Assessment of local predation risk: the role of subthreshold concentrations of chemical alarm cues

The ability to accurately assess local predation risk is critical to prey individuals, as it allows them to maximize threat-sensitive trade-offs between predator avoidance and other fitness related activities. A wide range of taxonomically diverse prey (including many freshwater fishes) relies on chemical alarm cues (alarm pheromones) as their primary information source for local risk assessment. However, the value of chemical alarm cues has been questioned due to the availability of additional sensory inputs (i.e., visual cues) and the lack of an overt antipredator response under conditions of low perceived risk. In this paper, we test the hypothesis that chemical alarm cues at concentrations below the point at which they elicit an overt behavioral response function to increase vigilance towards other sensory modalities (i.e., visual alarm cues). Shoals of glowlight tetras (Hemigrammus erythrozonus) exposed to the subthreshold concentration of hypoxanthine-3-N-oxide (the putative Ostariophysan alarm pheromone) did not exhibit an overt antipredator response in the absence of secondary visual cues (not different than the distilled water control). However, when exposed to the sight of a visually alarmed conspecific, they significantly increased the intensity of their antipredator response (not different from shoals exposed to the suprathreshold alarm cue). This study demonstrates that prey may benefit from responding to low concentration alarm cues by increasing vigilance towards secondary cues during local risk assessment, even in the absence of an overt behavioral response. By increasing vigilance towards secondary risk assessment cues in the presence of a low risk chemical cue, individuals are likely able to maximize the threat-sensitive tradeoffs between predator avoidance and other fitness related activities.