Discoveries in Marine Mammal Hearing

Early studies of marine mammals underwater were driven by awe, imagination, and the wealth they represented from oil, bone, and flesh. Aristotle recognized that these animals are mammals and wrote of their unusual acoustic abilities. Later, scientists erroneously categorized them as fishes until Linnaeus restored them to Mammalia. Not until the mid-twentieth century, when new tools for underwater acoustics appeared, was it revealed that marine mammals rely primarily on hearing for communicating, finding prey, and detecting predators. With more sophisticated technology in the 1950s, field recordings and behavioral work with captive marine mammals showed that they produce and detect sounds ranging from infrasonic to ultrasonic frequencies. Hearing underwater is accomplished by an essentially mammalian auditory system that re-adapted to water, and some species have ultrasonic inner ears like those of bats, but with unique fatty tissues for reception of water-borne sound. From 1960 onward, advances were made particularly in understanding the mechanisms of dolphin biosonar. Recent research initiated studies on hearing in free-ranging animals and broadened the field to more species, but many of the original puzzles remain unsolved and are still being investigated today.