Blue Whale

The Blue Whale (Balaenoptera musculus) is a marine mammal belonging to the suborder of baleen whales (called Mysticeti). At up to 32.9 metres (108 feet) in length and 172 metric tonnes (190 short tons) or more in weight, it is the largest animal ever to have existed.

Long and slender, the Blue Whale's body can be various shades of bluish-grey dorsally and somewhat lighter underneath. There are at least three distinct subspecies: B. m. musculus of the North Atlantic and North Pacific, B. m. intermedia of the Southern Ocean and B. m. brevicauda (also known as the Pygmy Blue Whale) found in the Indian Ocean and South Pacific Ocean. B. m. indica, found in the Indian Ocean, may be another subspecies. As with other baleen whales, its diet consists almost exclusively of small crustaceans known as krill.

Blue Whales were abundant in nearly all the oceans until the beginning of the twentieth century. For over 40 years, they were hunted almost to extinction by whalers until protected by the international community in 1966. A 2002 report estimated there were 5,000 to 12,000 Blue Whales worldwide, located in at least five groups. More recent research into the Pygmy subspecies suggests this may be an underestimate. Before whaling, the largest population was in the Antarctic, numbering approximately 239,000 (range 202,000 to 311,000). There remain only much smaller (around 2,000) concentrations in each of the North-East Pacific, Antarctic, and Indian Ocean groups. There are two more groups in the North Atlantic, and at least two in the Southern Hemisphere.

Taxonomy

Blue Whales are rorquals (family Balaenopteridae), a family that includes the Humpback Whale, the Fin Whale, Bryde's Whale, the Sei Whale and the Minke Whale. The family Balaenopteridae is believed to have diverged from the other families of the suborder Mysticeti as long ago as the middle Oligocene. However, it is not known when the members of those families diverged from each other.

The Blue Whale is usually classified as one of eight species of whale in the genus Balaenoptera; one authority placed it in a separate monotypic genus, Sibbaldus, but this was not accepted elsewhere. DNA sequencing analysis indicates that the Blue Whale is phylogenetically closer to the Sei Whale (Balaenoptera borealis) and Bryde's Whale (Balaenoptera brydei) than to other Balaenoptera species, and closer to the Humpback Whale (Megaptera) and the Gray Whale (Eschrichtius) than to the minke whales (Balaenoptera acutorostrata and Balaenoptera bonaerensis). If further research confirms these relationships, it will be necessary to reclassify the rorquals.

There have been at least 11 documented cases of Blue/Fin Whale hybrid adults in the wild. Arnason and Gullberg describe the genetic distance between a Blue and a Fin as about the same as that between a human and a gorilla. Researchers working off of Fiji believe they photographed a hybrid Humpback/Blue Whale.

The specific name musculus is Latin and could mean "muscular", but it can also be interpreted as "little mouse". Linnaeus, who named the species in his seminal Systema Naturae of 1758, would have known this and may have intended the ironic double meaning. Herman Melville called this species Sulphur-bottom in his novel Moby-Dick due to an orange-brown or yellow tinge on the underparts from diatom films on the skin. Other common names for the Blue Whale have included the Sibbald's Rorqual (after Sir Robert Sibbald), the Great Blue Whale and the Great Northern Rorqual. These names have fallen into disuse in recent decades.

Authorities classify the species into three or four subspecies: B. m. musculus, the Northern Blue Whale consisting of the North Atlantic and North Pacific populations, B. m. intermedia, the Southern Blue Whale of the Southern Ocean, B. m. brevicauda, the Pygmy Blue Whale found in the Indian Ocean and South Pacific, and the more problematic B. m. indica, the Great Indian Rorqual, which is also found in the Indian Ocean and, although described earlier, may be the same subspecies as B. m. brevicauda.

Description and behaviour

The Blue Whale has a long tapering body that appears stretched in comparison with the stockier build of other whales. The head is flat and U-shaped and has a prominent ridge running from the blowhole to the top of the upper lip. The front part of the mouth is thick with baleen plates; around 300 plates (each around one metre (3.2 ft) long) hang from the upper jaw, running 0.5 m (1.6 ft) back into the mouth. Between 60 and 90 grooves (called ventral pleats) run along the throat parallel to the body. These pleats assist with evacuating water from the mouth after lunge feeding (see feeding below).

