Blue whale

For other uses, see Blue whale (disambiguation).

The blue whale (Balaenoptera musculus) is a type of baleen whale. It is the largest marine mammal in the rorqual family Balaenopteridae. It can grow to about 30 meters (98–100 feet) long and weigh up to 190–200 tonnes. This makes it the largest animal known to have ever existed. Its long, slim body is greyish-blue on top and a little lighter underneath.

Four subspecies are recognized. They live in different parts of the world's oceans. One of them, the pygmy blue whale, lives in the Indian Ocean and South Pacific Ocean. Blue whales usually move between summer feeding areas near the poles and winter breeding areas near the tropics. Some groups stay in one area all year. Others move depending on their age or sex.

Blue whales are filter feeders, and they eat almost only krill. They usually travel alone or in small groups. Apart from the bond between mothers and calves, they do not have a clear social structure. Blue whales vocalize at a fundamental frequency of 8 to 25 Hz. Their calls may change depending on the region, the season, their behavior, and the time of day.

The blue whale was abundant in nearly all the Earth's oceans until the end of the 19th century. Whalers hunted it until it was nearly gone. In 1966, the International Whaling Commission banned all blue whale hunting. As of 2018, the International Union for Conservation of Nature has listed blue whales as endangered. They still face threats caused by people, such as ship strikes, pollution, ocean noise, and climate change.

Taxonomy

See also: Evolution of cetaceans

The genus name, Balaenoptera, means "winged whale." The species name, musculus, could mean "muscle" or a small "mouse." Carl Linnaeus may have chosen this playful name when he named the species in Systema Naturae. In 1692, Robert Sibbald found a whale that had washed ashore in the Firth of Forth in Scotland. His book Phalainologia Nova gave one of the first published descriptions of a blue whale. The name "blue whale" comes from the Norwegian word blåhval, which was coined by Svend Foyn. The Norwegian scientist G. O. Sars made it the common name in 1874. Whalers also called them "sulphur bottom" whales, because some bellies look yellow from a coating of diatoms. Herman Melville mentions these whales in his novel Moby-Dick. Blue whales are rorquals in the family Balaenopteridae. A 2018 study estimates that this family split from other families between about 10.48 and 4.98 million years ago, during the late Miocene. The oldest known blue whale fossil that looks like today's blue whales is a partial skull from southern Italy. It is from the early Pleistocene, about 1.5 to 1.25 million years ago. The Australian pygmy blue whale split off during the Last Glacial Maximum. This recent split, and especially the split of New Zealand blue whales, has left them with low genetic diversity.

Blue whales are known to interbreed with fin whales, and hybrids have been found in several places. In one case, a blue whale taken in 2018 by the Icelandic whaling company Hvalur hf. was tested with DNA. The results suggested a fin whale father, but they still need to be checked. Whole genome sequencing suggests that blue whales are most closely related to sei whales. Gray whales are their sister group. One study found significant gene flow between minke whales and the ancestors of blue and sei whales. A humpback and blue whale hybrid has also been reported in the South Pacific. Like many large rorquals, the blue whale is a cosmopolitan species. This means it is found all over the world, but it is mostly absent from the Arctic Ocean, the Mediterranean, Okhotsk, and the Bering Sea. At least four subspecies are traditionally recognized. Northern whales gather near New England, the Gulf of St. Lawrence, the Norwegian Sea, the West Indies, the Azores, the Alaska Gyre, the Aleutian Islands, the Gulf of California, the Costa Rica Dome, the Kamchatka Peninsula, and the Gulf of Alaska. Pygmy blue whales move between the Seychelles, the Amirante Islands, the Crozet Islands, the Prince Edward Islands, and the Mozambique Channel. Australian whales feed near Perth Canyon, the Great Australian Bight, and Bass Strait. New Zealand whales may use the Tasman Sea, the Lau Basin, the South Taranaki Bight, and North Island. Chilean blue whales may be a separate subspecies. They may overlap in the Eastern Tropical Pacific with whales from Antarctica, but interbreeding between them seems unlikely. A 2019 study of whales from the 1965/1966 whaling season supported their separate population status.

Description

The blue whale has a long, slim body and a wide, U-shaped head. It has long, thin flippers and a small, curved dorsal fin near its tail. Its wide, thin flukes sit at the end of its tail. Its upper jaw has 70–400 black baleen plates. Each plate is less than 1 m (3.3 ft) long. Its throat has grooves that let it stretch when it feeds. It has two blowholes that can shoot water 9.1–12 m (30–39 ft) into the air. Its skin is a mottled grayish-blue that looks blue underwater, and its belly is lighter. The belly can look yellowish because of diatoms in the water. For this reason, it was once nicknamed "sulphur bottom." Some albino blue whales are completely white.

