Evolution of synchronies in insect choruses

In many acoustic insects, neighbouring males adjust their calling rhythms and broadcast a temporally structured chorus ranging from an irregular mixture of alternation and synchrony to a nearly perfect synchrony wherein most males sing in unison. Irregular synchrony can arise where females ignore calls that follow a neighbour’s call by a brief delay, a perception that selects for competitive male adjustments to calling rhythm that increase leading calls and ultimately yield alternation and synchrony. Such chorusing emerges as an incidental property, and the collective temporal structure may have little significance for females and males. More perfect synchronies are expected where females must perceive distinct sound envelopes before approaching a chorus or a given male. Here, selection will favour cooperative male adjustments by which neighbours align their calls, as any male who does not adjust would reduce the group’s attractiveness and therefore his own. We tested these expectations in two sympatric bushcrickets, Ephippiger diurnus and Sorapagus catalaunicus, that exhibit irregular and more perfect synchrony, respectively. We found, however, that females in both species prefer leading calls and must hear distinct sound envelopes. We infer that fundamental differences in perception and singing influence the relative importance of competition and cooperation, producing the two synchronies.