ABSTRACT
Synthetic calls with a waveform periodicity of 300/sec and a bimodal spectrum attracted female green treefrogs as effectively as natural calls.
Effectiveness was markedly reduced if the relative amplitude of the two spectral peaks differed by as little as 10 dB.
Optimal frequency bands were 900-1100 Hz in the low range and 2700-3300 Hz in the high.
Addition of components at 1800 and 2100 Hz rendered the call less attractive.
In the absence of a bimodal stimulus, most females responded to a call containing a single spectral peak in the high or low range.
Mating call recognition in the green treefrog is compared with that in the bullfrog.
A second peak (37 dB down relative to the 900 Hi component) was present at 2700 Hz in the acoustic stimulus. This peak represented third harmonic distortion by the speaker-amplifier and was determined by analysing a recording of the stimulus made 2 m in front of the speaker (General Radio 1900 A wave analyser).
The standard call was not tested against calls in which the lower peak was varied because, except at 600 and 1200 Hz the waveform periodicity would be different as well. For example, the periodicity of a call composed of 800, 2700, 3000 Hz tones is 100/sec as opposed to the 300/sec periodicity of the standard call.
A call with a low frequency peak alone was effective if there was also sufficient sound energy, but not necessarily a peak, in the high frequency region.
One could argue that energy present at 2700 Hz due to third harmonic distortion of the 900 Hz call is sufficient to excite neurones tuned to this part of the spectrum, but this argument does not apply to experiments with the call having only the upper peak. Alternatively, auditory neurones in the periphery, although optimally tuned to one of the spectral peaks, nevertheless may be sensitive enough to respond to energy in the other peak as well. Neurophysiological studies and behavioural experiments using sounds played back at very low levels should resolve these questions.
