High-risk environments promote chemical disturbance signalling among socially familiar Trinidadian guppies.

Under predation threat, many species produce cues that can serve as crucial sources of information for social companions. For instance, chemical cues released when experiencing a disturbing event (i.e. 'disturbance cues'), such as a predator chase, can lead to antipredator avoidance and increased survival for nearby individuals. These chemicals also have potential to be produced as a voluntary signal for communicating threat to others. We found evidence for this hypothesis by manipulating the shoal familiarity of guppies from populations differing in background predation risk and then presenting their disturbance cues to unfamiliar conspecifics from the same populations. Receivers from low-risk sites increased shoal cohesion and decreased area use regardless of whether the disturbance cues were produced in donor groups where members were familiar or unfamiliar with each other. However, receivers from high-risk sites showed strong antipredator reactions towards disturbance chemicals produced in familiar groups and no response towards those produced in unfamiliar groups, suggesting that donors from high-risk sites may alter the quality or quantity of their disturbance cues to influence familiar individuals to enact predator defences. Because high-risk environments strengthen guppy social networks, these environments may facilitate reliance on chemical disturbance signalling to coordinate group defences with familiar individuals.