Function of a multimodal signal: a multiple hypothesis test using a robot frog

1. Multimodal communication may evolve because different signals may convey information about the signaller (content-based selection), increase efficacy of signal processing or transmission through the environment (efficacy-based selection), or modify the production of a signal or the receiver’s response to it (inter-signal interaction selection). 2. To understand the function of a multimodal signal (aggressive calls + toe flags) emitted by males of the frog Crossodactylus schmidti during territorial contests, we tested two hypotheses related to content-based selection (quality and redundant signal), one related to efficacy-based selection (efficacy backup), and one related to inter-signal interaction selection (context). For each hypothesis we derived unique predictions based on the biology of the study species. 3. In a natural setting, we exposed resident males to a robot frog simulating aggressive calls (acoustic stimulus) and toe flags (visual stimulus), combined and in isolat...