Rorquals are the largest group of baleen whales, with nine species in two genera. They include the largest animal that has ever lived, the Blue Whale, which can reach 150 tonnes, and two others that easily pass 50 tonnes; even the smallest of the group, the Northern Minke Whale, reaches 9 tonnes.
Rorquals take their name from the Norwegian word röyrkval, meaning "furrow whale". All members of the family have a series of longitudinal folds of skin running from below the mouth back to the navel (except the Sei Whale, which has shorter grooves). These are understood to allow the mouth to expand immensely when feeding. The "Minke" is allegedly named after a Norwegian whaler named Meincke, who mistook a Northern Minke Whale for a Blue Whale.
Rorquals are slender and streamlined in shape, compared with their relatives the right whales, and most have narrow, elongated flippers. They have a dorsal fin, situated far back on the body, near to the tail. Rorquals feed by gulping in water, and then pushing it out through the baleen plates with their tongue. They feed on crustaceans, such as krill, but also on various fish, such as herrings and sardines.
Gestation in rorquals lasts 11-12 months, so that both mating and birthing occur at the same time of year. Mothers give birth to a single young, which is weaned after 6-12 months, depending on species. Adults live in small herds, or 'pods' of two to five individuals.
Distribution is worldwide: the Blue, Fin, Humpback, and the Sei Whales are found in all major oceans; the Common (Northern) and Antarctic (Southern) Minke Whale species are found in all the oceans of their respective hemispheres; and either of Bryde's Whale and Eden's Whale occur in the Atlantic, Pacific, and Indian oceans, being absent only from the cold waters of the Arctic and Antarctic.
Most rorquals are fairly strictly oceanic: the exceptions are Bryde's Whale and Eden's Whale (which are usually found close to shore all year round) and the Humpback Whale (which is oceanic but passes close to shore when migrating). It is the largest and the smallest types - the Blue Whale and Antarctic Minke Whale - that occupy the coldest waters in the extreme south; the Fin Whale tends not to approach so close to the ice shelf; the Sei Whale tends to stay further north again. (In the northern hemisphere, where the continents distort weather patterns and ocean currents, these movements are less obvious, although still present.) Within each species, the largest individuals tend to approach the poles more closely, while the youngest and fittest ones tend to stay in warmer waters before leaving on their annual migration.
Most rorquals breed in temperate waters during the winter, then migrate back to the polar feeding grounds rich in plankton and krill for the short polar summer.
Taxonomically, the Balaenopteridae (rorqual) family is split into two subfamilies - Balaenopterinae and Megapterinae. Each subfamily contains one genus - Balaenoptera and Megaptera respectively. However, the phylogeny of the various rorqual species shows the current division is paraphyletic, and may need to be adjusted.
The discovery of new member of the Balaenopteridae family was announced in November 2003 - specimens of the Balaenoptera omurai, which looks similar to, if smaller than, the Fin Whale were found in Indo-Pacific waters.