A two-signal, two-receiver signalling system: Snake mobbing in the Arabian babbler

Mobbing is a collective antipredator behaviour in which animals approach and harass a threat while coordinating signals to alert conspecifics, recruit allies, or deter predators. In group-living Arabian babblers (Argya squamiceps), snake mobbing involves two distinct signals: acoustic "zwick" calls inaudible to the snake and visual wing-lifting displays. We elicited snake-mobbing events with dummy vipers and recorded babbler groups using a synchronized acoustic camera enabling individual caller identification and postural analysis. Our results reveal the different and similar roles of the two signals in babblers' multimodal mobbing behaviour. Recruitment order was independent of sex, age and rank. Individual investment in mobbing signaling decreased as group size increased, supporting the hypotheses of social buffering and predation risk dilution. Solitary individuals always called during wing-lifts, supporting the calls' recruitment function. Mobbing individuals responded faster (with signaling) to a joint visual-acoustic conspecific display than an only acoustic signal. When coupled, wing-lifts significantly altered vocal characteristics, decreasing call frequency and increasing call rate, thus strengthening the acoustic signal. Moreover, calls emitted toward the end of the wing-lift display exhibit stronger vocal modulation. Together, these findings demonstrate the multi-functional role of multi-modal mobbing signals during a risky cooperative task balancing social communication and predator deterrence.