The blue whale, Balaenoptera musculus, is the largest animal known to have lived, larger than any dinosaur yet described. It eats one of the smallest animals in the ocean, it produces some of the loudest sounds any animal makes, and in the twentieth century it was reduced by industrial whaling to a small fraction of its former numbers, from which it has only partly recovered.
The largest reliably measured blue whales are around thirty metres and 190 tonnes. Almost every comparison drawn from this is true and still fails to convey it: the tongue alone weighs as much as an elephant, the heart of a specimen recovered in Newfoundland in 2014 and preserved at the Royal Ontario Museum weighs about 180 kilograms, and the aorta is wide enough to put a head into.
A calf is born at around seven metres and two and a half tonnes and gains something in the region of ninety kilograms a day on milk with a fat content near forty per cent. It is the fastest growth of any animal.
Size on this scale is possible only in water. A land animal of this mass could not support itself or move blood against gravity through that height. Blue whales are also, unusually, still getting larger in evolutionary terms: baleen whale gigantism appears to be recent, arising within the last few million years, and is generally linked to the intensification of ocean upwelling that concentrated their food into dense patches worth the trouble of a huge body.
Blue whales eat krill, almost exclusively. They feed by lunging: accelerating into a swarm with the mouth open, engulfing a volume of water that can exceed the whale's own body volume, held by the pleated throat which expands enormously. The mouth closes, the tongue and throat muscles force the water out through the baleen, and the krill is retained.
A single lunge is metabolically expensive, and the whole strategy only pays where krill occurs in dense patches. Tag-based work published in 2021 revised intake estimates sharply upward, to as much as sixteen tonnes a day for the largest individuals in peak feeding season, roughly three times earlier figures.
That revision produced a genuine puzzle about the Southern Ocean. Removing the great whales should, on a simple grazing model, have left more krill. Krill instead declined. The leading explanation is the whale pump: whales feed at depth and defecate at the surface, releasing iron in a form phytoplankton can use in an ocean where iron is the limiting nutrient. The whales were fertilising the system that fed them, and removing them removed the fertiliser.
Blue whales produce calls below 20 hertz, at the edge of or beneath human hearing, at source levels around 188 decibels, among the loudest sounds made by any animal. Low frequency travels extraordinarily well in water, and these calls carry hundreds of kilometres.
Only males sing the long patterned songs, which suggests a reproductive function, though the details are not established. Populations have distinct song types, and those types are one of the main ways populations are told apart.
There is an unexplained trend attached. The frequency of blue whale song has been falling worldwide since recordings began in the 1960s, by a few tenths of a hertz per year, consistently across populations that never meet. Proposed explanations include recovering population density changing the need to project, changing ocean acidity affecting sound absorption, and a cultural drift in song. None is established, and it remains one of the more interesting open questions in cetacean biology.
The famous 52-hertz whale, recorded since 1989 singing far above the usual blue whale frequency and often called the loneliest whale in the world, is probably a hybrid or an anomalous individual. It is not evidence that it has no companions; recordings from multiple hydrophones have suggested more than one animal calling at that frequency.
Blue whales were largely unhunted through the age of sail: too fast, too large, and they sink when killed. The steam-powered catcher with a bow-mounted explosive harpoon, introduced by Svend Foyn in the 1860s, removed all three obstacles, and the pelagic factory ship in the 1920s removed the need to return to a shore station.
The result was the largest deliberate removal of animal biomass in history. Something over 380,000 blue whales were killed in the Southern Hemisphere in the twentieth century. The Antarctic population, once perhaps 200,000 or more, fell to a few hundred, on the order of one per cent.
Protection came in 1966 through the International Whaling Commission, though Soviet fleets continued illegal catches for years afterward and falsified their returns, a fact that only emerged fully after 1991 and which invalidated much of the catch record used to model recovery.
The species is Endangered, with a global population usually estimated between about 10,000 and 25,000. Recovery is uneven: the eastern North Pacific population off California is doing comparatively well and may be near its historical level, while Antarctic blue whales remain at a small fraction of pre-whaling numbers despite a measurable increase.
Current threats are ship strikes, which are a leading cause of death in shipping lanes off California, Sri Lanka and Chile; entanglement in fishing gear; noise that masks the low-frequency calls they use to find each other across ocean basins; and, further out, changes to krill distribution from warming and from the krill fishery itself.
The blue whale is the clearest demonstration that a species can be reduced by ninety-nine per cent and still exist, and that existing is not the same as recovering. Sixty years after protection, the population that took the worst of it is still counted in thousands where it was once counted in hundreds of thousands, because an animal that matures at around ten years and produces one calf every two or three years cannot rebuild quickly, whatever is done for it.
