The blue whale has one of the fastest-growing body sizes among living animals and is the largest animal known to have existed.
Blue whales begin life at roughly 2 to 3 tonnes and can gain around 90 kilograms per day during their first months. This rapid growth is supported by exceptionally energy-rich milk, which contains a high proportion of fat compared with milk from many land mammals.
After nursing, young blue whales shift toward feeding on krill. Seasonal migrations let them take advantage of productive feeding grounds, where dense krill swarms provide the energy needed to maintain their enormous bodies and reproduce.
The exact wording of growth comparisons matters: blue whales are not necessarily the fastest-growing organisms in every biological measure. Their notable record concerns the rapid increase in mass of a living animal. They also remain distinct from the whale shark, which is the largest fish but not a mammal.