Chorus structure in tarbush grasshoppers: inhibition, selective phonoresponse and signal competition

In species where sexually advertising males signal in groups, the timing of an individual's signals relative to those of neighbours may be a critical aspect of mating success. Temporal signal interactions and their relation to female attraction were examined in the tarbush grasshopper, Ligurotettix planum. The acoustic signals of neighbouring L. planum males form a crudely alternating chorus. Alternation within male pairs is effected by an (inhibitory resetting) mechanism that causes a male to refrain from calling for at least a 1·8-s interval beginning 0·2s after the onset of a neighbour's call. This mechanism is not equally applied to all neighbours. Rather, males are selectively inhibited by only their nearest one or two neighbours, whereas more distant, but audible, callers are ignored. Males also time their calls to occur shortly before calls at the ends of predictable recurring silent intervals. Consequently, males do not decrease their calling rates in high population density, and they compete effectively for females. Phonotaxis in L. planum females is characterized by preference for leading calls, a psychoacoustic feature that would select for timng mechanisms that avert the production of following calls in males. The inhibitory resetting mechanism averts such calls. Moreover, because males are only inhibited by nearest neighbours, their strongest competitors, they can avoid producing unattractive following calls and still maintain normal calling rates. By calling at the ends of silent intervals, males may actively compete with neighbours via ‘attempting’ to relegate the neighbours' calls to ineffective, from the neighbour's perspective, time intervals following the focal male's call.