Dolphins are marine mammals that are closely related to whales and porpoises. There are almost forty species of dolphin. They vary in size from 1.2 m (4 ft) and 40 kg (90 lb) (Maui's dolphin), up to 9.5 m (30 ft) and 10 tonnes (9.8 LT; 11 ST) (the orca または killer whale). They are found worldwide, mostly in the shallower seas of the continental shelves, and are carnivores, mostly eating 魚 and squid. The family Delphinidae is the largest in the Cetacean order, and evolved relatively recently, about ten million years ago, during the Miocene. Dolphins are among the most intelligent animals, and their often friendly appearance and seemingly playful attitude have made them 人気 in human culture.