Multimodal signaling in male and female foot-flagging frogs Staurois guttatus (Ranidae):: An alerting function of calling

In multimodal communication, individuals use several sensory modalities for information transfer. We report on novel observations of foot-flagging in the Bornean ranid frog Staurois guttatus that is temporally linked to advertisement calling. In addition, we document the first case of foot-flagging in a female anuran as well as additional visual displays in both males and females including arm-waving, vocal-sac pumping and open-mouth display. In males, advertisement calls and foot-flags were given throughout most of the day, suggesting that acoustic and visual signals form a multicomponent and multimodal display. We tested the efficacy-based alerting signal hypothesis of multimodal communication using acoustic playback experiments with males. This hypothesis predicts that an initial signal draws the receiver's attention to the location of a subsequent more informative signal. Several lines of evidence supported the alerting hypothesis. First, the latency between foot-flags and advertisement calls was significantly higher than that between advertisement calls and foot-flags, suggesting a functional linkage with calls drawing attention to foot-flags. Secondly, advertisement calling had a signaling function with males responding significantly more often with both calls and foot-flags compared with pre- and post-playback control periods. Finally, and most notably, all males tested turned towards the playback stimulus, suggesting that the advertisement call serves to focus their attention on subsequent signals. We discuss the potential of multimodal signaling for conspecific and heterospecific communication and the circumstances under which such a multimodal communication system could evolve.