The blue whale is the largest of all whales and is said to be the largest animal in Earths history. The average length of an adult blue whale is 79 to 88 feet and the average weight is 130 to 150 tons. Blue whales are blue to blue-gray and may sometimes appear to have a faint yellow skin color because of the many microorganisms that accumulate on their bodies as they travel through colder waters. Blue whales prefer cold waters and open seas and prefer not to venture near coastal areas. However, some whales have been seen off the coast of Peru.

The blue whale eats mostly krill (small shrimp-like crustaceans) by using its baleen plates lining its mouth to sieve krill from the water. It can consume several tons of krill daily. Researchers have believed that the blue whale only eats in the summer, seldom eating during the rest of the year. But recent research has suggested that some blue whales of the north may eat year-round and that blue whales found traveling with empty stomachs may stop eating during migration and resume feeding at wintering areas. Blue whales only mate in warmer waters and the gestation period lasts 10 to 11 months. The females give birth to only one calf in the spring which gives the calf the entire summer to grow a layer of blubber needed to protect it when traveling through colder waters. The calves nurse for around seven months.