Ontogeny of auditory brainstem responses in the bat, Phyllostomus discolor.

Hearing is the primary sensory modality in bats, but its development is poorly studied. For newborns, hearing appears essential in maintaining contact with their mothers and to develop echolocation abilities. Here we measured auditory brainstem responses (ABRs) to clicks and narrowband tone pips covering a large frequency range (5-90 kHz) in juveniles (p7 to p200) and adults of the bat, Phyllostomus discolor. Tone-pip audiograms show that juveniles at p7 are already quite responsive, not only below 20 kHz but up to 90 kHz. Hearing sensitivity increases further until about p14 and is then refined, possibly correlated with growth and differentiation of the animals' outer ears. ABR amplitudes decrease within the first 3-4 months, inversely correlated with the bat weight and forearm length. ABR Wave I latency decreases with increasing stimulation level. ABR duration (measured between Waves I and V) is longer in juveniles and shortens with age which may reflect temporal refinement of auditory brainstem neurons to accommodate the exceptional temporal precision required for effective echolocation. Overall our data show that P. discolor bats have good hearing very early in life. The current method represents a fast and minimally invasive way of characterizing basic hearing in bats.