Fear, concealment, and time of day interactively predict group size of a common ungulate

Abstract

AbstractAnimals commonly form groups with conspecifics. Hypotheses on the drivers of group size often (but not always) pertain to fear of predation and food availability or quality. However, for a more complete understanding of group forming, multiple, possibly interacting mechanisms should be considered. We used camera traps, detectability estimates from distance sampling, and hierarchical Bayesian modeling to index group size and test multiple additive and interacting group‐formation hypotheses in white‐tailed deer (Odocoileus virginianus), within Indiana, USA. We found a strong relationship between our index of group size and (1) a three‐way interaction between spatiotemporal pulses in fear of natural predation risk and anthropogenic development and (2) a three‐way interaction between the local cover (defined as the amount of vegetative cover in the immediate area of the camera), the area of concealment (defined as forests or wetlands) within the larger landscape, and time of day. Specifically, we documented larger groups in areas near anthropogenic development, in areas with high predator use intensity, and when predators were active. Additionally, we found larger groups during dawn and dusk when deer were most active in locations with sparse local cover when the area of concealment in the surrounding landscape was small, and within dense local cover when the area of concealment in the surrounding landscape was large. Our methods can aid future behavioral and ecological studies, as we present a clear and easily replicable strategy that infers group membership from spatiotemporal proximity, corrects for differences in detectability, and enables sampling across large spatial and long temporal scales. Our findings indicate that fear of natural predators and anthropogenic disturbance interactively explain variation in group size.