Blue Whale (Balaenoptera musculus): Size, Facts and Diet

What does a blue whale look like?

The blue whale has a long, slender, streamlined body and a broad, flat head that is U-shaped when seen from above. A single ridge runs from the two blowholes toward the tip of the snout. The skin is mottled blue-gray, darker on the back and paler underneath. Through the water it can look bright blue, which gave the species its name. Each whale’s mottling is unique, so researchers use photographs of it to recognize individuals.

The dorsal fin is tiny for such a huge animal, often only about 35 cm (14 in) tall, and sits far back near the tail. The flippers are long and slim, and the tail flukes are broad and triangular. When a blue whale breathes out, its blow rises as a tall, narrow column of spray up to 9–12 m (30–39 ft) high.

Under the chin and throat run roughly 60–90 deep grooves, called ventral pleats, which let the throat balloon out when the whale feeds. Instead of teeth, the upper jaw holds several hundred black baleen plates, each less than 1 m (3.3 ft) long. Some blue whales have a yellowish belly, caused by a film of microscopic algae called diatoms, which is why old whalers called them “sulphur-bottoms”.

How big is a blue whale?

Size varies between populations. Antarctic blue whales are the giants: mature females there average about 25–27 m (83–87 ft) long. Mature females in the North Atlantic and North Pacific average about 21–24 m (69–79 ft), and pygmy blue whales average about 21 m (70 ft). As in other baleen whales, females grow larger than males.

The longest reliably measured blue whales were about 29.9–30.5 m (98–100 ft). Whaling records list a few animals of 33 m (108 ft) or more, but these figures are doubted because whalers did not always measure carefully. The heaviest whale on record, a female caught in the Southern Ocean in 1947, was estimated at about 190 tonnes.

For bus, dinosaur and airliner comparisons, see How big is a blue whale?. Even its heart is the largest of any animal, though not the size of a car, as explained in How big is a blue whale’s heart?

Where do blue whales live?

Blue whales live in every ocean except the Arctic Ocean, although they are rare in enclosed seas such as the Mediterranean. They spend most of their lives in the open ocean but also gather along continental shelf edges and deep underwater canyons, where currents concentrate krill.

Many populations migrate between cold, food-rich feeding grounds toward the poles in summer and warmer waters nearer the tropics in winter, where they breed. The pattern is not fixed. Some whales stay in one region all year, some feed in their winter range, and the blue whales of the northern Indian Ocean appear to remain there year-round.

Scientists usually recognize four subspecies:

• Northern blue whale (B. m. musculus): North Atlantic and North Pacific.
• Antarctic blue whale (B. m. intermedia): Southern Ocean, the largest form.
• Pygmy blue whale (B. m. brevicauda): Indian Ocean and South Pacific.
• Northern Indian Ocean blue whale (B. m. indica): its status as a separate subspecies is uncertain.

Blue whales off Chile differ in genetics and song and may form a fifth subspecies that has not yet been formally named.

What do blue whales eat?

Blue whales are filter feeders that eat almost nothing but krill, small shrimp-like crustaceans. In the Southern Ocean they feed on Antarctic krill (Euphausia superba), while off California they take mainly Thysanoessa spinifera and North Pacific krill. The blue whales of the northern Indian Ocean are an exception and eat mostly small sergestid shrimp.

They catch their food by lunge feeding. A whale speeds toward a dense swarm, drops its jaw open and lets its pleated throat swell like a balloon, taking in water and krill that may weigh as much as the whale itself. It then closes its mouth and uses its tongue and throat to push the water out through the baleen, which traps the krill.

Estimates of how much they eat vary. Older studies suggested around 4 tonnes of krill a day, while a 2021 study based on tags estimated about 16 tonnes a day for eastern North Pacific blue whales during the feeding season. Learn more in What do blue whales eat?

How do blue whales behave and communicate?

Blue whales are usually seen alone or in pairs. Where krill is plentiful, loose gatherings of 50 or more can form, but apart from the bond between a mother and her calf they have no stable social groups. They normally cruise at 2–8 km/h (1–5 mph) and can sprint for short bursts. Feeding dives off California average about 10 minutes and reach around 200 m (660 ft); the deepest dive recorded from a tagged blue whale was 315 m (1,033 ft), and a pygmy blue whale has been tracked to 506 m (1,660 ft).

