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The ear is the sensory apparatus associated with hearing and, in its inner-ear components, balance. The outer and middle ear transmit sound mechanically toward the inner ear, where specialized sensory biological cells transduce movement into neural signals. Sound transmission and vestibular sensing are related anatomically but are different functions.

Fluid within the membranous inner ear. Temperature-dependent fluid properties motivate biomechanical proxies for extinct-animal physiology.

The inner ear contains fluid-filled sensory structures for hearing and balance. In the cochlea, motion transmitted from the middle ear creates a traveling disturbance along the basilar membrane. Movement of sensory hair cells bends their stereocilia, changing mechanically gated ion-channel opening and producing an electrical response.^[ref]

The mechanical response varies along the cochlea, so position contributes to frequency selectivity. The vestibular part of the inner ear instead detects head motion and orientation. The distinction is anatomical and functional: detecting a sound frequency and detecting a rotation of the head use related mechanosensory principles in different structures. Mechanical input, hair-cell response and activity in the associated sensory neurons are successive measurable stages.

The tympanic membrane and ossicular coupling system that transfers airborne vibration toward the fluid-filled inner ear. Its mechanical transformation helps address an acoustic impedance mismatch; it is not an ideal, lossless, frequency-independent amplifier.

A paleontologist who investigated inner ear biomechanics as evidence bearing on the origin of mammalian endothermy.^[ref]

The vestibular system senses head movement and orientation relative to gravity. Signals from the inner ear interact with visual and bodily information to support gaze and posture. A sensory signal about acceleration is distinct from an unconstrained measurement of absolute position.