A tale of two ears: development of binaural auditory processing in the big brown bat.

Big brown bat (Eptesicus fuscus) pups are born naked, blind, and with closed external ear canals. We examined the ears of newborn bat pups (n = 82; 42 females) and found that in a majority of pups (80%), the ear canals open asymmetrically. What remains unknown is whether this developmental asymmetry influences hearing. We recorded auditory brainstem response (ABR) waveforms in a subset of pups with both ears open by placing subdermal needle electrodes near the base of the auditory bullae and repeatedly presenting broadband acoustic clicks and averaging the evoked neural response. The resultant average ABR waveform represents the summated activity of neurons from various nuclei along the ascending auditory brainstem pathway. We compared monaurally- and binaurally-evoked hearing thresholds in pups at 2, 3, 4, 8, and 12 weeks and approximately 1 year of age. In pups < 4 weeks old, monaural thresholds were lower when stimulating the ear that opened first compared to the opposite ear; however, after 4 weeks of post-natal life there was no longer a threshold difference between the ears. In general, binaurally-evoked thresholds were lower than monaurally-evoked thresholds across development. We also measured sound lateralization performance in bats that using a two-alternative forced choice behavioural task and found pups showed a slight aural bias in sensitivity to noise bursts presented to the ear that opened first. Our data suggest that the initial asymmetry in ear opening and response physiology may help pups localize (or lateralize) sounds until their central auditory processing circuits mature.