Function of the sexually dimorphic ear of the American bullfrog,Rana catesbeiana: brief review and new insight

Abstract

SUMMARYThe dimorphic ear of the bullfrog, Rana catesbeiana, has long been enigmatic. The male's tympanic membrane (TM) area approximates twice the area of the female's; however, similar size differences in the area of the columellar footplate were not observed between the sexes. Hence, the male's hearing is expected to be more sensitive than the female's but this is not the case. Asking what offsets the advantage of the large TM, we applied a series of experiments to the auditory system. Male and female audiograms based on stimulation with airborne sound and on both multi-unit responses from the brain and alternating cochlear potentials ('microphonics') showed equal sensitivity and a small difference in frequency response; at low frequencies the male was more sensitive than the female. Amputating the columella and stimulating the stump with mechanical vibration showed that for an equal microphonic response, the male's footplate vibrated with lower amplitude than the female's footplate. Mechanically stimulating the TM of the intact ear replicated this result, excluding the involvement of the mechanical lever. The TM of the male weighs five times the TM of the female, and artificial loading of the TM of either sex greatly reduced the ear's sensitivity. Hence, the male's excessive area ratio (TM to columellar footplate) is offset by the heavier cartilage cushion on the male's TM, damping the TM's response to sound. This is corroborated by experimentally artificially loading the TM. The product of area ratio and footplate vibration amplitude would result in similar stimulation of the inner ear in the two sexes.

Auditory brainstem responses in the Chinese tiger frog Hoplobatrachus chinensis (Osbeck, 1765) (Anura: Dicroglossidae) reveal sexually dimorphic hearing sensitivity

“…Previous studies indicate that the greater tympanic membrane area ratio (tympanic membrane to columellar footplate) of males is offset by a heavier cartilage cushion on the male's tympanic membrane, thus damping the tympanic membrane's response to sound (Werner et al 2009). Thus, the fact that females' hearing is more sensitive at relatively lower frequencies than that of males may be due to the heavier cartilage cushion on the male's tympanic membrane.…”

“…Thus, the fact that females' hearing is more sensitive at relatively lower frequencies than that of males may be due to the heavier cartilage cushion on the male's tympanic membrane. This point needs to be studied further using methods such as additional artificial loading of the membrane in conjunction with microphonics experiments (Werner et al 2009). Thus, intersexual differences in the shape of the tympanic membrane in Chinese tiger frogs may be closely related functionally to intersexual differences of the tympanic membrane during the course of evolution (Hetherington 1994).…”

Functional basis of the sexual dimorphism in the auditory fovea of the duetting bushcricketAncylecha fenestrata

“…By comparing the biomechanical and electrophysiological frequency tuning with morphological properties along the crista acustica in A. fenestrata, our study provides evidence for an auditory fovea as a sex-specific adaptation that reflects the asymmetric signalling in the reciprocal acoustic communication of these duetting bushcrickets. Sexual dimorphism in hearing organs had been reported predominantly in insects [41][42][43][44][45] and anurans [46][47][48][49][50], but was also shown in geckos [51] and humans [52]. These anatomical differences often reflect sex-specific adaptations of the hearing system to improve the auditory sensitivity or selectivity for acoustic properties of conspecific mating signals or other sounds that have different behavioural relevance to males and females of the same species [23,53,54].…”

Sexually dimorphic auditory representation inAedes aegyptibrains

“…These differences often rely on dimorphisms in sensory processing systems, consisting of peripheral sensory organs and central neural circuits [2][3][4] . In many animals, sensory organs have diverged to tune responses to important cues for each sex [5][6][7][8][9][10][11] . The importance of sexual dimorphisms in central sensory processing for sex-specific behaviours has also been reported 4,[12][13][14] , though these insights are mainly limited to specific behaviours in certain model organisms.…”