The blue whale is the largest animal known to have ever lived. Some other animals, such as shastasaurid ichthyosaurs and the ancient whale Perucetus, may have been similar in size. However, those estimates are based on incomplete remains. Female blue whales are usually larger than males. The longest blue whale ever scientifically measured was caught in the Panama Canal. It was 29.9 m (98 ft) from nose to tail. Blue whales usually live around 80–90 years or more. Scientists can estimate a whale's age by counting the light and dark layers of earwax in its ear plug. One pair of layers forms each year. The oldest whale found this way was 110 years old.

Behavior and ecology

Blue whales usually live alone, but they are sometimes found in pairs. When there is plenty of food, they may gather in groups of more than 50. Some groups go on long migrations. They travel to summer feeding grounds near the poles and then to winter breeding grounds in warmer waters closer to the equator. They seem to use memory to find the best places to feed. Blue whales usually swim at 2–8 km/h (1.2–5.0 mph). They can swim much faster, at 32–48 km/h (20–30 mph), when they meet boats, predators or other whales. Their huge size makes it hard for them to breach.

Blue whales eat almost only krill. They catch it by lunge feeding. They swim fast toward the krill with their mouths open and take in a large amount of water. Then they push the water out through their baleen plates and swallow the krill that is left. In the Southern Ocean, they eat Antarctic krill (Euphausia superba). Off California, they mostly eat Thysanoessa spinifera and less often Euphausia pacifica. The Northern Indian Ocean subspecies (B. m. indica) mainly eats sergestid shrimp. Its droppings also showed fish, krill, amphipods, cephalopods and scyphozoan jellyfish.

Blue whales are thought to be ready to have young when they are between 5 and 15 years old. They appear to be polygynous, which means males compete for females. They mate from fall to winter. Gestation may last 10–12 months. Newborn calves are 6–7 m (20–23 ft) long and weigh 2–3 t. Calves may be weaned at 6–8 months old.

Blue whales make some of the loudest and lowest sounds of any animal. Their inner ears seem well suited to hearing low-frequency sounds. Their fundamental frequency for vocalizations ranges from 8 to 25 Hz, and their loudest calls reach 188 dB. Their songs differ between populations, and some have slowly become lower in tone.

Only orcas have been known to attack blue whales, and they often go after calves. Some whales in the Gulf of California have rake-like scars, which are a sign of orca encounters. The first known attack happened in 1977 off Baja California, Mexico. Groups of orcas have also been seen in the Eastern Tropical Pacific and chasing whales in Monterey Bay.

Blue whales in Antarctic waters can carry diatoms such as Cocconeis ceticola. These are usually removed in warmer waters. Barnacles such as Coronula diadema, Coronula reginae and Cryptolepas rhachianecti attach to their skin, and whale lice live in cracks in their skin. The copepod Pennella balaenopterae attaches to the blubber to feed. Intestinal parasites include trematode worms, tapeworm genera such as Tetrabothrius, and the thorny-headed worm Bolbosoma. In the North Atlantic, the single-celled organisms Entamoeba, Giardia and Balantidium have also been found.

Conservation

Scientists think there are about 5,000 to 15,000 adult blue whales in the world, and 10,000 to 25,000 in total, based on counts from 2015 to 2018. In 1926, there were at least 140,000 adult whales. Blue whales have been protected in parts of the Southern Hemisphere since 1939. In 1955, they received full protection in the North Atlantic under the International Convention for the Regulation of Whaling. This protection spread to the Antarctic in 1965 and the North Pacific in 1966. In the United States, the Endangered Species Act protects the species. Blue whales are listed as endangered under this law and the IUCN Red List. They are also listed on Appendix I of the Convention on International Trade in Endangered Species of Wild Fauna and Flora (CITES) and the Convention on the Conservation of Migratory Species of Wild Animals. Some groups, such as Antarctic blue whales, are critically endangered. It is very likely that the number of blue whales around the world has grown over the past few decades.

Blue whales were hard to catch at first because they were so large and fast. New harpoons made in the mid-1800s made hunting them easier. In 2017, scientists studied DNA from whale bones found at Icelandic sites. The tests showed that people in Iceland were hunting blue whales as early as the 9th century, around the time the settlement of Iceland began. Whaling was at its highest in 1930 and 1931. The International Whaling Commission banned hunting blue whales in 1966, but the Soviet Union continued to hunt them secretly into the 1970s.

Today, collisions with ships are a major danger to blue whales, especially off the west coast of the United States. In 2007, several blue whales died off California after being hit by ships. This was treated as an unusual event under the Marine Mammal Protection Act. Loud man-made noise underwater can also affect them. In the Southern California Bight, blue whales made quieter calls when sonar was in use.

Scientists do not yet know how pollutants affect blue whales. Because they feed low on the food chain, they are less likely to build up harmful chemicals in their bodies through bioaccumulation. Still, chemicals such as persistent organic pollutant (POP) compounds and DDT have been found in them. These chemicals can pass from mothers to their young through maternal transfer. Males in the Gulf of St. Lawrence had higher levels of these chemicals than females did.