Effect Of Intensive Noise On Auditory Response Of Concave-eared Frogs

The efficiency of acoustic communication in animals depends on the sound power, the environmental noise and sensitivity of receivers. Noise is a kind of sound that could damage to sense of hearing. For the animals living in the areas with high level ambient noise, their call signals could be overlapped with the frequency, amplitude and temporal characters of the noise, making the spreading efficiency of the animals acoustic signals decreased. Researches in fish, birds, monkey and cats have implied that high-level noise generally impaired acoustic communication in animals.Anuran amphibians, such as frogs and toads, are thought to be intermediate between fishes and amniote vertebrates. Sounds of anurans are typically simplex and the auditory systems are similar to those of amniote vertebrates. Hence, anurans serve as an attractive model for researching the animal acoustic communication. Anurans mainly rely on their calls for communication. The lower level spread efficiency of their acoustic signal caused by environmental noise will impact individual recognition, mate selection, territorial defense, population density, community structure, etc. Comparing to other species, anurans share a relatively limited habitat that always contains complex obstacles e.g. fast flowing stream, rain forest and grass. The acoustic communication of frogs burden more risk of noise interference.Since acoustic communication can be considerably impaired by environmental noise, it has been suggested that animals have evolved adaptations to counteract the masking effect of noise, such as Lombard effect in animal. Birds overcome the masking of noise by amplitude modulation, changing the duration, altering the rhythm and increasing the frequency of their song. The same phaenomenon were also found in mammals and anurans.Odorrana tormota, a frog living in torrent streams in Mt. Huangshan of China has been proved to be able to detect and respond to high-frequency signals up to ultrasound. Field playback experiment and recordings from TS indicated that this frog evolved high-frequency acoustic communication (≥20kHz) in signal exchanges with upper limit of35kHz (87dB SPL). In breeding season, which is between April and June, the ambient noise could be up to>90dB SPL due to the rainy weather. It is largely unknown how noise affects the sound communication of O.tormotalIn our research, we exposed males of O.tormota in three levels of noise (35,65,85dB SPL), to study how noise affect the acoustic communication of this frog. Our study includes two parts:in the first part, we carried out field playback experiment to display how different levels of noise (65and85dB SPL) affected the antiphonal response to female calls of males. In the second part, we recorded the TS response from midbrain to study the auditory evoked near-field potentials (AENFPs) and single unit spikes in three noise levels (35,65and85dB SPL).From the results of field play back experiment on21male frogs,13males responded to female calls with sound type of CF (consistent frequency) and8males with type of FM (frequency modulation) in both mid-level (65dB SPL) and high-level (85dB SPL) noise. The measurements revealed that for the same male, it antiphonally responded to the female call with longer duration, higher frequency, greater intensity and longer interval when exposed to high-level noise.From TS recordings, in three noises, the maximum amplitude, minimum latency and thresholds of AENFPs were found at7kHz-8kHz. We found neither significant differences in latency or in thresholds, or amplitudes (except at12kHz and19kHz). The frequency width at half value of the peak ranges from3kHz to12kHz for males in low-and mid-level noise (35and65dB SPL), while from5kHz to12kHz in high-level noise (85dB SPL). In high-levels noise, amplitude were decreased, latency prolonged and threshold raised compared to other two noises. Although the high-level noise constrained most frequency stimuli, it affected less for those with high frequencies. Results from single unit indicated the BEF were7kHz,17kHz and20kHz. The effects of three noise levels in single unit studies were similar to those in study of AENFPs. These results suggests that the high frequency sound signals facilitate the acoustic communication of O.tormota in noisy environment.