What is the largest mammal on Earth?

The blue whale outweighs every other mammal by a margin that is hard to picture: adults run to around thirty metres and well over a hundred tonnes, with a heart the size of a small car and a tongue that alone weighs as much as an elephant. Water is what makes that possible. On land an animal's weight has to be carried by bone and muscle, and the strength of a leg grows more slowly than the mass it supports, which puts a hard ceiling on land animals. A whale is neutrally buoyant, so its skeleton never has to hold it up, and it can keep growing as long as it can feed itself. It does that by lunge-feeding on krill, engulfing enormous volumes of water and straining it through baleen plates. Filtering a tiny, superabundant food source is far more efficient than hunting large prey, and it is the reason the biggest animals in the ocean eat some of the smallest.

Baleen is made of keratin, the same material as fingernails and hair.