Hearing and age-related changes in the gray mouse lemur.

Abstract

In order to examine auditory thresholds and hearing sensitivity during aging in the gray mouse lemur (Microcebus murinus), suggested to represent a model for early primate evolution and Alzheimer research, we applied brainstem-evoked response audiometry (BERA), traditionally used for screening hearing sensitivity in human babies. To assess the effect of age, we determined auditory thresholds in two age groups of adult mouse lemurs (young adults, 1-5years; old adults, ≥7years) using clicks and tone pips. Auditory thresholds indicated frequency sensitivity from 800Hz to almost 50kHz, covering the species tonal communication range with fundamentals from about 8 to 40kHz. The frequency of best hearing at 7.9kHz was slightly lower than that and coincided with the dominant frequencies of communication signals of a predator. Aging shifted auditory thresholds in the range between 2 and 50.4kHz significantly by 12-27dB. This mild presbyacusis, expressed in a drop of amplitudes of BERA signals, but not discernible in latencies of responses, suggests a metabolic age-related decrease potentially combined with an accompanying degeneration of the cochlear nerve. Our findings on hearing range of this species support the hypothesis that predation was a driving factor for the evolution of hearing in small ancestral primates. Likewise, results provide the empirical basis for future approaches trying to differentiate peripheral from central factors when studying Alzheimer's disease-like pathologies in the aging brain.