The dorsal fin is small, visible only briefly during the dive sequence. Located around three-quarters of the way along the length of the body, it varies in shape from one individual to another; some only have a barely perceptible lump, but others may have prominent and falcate dorsals. When surfacing to breathe, the Blue Whale raises its shoulder and blowhole out of the water to a greater extent than other large whales such as the Fin or Sei. This trait may be used by observers to differentiate between species at sea. Some Blue Whales in the North Atlantic and North Pacific raise their tail fluke when diving. When breathing, the whale emits a spectacular vertical single-column blow (up to 12 m (40 ft), typically 9 m (30 ft)) that can be seen from a great distance on a calm day. Its lung capacity is 5,000 litres (1320 US gallons). Blue whales have twin blowholes shielded by a large splashguard.

The flippers are three to four metres (10 to 13 ft) long. The upper sides are grey with a thin white border. The lower sides are white. The head and tail fluke are generally uniformly grey. The whale's upper parts, and sometimes the flippers, are usually mottled. The degree of mottling varies substantially from individual to individual. Some may have a uniform slate-grey colour all over, but others demonstrate a considerable variation of dark blues, greys and blacks, all tightly mottled.

Blue Whales can reach speeds of 50 km/h (30 mph) over short bursts, usually when interacting with other whales, but 20 km/h (12 mph) is a more typical traveling speed. When feeding, they slow down to 5 km/h (3 mph).

Blue Whales most commonly live alone or with one other individual. It is not known whether those that travel in pairs stay together over long periods or form more loose relationships. In locations where there is a high concentration of food, as many as 50 Blue Whales have been seen scattered over a small area. However, they do not form the large close-knit groups seen in other baleen species.

Size

Blue Whales are difficult to weigh because of their size. Most Blue Whales killed by whalers were not weighed whole, but cut up into manageable pieces first. This caused an underestimate of the total weight of the whale, due to the loss of blood and other fluids. Nevertheless, measurements between 150 and 170 tonnes (160 and 190 short tons) were recorded of animals up to 27 m (88 ft 6 inches) in length. The weight of a 30 m (98 ft) individual is believed by the American National Marine Mammal Laboratory (NMML) to be in excess of 180 tonnes (200 short tons). The largest Blue Whale accurately weighed by NMML scientists to date was a female that weighed 177 tonnes (196 short tons).

The Blue Whale is the largest animal ever to have lived. The largest known dinosaur of the Mesozoic Era was the Argentinosaurus, which is estimated to have weighed up to 90 tonnes (100 short tons), though a controversial vertebra of Amphicoelias fragillimus may indicate an animal of up to 122 tonnes (135 short tons) and 40–60 meters (130–200 ft). Furthermore, there are weight estimates for the very poorly known Bruhathkayosaurus ranging from 140 up to 220 tonnes, besides length estimates up to about 45 meters. The extinct fish Leedsichthys may have approached its size. However, complete fossils are difficult to come by, making size comparisons difficult. All these animals are considered to be smaller than the blue whale.

There is some uncertainty about the biggest Blue Whale ever found, as most data comes from Blue Whales killed in Antarctic waters during the first half of the twentieth century and was collected by whalers not well-versed in standard zoological measurement techniques. The longest whales ever recorded were two females measuring 33.6 m and 33.3 m (110 ft 3 in and 109 ft 3 in) respectively. The longest whale measured by scientists at the NMML was 29.9 m (98 ft).

A Blue Whale's tongue weighs around 2.7 tonnes, (3 short tons) and, when fully expanded, its mouth is large enough to hold up to 90 tonnes (100 short tons) of food and water. Despite the size of its mouth, the dimensions of its throat are such that a Blue Whale cannot swallow an object wider than a beach ball. Its heart weighs 600 kg (1,320 lb) and is the largest known in any animal. A Blue Whale's aorta is about 23 cm (9 in) in diameter. During the first seven months of its life, a Blue Whale calf drinks approximately 400 litres (100 US gallons) of milk every day. Blue Whale calves gain weight quickly, as much as 90 kg (200 lb) every 24 hours. Even at birth, they weigh up to 2,700 kilograms (6,000 lb)—the same as a fully grown hippopotamus.

