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 relies on two signals that differ in receivers: acoustic "zwick" calls inaudible to the snake and visual wing-lift displays perceived by both conspecifics and the snake, raising the question of whether each signal has one receiver or involves a more complex structure. We elicited snake-mobbing events with dummy vipers and recorded babbler groups using a synchronized acoustic camera enabling individual caller identification and postural analysis. We examined mob formation, the effect of group size on signalling, and the interplay between the two signal types. First signalling (recruitment) order was independent of sex, age and rank. Individual call rate decreased as group size increased, supporting the hypotheses of social buffering and predation risk dilution. Mobbing individuals responded faster (with signalling) to a joint visual-acoustic conspecific display and the onset of a visual display than to an acoustic-only signal. When coupled, wing-lifts significantly altered vocal characteristics, decreasing peak frequency and increasing call rate, thus strengthening the acoustic signal. Moreover, calls emitted toward the end of the wing-lift display exhibited a stronger drop in peak frequency. Together, these findings demonstrate the multi-functional role of multimodal mobbing signals during a risky cooperative task balancing social communication and predator deterrence.