Sex differences in the middle ear of the bullfrog (Rana catesbeiana)

Impedance transformation by the middle ear of terrestrial vertebrates is considered to be a function of the ossicular lever apparatus, which increases force and decreases velocity at the stapes footplate relative to the tympanic membrane, and the ratio of tympanic membrane area to stapes footplate area (the area ratio), which increases pressure at the stapes. A recent vibrometric study has found that the velocity ratio between tympanic membrane and stapes footplate is higher in male bullfrogs (Rana catesbeiana) than in females. Although the tympanic membrane area is known to be larger in male bullfrogs, the area ratio has apparently never been compared between the sexes. The hypothesis that a lower area ratio in males helps to compensate for the higher velocity ratio was investigated by comparing tympanic membrane, stapes footplate and operculum areas in frogs of both sexes. Contrary to expectation, the difference in stapes footplate area between males and females is very small, and area ratios are considerably higher in males. Area ratios increase with increasing body size in both sexes. Operculum area does not differ between the sexes. The apparent difference in impedance transformation between male and female frogs might be necessary due to different inner ear impedances, or might reflect serious inadequacies in the conventional models of middle ear function.