The Role of the Vocal Sac in Multimodal Signaling - an Integrative Study on Reed Frogs

During calling the anuran vocal sac serves as an air reservoir, in minimizing the loss of sound energy and in distributing calls omnidirectionally. The vocal sac`s role beyond acoustics had been overlooked for some time, but the diversity in vocal sac coloration and shape found in different species is striking and recently its visual properties have gained vast attention. Males of many reed frog species (Anura: Hyperoliidae) have a prominent colorful gular patch on their vocal sac, which is very conspicuous once the vocal sac is inflated. To date, the function of the gular patch remains unknown. In a pilot study undertaken in Uganda we found strong evidence that the gular patch is a gland producing volatiles emitted while calling. The description of this phenomenon would be the first record of the vocal sac as a source for chemical signals. Furthermore, the coloration of vocal sac and gular patch might be species specific and may serve as a visual signal component in calling males. Therefore, it is likely that hyperoliid frogs use a complex combination of acoustic, visual and chemical signals in species recognition and mate choice unknown in any anuran to date. Based on our preliminary observations we will study the multimodal communication in syntopic reed frog species in the field and test signal function in intra- and intersexual communication in manipulative experiments in the lab. Additionally, histological and biochemical methods will underpin our insights about function and functionality of the gular patch. We aim to use an integrative approach to answer the following main questions: What is the morphological structure of the gular patch and is it responsible for producing the volatile compounds? Which chemical compounds are emitted and are they species specific? What are the effects of the volatile compounds on conspecific males and females? How does vocal sac and gular patch coloration differ between syntopic species? How do acoustic, visual and chemical signals influence species recognition and mate choice in reed frogs?

The vocal sac present in males of most frog or toad species is generally associated with its role in call production. However, its function aside from acoustic communication was neglected and dynamic cues such as vocal-sac movement, were traditionally seen as a byproduct of sound production. However, the diversity in vocal-sac colouration and shape found in different frog species is striking and in recent years its visual properties were interpreted as important role in signalling. Males of many reed frog species (Anura: Hyperoliidae), have a prominently coloured patch on their vocal sac, which is especially conspicuous once the vocal sac is inflated. The function of the gular patch used to be speculated on, but remained unknown and was the central objective of this exploratory thesis. We aimed to highlight the unique reed frog vocal sac and its potential function in multimodal signalling in particular, and the fascinating diversity and complexity of anuran multimodal signalling in general. To investigate the role of the vocal sac and the reed frog gular patch in inter- and intra-specific communication, we integrated histological and biochemical analyses in the laboratory as well as measurements of signal parameters and associated receiver response to signal components during behavioural experiments in the field. We could demonstrate that the gular patch is a gland complex producing species-specific chemical cocktails. The scent bouquets are expelled by male reed frogs most likely during calling - an insofar undescribed signalling strategy 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 behaviour in which males tap their feet on the surface they are sitting on. The foot-tapping likely resembles an advertisement call in the visual and or seismic signal domain, which is produced to deter rival males and to attract females. Due to their complex signals, advertising males might be easier to find in a frog chorus, which could help them to spend less energy on extensive calling. Furthermore, the chances of survival for female reed frogs could be improved, as they might be able to approach an attractive male more efficiently which might result in a reduced predation risk. The insights into the mechanism and function of the vocal sac in acoustic, visual and chemical signalling of reed frogs found in this research project could greatly contribute to our understanding of the evolution of multimodal communication in the animal kingdom.