2007 Schools Wikipedia Selection. Related subjects: Mammals
A drawing of a Blue Whale seen on a Faroese stamp
Size comparison against an average human
Blue Whale range
The Blue Whale (Balaenoptera musculus) is a marine mammal belonging to the suborder of baleen whales. At up to 30 metres (98 feet) in length and 177 metric tonnes (196 short tons) or more in weight, it is believed to be the largest animal to have ever lived on Earth.
Blue Whales were abundant in most oceans around the world until the beginning of the twentieth century. For the first 40 years of that century they were hunted by whalers almost to extinction. Hunting of the species was outlawed 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 suggest this may be an under-estimate. Before whaling the largest population (202,000 to 311,000) was in the Antarctic but now there remain only much smaller (around 2,000) concentrations in each of the North-East Pacific, the Antarctic, and the Indian Ocean. There are two more groups in the North Atlantic and at least two in the Southern Hemisphere.
Taxonomy and evolution
Blue Whales are rorquals (family Balaenopteridae), a family that includes the Humpback Whale, the Fin Whale, the 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 these families diverged from each other.
The Blue Whale is usually classified as one of seven species of whale in the genus Balaenoptera; however, DNA sequencing analysis indicates that Blue Whales are phylogenetically closer to the Humpback (Megaptera) and the Gray Whale (Eschrichtius) than to other Balaenoptera species; should further research corroborate these relationships, it will be necessary to recognize the separate genus Sibbaldus for the Blue Whale.
There have been at least 11 documented cases of Blue/ Fin Whale hybrid adults in the wild. Aranson and Gullberg (1983) describe the genetic distance between a Blue and a Fin as about the same as that between a human and gorilla. Blue Whale/Humpback Whale hybrids are also known.
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 work of 1758, would have known this and, given his sense of humour, may have intended the ironic double meaning. Other common names for the Blue Whale have included the Sulphur-bottom, Sibbald's Rorqual, the Great Blue Whale and the Great Northern Rorqual. These names have fallen into disuse in recent decades.
Authorities classify the species into three subspecies: B. m. musculus, consisting of the north Atlantic and north Pacific populations, B. m. intermedia, the Southern Ocean population and B. m. brevicauda (also known as the Pygmy Blue Whale) found in the Indian Ocean and South Pacific. Some older authorities also list B. m. indica as a further separate subspecies in the Indian Ocean, but it is most likely that these blue whales are pygmy blue whales, and this designation does not therefore have a listing in the Red List of Threatened Species. Both subdivisions are still questioned by some scientists; genetic analysis may yet show there are just two true subspecies.
The Blue Whale has a long tapering body that appears stretched in comparison with the much stockier appearance of other whales. The head is flat and U-shaped and has a very prominent ridge running from the blowhole to the top of the upper lips. The front part of the mouth is thick with baleen plates; around 300 plates (each one metre long) hang from the upper jaw, running half a metre back into the mouth. Between 60 and 90 grooves (called ventral pleats) run along the throat parallel to the body. These plates 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. It varies in shape from one individual to another; some only have a barely perceptible lump, whilst other fins are quite prominent and falcate. It is located around three-quarters of the way along the length of the body. When surfacing to breathe, the Blue Whale raises its shoulder and blow hole region out of the water to a greater extent than other large whales (such as the Fin or Sei). This can often be a useful clue to identifying a species at sea. Whilst breathing, the whale emits a spectacular vertical single column blow (up to 12 m, typically 9 m) that can be seen from many kilometers on a calm day. Its lung capacity is 5,000 litres.
The flippers are three to four metres long. The upper side is grey with a thin white border. The lower side is white. The head and tail fluke are generally uniformly grey coloured whilst the back, and sometimes the flippers, are usually mottled. The degree of mottling varies substantially from individual to individual. Some may have a uniform grey colour all over, whilst 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 travelling speed. When feeding they slow down to 5 km/h (3 mph). Some Blues in the North Atlantic and North Pacific raise their tail fluke when diving. The majority, however, do not.
