The majority of aquatic mammals are carnivorous, taking advantage of the abundance of high-energy food found in the ocean. They have retained their air-breathing lung instead of developing gills so they must surface to breathe and hold their breath like a free diver underwater. Their nostrils have been moved to either the end of the snout or the top of the head to aid in breathing. Marine mammals differ from land mammals in that they feed exclusively in an aquatic environment and store their food internally as fat instead of storing it externally in a den or other hiding place. Manatees and dugongs feed exclusively on aquatic plants.
Marine mammals have excellent auditory senses.
They use sound for communication, navigation, orientation and identification of foreign objects. Reproduction in an aquatic environment is rather complex but still follows the 6 basic steps of all mammals. Step one is courtship. Courtship is heavily ritualized using visual, olfactory, auditory and tactile communication. Once a pair has established by courtship, copulation can occur.