Whales are a group of fully aquatic marine mammals that first appeared about 50 million years ago, during the Eocene epoch, and are still alive today. There are two major groups of whales, the baleen whales and the toothed whales, and their last common ancestor lived about 34 million years ago. The whales' closest land relatives are the hippopotamuses, from which they diverged about 54 million years ago.
Whales are fully aquatic mammals: they can feed, mate, give birth, nurse, and raise their young at sea. Whales range in size from the dwarf sperm whale, at 2.6 meters and 135 kilograms, to the blue whale at 29.9 meters and 190 tonnes, which is the largest known animal ever to have lived. At the same time, the sperm whale is the largest toothed predator on Earth.
Baleen whales lack teeth. They feed on krill and plankton (very small marine organisms), taking them in with large volumes of water and then expelling the water through their baleen plates. Their enormous heads can account for up to 40% of their body mass, and throat pleats allow their mouths to expand, enabling them to take in large volumes of water at once. Baleen whales also have a well-developed sense of smell.
Toothed whales, in contrast, have conical teeth adapted for catching fish or squid. Their acute hearing, both above and below the water’s surface, can allow some individuals to survive even when blind. Sperm whales are particularly well adapted to deep diving in pursuit of squid and other prey.