Blue whales make some of the loudest and lowest sounds of any animal. Their calls have main frequencies of about 8–25 hertz, at or below the lowest pitch human ears can hear, and reach source levels of about 188 decibels. Such low sounds travel well in seawater and can be picked up hundreds of kilometers away. The long, patterned songs off California are sung by males and probably play a role in mating.

Each population has its own song type, which lets scientists track whales with underwater microphones. The pitch of some songs has been falling for decades: off California, it dropped by about 31% between the early 1960s and the early 2000s. Why this is happening is still debated.

Reproduction, calves and lifespan

Blue whales mate from late fall into winter, mostly in warmer waters. Males follow females and may chase off rivals. After a pregnancy of 10–12 months, the mother gives birth to a single calf that is already 6–7 m (20–23 ft) long and weighs 2–3 tonnes. Feeding on very rich milk, a calf gains about 90 kg (200 lb) a day and is weaned at around 6–8 months, when it is about 16 m (53 ft) long.

Females usually give birth every 2–3 years. Blue whales become sexually mature at roughly 5–15 years old, with females averaging about 10 years. Mothers with young calves are seldom seen, but the Gulf of California in Mexico is a known nursery area from December to March.

Blue whales are long-lived. Their lifespan is estimated at around 80–90 years. Scientists age dead whales by counting yearly layers in their waxy earplugs, much like tree rings, and the oldest blue whale aged this way was about 110 years old.

Predators and threats

Orcas are the only known natural predator, and they mainly target calves. Many blue whales carry rake-like tooth scars from orca encounters. The first kills by orcas to be directly observed happened off southwestern Australia in 2019 and 2021; one of the victims was estimated at 18–22 m (59–72 ft) long.

The main threats today come from people. Collisions with large ships are a significant cause of death, especially off the U.S. West Coast and southern Sri Lanka, where feeding areas overlap busy shipping lanes. Off the U.S. West Coast, 17 blue whales were known or suspected to have been killed by ships between 1998 and 2019. Other risks include entanglement in fishing gear, underwater noise from ships, sonar and seismic surveys, pollution, and climate change, which can shift or reduce the krill swarms the whales depend on.

Conservation status

The blue whale is listed as Endangered on the IUCN Red List and under the U.S. Endangered Species Act. Explosive harpoons, fast steam-powered catcher boats and factory ships made it possible to hunt them from the late 19th century, and catches soared in the 20th century. Around 350,000–360,000 blue whales were killed in the Antarctic alone in the first half of the century, with about 30,000 taken in the single 1930–31 season.

The International Whaling Commission banned all blue whale hunting in 1966, although illegal Soviet catches continued for some years. Today the world population is estimated at about 10,000–25,000 animals, of which 5,000–15,000 are mature, compared with at least 140,000 mature whales in 1926. Numbers are rising, but the Antarctic subspecies is still listed as Critically Endangered. Regional figures are in How many blue whales are left?

Myths and facts

Myth: blue whales are bright blue. In air they look mottled gray-blue; only through the water do they appear vividly blue.

Myth: a blue whale’s heart is the size of a car. The largest heart ever measured weighed about 180–200 kg (400–440 lb) and was about 1.5 m (5 ft) tall, huge but far smaller than a car.

Fact: the scientific name may be a joke. Balaenoptera means “winged whale”, while musculus can mean either “muscular” or “little mouse”. Carl Linnaeus, who named the species in 1758, may have intended the pun.

Fact: blue whales sometimes hybridize. Blue whales occasionally interbreed with fin whales, and hybrids have been confirmed by DNA tests in the North Atlantic. A 2024 genome study found that about 3.5% of the North Atlantic blue whale genome came from fin whales. For how the biggest whale compares with the biggest fish, see whale shark vs blue whale.

Where can you see blue whales?

Blue whales can be watched from boats in several places. In North America, the best-known sites are Monterey Bay and the Santa Barbara Channel in California in summer and fall, the Gulf of St. Lawrence in Quebec, and the Gulf of California in Mexico in late winter. In Europe, they are seen off the Azores in spring and occasionally around Iceland. Mirissa, in southern Sri Lanka, is famous for winter sightings.

Responsible tour operators keep their distance, slow down near whales and never chase them, which helps protect animals that are still recovering from whaling.