Blue Whales most commonly live alone or with one other individual. It is not known whether those that travel in pairs stay together over many years or form more loose relationships. In areas of very high food concentration, as many as 50 Blue Whales have been seen scattered over a small area. However, they do not form large close-knit groups as seen in other baleen species.
The Blue Whale is believed to be the largest animal ever to have lived on Earth. The largest known dinosaur of the Mesozoic era was the Argentinosaurus, which is estimated to have weighed up to 90 tonnes (100 short tons). There is some uncertainty about the biggest Blue Whale ever found. 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. However, there are some disputes over the reliability of these measurements. The longest whale measured by scientists at the American National Marine Mammal Laboratory (NMML) was 29.9 m long (98 ft) — about the same length as a Boeing 737 aeroplane or three double-decker buses.
A Blue Whale comes in first place when judging animal size. A Blue Whale's tongue is so big that it is about the size of an elephant and 50 humans could stand in its mouth. Its heart is close to the size of a small car and is the largest known in any animal. A human baby could squeeze into a Blue Whale's aorta, which is about 9 inches in diameter. During the first 7 months of its life, a Blue Whale calf drinks approximately 400 litres (100 US gallons) of milk every day. Blue Whale calves gain weight as quickly as 90 kg (200 pounds) every 24 hours. Even at birth, they weigh up to 2700 kg (6000 lb) – the same as a fully-grown hippopotamus.
Blue Whales are very difficult to weigh because of their massive size. Most Blue Whales killed by whalers were not weighed as a whole, but cut up into manageable pieces before being weighed. This caused an underestimate of the total weight of the whale, due to loss of blood and other fluids. Nevertheless, measurements between 150 to 170 tonnes (160 and 190 short tons) were recorded of animals up to 27 m (88 ft 6 in) in length. The weight of a 30 m (98 ft) individual is believed by the 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).
Mating starts in late autumn, and continues to the end of winter. Little is known about mating behaviour or even breeding grounds. Females typically give birth at the start of the winter once every two to three years after a gestation period of ten to twelve months. The calf weighs about two and a half tonnes and is around 7 m in length. 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 long (or more in the southern hemisphere). Females are larger still, reaching sexual maturity around 21 m or around the age of five.
Scientists estimate that Blue Whales can live for at least eighty years; however, since individual records do not date back into the whaling era, this will not be known with certainty for many years yet. The longest recorded study of a single individual is thirty-four years, in the north-east Pacific (reported in Sears, 1998). The whales' only natural predator is the Orca. Calambokidis et al (1990) report that as many as 25% of mature Blue Whales have scars resulting from Orca attack. 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 quite a public event. 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.
|Multimedia relating to the Blue Whale
Note that the whale calls have been sped up 10x from their original speed.
The Blue Whale is the loudest animal in the world. Estimates made by Cummings and Thompson (1971) and Richardson et al (1995) suggest that source level of sounds made by Blue Whales are between 155 and 188 decibels when measured at a reference pressure of one micropascal at one metre. By comparison, a pneumatic drill is about 100 dB. A human, however, would likely not perceive the Blue Whale as the loudest of all animals. 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.
Scientists do not know why Blue Whales vocalize. Richardson et al (1995) discuss six possible reasons:
- Maintenance of inter-individual distance
- Species and individual recognition,
- Contextual information transmission (e.g., feeding, alarm, courtship)
- Maintenance of social organization (e.g., contact calls between females and males)
- Location of topographic features
- Location of prey resources
Population and whaling
The hunting era
Blue Whales are not easy to catch, kill, or retain. Their speed and power meant that they were often not the target of early whalers who instead targeted Sperm and Right Whales. As the populations of these other species declined, whalers increasingly hunted the largest baleen whales, including the Blue Whale. In 1864 Norwegian Svend Foyn equipped a steamboat with harpoons specifically designed for catching large whales. Although initially cumbersome and with a low success rate, the harpoon became the weapon of choice for whale hunting, and by the end of the nineteenth century, the population of Blue Whales in the North Atlantic had declined significantly.
