Eavesdropping : how do vervet monkeys perceive the alarm calls of other species?.

Abstract

Perceived predation risk has a large impact on how prey species utilise landscapes. In an effort to reduce predation risk, individuals tend to utilise safer areas more than unsafe areas. How perceived predation risk affects the utilisation of landscapes by animals is termed a “landscape of fear”. Vervet monkeys (Chlorocebus pygerythrus) have a landscape of fear that operates in both horizontal and vertical planes. Within this landscape, vervets perceive the safest area to be up in a tree, under the canopy. To reduce predation risk, vervets use various predator-specific alarm calls and have been found to eavesdrop on the alarm calls of other species (e.g. birds). In this study, I explored whether vervet monkeys were able to associate eavesdropped alarm calls with specific predator types (i.e. aerial and terrestrial) as they do with their own predator specific alarm calls. To do this, I first quantified the three-dimensional landscape of fear for vervet monkeys by measuring giving up densities in artificial patches. I then used playbacks of the vervets’ aerial and terrestrial predator alarm calls, the alarm call of a red-backed shrike, and a mixed-species flock mobbing call to manipulate perceived predation risk. By comparing changes in foraging intensity within the patches, I quantified the specific reactions of the vervet monkeys to aerial and terrestrial predators. In addition, I found that the monkeys did not eavesdrop on the red-backed shrike call. However, the vervets did eavesdrop on bird mobbing calls, and associated the calls with the location of the potential treat and reacted as if it was a particular predator type. Specifically, the vervets reacted to mobbing calls played from up in a tree the same way as they did if an aerial predator was present, and calls from the ground as if a terrestrial predator was present. Thus, this suggests that they were able to associate a non-functional referential call (i.e. the mobbing call) with specific information, gathered from the location of the calls, and interpreted it in a referential manner. Moreover, intensity of these reactions (as measured by total feeding effort) indicated that vervets saw aerial predators as a greater threat compared to terrestrial predators. Ultimately, my results suggest that vervets can associate eavesdropped calls with specific predators, and this likely provides a fitness benefit in a dangerous and unpredictable world.