A male seal is called a bull. A female is called a cow. Young are called pups. A group is called a herd, rookery or harem. 'Pinniped' means 'fin-footed.' Walruses ('odobenids') are neither seals nor sea lions and have no external ear flap but can walk on all fours. 'Odobenids' are the third grouping of 'pinnipeds'.

Seals are carnivorous 'pinnipeds' while manatees and dugongs are called plant-eating 'sirenians'. The difference between a sea lion and a seal is that the former has flaps over its ears and large front flippers that allow it to maneuver around on land more easily. Seals and sea lions have many similarities with land carnivores like dogs and cats such as a long snout and sharp teeth and are thought to have developed from land mammals about 20 million to 25 million years ago.

There are 34 species of seal, all of which mate and give birth on land and feed at sea. They fall into two categories: true seals and eared seals. True seals ('phocids') have no external ear and use their fishtail-like rear flippers for swimming. They are not very mobile on land. They hunch along caterpillar fashion using their whole bodies. Their clawed foreflippers do not offer much help getting around on land.

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