Anatomical and physiological evidence for auditory specialization in the mountain beaver (Aplodontia rufa).

On examination of brain stem auditory nuclei in a comparative series of over 100 mammals, the mountain beaver (Aplodontia rufa) was discovered to have a very large and unique cochlear nuclear complex. The dorsal cochlear nucleus in this species is 4–7 times larger than in any of 17 other rodent species examined. While there is a very small laminated region of the nucleus, its main bulk lacks the laminar organization so characteristic of the nucleus in other mammals. The cochlear granule field is extraordinarily large, comprising nearly half of the volume of this enlarged cochlear nuclear complex. The ventral cochlear nucleus is similar in size and organization to the nucleus in other mammals. Study of single units in the specialized dorsal cochlear nucleus revealed that many neurons responded to exceptionally low frequency stimuli (below 10 Hz). Thresholds of neurons were relatively high and response areas relatively flat in comparison with responses of neurons in the ventral nucleus. Neurons isolated within the large granule cell field could not be excited by tonal or other simple acoustic stimuli. These data suggest that part of the unique auditory system in the mountain beaver is specialized for the detection of slow changes in air pressure.