Rorquals (Balaenopteridae)

(Balaenopteridae)

Class: Mammalia

Order: Cetacea

Suborder: Mysticeti

Family: Balaenopteridae

Thumbnail description
Large-sized whales with furrows or pleats on their undersides, and baleen instead of teeth. Unlike other baleen whales, rorquals have pointed dorsal fins, longer, streamlined bodies, and relatively small heads

Size
32–102 ft (10–31 m); 22,000–400,000 lb (9,980–181,440 kg)

Number of genera, species
2 genera; 7 or 8 species

Habitat
Marine, estuaries, aquatic, deep, benthic, and pelagic

Conservation status
Endangered: 3 species; Vulnerable: 1 species; Lower Risk/Conservation Dependent: 1 species; Lower Risk/Near Threatened: 1 species; Data Deficient: 1 species

Distribution
All oceans and adjoining seas of the world

Evolution and systematics

The oldest fossil whales are often grouped together in a taxon known as the archaeocetes. They exhibit several features that modern whales lack, such as teeth of different types and nostrils near the tip of the nose. For many years, paleontologists thought that whales evolved from a group of now-extinct, wolf-like hoofed mammals called mesonychians. The similarities in the shape and construction of the skull and in the shape of the teeth were the best evidence for this. However, several phylogenetic studies of gene sequences of living mammals have argued that whales are most closely related to artiodactyls, which are the hoofed mammals with an even number of toes, such as cows, pigs, camels, deer, and hippopotamus. In fact, the genetic evidence suggests that whales are most closely related to hippos and are thus actually a subgroup of artiodactyls. One of the main synapomorphies (a shared character that originated in their last common ancestor) linking all living and extinct artiodactyls is an anklebone that has rounded, pulley-shaped joints on each end. This anklebone allows only front-to-back motion between the leg and ankle, and the ankle and toes. The front-and-back motion is well suited for efficient running. Recent descriptions of the ankle bones of Eocene whale species from Pakistan and India show that whales have the double-pulley anklebone of artiodactyls, suggesting that the ancestor of whales and hippos may have ventured into the water more than 55 million years ago.

In 1978, the earliest known well-preserved cetacean, a 52-million-year-old skull, was discovered in Pakistan. The new bones, dubbed Pakicetus, proved to have key features that were transitional between terrestrial mammals and the earliest true whales, including an ear that was modified for directional hearing underwater. An amphibious animal, Pakicetus was found in near-shore marine sediments. Basilosurus is another Eocene archaeocete, from the Gulf Coast, which retained tiny hind limbs that projected from the body, although there was no joint between the pelvic bones and the vertebrae.

By the late Oligocene, the two modern lineages of cetaceans had evolved from archaeocete ancestors. The late Oligocene whale, Aetiocetus from Oregon, has skull and jaw features typical of baleen whales and is considered the earliest mysticete, yet it also bares a full set of teeth. By the late Miocene, rorquals were relatively common fossils in many marine deposits.

Gray created this family (the Balaenopteridae) in 1864 to include all the rorqual whales—defined as those that have a number of throat grooves. All except the humpback whale (Megaptera novaeangliae) share a strong family resemblance.

Physical characteristics

Rorqual whales are relatively streamlined in appearance and have pointed heads and small pointed fins. They can be distinguished from other whales by many (25–200) deep groves along their throats that expand when they feed. The tongue is soft and fleshy, well adapted for licking food off their baleen. There are eight species of rorqual whales: humpback whale (Megaptera novaeangliae), fin whale (Balaenoptera physalus), Bryde's whale (Balaenoptera edeni), blue whale (Balaenoptera musculus), northern minke (Balaenoptera acutorostrata), Antarctic minke (Balaenoptera bonaerensis), and sei whale (Balaenoptera borealis). Balaenoptera edeni may represent two species, B. edeni and B. brydei, though this is not yet determined.

