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The effect of ultrasound on the attractiveness of acoustic mating signals

Abstract. Previous laboratory studies (Nolen & Hoy 1986b) have shown that the phonotactic responses of flying crickets are influenced by the relative intensities of attractive (mating signal) and repulsive (predator) stimuli. At the functional level, these results suggest that predator cues (ultrasound) can change the attractiveness of a calling song. Using extracellular recordings from cervical connectives it was shown that, like other field crickets, Gryllus rubens (south‐eastern field cricket) is sensitive to ultrasound. This ultrasonic sensitivity has probably evolved in response to predation pressure from echolocating bats. Using acoustic playback under field conditions, it was tested whether the relative attractiveness of two male calling songs was influenced by the simultaneous broadcast of ultrasound. A simulated male calling song of G. rubens was broadcast at two different intensities (109 and 103 dB) from two sound traps that caught flying crickets attracted to the songs. Simulated bat ultrasound was broadcast simultaneously with the high‐intensity calling song (109 dB) and the relative catch in each of the two traps was measured. The intensity of the ultrasound was varied on different nights. The relative attractiveness of the high‐intensity sound trap decreased significantly as the intensity of the ultrasound broadcast with it was increased. For the lowest of the ultrasound broadcast levels, the relative attractiveness did not differ from that expected for two calling songs broadcast without ultrasound. Thus, increased levels of simulated predation risk decreased the attractiveness of the calling song associated with it. These are the first field experiments to show that predation risk in the form of simulated bat ultrasound influences the phonotactic behaviour of flying crickets.