Hearing conspecific vocal signals alters peripheral auditory sensitivity

Abstract

We investigated whether hearing advertisement calls over several nights, as happens in natural frog choruses, modified the responses of the peripheral auditory system in the green treefrog, Hyla cinerea. Using auditory evoked potentials (AEP), we found that exposure to 10 nights of a simulated male chorus lowered auditory thresholds in males and females, while exposure to random tones had no effect in males, but did result in lower thresholds in females. The threshold change was larger at the lower frequencies stimulating the amphibian papilla than at higher frequencies stimulating the basilar papilla. Suprathreshold responses to tonal stimuli were assessed for two peaks in the AEP recordings. For the peak P1 (assessed for 0.8-1.25 kHz), peak amplitude increased following chorus exposure. For peak P2 (assessed for 2-4 kHz), peak amplitude decreased at frequencies between 2.5 and 4.0 kHz, but remained unaltered at 2.0 kHz. Our results show for the first time, to our knowledge, that hearing dynamic social stimuli, like frog choruses, can alter the responses of the auditory periphery in a way that could enhance the detection of and response to conspecific acoustic communication signals.

Keywords: Hyla cinerea; audition; hearing; lek; plasticity; social signals.

Figure 1.

Examples of stimuli used for the nightly sound exposure during a 10 day period. (a) The waveforms of the stimuli and (b) the spectrograms of the stimuli. The tones had the same repetition rate, duration and amplitude envelope as the frog chorus.

Figure 2.

Example traces of AEPs to a low-frequency (1.6 kHz) and high-frequency (4 kHz) stimulus at several stimulus intensities. The amplitude of P1 was measured from the vertex of the first positive peak (marked P1) to the subsequent negative trough. P2 amplitude was measured from the vertex of the second positive peak to the subsequent negative trough.

Figure 3.

Auditory thresholds of animals in the (a) chorus group and (b) tone groups before and after 10 days of nightly stimulus exposure. Thresholds (±s.e.) are plotted as a function of frequency.

Figure 4.

(a) Amplitude (±s.e.) of peak one (P1) of the AEP as a function of stimulus, time, and frequency. The AEP amplitudes are average across intensity. (b) Amplitude (±s.e.) of peak two (P2) of the AEP as a function of stimulus, time and frequency. The difference in P2 amplitude between the two groups before exposure is probably due to the relative numbers of males and females in each group, as females had a higher average P2 amplitude.