Selective attention and the spatio-temporal structure of orthopteran choruses

Temporally structured choruses in which neighbouring males alternate or synchronize their calls are common among rhythmically singing Orthoptera. In many cases, chorusing appears to be driven ultimately by psychoacoustic precedence effects that influence females to orient toward leading male calls and to ignore males whose call onsets follow their neighbours' onsets by a critical interval, 0-70 msec in some species but as long as 0.2-2.0 sec in others. When such preferences occur, call timing mechanisms with which males reduce their production of following calls are favored by selection. These timing mechanisms are observed among rhythmically calling species, and they may generate the emergence of group synchrony or alternation as a byproduct of local pairwise signal interactions. Where males are selected to adjust call timing and increase their incidence of leading calls, they confront a dilemma if density is high: Adjusting call rhythm in response to all singing neighbours may severely reduce the calling rate, whereas forgoing rhythm adjustment may lead to a preponderance of ineffective following calls. Simulations and laboratory experiments demonstrate that calling males may solve this problem by selectively attending to only a subset of neighbours. We studied three orthopteran species, Ligurotenix planum, Ephippiger ephippiger (both alternating chorusers), and Neoconocephalus spiza (synchronous choruser), in the field to determine the extent to which selective attention occurs in natural populations and structures chorusing. These three species were chosen because previous studies demonstrated moderate to strong precedence effects in females and timing adjustments in males with which they reduced production of following calls; moreover, controlled experiments indicated that selective attention influenced interactions among calling males in one of the species. As predicted, our studies of natural choruses showed that males in all three species maintained high call rates by attending to only some of their neighbours. Attended neighbours were generally the nearest, and loudest, ones, but other rules with which attention is applied may also occur. In L. planum and E. ephippiger, males generally attended to a single calling neighbour, but N. spiza males often attended to several. We propose that reduced selectivity in N. spiza reflects the synchronous nature of its chorusing, implying that a group effect emerging incidentally can influence via feedback the individual behaviour yielding that collective activity.