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The blue whale is the largest mammal ever to have lived on Earth. Ironically, it feeds on some of the smallest ocean life, plankton. Although blue whales feed in deep water, they are still mammals, and must come to the surface to breathe. They exhale air in a cloud of pressurized steam that rises straight up for about 20 feet.

Like other marine mammals, blue whales are descended from early land animals. Millions of years ago, the richness of life in the sea lured them to the water, aquatic life gradually changed their physical characteristics. Today, they spend most of their time in the Arctic and Antarctic oceans, where plankton is plentiful. In winter, the whales migrate to the warm waters of the tropics. But food in the tropics is scarce, so the whales depend heavily on their thick layer of blubber for nourishment.

In the Antarctic, blue whales feed on vast quantities of a plankton called krill. In Arctic water, they feed on only three species of crustacean plankton. Plankton is up to 30 times more abundant in the Arctic and Antarctic than it is in the warmer waters of the tropics In spite of its bulk, the blue whale can reach speeds of 10 to 15 knots, however, it catches most of its food by diving. It can dive to depths of 1,650 feet and lie submerged for up to two hours.

Blue whales form close ties with one another and are often seen in groups of two or four. Mating takes place in the warm waters of the tropics, where the young are born. The mother gives birth to a single calf with the assistance of other females, who help her deliver the calf and then nudge the newborn to the surface, where it takes its first breath of air. At birth, the calf measures about 23 feet long and weighs 16,000 pounds. The baby is suckled in the water, drinking more than 160 gallons of milk a day. At 7 months old, it is able to catch its own food.

Because of its great size, the blue whale was a prime target for the whaling industry. Its body was a source of oil, and the baleen was used to make women’s corsets. Since 1986, commercial whaling has largely ceased, and blue whales now show signs of breeding success. Still, it will take a century of protection before they are out of danger of extinction.