The combined effects of elevated predation risk and anthropogenic noise on dwarf mongoose vigilance behaviour.

Anthropogenic noise is a pervasive pollutant in the world's ecosystems, with numerous studies demonstrating negative physiological, developmental and behavioural impacts across taxa. However, research has tended to focus on anthropogenic noise in isolation; many species often experience this pollutant in conjunction with other anthropogenic and natural stressors. Here, we used a field experiment to investigate the combined effects of a sequential elevation in perceived predation risk followed by exposure to road noise on the vigilance behaviour of dwarf mongooses (Helogale parvula). As expected, both alarm-call playback (simulating a greater predation risk) and road-noise playback independently led to more vigilance compared to close-call and ambient-sound (control) playbacks, respectively. The two stressors had an equivalent effect on total vigilance, lending support to the risk-disturbance hypothesis. The combination of the two stressors did not, however, generate a significantly different amount of vigilance compared to road-noise playback alone. Thus, our experiment provides further evidence that anthropogenic noise can influence the vigilance-foraging trade-off but no indication of an additive or synergistic effect when combined with the natural stressor of elevated predation risk. Further investigation of combined-stressor effects is critical if we are to understand the true impacts of anthropogenic disturbances on species and communities.