Banded mongoose recruitment calls convey information about risk and not stimulus type

Many species use specific vocalizations that attract conspecifics to food sources or that recruit other group members to inspect secondary predator cues or to mob predators. However, little is known regarding the variation in the acoustic structure of such calls and the associated meaning. In the cooperatively breeding banded mongoose, Mungos mungo, we investigated calls produced in the context of group recruitment to three different stimulus types: (1) secondary predator cues; (2) snakes; and (3) members of rival groups. Calls produced in response to predator faeces differed from the harsher variants elicited by snakes and rival groups, but the latter two elicited similar calls. Only a few calls of intermediate acoustic structure were produced in response to the different stimulus categories. Playbacks of calls elicited by the different stimuli caused individuals to approach the speaker in the same way. However, calls elicited specifically by snakes and rival mongooses resulted in a higher proportion of receivers responding and a faster receiver response than calls elicited by faeces. This suggests a graded rather than a discrete recruitment call system. Thus, despite obvious differences in stimulus type and the purpose of recruitment, the acoustic structure of calls conveys information about the risk of the encountered stimulus and not the stimulus type itself, allowing receivers to adjust their response according to the urgency. Since calling continued for prolonged periods after all group members had gathered around the callers, these calls may also function to deter rivals and to coordinate the subsequent group response.