century. Today, despite signs of recovery in some regions, they remain classified as an endangered species and continue to face multiple anthropogenic threats.

Taxonomy and Nomenclature

The blue whale belongs to the family Balaenopteridae, the rorquals, a group characterised by pleated throat grooves and lunge-feeding behaviour. The genus name Balaenoptera means “winged whale”, while the species epithet musculus may refer either to “muscle” or the Latin term for “little mouse”, a possible pun by Carl Linnaeus.
Early descriptions include Robert Sibbald’s seventeenth-century account of a stranded specimen from Scotland. The common name “blue whale” is derived from the Norwegian blåhval, adopted widely in the late nineteenth century. Historical whalers also used the term “sulphur-bottom” for individuals whose undersides bore a yellowish hue caused by diatom accumulation.
Multiple scientific names were historically applied to the species, reflecting uncertainties in early taxonomic classification. Through morphological and genetic studies, the species is now recognised under Balaenoptera musculus, with several subspecies.

Evolution and Genetic Relationships

Phylogenetic studies suggest that rorquals diverged from other baleen whale families during the late Miocene. Fossil evidence of anatomically modern blue whales dates to the Early Pleistocene. The pygmy blue whale, a smaller subspecies found in the Indo-Pacific region, appears to have diverged during the Last Glacial Maximum, resulting in relatively low genetic diversity.
Whole-genome sequencing indicates that blue whales are most closely related to sei whales, with grey whales forming their nearest sister lineage. Genetic analyses also reveal historical gene flow between blue, sei and minke whales, demonstrating complex evolutionary interactions within the Balaenopteridae.

Hybridisation

Blue whales are known to hybridise with fin whales, producing viable offspring. Historical accounts from Iceland and the North Pacific document unusual individuals with characteristics of both species. Modern genetic testing has verified several hybrids, including fertile adults, and has detected hybrid meat in some market samples.
A 2024 genome study found that approximately one-third of the North Atlantic blue whale genome shows evidence of introgression from fin whales. This gene flow appears to have occurred after the divergence of northern and southern populations. Occasional reports also suggest rare hybrids between blue whales and other rorquals, such as humpbacks.

Subspecies and Population Structure

At least four subspecies of blue whale are broadly recognised:

• B. m. musculus – North Atlantic and North Pacific
• B. m. intermedia – Southern Ocean
• B. m. brevicauda – pygmy blue whale of the Indian and South Pacific Oceans
• B. m. indica – Northern Indian Ocean

Further regional distinctions – often referred to as stocks – exist within these subspecies. Notable groups include populations in the Gulf of St Lawrence, eastern North Pacific, central-western Pacific and waters off Chile, the latter potentially representing a distinct subspecies.
Migration patterns vary. Many populations migrate seasonally between high-latitude feeding grounds and tropical breeding areas. Others show evidence of partial, age-based or sex-based migration, and some groups appear resident year-round within productive coastal regions.

Anatomy and Morphology

Blue whales possess a long, slender body and a relatively small dorsal fin set far back on the body. Their head is broad and U-shaped, with a prominent ridge running from the blowholes to the snout. The baleen plates, used for filter feeding, line the upper jaw.
They have two blowholes and powerful tail flukes that generate slow but forceful propulsion. Colouration varies from mottled slate-blue to greyish white, with lighter patches across the flanks.

Feeding Ecology and Behaviour

Blue whales are specialist filter feeders. Their diet consists almost entirely of krill, small shrimp-like crustaceans. Using lunge-feeding techniques, they take in large volumes of water and then force it through their baleen plates, trapping krill.
Feeding intensity is highest in summer in cold, nutrient-rich waters. During winter, fasting is common as whales migrate to warmer breeding grounds. Blue whales typically travel alone or in pairs, though small aggregations form in feeding areas. Social structures are fluid, with the strongest and most consistent bond being between mothers and calves.

Vocalisations and Communication

Blue whales produce extraordinarily low-frequency calls, typically between 8 and 25 Hz, capable of travelling vast distances through the ocean. These calls vary by region, season and behavioural context, and are used for communication during feeding, navigation and breeding. Their calls are among the loudest produced by any animal.

Predation and Natural Threats

Orcas (killer whales) are the only confirmed natural predators of blue whales. Attacks usually target calves or weakened adults. Apart from predation, natural mortality may result from disease, starvation or environmental fluctuations.

Human Impacts and Conservation Status

Prior to industrial whaling, blue whales were abundant across the world’s oceans. Nineteenth- and twentieth-century commercial whaling reduced global populations by nearly 97 per cent, with some regions experiencing near-extirpation. The International Whaling Commission banned the hunting of blue whales in 1966, allowing gradual but uneven recovery.
Today, blue whales remain listed as Endangered by the IUCN. They face numerous modern threats, including:

• Ship strikes in busy maritime zones
• Entanglement in fishing gear
• Chemical pollution and bioaccumulation
• Underwater noise from ships, sonar and industrial activity
• Climate change effects on krill distribution and ocean productivity