Ultrasonic communication and sexual selection in wax moths: female choice based on energy and asynchrony of male signals

Abstract

Pair formation in the lesser wax moth, Achroia grisella, is facilitated by male ultrasonic signals and female phonotaxis towards those signals. The signals are generated by the action of a pair of tymbals located on the tegulae and consist of pairs of approximately 100-μs pulses of 70–130 kHz sound. A brief (<400 μs) silent interval normally separates the two pulses in a pair, and 100–120 pairs of pulses are produced per second. Playback experiments using edited male signals showed that females preferentially oriented towards calls that were louder, delivered with higher rates and more evenly spaced pairs of pulses, and included longer pulse lengths and longer silent intervals within pairs of pulses. Preferred rates, lengths and silent intervals were greater than mean values in the male population, indicating the potential for directional selection and female choice. Preferences for pulse amplitude, rate and length were based on acoustic energy and power of the male signals, whereas the preference for long silent intervals was not. Female evaluation of pulse length and asynchrony interval shows that the insects can distinguish signal events as brief as 150 μs. Achroia grisellafemales do not continue to increase their response levels as rates are elevated beyond 142 pairs of pulses per second, which may reflect a neural constraint. Similarly, the preference for long silent intervals may reflect a mechanical constraint in which the tympanum responds less strongly to pairs of pulses separated by short intervals, thereby reducing the probability that an action potential is evoked.