Cetaceans (Whales, Dolphins and Porpoises)

This group gathers Mammals subcategory members belonging to the infraorder Cetacea, the fully aquatic whales, dolphins and porpoises, which never leave the water and have lost their hind limbs entirely over the course of their evolution from land dwelling, even-toed hoofed ancestors. It spans the baleen whales, which filter small prey such as krill through comb-like plates in the mouth rather than teeth, including the blue whale, the largest animal known to have ever lived, the humpback whale and the gray whale, and the far more numerous toothed whales, which include every living dolphin and porpoise species, the sperm whale, the narwhal and beluga, and the orca, itself technically the largest member of the dolphin family despite its common name. A freshwater lineage, the river dolphins, is also placed here, having independently evolved a similar body plan in several of the world's major river systems.