Little effect of natural noise on high-frequency hearing in frogs, Odorrana tormota.

Ambient noise influences acoustic communication in animals. The concave-eared frogs (Odorrana tormota) produce high-frequency sound signals to avoid potential masking from noise. However, whether environmental noise has effect on the high-frequency hearing of frogs is largely unclear. By measuring the auditory evoked near-field potentials (AENFPs) from the torus semicircularis of the midbrain at frequencies 1-23 kHz in the presence of three noise levels, we found no significant difference in the peak-to-peak amplitude, threshold and latency of AENFP between low-level (35 dB SPL) background noise and mid-level (65 dB SPL) broadcast natural noise. For a natural noise level of 85 dB SPL, AENFP amplitude decreased and threshold and latency increased at frequencies 3-13 kHz. Spike counts evoked by stimuli at the best excitatory frequency under 85 dB SPL natural noise exposure were lower in 7-kHz CF neurons than in exposures to 35 and 65 dB SPL noise. However spike counts were similar for 14- and 20-kHz CF neurons at the three exposure levels. These findings indicate that environmental noise does not mask the responses of high-frequency tuned auditory neurons, and suggest that the acoustic communication system of O. tormota is efficiently adapted to noisy habitats.