Acoustic Dueling in Tarbush Grasshoppers: Settlement of Territorial Contests via Alternation of Reliable Signals

A field study of disputes over ownership of mating territories by male tarbush grasshoppers (Ligurotettix planum) revealed that most contests were settled ritualistically via the antiphonal exchange of acoustic signals. Males used a special aggressive signal, referred to as the “shuck” call, in these encounters. Individuals that never produced shuck calls invariably departed the contested sites, and a playback experiment showed that shuck calls elicited higher response levels from males than sexual advertisement calls. Approximately 1/5 of all territorial contests escalated to biting, grappling, or kicking. The outcomes of territorial contests that were settled via purely acoustic encounters could not be predicted by the size of the participants. Prior residency was a predictor of winning in some contests, but often the competing males did not possess clear intruder/resident roles. However, a signal parameter that was a combined measure of the rate of shuck calls and their mean length predicted the winner of most acoustic encounters. Contests that escalated, though, were generally distinguished by participants that displayed comparable measures of all signal parameters save call length. A tradeoff occurred between call rate and length in most individuals. This, as well as the finding that call length did predict winning in escalated contests, implies that these signal parameters may reflect an individual's strength or motivation. Contests in which the signal parameters of the males were similar tended to be prolonged and escalated. This relationship suggests that individuals assess the signals of their rivals and depart if they cannot match their call rate and length. That both call rate and length are assessed may enhance the reliability with which signals can be used to predict a rival's fighting ability in the event of escalation. Various simple mechanisms for signal assessment are proposed. A second playback experiment suggested that the insects heard less effectively while they called. Timing mechanisms that preclude the overlap of calls during contests occur, and they may have evolved because of the importance of precise assessment of the signals of rivals. Such mechanisms circumvent the potential impairment in perception noted above and also generate a conspicuous feature of L. planum contests — mutual alternation of signals.