Blue whale

Blue whales (Balaenoptera musculus) are the largest animals that have ever lived on Earth. They can grow to more than 30m, but are usually between 24 and 28m. A large blue whale can weigh around 178,000kg.

Female blue whales are generally larger than males. A blue whale heart is the size of a Volkswagen Beetle and pumps nine tonnes of blood through the massive body. A blue whale aorta (the main blood vessel) is so large a human can crawl through it.

Blue whales are a lightly mottled blue-grey, with light grey or yellow-white undersides. The yellowish ventral colouring is due to the accumulation of diatoms in colder water, and has inspired the nickname "the sulphur bottom whale". The whale has a broad flat head and a small dorsal fin on its back quarter. Blue whales are believed to have excellent hearing, especially at low frequencies, which is valuable in the dark ocean environment.

They are the loudest animals on Earth and louder than a jet engine: their calls reach 188 decibels, while a jet reaches 140 decibels. Their low frequency whistle can be heard for hundreds of miles, and is probably used to attract other blue whales.

Where are they found?
Blue whales live in every ocean. Like many other baleen whales, they feed in cool waters at high latitudes, and migrate to warmer temperate and tropical waters to breed and give birth.

What do they eat?
Blue whales strain their food from the water through baleen. Blue whales feed nearer to the surface than some other whales, straining their food from the upper layers of the ocean. Their diet consists of krill and copepods and it takes 1000kg of food to fill a blue whale's stomach.

How long do they live?
Blue whales begin mating when they are between five and 10-years-old. Females become pregnant every two to four years and gestation lasts 11 months. The calf of a blue whale is about 7m long at birth and weighs 2.3 tonnes. Feeding on their mother's rich milk, calves can put on 90kg each day while nursing. This continues for seven months while whales migrate to the feeding grounds where the calf is weaned and can fend for itself. By this time the blue whale calf has grown to 16m and 20.7 tonnes. Mother and calf form a very close bond, with the baby swimming in close contact with its mother.
The blue whale has no teeth so it is difficult to tell its age (teeth can be used to estimate age in other mammals). However, scientists believe they live until they are at least 50. Blue whales are sometimes preyed on by packs of killer whales.

Blue whales and hunting
Historically, blue whales were hunted for baleen, blubber for oil, and meat. From hunting records, scientists have hypothesized that there were once more than 250,000 blue whales, but that there are now only 5,000 left, mostly in the Northern Hemisphere. The blue whale is an endangered species and hunting them has been banned since 1965 when they were protected by the International Whaling Commission. The whale has shown encouraging signs of recovery and populations have been increasing in the North Pacific and North Atlantic Oceans. In the Antarctic, however, where the largest catches were made, there is still no evidence of recovery.

Conservation concerns
Like other large whales, blue whales are threatened by environmental change including habitat loss, toxics, and climate change in the Antarctic. Blue whales can also be harmed by ship strikes and by becoming entangled in fishing gear.