Figure 1
From: Aquatic birds have middle ears adapted to amphibious lifestyles
(a) Three-dimensional rendering of a typical terrestrial bird ear built from a micro-CT scan of a rock dove Columba livia, highlighting the tympanic membrane (orange), the trifurcated and cartilaginous extracolumella (blue), the columella bone (black), the inner ear (grey), the round window (white arrow) and cochlear aqueduct (black arrow). (b) Hypothesized anatomical differences between a bird ear typical of terrestrial species and thus adapted for aerial hearing (top row) versus one adapted for underwater hearing (bottom row). From aerial to aquatic, we expect (i) a reduction of the tympanic membrane-to-columella footplate area ratio (shown as a reduction in tympanic membrane area), (ii) a reduced offset (thin grey line) of the columella from the center of the eardrum, resulting in a smaller lever ratio (l1 and l2 indicate the lever arms), (iii) a flattened tympanic membrane, quantified as the height of the umbo and the angles at the periphery relative to the base plane of the tympanic membrane, (iv) a reduced area of the round window to raise total inner ear impedance, (v) a shorter extrastapedius, here used as a proxy of overall extracolumella stiffness (dotted line), (vi) hypertrophication of the columella, and (vii) enlargement of the cochlear aqueduct (represented by a curved indentation). (c) Summary of hypothesized differences in the ear of terrestrial (left) vs aquatic birds (right), combined with expectation of reduced cranial air and connectivity, interaural canal and interbullar passage between the two ears.
