Unlike fishes, secondary swimmers (terrestrial animals that returned to an aquatic environment) have no such specific adaptations to the buoyancy problem. They all rely on simple density adaptations to help them. For example, the bones of diving birds are less pneumatic, and their air sacs are reduced (loons, penguins). Mammals that dive deep may hyperventilate before submerging, but they do not fill their lungs. Indeed, they may exhale before diving. Deep-diving whales have relatively small lungs. Sirenians, which may feed while resting on the bottom or standing on their tails, have unusually heavy skeletons; their ribs are swollen and solid. Likewise, the skeleton of the hippopotamus is also unusually heavy. The presence of blubber in marine mammals also contributes to their overall density, and walruses (Odobenidae) have two large air pouches extending from the pharynx, which can be inflated to act like a life preserver to keep the animals’ head above water while sleeping.
The largest group of marine mammals, the cetaceans, is also the group that has made the most complete transition to aquatic life. While most other marine mammals return to land at least part of the time, cetaceans spend their entire lives in the water. Their bodies are streamlined and look remarkably fish-like. Interestingly, even though all marine mammals have evolved from very different evolutionary groups, there are certain similarities in lifestyle and morphology, and they are considered good examples of the principle of convergence. Convergent evolution is the process by which creatures unrelated by evolution develop similar or even identical solutions to a particular problem; in this case, life in water.