Dolphins are marine mammals that are closely related to whales and porpoises. There are almost forty species of dolphin in seventeen genera. They are found worldwide, mostly in the shallower seas of the continental shelves, and are carnivores, mostly eating fish and squid. The family Delphinidae is the largest in the Cetacean order, and relatively recent: dolphins evolved 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 popular in human culture. A group of dolphins is called a "school" or a "pod". Male dolphins are called "bulls", females "cows" and young dolphins are called "calves".