Sensitivity, range and temperature dependence of hearing in the grass frog and fire-bellied toad

Multi-unit recordings from the torus semicircularis of the fire-bellied toad (Bombina bombina L.) and the grass frog (Rana t. temporaria L.) were used to obtain threshold vs. frequency curves for these anurans. The effect of body temperature on these audiograms was tested over a range of 10-28° C for the toad and 5-20° C for the frog. The range of frequencies audible to the fire-bellied toad at a body temperature of 21° C extends to 2400-3000 Hz. Threshold is relatively low in three regions: 300-450 Hz, 700-900 Hz and 1200-1700 Hz. The auditory system is most sensitive in the low frequency region. The audiograms of both species depend greatly on temperature. As temperature is increased sensitivity is enhanced, particularly at low and intermediate frequencies. Grass frogs are maximally sensitive at temperatures as low as 15° C, whereas the auditory threshold of fire-bellied toads continue to fall as the temperature is raised from 16° C to 22° C. Hearing evidently is adapted to different temperature ranges in the two species, and these correspond to the temperatures at which the animals engage in mating behavior.