SEXUAL DIFFERENCES IN THE TYMPANIC FREQUENCY RESPONSES OF THE AMERICAN BULLFROG (RANA-CATESBEIANA)

Adult male and female American bullfrogs (Rana catesbeiana) have sexually dimorphic middle ears. The most conspicuous feature is the relatively large tympanum of adult males. The acoustic responsiveness of the tympana of adult male and female bullfrogs was studied with a laser vibrometer. Clear sex-related differences in middle ear frequency responses were observed between the sexes. The middle ear of adult males was generally more sensitive, but, more significantly, it showed a distinct peak in sensitivity at very low frequencies around 200 Hz that was entirely lacking in the female middle ear. The male middle ear therefore is especially responsive to the distinct low-frequency peak in the male mating call centered around 200-300 Hz. The enlarged middle ear of male bullfrogs therefore may have evolved as a result of selective pressure to improve detection of the mating vocalizations of other males. Female bullfrogs lack tympanic sensitivity to the low-frequency peak the male mating call, and presumably respond more to the higher frequency components of call.