Multimodal signaling improves mating success in the green tree frog (Hyla cinerea), but may not help small males

Many anuran amphibians are challenged with the detection of courtship signals in noisy chorus environments. Anurans and other animals partially solve this discrimination challenge by employing auditory mechanisms such as grouping sounds by frequency, time, or spatial location. Animals are also known to employ visual cues as a mechanism of improving auditory signal detection. In this study, we examined the effect of acoustic and visual stimuli on female mate choice preferences in the green tree frog, Hyla cinerea. We used a series of two choice playback tests and added a robotic frog, with an inflatable vocal sac, to test interactions among visual and acoustic signal components. Females preferred vocalizations with faster call rates (i.e., high energy cost) and lower call frequencies (i.e., larger males). When call properties were held equal, females discriminated against an acoustic only stimulus in favor of the combined acoustic/visual multimodal signal. A visual component did not, however, increase the attractiveness of an otherwise unattractive (high-frequency) acoustic signal. Thus, female green tree frogs integrate the visual display into the acoustic communication system and males that are visually accessible can increase their probability of mating success. Visual accessibility, however, is unlikely to improve mating success for small males (high-frequency callers). Animal communication signals are often complex and communicated in multiple sensory channels (e.g., auditory + visual). Female choice is known to be an important mechanism driving signal evolution. Thus, for complex mating signals, a first step in understanding their evolution is to test how females respond to various combinations of components. Here, we tested female mate choice in the green tree frog, H. cinerea, using a combination of audio playbacks and a robotic frog as the visual component. When the audio signal was standardized, females preferred a signal enhanced by a robotic frog. The robotic frog did not increase female responses to an unattractive call (indicative of a small male), however. These results suggest that visual accessibility can improve a male's chance of mating, but this advantage is context dependent and does not extend to smaller males.