Rorqual

Rorquals /ˈrɔːrkwəl/ (Balaenopteridae) are the largest group of baleen whales, a family with nine extant species in two genera. They include what is believed to be the largest animal that has ever lived, the blue whale, which can reach 180 tonnes (200 short tons), and the fin whale, which reaches 120 tonnes (130 short tons); even the smallest of the group, the northern minke whale, reaches 9 tonnes (9.9 short tons).

Rorquals[1]
Temporal range: Miocene–Recent[2]
Humpback whale, Megaptera novaeangliae
Scientific classification
Kingdom: Animalia
Phylum: Chordata
Class: Mammalia
Order: Artiodactyla
Infraorder: Cetacea
Superfamily: Balaenopteroidea
Family: Balaenopteridae
Gray 1864
Genera

Balaenoptera
Megaptera and see text

Rorquals take their name from French rorqual, which derives from the Norwegian word røyrkval, meaning "rorqual-whale" (the first element røyr originated from the Old Norse name for this type of whale, reyðr,[3][4] probably related to the Norse word for "red").[5] The family name Balaenopteridae is from the type genus, Balaenoptera.

Characteristics

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 and common minke whale, which have shorter grooves). These furrows allow the mouth to expand immensely when feeding, "permitting them to engorge great mouthfuls of food and water in a single gulp".[6] These "pleated throat grooves" distinguish balaenopterids from other whales.[6]

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 about two-thirds the way back. 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.[7]

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 weaned after 6–12 months, depending on species.[7] 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.

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; 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 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 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
Humpback feeding on young pollock off Alaska

As well as other methods, rorquals obtain prey by lunge-feeding on bait balls.[8] 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.[8]

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.[9] Furthermore, their large nerves are flexible so that they can stretch and recoil.[10] 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.[10] According to Potvin and Goldbogen, lunge feeding in rorquals represents the largest biomechanical event on Earth.[11]

Taxonomy
Cladogram of the family Balaenopteridae using complete mtDNA sequences and short interspersed repetitive element (SINE) insertion data.
Skeleton of the extinct "Megaptera" hubachi at the Museum für Naturkunde, Berlin

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.[12] 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.

Alternative generic taxonomy for living rorquals

In 2012, the following alternate taxonomy was presented:[14]

A 2018 genetic study suggests that gray whales (Eschrichtius robustus) be counted among the rorquals.[15]

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References

Notes
  1. Mead, J.G.; Brownell, R. L. Jr. (2005). "Order Cetacea". In Wilson, D.E.; Reeder, D.M (eds.). Mammal Species of the World: A Taxonomic and Geographic Reference (3rd ed.). Johns Hopkins University Press. pp. 723–743. ISBN 978-0-8018-8221-0. OCLC 62265494.
  2. "Family Balaenopteridae Gray 1864 (rorqual)". Fossilworks. Retrieved 9 April 2018.
  3. "rorqual". Merriam-Webster Dictionary.: "from Norwegian rørhval, from Old Norse reytharhvalr, from reythr rorqual + hvalr whale".
  4. "rorqual". Oxford English Dictionary (3rd ed.). Oxford University Press. September 2005. (Subscription or UK public library membership required.): "ad. Norw. røyrkval, repr. ON. *røyðar-, OIcel. reyðar-hvalr, f. reyðurreyðr the specific name + hvalr whale."
  5. The Century Dictionary. 1895. p. 5228.
  6. Minasian, Stanley M.; Balcomb, Kenneth C.; Foster, Larry, eds. (1984). The World's Whales: The Complete Illustrated Guide. New York: The Smithsonian Institution. p. 18. ISBN 978-0-89599-014-3.
  7. Gambell, Ray (1984). Macdonald, D (ed.). The Encyclopedia of Mammals. New York: Facts on File. pp. 222–225. ISBN 978-0-87196-871-5.
  8. Reeves, RR; Stewart, BS; Clapham, PJ; Powell, JA (2002). National Audubon Society Guide to Marine Mammals of the World. Chanticleer Press. ISBN 978-0-375-41141-0.
  9. Pyenson, N.D.; Goldbogen, J.A.; Vogl, A.W.; Szathmary, G; Drake, R.L.; Shadwick, R.E. (2012). "Discovery of a sensory organ that coordinates lunge feeding in rorqual whales". Nature. Nature Publishing Group. 485 (7399): 498–501. Bibcode:2012Natur.485..498P. doi:10.1038/nature11135. PMID 22622577.
  10. McSpadden, Kevin (5 May 2015). "Gigantic Whales Eat Thanks To 'Bungee-Cord' Nerves". TIME.com. Retrieved 6 May 2015.
  11. Potvin, J; Goldbogen, J.A. (2009). "Passive versus active engulfment: verdict from trajectory simulations of lunge-feeding fin whales Balaenoptera physalus". J. R. Soc. Interface. 6 (40): 1005–1025. doi:10.1098/rsif.2008.0492. PMC 2827442. PMID 19158011.
  12. Deméré, T.A.; Berta, A.; McGowen, M.R. (2005). "The taxonomic and evolutionary history of fossil and modern balaenopteroid mysticetes". Journal of Mammalian Evolution. 12 (1/2): 99–143. doi:10.1007/s10914-005-6944-3.
  13. Thalassotherii in the Paleobiology Database. Retrieved October 2013.
  14. Hassanin, A.; Delsuc, F.; Rpiquet, A.; Hammer, C.; Vuuren, B. J.; Matthee, C.; Ruiz-Garcia, M.; Gatzeflis, F.; Areskoug, V.; Nguyen, T. T.; Couloux, A. (2012). "Pattern and timing of diversification of Cetartiodactyla (Mammalia, Laurasiatheria), as revealed by a comprehensive analysis of mitochondrial genomes". Comptes Rendus Biologies. 335 (1): 32–50. doi:10.1016/j.crvi.2011.11.002. PMID 22226162.
  15. Árnason, Úlfur; Lammers, Fritjof; Kumar, Vikas; Nilsson, Maria A.; Janke, Axel (2018). "Whole-genome sequencing of the blue whale and other rorquals finds signatures for introgressive gene flow". Science Advances. 4 (4): eaap9873. Bibcode:2018SciA....4.9873A. doi:10.1126/sciadv.aap9873. PMC 5884691. PMID 29632892.

Sources
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