Rorquals range in size from 32 to 102 ft (10–31 m) and weigh 22,000–400,000 lb (9,980–181,440 kg). Females grow slightly larger than males.

Major distinctive features of the rorquals are a white right lower lip and a white edge on the upper jaw in fin whales; a single rostral ridge extending from the base of the blowhole in sei whales; three prominent ridges on the rostrum (upper jaw or snout) in Bryde's whale; and a triangular-shaped rostrum with a single prominent ridge in the minke whales. The blue whale, the largest of all whales, possesses a small dorsal fin, a flat rostrum that appears U-shaped when viewed from above, and a tall, dense spout. In the Antarctic, a yellowish film of diatoms is often present on the ventral and lateral surfaces of these whales, prompting the whaler's term "sulfur-bottom."

Distribution

Bryde's whale is found in tropical and temperate waters around the world. They are especially abundant in areas of high food productivity. In the western Pacific, Bryde's whale occurs from Japan to New Zealand, and in the eastern Pacific, from Baja California to Chile. In the northeast Pacific, they move between Bonin Islands and the coast of Japan, west Kyushu, and further north. In the Atlantic, the species is reported from Virginia to the Gulf of Mexico and the Caribbean, south to Brazil. In the east, reports range from the Canaries and Morocco south to the Cape of Good Hope. In the Indian Ocean, their north-south range is from the Persian Gulf to the Cape of Good Hope, and from Myanmar to Australia.

Minke whales, the most widespread of the rorquals, are found in tropical, temperate, and polar waters of both hemispheres. The species is frequently seen in inshore northern and western coastal waters of the United Kingdom, and occasional records have been reported from the channel coast of mainland Europe, the Mediterranean, and the Azores and Portugal. In the Pacific, they range from the tropics (Vietnam, Baja California) to the Bering Sea. During summer, minke whales are found from temperate waters all the way up to the ice pack. Their winter movements are poorly known; some may stay in temperate waters year-round, and there is recent acoustic evidence that some minke whales in the North Atlantic may move into tropical waters in the Caribbean during winter. Within their range, they are widely distributed, and are found over a more widespread area than their larger relatives.

Fin whales migrate to polar waters in summer for feeding and return to warmer seas in winter for breeding. Photo-identification work indicates that fin whales in the North Atlantic have been detected to move throughout the New England/Nova Scotia region, but have never been sighted off of Newfoundland or the Gulf of St. Lawrence. Similarly, genetic work indicates that fin whales in the Mediterranean, on the other side of the North Atlantic, are a separate population. No wintering concentration area is known anywhere in the world; the speculation is that these animals go to deep waters and disperse. There is a year-round resident group in the Gulf of California in Mexico.

In the eastern North Pacific, fin whales winter from at least central California southward, and summer from central Baja California into the Chukchi Sea. In the western North Pacific they winter in the Philippine Sea, including concentrations in the East China Sea and the Sea of Japan. In the western North Atlantic, they winter from the ice-edge south to Florida and the Greater Antilles, and into the Gulf of Mexico, primarily in offshore waters. They summer from below the latitude of Cape Cod to the Arctic Circle. They are present in the Mediterranean Sea and in the eastern North Atlantic from the Strait of Gibraltar to southwestern Norway in winter. Although fin whales are in the Mediterranean Sea year-round, they apparently migrate to more northerly waters along the eastern European coasts. In the southern hemisphere, fin whales migrate from summering grounds in the Antarctic past New Zealand into the southwestern Pacific, along South America to Peru on the west coast and Brazil on the east coast, to the central Atlantic off the west coast of Africa, and to the southern Indian Ocean.

Sei whales are largely oceanic, and widely distributed in temperate and polar waters of both hemispheres. Blue whales are found closer to shore, often near deep coastal canyons. Humpback whales migrate between their breeding and feeding grounds, moving mainly along the continental coasts in the Northern and Southern Hemispheres.

