Think for a moment about marine mammals: seals, walruses, dugongs and whales. Seals
and walruses are primarily cold-water species that eat mostly fish and can spend part of
their time on land (or ice). Dugongs and manatees are tropical herbivores and are entirely
aquatic. Toothed whales and baleen whales are also entirely aquatic. All of these
species share many similarities such as streamlined bodies and flippers. But they are all
clearly mammals: they have fur, bear live young, lactate, and their skeletons match many
features of terrestrial mammals.