The two main species intermingled in a brood of the 17-year cicada (Magicicada) have distinctive sound-making patterns and correspondingly different hearing abilities. Thus, they are acoustically isolated for mating purposes. Their simultaneous emergence and community "singing" has the important advantage of repelling predators.

Adults of three species of the 17-yr cicada, Magicicada septendecim, M. cassini, and M. septendecula, emerge from the ground together over a three-week period to mate and lay eggs. The males produce sounds (bursts of 1.0–1.5 kHz for septendecim and 4.0–6.0 kHz for cassini and septendecula) when congregating and when disturbed. Neural responses from within the chitinous capsule of the auditory receptors showed greatest sensitivity between 1.0 and 3.0 kHz in male and female septendedim and between 3.5 and 7.0 kHz in both sexes of cassini and septendecula. The tympanic membrances of the males are partially collapsed by muscles which contract during sound emission, reducing auditory sensitivity by 5–15 dB. Behavioral audiograms of males indicate best hearing in septendecim at 1.2–1.8 kHz and 4.5–6.0 kHz in cassini. Each species is very insensitive to frequencies used by the other. The sound level (80–95 dB SPL) around infested trees probably discouraged predation by driving away birds.

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.

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