Development of hearing in the big brown bat.

We studied the development of hearing in newborn pups of the big brown bat, Eptesicus fuscus. In the majority of pups, the opening of both outer auditory canals occurred on or before postnatal day (PND) 7, but in some, it extended to PND 11. Using repeated auditory brainstem response (ABR) recordings, we tracked the progressive development and maturation of auditory sensitivity in 22 E. fuscus pups every 3 days, from PND 10 to PND 31, with additional recordings in a subset of bats at 2 months, 3 months and 1 year of life. There was a profound increase in auditory sensitivity across development for frequencies between 4 and 100 kHz, with the largest threshold shifts occurring early in development between PND 10 and 19. Prior to PND 13-16 and when pups were still non-volant, most bats were unable to hear frequencies above 48 kHz; however, sensitivity to these higher ultrasonic frequencies increased with age. Notably, this change occurred near the age when young bats started learning how to fly and echolocate.