Visual displays enhance vocal duet production and the perception of coordination despite spatial separation of partners

Abstract

Many group-living animals use precisely coordinated vocalizations to signal group quality and cohesion. However, the timing of these signals depends not only on the precision of signallers, which has been studied in detail, but also on the geometric location of both signallers and receivers. This is because the timing of notes within signals is affected by the speed of sound and relative location of senders and receivers, yet these spatial effects have not been examined experimentally. We studied how spatial separation of signallers affects production of and response to vocal duets in Australian magpie-larks, Grallina cyanoleuca. We specifically tested whether the visual displays that accompany duets reduce spatial effects on the production of natural duets and enhance the listener's perception of note coordination within experimental, multimodal playbacks of duets. Visual displays are not subject to time lags on a biological scale, because of the speed of light, and so might be used to compensate for the slow speed of sound during both production and reception of signals. Observations of natural duets suggest that the visual component of these multimodal displays reduces the effect of partner separation on duet tempo, which would otherwise decline linearly with separation, and enhances the regularity of duets. Acoustic playbacks with robotic models showed in turn that visual displays reduced the effect of spatial separation of partners on the response to their duets. Spatial separation of robots led to temporal offset within duets from the perspective of the listener, yet the visual display ameliorated the negative effect of offset on response. In addition, a greater tempo of vocal duets, which is promoted by visual coordination during production, increased response to duets. We conclude that the visual component of duet displays helps compensate for the effects of spatial separation of partners during the production and response to duets.