From the biologist who produced The Year of the Whale, The Year of the Seal, and other works comes a charming introduction to the whole aquarium of sea mammals: six groups consisting of sea otters, walking seals, crawling seals, sirenians (those mythical-sounding manatees and dugongs), toothed cetaceans (porpoises and some varieties of whales), and baleen cetaceans (other whales). Scheffer traces their evolutionary return to the sea and their remarkable adaptations to that environment: the loss of legs and streamlining of head into body, the development of insulating and fatty layers to maintain warm temperature, and, of particular interest, the respiratory and circulatory changes that make it possible for some sea mammals to make hourlong deep dives. (Shifts of blood from the periphery to supply brain and other vital organs, and lung collapse and deposition of gases in non-absorbable cavities, are among the specializations behind such dives.) Chapters on behavior, migration, intelligence, mating, predators, and so on round out the natural history. Much of the newer information is based on the observation of the animals in near-natural aquarium environments or at sea, using tagging, telemetry equipment, or other non-harmful intervention to track individual animals. Short factual sentences interlaced with anecdotes suit the book to high school or even junior high school readers.