Animals detect sound energy through a process known as hearing, which involves several stages:
1. Capture of Sound Waves: Sound energy travels in the form of waves through the air, water, or other medium. These waves first reach the animal's outer ear (in species that have this structure), which is shaped in a way that helps to gather the sound waves and funnel them into the ear canal.
2. Transmission to the Middle Ear: The sound waves travel down the ear canal to the eardrum, causing it to vibrate. These vibrations are transferred to three tiny bones in the middle ear (the ossicles, which in mammals are known as the malleus, incus, and stapes).
3. Conversion to Fluid Waves in the Inner Ear: The last of these bones connects to the cochlea, a fluid-filled structure in the inner ear. The vibrations of the ossicles create waves in the fluid of the cochlea.
4. Detection of Fluid Waves by Hair Cells: Inside the cochlea, there are specialized sensory cells called hair cells. The fluid waves cause the "hairs" (stereocilia) on these cells to move. The movement of the stereocilia triggers an electrical signal in the hair cell.
5. Transmission to the Brain: The electrical signals from the hair cells are carried by the auditory nerve to the brain, which interprets these signals as sound.
This process can vary somewhat among different animal species. For example, many aquatic animals have adaptations for hearing underwater, where sound waves behave differently than they do in air. But the basic principles of capturing sound energy and converting it to electrical signals that the brain can interpret are the same.