DEGENERATE HEARING AND SOUND LOCALIZATION IN NAKED MOLE RATS (HETEROCEPHALUS-GLABER), WITH AN OVERVIEW OF CENTRAL AUDITORY STRUCTURES

Behavioral tests of absolute sensitivity and sound localization in Afr ican naked mole rats show that, despite their communal social structur e and large vocal repertoire, their hearing has degenerated much like that of other subterranean species. First, their ability to detect sou nd is limited, with their maximum sensitivity being only 35 dB (occurr ing at 4 kHz). Second, their high-frequency hearing is severely limite d, with their hearing range (at 60 dB sound pressure level [SPL]) exte nding from 65 Hz to only 12.8 kHz. Third, determination of the effect of duration on noise thresholds indicates that, compared with other an imals, mole rats require a sound to be present for a much longer durat ion before reaching asymptotic threshold. Finally, they are unable con sistently to localize sounds shorter than 400 ms and cannot accurately localize sounds of longer duration, raising the possibility that they are unable to use binaural locus cues. Thus, it seems that the essent ially one-dimensional burrow system of a subterranean habitat produces severe changes in hearing comparable to the changes in vision that re sult from the absence of light. To explore the relation between vision and sound-localization acu , retinal ganglion cell densities were det ermined. The results indicate that naked mole rats have a broad area o f best (albeit poor) vision, with maximum acuity estimated at 44 cycle s/degree. That mammals with wide fields of best vision have poorer sou nd-localization acuity than those with narrower fields is consistent w ith the thesis that a major function of sound localization is to direc t the gaze to the source of a sound. However, the fact that subterrane an mammals have little use for vision in a lightless environment sugge sts that they represent an extreme case in this relationship and may e xplain the fact that, unlike surface-dwelling mammals, they have virtu ally lost the ability to localize brief sounds. Finally, despite their very limited auditory abilities, the major brainstem auditory nuclei, although relatively small, appear to be present.