Responses of Redfronted Lemurs to Experimentally Modified Alarm Calls: Evidence for Urgency-Based Changes in Call Structure

Abstract The aim of this study was to investigate how information about the affective state is expressed in vocalizations. Alarm calls can serve as model systems with which to study this general question. Therefore, we examined the information content of terrestrial predator alarm calls of redfronted lemurs ( Eulemur fulvus rufus ), group-living Malagasy primates. Redfronted lemurs give specific alarm calls only towards raptors, whereas calls given in response to terrestrial predators (woofs) are also used in other situations characterized by high arousal. Woofs may therefore have the potential to express the perceived risk of a given threat. In order to examine whether different levels of arousal are expressed in call structure, we analysed woofs given during inter-group encounters or in response to playbacks of a barking dog, assuming that animals engaged in inter-group encounters experience higher arousal than during the playbacks of dog barks. A multivariate acoustic analysis revealed that calls given during group encounters were characterized by higher frequencies than calls given in response to playbacks of dog barks. In order to examine whether this change in call structure is salient to conspecifics, we conducted playback experiments with woofs, modified in either amplitude or frequencies. Playbacks of calls with increased frequency or amplitude elicited a longer orienting response, suggesting that different levels of arousal are expressed in call structure and provide meaningful information for listeners. In conclusion, the results of our study indicate that the information about the sender's affective state is expressed in the structure of vocalizations.