They evolved from terrestrial animals to an entirely aquatic life form that is completely separated from the land in all aspects of biology. Cetaceans live, breed, rest, and carry out all of their life functions in the water. The phylogenetic relationships discussed above are in the currently accepted form but many biologists still classify the Mysticetes and Odontocetes as separate orders. Many of these characteristics are adaptations to reduce drag for fast swimming in an aquatic environment. Protuberances such as external ears or genitals would create turbulence and would be very inefficient for an animal in the water.
Many cetaceans can dive for extended periods of time to great depths.
Some cetaceans are thought to be the most intelligent non-primates and many have proportionately large brains. They also have remarkably efficient lungs and circulatory systems. The baleen whales feed primarily on plankton and krill. These plates are used to strain food from the water. Different species have different density and coarseness of baleen due to their different food sources.