Neural basis for sexual differences in the auditory system of the treefrog, Eleutherodactylus coqui

Abstract

The males of the species E. coqui produce a two-note call; the first note (“co”) consists of a constant tone of duration 100 msec and frequency 1.1 kHz, and the second note (“qui”) sweeps upward in frequency from 1.8–2.2 kHz in about 170 msec. Acoustic playback experiments with males and females revealed a behavioral difference in the response of the males and females to the two-note call. These experiments showed that the first note functions in male-male interactions and probably serves to maintain spacing between adjacent males, but does not attract a female. The second note serves to attract conspecific females and apparently plays no role in male-male encounters. An electrophysiological study of the response properties of auditory fibers was conducted in the VIIIth nerve of males and females to determine whether this selectivity could occurs as early as the peripheral auditory system. The results of this study revealed that the functional dichotomy of the two notes in the male's call is reflected by a difference in the distribution of the best excitatory frequencies of primary auditory neurons for the males and females. Furthermore, the Q of the high frequency units showed a distinct difference between the sexes, thus confirming that the basilar papilla in the inner ear is tuned differently in males and females. Eleutherodactylus is the first known example of a vertebrate in which the peripheral auditory system shows a sexual difference. [Supported by NIH Grant NS-09244.]