PSYCHOACOUSTICS OF FEMALE PHONOTAXIS AND THE EVOLUTION OF MALE SIGNALINTERACTIONS IN ORTHOPTERA

A series of playback experiments conducted in a field arena showed tha t female tarbush grasshoppers, Ligurotettix planum (Orthoptera Acridid ae), were attracted to male calls and that when given the choice of ca lls that differed only in relative timing, females oriented toward the leading calls. This psychoacoustic feature, known as a ''precedence e ffect,'' 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 lat ter situation, females even failed to exhibit phonotaxis. Previous wor k 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 ti me constants in this inhibitory resetting mechanism evolved under sele ction pressure from the female precedence effect. Intermale signal int eractions occur in many acoustic orthopterans and anurans, and we pred ict that female precedence effects, presently known in only a few spec ies, will be revealed as responsible for various of these interactions . As in L. planum and another orthopteran species (Neoconocephalus spi za) in which both male signal interactions and female psychoacoustic p references have been studied, inhibitory intervals in male interactive calling are expected to be congruent with or to exceed the lengths of precedence effects.