  The viscous liquid surrounding a hair bundle dissipates energy and dampens
oscillations, which poses a fundamental physical challenge to the high
sensitivity and sharp frequency selectivity of hearing. To identify the
mechanical forces at play, we constructed a detailed finite-element model of
the hair bundle. Based on data from the hair bundle of the bullfrog's sacculus,
this model treats the interaction of stereocilia both with the surrounding
liquid and with the liquid in the narrow gaps between the individual
stereocilia. The investigation revealed that grouping stereocilia in a bundle
dramatically reduces the total drag. During hair-bundle deflections, the tip
links potentially induce drag by causing small but very dissipative relative
motions between stereocilia; this effect is offset by the horizontal top
connectors that restrain such relative movements at low frequencies. For higher
frequencies the coupling liquid is sufficient to assure that the hair bundle
moves as a unit with a low total drag. This work reveals the mechanical
characteristics originating from hair-bundle morphology and shows
quantitatively how a hair bundle is adapted for sensitive mechanotransduction.