Hunting of Blue Whales rapidly increased around the world, and by 1925, the United States, Britain and Japan had joined Norway in chasing whales on 'catcher boats' that caught the whales and handed them onto huge 'factory ships' for processing. In 1930/1931, these ships killed 29,400 Blue Whales in the Antarctic alone. By the end of World War II populations had been significantly depleted, and in 1946 the first quotas restricting international trade in whales were introduced. These were ineffective because of the lack of differentiation between species. Rare species could be hunted equally with those found in relative abundance. By the time Blue Whale hunting was finally banned in the 1960s and illegal USSR whaling halted in the 1970s by the International Whaling Commission, 330,000 Blue Whales had been killed in the Antarctic, 33,000 in the rest of the Southern Hemisphere, 8,200 in the North Pacific, and 7,000 in the North Atlantic and the world population had been reduced to less than 1% of its total one hundred years before.
Population and distribution today
Since the whaling ban, it is not well known whether the global Blue Whale population is increasing or remaining stable. In the Antarctic, best estimates show a significant increase since the end of illegal Soviet Union whaling, but numbers remain at under 1% of their original levels. It has also been suggested that Icelandic and California populations are increasing but these increases are not statistically significant. The total world population was estimated to be between 5,000 and 12,000 in 2002 although there is great uncertainty in available estimates for many areas. The Blue Whale remains listed as "endangered" on the IUCN Red List of threatened species as it has been since the list's inception. The largest known concentration, consisting of about 2,000 individuals, is the North-East Pacific population that ranges from Alaska to Costa Rica but is most commonly seen from California in summer. This group represents the best hope for a long-term recovery in Blue Whale population. Sometimes this population strays over to the North-West Pacific; infrequent sightings between Kamchatka and the northern tip of Japan have been recorded.
In the Southern Hemisphere, there appear to be two distinct subspecies, the Antarctic blue whale and the poorly-understood Pygmy blue whale, found in Indian Ocean waters. Recent abundance estimates for the Antarctic subspecies range from 1100 to 1700. Research into the number of Pygmy Blues is on-going - the most recent reports suggest that there may be as many as 10,000 animals. If this is confirmed it will lead to a substantial upward revision in the global numbers.
Migratory patterns of these subspecies are not well known. For example, pygmy blue whales have been recorded in the northern Indian Ocean (Oman, Maldives, Sri Lanka) where they may form a distinct resident population. In addition, the population of Blue Whales occurring off Chile and Peru may also be a distinct population. Some Antarctic blue whales approach the eastern South Atlantic coast in winter, and occasionally their sounds are heard off Peru, Western Australia, and in the northern Indian Ocean. In Chile, the Cetacean Conservation Centre, with support from the Chilean Navy, is undertaking extensive research and conservation work on a recently discovered feeding aggregation of the species off the coast of Chiloe Island.
In the North Atlantic, two stocks are recognized. The first is found off Greenland, Newfoundland, Nova Scotia and the Gulf of Saint Lawrence. This group is estimated to total about 500. The second, more eastern group is spotted from the Açores in Spring to Iceland in July and August; it is presumed that the whales follow the Mid-Atlantic Ridge between the two volcanic islands. Beyond Iceland, Blue Whales have been spotted as far north as Spitsbergen and Jan Mayen though such sightings are rare. Scientists do not know where these whales spend their winters. The total North Atlantic population is between 600 and 1500.
Human threats to the potential recovery of Blue Whale populations include the accumulation of polychlorinated biphenyl (PCB) chemicals within the whale's blood, causing poisoning and premature death, and the ever-increasing amount of noise created by ocean traffic. This noise drowns out the noises produced by whales (see whale song), which may make it harder for whales to find a mate.
Efforts to calculate the Blue Whale population more accurately are supported by marine mammologists at Duke University who maintain the OBIS-SEAMAP (Ocean Biogeographic Information System - Spatial Ecological Analysis of Megavertebrate Populations), a collation of marine mammal sighting data from around 130 sources.