Vocal sacs do not function in multimodal mate attraction under nocturnal illumination in Cope's grey treefrog

Abstract

Multimodal communication signals consist of two or more distinct components produced in different sensory modalities and transduced by receivers using multiple sensory systems. One evolutionary trajectory by which incipient multimodal signals may arise is when receivers are selected to attend both to a well-established signal and a cue in a different sensory modality associated with that signal's production. Previous studies of frogs suggest movement of the male's vocal sac, which is inextricably tied to vocal production in most species, functions as the dynamic visual component of a multimodal mate attraction signal that modulates female responses to sexually advertising males. Most of this work, however, has presented multimodal stimuli using video playbacks or artificially illuminated robots in laboratory settings, which leaves open the question of whether the vocal sac functions in multimodal signalling under more natural nocturnal illumination. In this study of Cope's grey treefrog, Hyla chrysoscelis , a nocturnally breeding species, we tested the hypothesis that vocal sacs are a dynamic visual component of a multimodal mate attraction signal that influences female responses to sexually advertising males. Using robotic frogs as stimuli, we performed multimodal playback experiments outdoors under nocturnal illumination. We found no evidence that vocal sacs were attractive to females or that they influenced the responses of females when acoustic information was rendered less certain due to a degraded signal structure or background noise. While these negative results may reflect genuine species differences, they also corroborate a negative result from one of the only previous studies conducted under natural nocturnal illumination to investigate frog vocal sacs as the visual component of a putative multimodal mate attraction signal (Taylor et al., 2007, Animal Behaviour, 74 , 1753–1763). We consider possible proximate and ultimate explanations for our results and critically review previous research on multimodal mate attraction in nocturnal frogs. • Multimodal signals stimulate multiple receiver sensory organs. • Diverse animals use multimodal signals in many contexts, including mate choice. • Frog vocal sacs previously implicated as components of multimodal sexual signals. • Robotic playbacks under natural, nocturnal illumination suggest otherwise. • Current methods may mismeasure multimodal mate choice in nocturnal animals.