Predation risk sensitivity and the spatial organization of primate groups: a case study using GIS in lowland Woolly...

Predation risk is thought to be a potent force influencing intragroup cohesion, and the level of risk experienced by an individual is expected to vary with both group size and spatial position within a group. Smaller-bodied and less-experienced individuals are presumed to be more vulnerable to predators, suggesting that within-group spatial organization should show size- and age-dependent patterns in predator sensitive positioning. However, such effects have been difficult to evaluate for arboreal primates living in large groups. We conducted a preliminary study using a novel, spatially explicit method of assessing group spatial organization using GIS data in two groups of wild lowland Woolly monkeys, in which one group had a membership roughly twice as large as the second. In the larger group, group spread was more diffuse and large adult males were more frequently on the outskirts of the group than other age/sex classes, while immatures and females with dependents were more often in the center. Leaf cover around an individual-presumed to index an animal's perception of risk-increased significantly with distance from the group center for all immatures, although they were typically under lower leaf cover than adults; the number of groupmates in proximity also had an effect, but nearest neighbor distance did not. These differences were not detectable in the smaller group. This preliminary study suggests that thorough studies of spatial organization and predation risk sensitivity in arboreal primates are possible and could yield valuable information on how gregarious individuals offset ecological risks through social spacing.