Fin whales are enormous animals, with the largest individuals measuring nearly 90 feet in length and weighing 80 tons. Something that large should be conspicuous, especially in the coastal waters where fin whales spend much of their time. But the species’ propensity to disperse to open water and steep declines in its numbers in the 20th century have rendered it a rare sight. And so, relative to its famous baleen cousins, the blue whale and the humpback, the fin whale is lesser known, and its behavior little understood.
Fin whales (Balaenoptera physalus) are large from birth. In the last couple months of gestation, in one of the fastest fetal growth spurts known to animals, they more than double in size, growing to about 21 feet and two tons by the time they are born. Within five or six years, most fin whales have reached their adult size, and their distinguishing features have become pronounced. These features include a prominent ridge that stretches from the tip of the upper jaw to the blowhole, a pointed or hooked dorsal fin, and a sharp ridge that runs along the top edge of the lower back.
Fin whales also have remarkably long and trim bodies, which contrast with the stockier build of blue whales and humpbacks. In fact, although some blue whales may weigh more than twice as much as the largest fin whales when full-grown, blue whales are about the same length as or only slightly longer than fin whales. The latter's slender, hydrodynamic profile allows it to explode in bursts of speed of as many as 25 knots, making it one of the fastest whales in the world. That speed is especially useful for feeding, when a rapid lunge into a school of prey, with mouth wide open, allows for the swift intake of food.
While fin whales feed primarily on krill, they also enjoy small fish, such as capelin, herring, and sandlance. Like some other whales that feed on schooling fish, fin whales will circle their prey to encourage the fish to gather into a tight group. They then lunge into the school with mouth agape, engulfing both fish and water. The whales' baleen filters the water, trapping the fish in the mouth.
Although it is not known with certainty, fin whales may also make use of their asymmetrical coloration when feeding. The asymmetry affects the lower right and left jaws, with the right side being gray or white and the left black or dark brown; this coloration is repeated in the fin whale's baleen. When lunging on a school of fish from above, fin whales may do so on their right sides, thereby showing the white jaw to their prey and thus blending in with light from the sky above. This may confuse the fish just long enough to allow the whales to capture a larger quantity than they would otherwise.
Fin whales inhabit the world’s major oceans but occur most frequently in temperate and polar waters. But relatively little is known about their movements. For instance, while they occur over a wide range of latitudes throughout the year, suggesting that they do not migrate, some groups appear to move into winter or summer ranges occupied by other groups within their latitudinal range. Tracking the movements of fin whales is made difficult by their tendency to swim alone or in small groups dispersed over large areas and by their occasional mingling with blue whales.
For much of its coexistence with whaling vessels piloted by humans, the fin whale's greatest asset has been its speed. But with the appearance of faster vessels and stronger harpoons in the 20th century, fin whales could no longer escape man. The result was the persecution of the species, to near extinction, particularly in the Southern Hemisphere.
Today the fin whale is listed as endangered. But while populations in Antarctica still suffer low numbers, those in the North Atlantic appear to be recovering, aided in part by the female fin whale's ability to bear offspring every two or three years for the greater part of her 80-year-long life.
Kara Rogers is a freelance science writer and senior editor of biomedical sciences at Encyclopaedia Britannica, Inc. She is a member of the National Association of Science Writers and author of Science Up Front on the Britannica Blog. She holds a Ph.D. in Pharmacology/Toxicology, but enjoys reading and writing about all things science. You can follow her on Twitter at @karaerogers.