Deafness in an auditory specialist, the big brown bat (Eptesicus fuscus)

•We tested hearing and performed cochlear histology in big brown bats suspected of having hearing loss.•Auditory brainstem response and distortion product otoacoustic emission measurements indicated deafness in a subpopulation of big brown bats that were housed and bred in captivity.•Analysis of cochlear tissue revealed extensive loss of hair cells, supporting cells, and spiral ganglion neurons in deaf bats.•These results are surprising in light of common assumptions that bats do not develop hearing loss. Bats are long-lived animals that show presumed resistance to noise-induced and age-related hearing loss, which has been attributed to their dependence on sound processing for survival. Echolocation and basic auditory functions have been studied extensively in the big brown bat (Eptesicus fuscus), an insectivorous microchiropteran species. We conducted hearing tests and analysis of cochlear sensory cells in a group of big brown bats that exhibited anomalies in behavioral sonar tracking experiments and/or lacked neural responses to acoustic stimulation in subcortical auditory nuclei. We show for the first time the presence of profound deafness and extensive cochlear damage in an echolocating bat species. Auditory brainstem responses were abnormal or absent in these bats, and histological analyses of their cochleae revealed extensive loss of hair cells, supporting cells, and spiral ganglion neurons. The underlying cause of deafness is unknown.