The blue whale (Balaenoptera musculus) is a marine mammal belonging to the baleen whales (Mysticeti). At 30 metres (98 ft) in length and 170 tonnes (190 short tons) or more in weight, it is the largest known 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 almost exclusively of small crustaceans known as krill.
Blue whales were abundant in nearly all the oceans on Earth until the beginning of the twentieth century. For over a century, 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 eastern North Pacific, Antarctic, and Indian Ocean groups. There are two more groups in the North Atlantic, and at least two in the Southern Hemisphere.
The blue whale is the largest animal ever known to have lived. The largest known dinosaur of the Mesozoic Era was Argentinosaurus, which is estimated to have weighed up to 90 metric tons (99 short tons).
Blue whales are difficult to weigh because of their size. As is the case with most large whales targeted by whalers, adult blue whales have never been 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. As a whole, blue whales from the Northern Atlantic and Pacific appear to be smaller on average than those from sub-Antarctic waters. Nevertheless, measurements between 150–170 metric tons (170–190 short tons) were recorded of animals up to 27 metres (89 ft) in length. The weight of an individual 30 metres (98 ft) long is believed by the American National Marine Mammal Laboratory (NMML) to be in excess of 180 metric tons (200 short tons). The largest blue whale accurately weighed by NMML scientists to date was a female that weighed 177 metric tons (195 short tons).
There is some uncertainty about the biggest blue whale ever found, as most data came from blue whales killed in Antarctic waters during the first half of the twentieth century, which were collected by whalers not well-versed in standard zoological measurement techniques. The heaviest whale ever recorded weighed in at approximately 190 metric tons (210 short tons). The longest whales ever recorded were two females measuring 33.6 metres (110 ft) and 33.3 metres (109 ft), although in neither of these cases was the piecemeal weight gathered. The longest whale measured by scientists at the NMML was a 29.9 metres (98 ft), female caught in the Antarctic by Japanese whalers in 1946–47. Lieut. Quentin R. Walsh, USCG, while acting as whaling inspector of the factory ship Ulysses, verified the measurement of a 30 m (98 ft) pregnant blue whale caught in the Antarctic in the 1937–38 season. The longest reported in the North Pacific was a 27.1 metres (89 ft) female taken by Japanese whalers in 1959, and the longest reported in the North Atlantic was a 28.1 metres (92 ft) female caught in the Davis Strait.
Due to its large size, several organs of the blue whale are the largest in the animal kingdom. A blue whale’s tongue weighs around 2.7 metric tons (3.0 short tons) and, when fully expanded, its mouth is large enough to hold up to 90 metric tons (99 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 kilograms (1,300 lb) and is the largest known in any animal. A blue whale’s thoracic aorta is estimated to be 23 centimetres (9.1 in) in diameter. During the first seven months of its life, a blue whale calf drinks approximately 400 litres (110 US gal) of milk every day. Blue whale calves gain weight quickly, as much as 90 kilograms (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 have relatively small brains, only about 6.92 kilograms (15.26 lb) , about 0.007% of its body weight. The male blue whale penis is the largest of any living organism