Blue whales are the largest known animals ever to live on our planet. They can weigh up to 330,000 pounds (165 tons), reach lengths over 100 feet, and like many other baleen whales, female blue whales are generally larger than males. Blue whales have a long body and generally slender shape. Their mottled blue-gray color appears light blue under water—hence their name, the blue whale. The mottling pattern is variable and can be used to identify individuals. Blue whales sometimes swim in small groups but are more often found alone or in pairs. They generally spend summers feeding in polar waters and undertake lengthy migrations towards the equatorial waters as winter arrives. Blue whales are found in all oceans except the Arctic Ocean and there are five currently recognized subspecies of blue whales (Northern, Antarctic, Pygmy, Northern Indian Ocean, & Chilean). The primary diet of blue whales is krill, but fish and tiny crustaceans may occasionally be part of the blue whale’s diet. When blue whales hunt for food, they filter feed by swimming toward large schools of krill with their mouth open and closing their mouths around the krill while inflating their throat pleats. Once closed, blue whales then push the trapped water out of their mouth with their tongue and use their baleen plates to keep the krill trapped inside. Some of the biggest individuals may eat up to 6 tons of krill a day. Blue whales typically swim at about 5 miles an hour while they are feeding and traveling, but can accelerate to more than 20 miles an hour for short bursts. They are among the loudest animals on the planet, emitting a series of pulses, groans, and moans, and it is thought that in the right oceanographic conditions, sounds emitted by blue whales can be heard by other whales up to 1,000 miles away..