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Rorquals () are the largest clade of , comprising the family Balaenopteridae, which contains nine extant taxon in two genus. They include the largest known animal that has ever lived, the blue whale, which can reach , and the fin whale, which reaches ; even the smallest of the group, the northern minke whale, reaches .
Rorquals take their name from French language rorqual, which derives from the Norwegian word : the first element røyr originated from the Old Norse name for this type of whale, reyðr,: " røyrkval, Old Norse * røyðar-, Old Icelandic reyðar-hvalr, the specific name + hvalr whale." probably related to the Norse word for "red", and the second from the Norse word hvalr meaning "whale" in general. The family name Balaenopteridae is from the type genus, Balaenoptera.
Rorquals are slender and streamlined in shape, compared with their relatives the , and most have narrow, elongated flippers. They have a dorsal fin, situated about two-thirds the way back. Most rorquals feed by gulping in water, and then pushing it out through the baleen plates with their tongue; the exception is the gray whale, which gulps in and filters large amounts of marine sediments from the seafloor. They feed on crustaceans, such as krill, but also on various fish, such as and .
Gestation in rorquals lasts 11–12 months, so that both mating and birthing occur at the same time of year. Cows give birth to a single calf, which is weaning after 6–12 months, depending on species. Of some species, adults live in small groups, or "pods" of two to five individuals. For example, humpback whales have a fluid social structure, often engaging behavioral practices in a pod, other times being solitary.
Most rorquals are strictly oceanic: the exceptions are the gray whale, Bryde's whale, Eden's whale, and Rice'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 , 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 tropical waters during the winter, then migrate back to the polar feeding grounds rich in plankton and krill for the short polar summer.
Rorquals have a number of anatomical features that enable them to do this, including bilaterally separate , throat pleats that can expand to huge size, and a unique sensory organ consisting of a bundle of that helps their brains to coordinate the engulfment action. Furthermore, their large nerves are flexible so that they can stretch and recoil. In fact, they give rorquals the ability to open their mouths so wide that they would be capable of taking in water at volumes greater than their own sizes. These nerves are packed into a central core area that is surrounded by elastin fibers. Opening the mouth causes the nerves to unfold, and they snap back after the mouth is closed. According to Potvin and Goldbogen, lunge feeding in rorquals represents the largest Biomechanics event on Earth.
The discovery of a new species of balaenopterid, Omura's whale ( Balaenoptera omurai), was announced in November 2003, which looks similar to, but smaller than, the fin whale; individuals of this species were found in Indo-Pacific waters. The discovery of the highly endangered Rice's whale was announced in 2021 after a genetic study found it to be distinct from Bryde's whale; this species is known from a small portion of the northeastern Gulf of Mexico.
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