Multimodal signal compensation: do field crickets shift sexual signal modality after the loss of acoustic communication?

Several hypotheses could explain the evolution of multimodal signals. One possibility is that such signals allow for communication even when one signalling modality is temporarily unavailable. However, little is known about the consequences of the permanent evolutionary loss of a signal modality. We used the field cricket Teleogryllus oceanicus to test the hypothesis that the loss of one mode of signalling can be accommodated by flexibly switching to another pre-existing modality. Field crickets use cues that carry social information in the form of both long-range acoustic signals and short-range cuticular hydrocarbons (CHCs), but males in some T. oceanicus populations have permanently lost the ability to sing because of a morphological mutation erasing sound-producing structures on their wings. In assays testing responsiveness to substrate-borne CHCs, T. oceanicus females responded to the presence of male, but not female, CHCs, which is consistent with known sexual dimorphism in field cricket chemical cues. However, we found no evidence for signal compensation in male crickets that have experienced an evolutionary loss of acoustic signals: females did not differentially respond to the CHCs of constitutively silent males compared to those of normal males. The ability of organisms to shift adaptively from one signalling modality to another following the evolutionary loss of a signal is likely to be constrained by both the degree to which signal production and receiving is flexible and the existence of suitably preadapted alternative modalities.