Whales evolved from animals on land (early relatives of hippos) over a period of 50 million years, slowly gaining their ability to hear sound underwater. To perceive and make sense of these complex auditory signals, they have evolved a complex auditory system.

Underwater clicks, songs and echolocation allow them to communicate with one another, 'see' and navigate their worlds, and find food. Sound is the dominant sense for whales, and they use their ears like we use our eyes. This 1936 blue whale specimen from the Gulf of Alaska (above: preserved in alcohol, below: CT scan) is part of a rare Smithsonian collection of whale fetuses that were collected in association with commercial whaling operations in the early to mid-20th century, strandings and fisheries by-catch.