Bone-conduction hearing in elephants and humans: a middle-ear comparative study

The elephant's middle ear (ME) boasts ossicles approximately nine times more massive than humans', with eardrums seven times larger. These anatomical features contribute to enhanced low-frequency sensitivity to air-conducted (AC) sound, but their contribution to bone-conduction (BC) hearing is less understood. To characterize BC hearing in elephants, we employed 3D laser Doppler vibrometry to measure ossicular motion in cadaveric temporal bones from both African and Asian elephants, as well as humans in response to BC stimulation. We analyzed velocity measurements projected along the piston direction at the umbo, stapes, and cochlear promontory across a frequency range of 12-11,000 Hz. Differential velocity (V_diff) was calculated as the difference in the velocity of the piston (V_pist) and the velocity of the promontory (Vprom) divided by the velocity of the promontory [(V_pist-V_prom)/V_prom]. V_diff plateaued above the BC resonant frequency-1.2 kHz for humans, reflective of prior studies, and 400 Hz for elephants. Below their respective resonant frequencies, the stapes V_diff magnitude for elephants was three to four times greater than that of humans, and both decreased at approximately 12 dB/octave until about 200 Hz. Surprisingly, both elephant and humans exhibited a plateau in their stapes V_diff below 200 Hz, rather than continuing to decrease at the 12 dB/oct rate. This suggests that both species have better low-frequency sensitivity for bone conduction than previously predicted. These results indicate that the larger ossicular and eardrum mass distributions play a crucial role in lowering BC resonance and enhancing low-frequency sensitivity. Elephants may further enhance BC hearing below about 200 Hz by contracting a skeletal muscle that occludes the ear canal, achieving a similar result to the occlusion effect in humans. These mechanisms may explain elephants' ability to detect ground-borne acoustic signals over long distances.