Finely tuned choruses: bush crickets adjust attention to neighboring singers in relation to the acoustic environment they create

Abstract

When acoustically advertising animals call in dense clusters, problems in signal efficacy often arise. These problems are particularly acute in species where females ignore males who call immediately following a neighbor and males adjust call timing to avoid broadcasting following calls: Males may forego such adjustments and produce many ineffective calls, they may attend to all neighbors and call at a reduced rate, or they may selectively attend to certain neighbors, likely those who are nearby and/or more intense. We studied the problem of group calling in Ephippiger diurnus, a European bush cricket distributed in genetically isolated populations that vary considerably in male song and chorusing and in female preference for male song. Female E. diurnus ignore following male calls, and males adjust their call timing but only with respect to several loud neighbors. We found that males were more selective in attending to only their nearest neighbor in a population where chorusing yields sound during a high proportion of a collective singing bout, and more indiscriminate in attending to several neighbors where chorusing yields more intermittent sound. Such fine tuning can maintain a relatively high calling rate and may be generated by positive feedback loops operating between individual and group-level calling traits. When animals sing in the company of conspecifics, individual singers often adjust their call timing such that they do not immediately follow neighbors. These adjustments become problematic in dense choruses, as adjusting for all neighbors could lead to a marked reduction in call rate. Consequently, some degree of selective attention, most likely to nearby and/or loud neighbors, is expected. We confirmed this expectation in the chorusing bush cricket E. diurnus, but we also found that the degree of attention varied among populations. In particular, singers were most selective, attending to only a single neighbor, where chorusing generated rather continuous sound and more indiscriminate attention would have led to sporadic calling. Thus, choruses appear to be finely tuned and controlled by feedback loops in which individual singers generate a collective display that, in turn, influences the singing behavior of those very same individuals.