In order for an animal to hear sound they need a way of sensing its energy and translating that energy to nerve impulses.
Animals have different sensors based on their environment.
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Ear Drum
Terrestrial animals have tympanic membranes that respond to pressure variations in air and convert them to mechanical movement sensed by hair cells and auditory nerves in the ear.
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Lateral Line
Many marine animals have “mechanoreceptors” that sense motion, including acoustical particle motion through cilia or “hair cells” on their body, like the line down teh sides of a fish.
© Encyclopaedia Britannica, Inc.
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Motion Sensor
Many fish have free moving bones or “otoliths” in their heads that help sense body motion including motion caused by acoustical particle motion.
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Acoustical lipids
Dolphins and porpoises have fatty “acoustical lipids” in their jaws that receive sounds and channel the sounds to their inner ears.
ILLUSTRATION: whaleresearch.com