Rorqual

Distribution and habitat

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; 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; and the gray whale is found in the northern Pacific Ocean, although it was also found in the Atlantic Ocean in historic times. Rice's whale has the smallest distribution of rorquals and possibly baleen whales in general, being endemic to a small portion of the Gulf of Mexico west of the Florida peninsula and south of Alabama and the Florida panhandle, although it likely formerly had a much wider distribution in the Gulf.

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 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 tropical waters during the winter, then migrate back to the polar feeding grounds rich in plankton and krill for the short polar summer.

Feeding habits

As well as other methods, rorquals obtain prey by lunge-feeding on bait balls. Lunge feeding is an extreme feeding method, where the whale accelerates to a high velocity and then opens its mouth to a large gape angle. This generates the water pressure required to expand its mouth and engulf and filter a huge amount of water and fish. In contrast to typical lunge feeding, Gray whales employ a unique suction feeding strategy to capture prey near or on the sea bottom.

Rorquals have a number of anatomical features that enable them to do this, including bilaterally separate mandibles, throat pleats that can expand to huge size, and a unique sensory organ consisting of a bundle of mechanoreceptors 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 biomechanical event on Earth.

Taxonomy

Formerly, the rorqual family Balaenopteridae was split into two subfamilies, the Balaenopterinae and the Megapterinae, with each subfamily containing one genus, Balaenoptera and Megaptera, respectively. However, the phylogeny of the various rorqual species shows the current division is paraphyletic, and in 2005, the division into subfamilies was dropped. Two genetic studies, one in 2018 and one in 2020, suggest that the gray whale (Eschrichtius robustus) be counted among the rorquals, being more derived than the two minke whales but basal to the humpback whale, fin whale, and the other taxa classified in Balaenoptera.

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.

Extant species

• Family Balaenopteridae: Rorquals
  • Balaenoptera
    • Fin whale, Balaenoptera physalus
      • Northern fin whale, Balaenoptera physalus physalus
      • Southern fin whale, Balaenoptera physalus quoyi
    • Sei whale, Balaenoptera borealis
    • Bryde's whale, Balaenoptera edeni
      • Eden's whale, Balaenoptera edeni edeni
    • Rice's whale, Balaenoptera ricei
    • Blue whale, Balaenoptera musculus
      • Pygmy blue whale, Balaenoptera musculus brevicauda
    • Common minke whale, Balaenoptera acutorostrata
    • Antarctic minke whale, Balaenoptera bonaerensis
    • Omura's whale, Balaenoptera omurai
  • Megaptera
    • Humpback whale, Megaptera novaeangliae

Fossil genera

• • †Archaebalaenoptera
  • †Archaeschrichtius
  • †Cetotheriophanes
  • †Diunatans
  • †Eschrichtioides
  • †Gricetoides
  • †Incakujira
  • †Miobalaenoptera
  • †Nehalaennia
  • †Parabalaenoptera
  • †Plesiobalaenoptera
  • †Plesiocetus
  • †Praemegaptera
  • †Protororqualus

Alternative generic taxonomy for living rorquals

In 2012, the following alternate taxonomy was presented:

• Balaenoptera
  • Fin whale, Balaenoptera physalus
• Megaptera
  • Humpback whale, Megaptera novaeangliae
• Pterobalaena
  • Common minke whale, Pterobalaena acutorostrata
  • Antarctic minke whale, Pterobalaena bonaerensis
• Rorqualus
  • Sei whale, Rorqualus borealis
  • Bryde's whale, Rorqualus brydei
  • Eden's whale, Rorqualus edeni
  • Blue whale, Rorqualus musculus
  • Omura's whale, Rorqualus omurai