Multimodal signaling in anurans

The proposed project aims to explore selection forces responsible for divergent and convergent evolution of multimodal signaling in four species of foot-flagging anurans (Staurois tuberilinguis, S. guttatus, Hylodes dactylocinus, and Micrixalus saxicolus) from Asia and South America. All species occur along noisy streams, and perform visual displays (foot flagging) and acoustic signals, thus making them ideal organisms to study traits affecting signal efficacy across two communication channels. Environmental conditions (e.g. intense background noise) shape signal design to enhance efficacy and affect microhabitat preferences. Natural selection favors signals that maximize the signal-to-noise ratio and minimize signal degradation. Masking of receiver perception and detection in one communication channel can be compensated by the use of another signal modality. To understand the evolutionary relationship of signal structure and behavioral traits, we propose using a multispecies electro mechanic frog model (EMF) which integrates acoustic and visual signals. By presenting sequences of either uni- or bimodal acoustic and visual signals through the EMF and quantifying receiver`s response, we propose to test hypotheses that account for multimodal signal functions in the evolution of frog audiovisual displays. Furthermore we shall enquire how sensory drive affects habitat selection and signal design in two closely related species occurring in sympatry. To understand divergent signal evolution in the optical channel, we will describe the light characteristics of the frogs` habitats, measure reflectance spectra of toe webbings and obtain data on the visual system of the two Staurois species by microspectrophotometry. This project will provide the unique opportunity to study a group of distantly related species in which bimodal signaling has evolved convergently on different continents under similar environmental conditions as well as two closely related species that occur sympatric and utilize different microhabitats along the same stream. This project will give us a better understanding on how various selection forces have driven the evolution of multimodal signal design.

The predominant communication mode of anuran amphibians are vocalizations, however intraspecific communication may involve multimodal (acoustic and visual) cues or signals in many more species than previously thought. Visual signals may act as an additional or a complementary mode of communication in noisy environments. Foot flagging as a striking form of visual signalling behaviour has evolved in at least in 16 species from 5 different families mainly living along fast-flowing streams, which generate continuous broadband back-ground noise. To better understand the function of foot flagging as multimodal signal component, we studied three Asian species from two families and performed cue-isolation experiments in the field.The Bornean species Staurois parvus and S. guttatus avoid acoustic interference of ambient stream noise by using call frequencies less masked by the background and utilize ac-companying visual signals to announce the readiness to defend calling sites, interestingly this behavior can be observed already in juveniles bred in the Vienna Zoo. Micrixalus saxicola from the Western Ghats (India) occurs along less noisy streams and acoustic signals are rather masked by chorus noise from conspecifics than by abiotic noise. Males use a variety of visual signals including foot-flagging and tapping during agonistic behaviour. Receiver responses from acoustic- vs. multimodal playback presentations provide evidence that the vocal sac acts as an additional visual cue. Comparisons of signal response to acoustic, visual and multimodal stimuli and analysis of signal brightness of foot webbings in M. saxicola and S. parvus further highlight differences in the magnitude and significance of signalling behaviour in the respective species. We suggest that foot-flagging displays ritualized from foot kicks, a frequently observed fighting technique in M. saxicola, to reduce physical attacks. The behavioural differences presumably represent a continuum in the evolution of foot displays resulting from ecological divergence and sexual selection, whereas signals in M. saxicola represents a nascent state in the evolution of visual communication. Together these investigations allow us to draw conclusions on signal efficacy and function and help to better understand the evolution of multimodal communication in anuran.