Habitat

Rorquals are found in open seas, but mainly over continental shelves, and sometimes in bays, inlets, and estuaries.

Behavior

The social organization of rorquals is still poorly understood. Groups generally only include two to five individuals, although larger, temporary aggregations occur on rich feeding grounds and during the breeding season. Rorquals are not deep divers, generally feeding within 330 ft (100 m) of the surface.

Because of the loud, low-frequency sounds made by fin whales, animals may remain in vocal contact over long distances, making it difficult to know when whales are or are not associated. The fin whale is sometimes called the "greyhound of the sea" because of its fast swimming speed; it can swim up to 23 mph (37 km/hr) in short bursts.

Some minke whales undertake lengthy migration, totaling 5,590 mile (9,000 km) or more, but others may move little. There are many regions where minkes are found year-round. However, seasonal variation in abundance and distribution suggests that the whales probably do undergo some migration, from higher latitudes in summer to lower latitudes in winter. Pregnant females seem to move farther north in summer than lactating and immature females, but in some temperate waters these animals are present year-round.

Long migrations are probably not typical of Bryde's whales, although there are indications that some animals may shift towards the equator in winter and toward more temperate waters in summer. They are not, however, one of the species that frequents the Antarctic Ocean each year. Bryde's whales mainly make local seasonal movements, and may form resident populations in some regions. In certain areas, for example, off South Africa, two forms of the species are found: one is resident year-round, and found within 25 miles (40 km) of the coast, and the other generally occurs about 62 miles (100 km) from the shore and appears there in autumn and spring. The offshore form undertakes north-south migrations as it follows shoals of fish throughout the year. In the northwest Pacific, Bryde's whale moves from the Bonin Islands north to the coast of Japan, a seasonal migration of only about 342 miles (550 km). In the Gulf of California, Bryde's whales probably make limited north-south migrations on a local scale, following sardine and herring concentrations. They seem to be relatively resident in the area year-round.

Minke whales are more likely to be seen close-up than other rorquals, as they often approach boats, especially stationary vessels, and are notoriously inquisitive. They are fast moving and may swim at speeds in excess of 13 miles (20 km) per hour. The surfacing and blow rates of minke whales tend to be less regular than those of the large baleen whales, and may be affected by the presence of vessels, time of day, activity of the animal, and/or the environmental conditions. A typical dive sequence is five to eight blows, at intervals less than one minute, followed by a dive, lasting from two to six minutes, although minkes can stay underwater for 20 minutes or longer. They are also known to breach more often than other baleen whales, leaping clear of the surface and reentering the water head-first or with a splash.

Bryde's whales also seem to breach often and, when feeding, often change direction and splash and roll around at the surface. They generally take four or five short breaths before starting a long dive. They may dive for 20 minutes or so, and rarely show their tail flukes as they dive. They often surface steeply, like the fin whale, with the blow becoming visible well before the dorsal fin is exposed. They have been observed to exhale underwater, surfacing with little or no visible blow. They are known to be inquisitive and sometimes approach boats, circling them or swimming alongside. Its lifespan is approximately 50 years.

Rorquals produce four types of sounds: low-frequency moans, including the "songs" of the humpback whale; gruntlike thumps and knocks of short duration; high-frequency chirps, cries, and whistles; and low-frequency clicks or pulses. Though few sounds are known to be linked with specific behaviors, it is thought that the sounds are social, for greeting, courtship, threat, individual identification, and other purposes. The origin of these sounds is suspected to be the larynx, although whales have no vocal cords.

Humpback whale "songs" are sung only by solitary males, and they all sing the same song during each breeding season. The specific function of the song is not known, but likely communicates information to other male and female humpbacks. When the solitary males join social groups they no longer sing their song.

Feeding ecology and diet

Rorquals feed with the help of their baleen, curtains of horny fronds hanging from the top of their giant, bowed upper jaws. Most rorquals drop their pleated lower jaws and engulf schools of small fish or invertebrates. Closing their mouths, they ram their tongues against the baleen, squeezing out the water through their lips, while leaving the food behind. Dives for food rarely last longer than 10 minutes, to depths less than 660 ft (200 m).

