Many animals communicate using composite signals that can be perceived by more than one sensory modality in their intended receivers (Candolin, 2003;Mitoyen et al., 2019;Rowe & Halpin, 2013). While the ultimate causes for the evolution of such multimodal communication have been a focus of behavioural ecology research for more than two decades (Bro-Jørgensen, 2010;Rowe, 1999), a majority of empirical studies assume that the alternative modalities have independent effects in shaping receiver responses (but see Hughes, 1996; Taylor, Klein, & Ryan, 2011; Vicente & Halloy, 2017). In the absence of inter-signal interaction, two general types of selection account for the evolution of multimodal signals. Selection may act on the information content of signals of different modalities (content-based selection) or on the degree to which these signals are effectively transmitted, perceived and processed by receivers (efficacy-based selection; Hebets & Papaj, 2005). Nonetheless, an alternative form of selection may arise when the behavioural response elicited by a signal modality depends on the presence of another signal, which stimulates a different sensory system (Hebets & Papaj, 2005).