Psychoacoustics of female phonotaxis and the evolution of male signal interactions in Orthoptera

Abstract

A series of playback experiments conducted in a field arena showed that female tarbush grasshoppers, Ligurotettix planum (Orthoptera Acrididae), were attracted to male calls and that when given the choice of calls that differed only in relative timing, females oriented toward the leading calls. This psychoacoustic feature, known as a “precedence effect,” occurred when a 0.2 or 1.0-sec silent interval separated the leading and following calls. Preference for leading calls disappeared at separations longer than 2 sec and when calls overlapped; in the latter situation, females even failed to exhibit phonotaxis. Previous work demonstrated that neighboring L. planum males time their calls in an alternating fashion and that they achieve this chorusing format with an “inhibitory resetting” mechanism that averts calling during the 2-sec interval following onset of a neighbor's call. We propose that time constants in this inhibitory resetting mechanism evolved under selection pressure from the female precedence effect. Intermale signal interactions occur in many acoustic orthopterans and anurans, and we predict that female precedence effects, presently known in only a few species, will be revealed as responsible for various of these interactions. As in L. planum and another orthopteran species (Neoconocephalus spiza) in which both male signal interactions and female psychoacoustic preferences have been studied, inhibitory intervals in male interactive calling are expected to be congruent with or to exceed the lengths of precedence effects.