Scientists call true seals, eared seals, and walruses "pinnipeds"—literally, "feather-foot."
Eared seals (sea lions and fur seals) differ from true seals in having external ear flaps and the ability to use their flippers for walking on land. Also, eared seals tend to use their front flippers for power while swimming; true seals use their rear flippers.
Pinnipeds, which arose from land mammals, have evolved slick, torpedo-shaped bodies ideal for gliding through water.
Seals such as the gray seal (Halichoerus grypus) can sleep underwater, and can even surface to breathe without awakening.
California sea lions (Zalophus californianus), which live off the Pacific coast, from British Columbia to Baja California, come ashore in huge groups from May to August. Males fight to establish territories on the beach, then maintain the boundaries with displays of barking, head-shaking, and ritualized fighting. Females come ashore shortly after the males to give birth to pups conceived the previous season. Each bull then mates with as many females in his territory as possible.
Zoo scientists discovered that hooded seals (Cystophora cristata) nurse for only four days—the shortest known lactation period of any mammal. Drinking milk that is 60-percent fat, each pup consumes about 65,000 calories a day, gaining 60 pounds and doubling its birth weight before weaning.