Hearing is the detection of sound. Both modern land mammals, including humans, and marine mammals evolved from ancestors that had air-adapted ears. So, many of the structures of the ear in both land and marine mammals are similar. Some marine mammals that live exclusively in water, like whales and manatees, hear very well in water but hear poorly, if at all, in air. Marine mammals that live on land at least part of the time, such as seals, sea lions and walruses (pinnipeds), otters and polar bear, have ears that are amphibious and can hear in both air and water.
The basics of hearing are the same in both land and marine mammals. Hearing is the result of the combined activity of the ear's three basic divisions: (1) the outer ear collects and directs sound, (2) the middle ear filters and amplifies the acoustic energy to the inner ear, and (3) the inner ear transforms the acoustic energy to electrical signals (neural impulses) that are processed by the brain.
The explanation of marine mammal hearing is divided into the following three sections: