The Blue whale (Balaenoptera musculus) is a marine mammal and a baleen whale. It is the largest animal known to have ever existed. The Blue whale was once abundant in nearly all the Earth's oceans until the end of the 19th century. It was hunted almost to the point of extinction by whalers until the International Whaling Commission banned all blue whale hunting in 1966. It continues to face numerous man-made threats such as ship strikes, pollution, ocean noise, and climate change.
The Blue whale has a huge long, slim, narrow body. Its skin is hairless and smooth, and is grayish blue, lighter on the underside, with a series of grooves on the throat. Its large tail is straight, splitting at the end into two rubbery flukes. Blue whales are part of the "baleen whales" group, having, instead of teeth, up to 395 hard, bristly baleen plates that descend from the upper jaw. These plates are used to filter seawater for food. They have two blow-holes that expel stale air and seawater out of the top of their heads when they come up to the surface to breathe.
Blue whales are cosmopolitan species found in every ocean in the world except the Arctic, but absent from some regional seas such as the Mediterranean, Red, Baltic, Okhotsk, and Bering seas. They take long migrations, traveling to their summer feeding grounds towards the poles and then heading to their winter breeding grounds in more equatorial waters.
Blue whales are generally solitary creatures that may spend some time in pairs and occasionally come together in casual groups to feed. They communicate with each other using a variety of sounds or songs, including squeaks, hums, and rumbles, mainly during the breeding season, which is in winter. The sounds Blue whales make are extremely loud, the loudest of any animal on the planet, and they have been recorded at higher than 180 decibels. They use their tails to perform deep dives, as by lifting their tail above the water's surface, they muster enough power to dive up to 200 meters straight down into the sea. Blue whales feed through lunge feeding; they swim toward krill at high speeds as they open their mouths up to 80°. While pursuing krill patches, Blue whales maximize their calorie intake by increasing the number of lunges while selecting the thickest patches. This provides them enough energy for everyday activities while storing additional energy necessary for migration and reproduction.
Blue whales are monogamous, mating during winter or early spring in warmer, tropical waters. Gestation lasts for almost a year, with the female giving birth to one calf when she returns to the region the next year. Having spent all summer feeding in the cold, rich waters of the Antarctic, the mothers eat almost nothing during the time they are providing their young with milk. Baby blue whales stay with their mother for at least a year and are weaned at about 8 months old. Blue whales can start reproducing between the age of 10-15 years old, producing a calf every 2 or 3 years.
Blue whales have almost no natural predators, due to their enormous size. The calves may be at risk of predation by large sharks and orcas. Humans hunted them so much throughout the 20th century that they almost became extinct. Globally, there appear to be no major threats to Blue whales at present. They can be subject to some ship strikes and entanglements but reported cases are few.
According to IUCN Red List, the global population size of the Blue whale is 5,000-15,000 mature individuals. Currently, this species is classified as Endangered (EN) on the IUCN Red List, but its numbers today are increasing.
As with other large baleen whales, Blue whales are significant predators of krill and control their populations.