What makes a multimodal signal attractive? A preference function approach

Abstract

Courtship signals are often complex and include components within and across sensory modalities. Unfortunately, the evidence for how multimodal signals affect female preference functions is still rather limited. This is an important scientific gap because preference function shape can indicate which male traits are under the strongest selection. We modelled how preference function shape can be altered under 4 scenarios of varying signal content, including both redundant and non-redundant signals. The model was tested with the brown-headed cowbird (Molothrus ater); we manipulated male song attractiveness and visual display intensity, and assessed female preferences in an audiovisual playback study. We found that the intensity of a visual display can modify how attractive a song is for females. This indicates that the visual and acoustic male signal components are non-redundant and modulate each other. Our study shows a change in the direction of female preference functions for one signalling modality resulting from changes in the attractiveness of the other modality. Overall, our findings suggest that male signals in this species may not be under the typical directional selection documented in other species, but rather selection may favour males that possess a range of different signals that can be used strategically during different social contexts.