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The blue whale (Balaenoptera musculus) is a marine mammal belonging to the baleen whale parvorder Mysticeti. Reaching a maximum confirmed length of 29.9 metres (98 ft) and weighing up to 199 tonnes (196 long tons; 219 short tons), it is the largest animal known to have ever existed. The blue whale's long and slender body can be various shades of greyish-blue dorsally and somewhat lighter underneath.


Blue whales have long, slender mottled grayish-blue bodies, although they appear blue underwater. The mottling pattern is highly variable. Individuals have a unique pigmentation pattern along the back in the region of the dorsal fin which can be used for the purpose of identification.

Additional distinguishing features of the blue whale include a broad, flat head, which appears U shaped from above; 270–395 entirely black baleen plates on each side of their upper jaw; 60–88 expandable throat pleats; long, slender flippers; a small—up to 33 centimetres (13 in)— falcate dorsal fin positioned far back toward the tail; a thick tail stock; and a massive, slender fluke.

Their pale underside can accumulate a yellowish coat of diatoms, which historically earned them the nickname "sulphur bottom". The blue whale's two blowholes create a tall, columnar spray, which can be seen 9.1–12.2 metres (30–40 ft) above the water's surface.


The blue whale is the largest known animal to have ever existed. The International Whaling Commission (IWC) whaling database reports 88 individuals longer than 30 metres (98 ft), including one of 33 metres (108 ft), but problems with how the measurements were taken suggest that any longer than 30.5 metres (100 ft) are suspect. The Discovery Committee reported lengths up to 31 metres (102 ft); however, the longest scientifically measured individual blue whale was 30 metres (98 ft) from rostrum tip to tail notch. Female blue whales are larger than males. Hydrodynamic models suggest a blue whale could not exceed 108 ft (33 m) because of metabolic and energy constraints. The highest recorded weight for the species is 199 tonnes (196 long tons; 219 short tons).

The average length of sexually mature female blue whales is 22.0 metres (72.1 ft) for Eastern North Pacific blue whales, 24 metres (79 ft) for central and western North Pacific blue whales, 21–24 metres (68–78 ft) for North Atlantic blue whales, 25.4–26.3 metres (83.4–86.3 ft) for Antarctic blue whales, 23.5 metres (77.1 ft) for Chilean blue whales, and 21.3 metres (69.9 ft) for pygmy blue whales.

In the Northern Hemisphere, males weigh an average 100 tonnes (220,000 lb) and females 112 tonnes (247,000 lb). Eastern North Pacific blue whale males average 88.5 tonnes (195,000 lb) and females 100 tonnes (220,000 lb). Antarctic males average 112 tonnes (247,000 lb) and females 130 tonnes (290,000 lb). Pygmy blue whale males average 83.5 tonnes (184,000 lb) to 99 tonnes (218,000 lb). The weight measured of the heart from a stranded North Atlantic blue whale was 0.1985 tonnes (438 lb), the largest known in any animal.