Foraging in a 3-D world: how does predation risk affect space use of vervet monkeys?

One of the most important factors influencing the foraging of animals is predation risk. As a result, animals do not utilize landscapes uniformly. Vervet monkeys (Chlorocebus aethiops) use both arboreal and terrestrial habitats, and thus forage across a 3-dimensional landscape. To explore how both aerial and terrestrial predators influenced small-scale foraging patterns, we measured giving-up densities (GUDs) in artificial food patches across a woodland grassland interface. Patch transects ran vertically through trees, and horizontally into open grasslands. As vervets move up trees to avoid terrestrial predators and down out of the canopies to avoid aerial predators, we predicted that there would be a safe zone with higher patch use extending from just below the canopy to the base of the tree. Lower GUDs in this area showed this to be the case. Another factor that influenced patch use was group size. The different sized groups displayed a similar overall pattern of patch selection (i.e., they all fed more in specific patches). However, larger groups fed more intensively in all patches. As a result, larger groups achieved lower GUDs in all patches compared to the smaller groups both vertically and horizontally. Possible explanations include differences in predator abundance between the locations of the groups, group benefits lowering predation risk, competition forcing individuals to feed in less safe patches, or a combination of these factors. Ultimately, our results indicate that predation risk from both aerial and terrestrial predators shapes the vervets' 3-dimensional landscape of fear.