The vocal sac present in males of most anuran species is generally associated with its role in advertisement call production. However, its function aside from acoustic commu-nication was neglected for some time and cues such as vocal sac movement, were tra-ditionally seen as an epiphenomon of sound production. The diversity in vocal sac col-ouration and shape found in different species is striking and recently its visual properties were awarded an important role in signalling. Males of many

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... frog species (Anura: Hyperoliidae), have a prominent colourful patch on their vocal sac, which is very conspicuous once the vocal sac is inflated. The function of the gular patch makes up the central objective of this exploratory thesis. I aim to highlight the unique hyperolid vocal sac and its potential function in multimodal signalling in particular, and the fascinating diversity and complexity of anuran multimod-al signalling in general. To investigate the role of the vocal sac and the hyperolid gular patch in inter- and intra-specific communication, I integrated a literature research, histo-logical and biochemical analyses in the laboratory as well as measurements of signal parameters and associated receiver response to signal components in cue-isolation experiments in the field. I demonstrate that the gular patch is a gland complex producing species-specific chemi-cal cocktails which are most likely expelled during calling - a so far undescribed mode of signalling in frogs. Furthermore, field experiments testing receiver-response to uni- and multimodal signal stimuli showed that the colourful gular patch does not act as a visual signal in male-male communication, but revealed a previously undescribed foot-tapping behaviour. In two chapters of my thesis I sum up the current knowledge on uni- and multimodal signalling strategies in anurans and review the versatile role of the vocal sac in the anu-ran communication system. Finally I discuss the mechanism and function of the vocal sac in acoustic, visual and chemical sig [...]