Selectivity of the peripheral auditory system of spadefoot toads (Scaphiopus couchi) for sounds of biological significance

Summary Scaphiopus couchi is a primitive anuran whose vocal repertoire consists of a mating call and a release call. The two calls are distinct and differ in trill rate. Reception of airborne sound is achieved by means of a poorly differentiated region of skin on the head which serves as an eardrum.Whereas more modern anurans possessthree distinct types of auditory nerve fibers, spadefoot toads possess onlytwo types: a low-frequency-sensitive group which exhibits tone-on-tone inhibition and a high-frequency-sensitive group which is not inhibitable. The sharpness of frequency tuning of primary fibers in each group is comparable to more advanced vertebrate species. While the response properties of auditory fibers in the high-frequency-sensitive group are well matched to the spectral and temporal features in the spadefoot's mating call and release call, the low-frequency-sensitive fibers do not respond to these calls. Instead they may be involved in detection of bodily transmitted sounds during clasping, as well as other low-frequency sounds in the environment. The two groups of auditory fibers probably derive from separate auditory organs within the inner ear. Thresholds of auditory nerve fibers in spadefoot toads are relatively poorer than in more advanced anurans, which likely is due to their less developed eardrum. The role of tone-on-tone inhibition in the peripheral auditory system is questioned with regard to its significance in processing sounds of biological value.We wish to dedicate this paper to Jasper J. Loftus-Hills who was killed in a tragic accident near Austin, Texas on June 11, 1974. His post-doctoral appointment in our laboratory and his assistance in collecting spadefoot toads in the field recall fond memories.We also wish to thank R. Sage for helping us collect animals and W. F. Blair for supplying tape recordings ofScaphiopus mating calls. The assistance of J. Paton in photographing the animal in Fig. 1 is gratefully appreciated. This research was supported by the U.S. Public Health Service (NIH Research Grant NS-09244) and the National Science Foundation (Grant GB-18836); travel expenses involved in collecting animals were supported by a Cornell University Research Grant.