The main food of rorquals is various species of krill (euphasiids). Rorquals also eat animals such as small squid, lantern fish (Myctophidae), and certain amphipods. Sei whale prefer copepods if available. Bryde's whales feed on krill in pelagic waters and fish in coastal areas. Blue whales feed almost exclusively on swarms of krill. However, off Baja California, Mexico, they also eat seagoing crabs in the winter. In the Antarctic, daily food consumption for a single blue whale is up to 8 tons (7.3 tonnes) of krill.

In certain feeding areas, one or two humpbacks swim in an upward spiral around swarms of krill found on or below the surface. As they circle the krill, they expel a chain of bubbles from their blowholes. The rising bubbles form a "bubble net" that forces the krill to mass to the center of the bubbles.

Because of their relatively small size, and lowered energetic needs, minke whales consume a wider variety of fish than the larger fin and humpback whales. At times, they may even take single larger fish rather than large quantities of smaller fish. Feeding minke whales are often seen near the surface chasing fish. The species has been reported to feed in one of two ways: lunge feeding or "bird association" feeding (depending mainly on the feeding areas). Most individuals seem to specialize in just one of these methods. Bird-associated foraging exploits the concentration of fish fry below flocks of feeding gulls and auks, while lunge feeding consists of the whale actively concentrating the prey against the air-water interface with no feeding birds involved. The minke whale is often seen turning on its side when lunge feeding. In the North Pacific, the minke whale feeds on krill and sand lance.

Bryde's whale often exploits the activities of other predators, swimming through and engulfing the fish they have herded. Therefore, it is frequently found in areas of high fish abundance, along with seabirds, seals, sharks, and other cetaceans. When feeding, Bryde's whales often roll onto their sides or churn the water at the surface by pinwheeling or halfheartedly breaching. They also frequently accelerate and change direction suddenly. So their movements when feeding look more like those of feeding dolphins than those of the other large whales. Bryde's whales feed actively year-round.

Reproductive biology

The entire reproductive cycle is correlated with the migrations of the whales between rich feeding areas and breeding/calving grounds. Mating usually occurs with the pair swimming on their side, belly to belly. The male's testes are retained permanently inside the abdominal cavity. Most rorquals have a polygamous mating system, with both males and females being promiscuous.

The gestation period is a year or slightly longer in all rorquals, except in minkes, which is about 10 months. The mean length at birth ranges from 9 ft (2.7 m) for the minke whale to about 23 ft (7m) for the blue whale. Only a single young is born. The teats of the mammary glands are found within paired slits on either side of the female reproductive opening. Contact with the teats during suckling causes the milk to spurt freely into the mouth of the calf. The milk of rorquals is unusually high in fat content: 30–53%. The high-fat content likely accounts for the rapid growth of the calf during the suckling period (usually one year), during which it can increase its body weight five to eight times. The larger, mature rorquals breed every two to three years; smaller minke whales breed almost every year. Sexual maturity is attained in both sexes between five and 15 years. Whales in depleted populations attain sexual maturity at an earlier age than those in populations at their carrying capacity. Rorquals average a life span of 50 to 80 years.

Conservation status

In 1946, 20 whaling nations set up the International Whaling Commission (IWC) in an attempt to regulate whale hunting to stop over-fishing. It collected data on the number of whales, though the numbers came mostly from the whalers themselves. The commission set annual quotas for the number of whales to be killed. These quotas, however, were nonbinding and could not be enforced. Furthermore, some whaling nations did not belong to the IWC. The blue whale, for example, was not completely protected by the IWC until the 1965–1966 season, long after its numbers had been drastically reduced. And even under the protection of the IWC, blue whales were hunted at least until 1971 by the fleets of countries that did not belong to the IWC.

