The Blue Whale (Balaenoptera musculus) is a marine mammal and the largest animal known to have ever existed, reaching up to 29.9 m (98 ft) in length and weighing as much as 199 t (219 short tons). Its body is long and slender, displaying varying shades of greyish-blue above and lighter below. There are four recognized subspecies: B. m. musculus in the North Atlantic and Pacific, B. m. intermedia in the Southern Ocean, B. m. brevicauda (pygmy blue whale) in the Indian Ocean and South Pacific, and B. m. indica in the Northern Indian Ocean. A possible fifth subspecies exists off the coast of Chile.
Blue whale populations migrate between summer feeding areas near the poles and winter breeding grounds near the tropics. They primarily feed on krill and are generally solitary or found in small groups, with mother–calf bonds being their main social structure. Blue whales vocalize at frequencies between 8 to 25 Hz, and orcas are their only natural predators.
Once abundant in Earth’s oceans, blue whales were hunted nearly to extinction until the International Whaling Commission banned their hunting in 1966. As of 2018, they are listed as Endangered by the International Union for Conservation of Nature and face threats from ship strikes, pollution, ocean noise, and climate change.
Physically, blue whales have a slender body with a broad U-shaped head, elongated flippers, a small dorsal fin, and large tail flukes. They possess 70–395 black baleen plates and 60–88 throat grooves that expand during feeding. Their skin appears mottled grayish-blue, looking blue underwater, and their underbelly can appear yellowish due to diatoms. Notably, male blue whales have the largest penis in the animal kingdom, measuring about 3 m (9.8 ft) long.
The blue whale is the largest known animal ever, with some estimates suggesting ancient creatures like the shastasaurid ichthyosaurs and Perucetus could match its size. Perucetus may even have been heavier, but its estimated size is debated due to fragmentary remains. On land, large sauropods like Bruhathkayosaurus and Maraapunisaurus could rival blue whales, although these estimates also rely on incomplete specimens.
The International Whaling Commission reports 88 blue whales longer than 30 meters (98 ft), with the longest scientifically measured individual at 30 meters. Female blue whales tend to be larger than males. Hydrodynamic models suggest blue whales likely can’t exceed 33 meters (108 ft) due to energy constraints.
Average lengths for mature females vary by region: 22.0 meters for Eastern North Pacific blue whales; 24 meters for the central and western North Pacific; 21–24 meters for North Atlantic; 25.4–26.3 meters for Antarctic; 23.5 meters for Chilean; and 21.3 meters for pygmy blue whales. Males weigh around 100 metric tons in the Northern Hemisphere, while females average 112 metric tons.
In 2024, research estimated a 25-meter blue whale weighs 101–119 tonnes, and a 30-meter individual weighs 184–205 tonnes. The largest blue whale recorded was 33 meters long, estimated to weigh 252–273 tonnes. A preserved fetus collected in 1922 provides insights into blue whale development, measuring approximately 133 mm at a young gestational age.
The blue whale is typically solitary but can occasionally be found in pairs or larger groups when food is abundant. They migrate long distances between summer feeding grounds in polar regions and winter breeding grounds in warmer waters. They have excellent memory for locating feeding areas and can display various migration strategies.
Swimming at an average speed of 2–8 kilometers per hour, blue whales can reach speeds of 32–36 kilometers per hour when threatened. Their maximum dive depth is recorded at 315 meters, with heart rates dropping as low as 2 bpm during deep dives and rising to 37 bpm upon surfacing.
Their diet mainly consists of krill, capturing it through lunge feeding, where they can engulf up to 220 metric tons of water. To meet their energetic needs, blue whales consume about 1,120 kilograms of krill daily, equivalent to 4 tons. While they predominantly feed on Antarctic krill in the Southern Ocean, the Northern Indian Ocean subspecies mainly consumes sergestid shrimp and other marine organisms.
Blue whales seem to avoid competition with other baleen whales by varying their feeding times, locations, and prey. Their feeding behavior may also change in response to environmental disturbances or human activity, with studies indicating that stress can influence their diet and behavior.
The age of sexual maturity for blue whales is estimated at 5–15 years. In the Northern Hemisphere, females reach maturity at 21–23 meters (69–75 ft) and males at 20–21 meters (66–69 ft). In the Southern Hemisphere, females mature at 23–24 meters (75–79 ft) and males at 22 meters (72 ft). Male pygmy blue whales average 18.7 meters (61.4 ft) at maturity, while females are about 21.0–21.7 meters (68.9–71.2 ft) and around 10 years old. Physical maturity is assumed to occur by age 35. Mating behavior is not well understood, but blue whales are believed to be polygynous, with males competing for females during the mating season from fall to winter.
Pregnant females consume about 4% of their body weight daily, and gestation lasts 10–12 months, with calves measuring 6–7 meters (20–23 ft) and weighing 2–3 metric tons at birth. Blue whales likely produce 220 kilograms (490 lb) of milk daily for their calves, which may be weaned at 6–8 months, reaching a length of 16 meters (53 ft) and gaining roughly 37,500 pounds (17,000 kg) during weaning. Interbirth intervals last 2–3 years, averaging 2.6 years in pygmy blue whales, and mother-calf pairings are infrequent due to mothers birthing and weaning away from summer feeding grounds.
