By considerable measure, the largest known animal on Earth is the blue whale. Mature blue whales can measure anywhere from 75 feet (23 m) to 100 feet (30.5 m) from head to tail, and can weigh as much as 150 tons (136 metric tons). That's as long as an 8- to 10-story building and as heavy as about 112 adult male giraffes! These days, most adult blue whales are only 75 to 80 feet long; whalers hunted down most of the super giants. Female blue whales generally weigh more than the males. The largest blue whale to date is a female that weighed 389,760 pounds (176,792 kg).
A blue whale's head is so wide that an entire professional football team -- about 50 people -- could stand on its tongue. Its heart is as big as a small car, and its arteries are wide enough that you could climb through them. Even baby blue whales dwarf most animals. At birth, a blue whale calf is about 25 feet (7.6 m) long and weighs more than an elephant. And they do grow up fast: During the first 7 months of its life, a blue whale drinks approximately 100 gallons (379 liters) of its mother's milk per day, putting on as much as 200 pounds (91 kg) every 24 hours. An adult blue whale can eat more than 4 tons (3.6 metric tons) of krill, a tiny shrimp-like creature, every day.
This puts blue whales well above any known land mammal in terms of size. Most people believe that the largest animals to ever exist on Earth were the dinosaurs. However, one of the largest land dinosaurs, the sauropod Argentinosaurus, weighed only about 180,000 pounds (81,647 kg). That's little more than half the size of an adult blue whale. It makes a lot of sense that the world's largest animal would be a sea creature. Land animals have to support their own weight, whereas sea creatures get some help from the water.
It is believed that at one time there were more than 200,000 blue whales. There are only about 10,000 blue whales now -- they've been on the endangered list since the mid-1960s -- and the population is not expected to recover.