Evolution of chorusing displays in acoustic animal species – EVOLCHOR

In many species of acoustic animals, males exhibit specialized patterns of collective display. These displays, termed ‘choruses’, range from advertisement signaling during a particular time of the day to precisely adjusted phase relationships between the call rhythms of neighboring individuals. Call rhythms are controlled by central nervous oscillators, and neighbors adjust their signaling via mutual phase delays or via actual changes in the free-running rhythm of their oscillators. These adjustments often generate collective patterns of call synchrony or alternation. Such patterns may enhance species recognition, maximize the ability of a local group to attract females, or help to evade natural enemies. Alternatively, synchrony and alternation may emerge from simple pairwise competitions between neighboring males jamming each other's signals. The mechanisms that effect acoustic signal jamming can be selected for by psychoacoustic factors, such as precedence effects in which female receivers prefer male signals that lead their neighbors’ signals by a brief interval.
Whereas we currently possess considerable knowledge of the neural mechanisms with which animals control their signal interactions, we have relatively little understanding of the evolution of the collective phenomena that are observed. In particular, the proposition that chorusing displays arise as emergent phenomena generated from basic psychoacoustic effects in receivers has not been critically tested in a way that would eliminate alternative hypotheses. Variation in chorusing among populations, as well as parallel variation at the level of signal interactions and receiver preferences, has received scant attention. Moreover, very few studies have analyzed the connectedness between behavior occurring at these different levels: individual participant, pairwise interaction between neighbors, and displaying group. Thus, fundamental questions on the evolution of signaler and receiver behavior within collective displays, on the generation of temporally structured displays by large groups of signalers, and on potential feedback loops in which group-level traits may influence the evolution of individual behavior remain largely unanswered.
Here, we propose a study on the evolution of chorusing displays in an acoustic insect, Ephippiger diurnus, found in southern France. Neighboring males adjust their call rhythms with a phase-delay mechanism, and alternating or synchronous choruses are observed. Receptive females orient toward singing males, and preferences for longer calls, for faster call rhythms, and for leading calls are found in many populations. The last feature suggests that E. diurnus choruses could represent emergent phenomena as described above. E. diurnus are flightless and are distributed in isolated patches between which gene flow may be low. Recordings of male song and tests of female preference for male song indicate considerable inter-patch variation in signal, signal interaction, and preference characters.
Our study will make use of the special features in E. diurnus communication and population genetic structure and thereby present the first comprehensive analysis of the proposition that the structure of communal display represents an emergent property rather than an adaptation per se that is directly controlled by the individual participants. It will also address the novel hypothesis that a group-level display emerging as such from the combined activity of many contributing individuals may then create a sensory environment that in turn influences evolution of behavior at the individual level. Thus, the study will also be the first to test the possibility that feedback loops exist between different hierarchical levels of animal signaling behavior, i.e. group and individual, and ultimately temper or exaggerate the evolution of communal display.