Under mounting pressure from conservationists, the IWC gradually banned the hunting of other whales. The United States Congress separately passed the Marine Mammal Protection Act of 1972, which bans the hunting of all marine mammals (except in the traditional fisheries of Alaskan natives) and the importation of their products. By 1974, the IWC had included the blue whale and the humpback whale under its protection. Minke whales, sei whales, and fin whales were still being hunted in large numbers, but worldwide catches began to dwindle. Catches fell from 64,418 in 1965 to 6,623 in 1985. A moratorium on all commercial whaling was finally declared by the IWC in 1985, a move considered long overdue by conservation groups. Japan, Iceland, and Norway, however, opted in 1988 to continue to hunt minke, fin, and sei whales, a fishery permitted by the IWC under the controversial guise of "scientific whaling."

The fin whale, blue whale, and sei whale are listed as Endangered by the IUCN; the humpback whale is Vulnerable; the northern minke whale is Lower Risk/Near Threatened; the Antarctic minke whale as Lower Risk/Conservation Dependent; and Bryde's whale as Data Deficient.

Significance to humans

Historically, demand for whale blubber, mostly oil used in the manufacture of margarine, soaps, and lubricants, and as a substitute for kerosene, was high. By the 1980s, artificial substitutes had been found for whale oil. Whale meat, however, remains valued as both a pet food and as human food, mostly in the kujiraya, or whale-meat bars, of Japan. About 70% of the cetacean products from market surveys in Japan and South Korea have proven to be from minke whales (about 19% Balaenoptera acutorostrata and 51% B. bonaerensis). The Japanese surveys included collections organized by Earthtrust, Whale and Dolphin Conservation Society, TRAFFIC Japan, and Greenpeace Germany from 1999 and 1998.

Blue whales, the largest of them all, were especially sought for their blubber. A large specimen yielded more than 9,000 gal (34,000 l) of oil. It has been estimated that more than 200,000 blue whales were taken worldwide between 1924 and 1971, close to 30,000 during the 1930–1931 whaling season alone. Soon catches pushed way above optimal yield level. As many as 80% of all blue whales caught by 1963 were sexually immature, meaning that there were even less individuals in the ocean to perpetuate the species.

Fin whales, the second largest of all whales, became the next major target as blue whales became scarcer. The 1950s and early 1960s saw annual catches of 20,000–32,000 fin whales per year, mostly from Antarctica. As their stocks dwindled, whalers shifted once again in the mid-1960s, this time to the smaller sei whale.

Whale watching has become, in many cases, an economically beneficial alternative to hunting. In 2000, it attracted some nine million enthusiasts in 87 countries, and generated a record-breaking $1 billion in revenue, according to the World Wildlife Fund. The income earned by the industry has doubled in only six years. In Iceland, whale-watching passenger numbers have grown from just 100 in 1991 to 44,000 in 2000.

Species accounts

Resources

Ellis, Richard. The Book of Whales. New York: Knopf, 1985.

Evans, Peter G. H. The Natural History of Whales and Dolphins. New York: Facts On File, 1987.

Baskin, Yvonne. "Blue Whale Population May Be Increasing Off California." Science 260, 5106 (1993): 287.

Olsen, E., and O. Grahl-Nielsen. "Blubber Fatty Acids of Minke Whales: Stratification, Population Identification and Relation to Diet." Marine Biology 142 (2003): 13.

Tennesen, Michael. "Biggest Animal that Ever Lived. Scientists Attempt to Solve the Many Riddles Posed by the Blue Whale." International Wildlife 31, no. 2 (2001): 22–29.

Young, Emma. "Minke Whales Are Out for the Count." New Scientist 170, 2295 (2001): 12.

International Whaling Commission. Website:

Society for Marine Mammalogy. Web site:

The Whale Center of New England. Web site:

Whale Net. Internet Marine Mammal Resource List. Web site:

[Article by: Gretel H. Schueller]