Feeding

Blue Whales feed almost exclusively on krill, though they also take small numbers of copepods. The species of this zooplankton eaten by Blue Whales varies from ocean to ocean. In the North Atlantic, Meganyctiphanes norvegica, Thysanoessa raschii, Thysanoessa inermis and Thysanoessa longicaudata are the usual food; in the North Pacific, Euphausia pacifica, Thysanoessa inermis, Thysanoessa longipes, Thysanoessa spinifera, Nyctiphanes symplex and Nematoscelis megalops; and in the Antarctic, Euphausia superba, Euphausia crystallorophias and Euphausia valentin.

An adult Blue Whale can eat up to 40 million krill in a day. The whales always feed in the areas with the highest concentration of krill, sometimes eating up to 3,600 kg (8,000 lb) of krill in a single day. This daily calorie requirement of an adult Blue Whale is in the region of 1.5 million. This means that they typically feed at depths of more than 100 m (330 ft) during the day and only surface-feed at night. Dive times are typically 10 minutes when feeding, though dives of up to 20 minutes are common. The longest recorded dive is 36 minutes (Sears 1998). The whale feeds by lunging forward at groups of krill, taking the animals and a large quantity of water into its mouth. The water is then squeezed out through the baleen plates by pressure from the ventral pouch and tongue. Once the mouth is clear of water, the remaining krill, unable to pass through the plates, are swallowed. The Blue Whale also incidentally consumes small fish, crustaceans and squid caught up with krill.

Life history

Mating starts in late autumn and continues to the end of winter. Little is known about mating behaviour or breeding grounds. Females typically give birth once every two to three years at the start of the winter after a gestation period of ten to twelve months. The calf weighs about two and a half tonnes (2.75 short tons) and is around 7 m (23 ft) in length. Blue Whale calves drink 380–570 litres (100–150 US gallons) of milk a day. Weaning takes place for about six months, by which time the calf has doubled in length. Sexual maturity is typically reached at eight to ten years, by which time males are at least 20 m (66 ft) long (or more in the Southern Hemisphere). Females are larger still, reaching sexual maturity at around the age of five, by which they are about 21 m (69 ft) long.

Scientists estimate that Blue Whales can live for at least 80 years; however, since individual records do not date back into the whaling era, this will not be known with certainty for many years. The longest recorded study of a single individual is 34 years, in the north-east Pacific (reported in Sears, 1998). The whales' only natural predator is the Orca. Studies report that as many as 25% of mature Blue Whales have scars resulting from Orca attacks. The rate of mortality due to such attacks is unknown.

Blue Whale strandings are extremely uncommon, and, because of the species' social structure, mass strandings are unheard of. However, when strandings do occur, they can become the focus of public interest. In 1920, a Blue Whale washed up near Bragar on the Isle of Lewis in the Outer Hebrides of Scotland. It had been shot in the head by whalers, but the harpoon had failed to explode. As with other mammals, the fundamental instinct of the whale was to try to carry on breathing at all costs, even though this meant beaching to prevent itself from drowning. Two of the whale's bones were erected just off a main road on Lewis and remain a tourist attraction.

Vocalizations

Estimates made by Cummings and Thompson (1971) suggest that source level of sounds made by Blue Whales are between 155 and 188 decibels when measured relative to a reference pressure of one micropascal at one metre. All Blue Whale groups make calls at a fundamental frequency of between 10 and 40 Hz, and the lowest frequency sound a human can typically perceive is 20 Hz. Blue Whale calls last between ten and thirty seconds. Additionally, Blue Whales off the coast of Sri Lanka have been recorded repeatedly making "songs" of four notes duration lasting about two minutes each, reminiscent of the well-known Humpback Whale songs. Researchers believe that as this phenomenon has not been seen in any other populations, it may be unique to the B. m. brevicauda (Pygmy) subspecies.

The reason for vocalization is unknown. Richardson et al. (1995) discuss six possible reasons:

1. Maintenance of inter-individual distance
2. Species and individual recognition
3. Contextual information transmission (e.g., feeding, alarm, courtship)
4. Maintenance of social organization (e.g., contact calls between females and males)
5. Location of topographic features
6. Location of prey resources

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