title: Moving in groups: how density and unpredictable motion affect predation risk.
abstract: One of the most widely applicable benefits of aggregation is aper capitareduction in predation risk. Many factors can contribute to this but, for moving groups, an increased difficulty in tracking and targeting one individual amongst many has received particular attention. This "confusion effect" has been proposed to result from a bottleneck in information processing, a hypothesis supported by both modelling and experiment. If the competition for limited attention is localised to the particular part of the visual field where the target is located, prey density is likely to be the key factor rather than group numbersper se. Furthermore, unpredictability of prey movement may enhance confusion, but both factors have received insufficient attention from empiricists: undoubtedly because of the difficulty of experimental manipulation in natural systems. We used a computer-based target tracking task with human subjects to manipulate effects of number and density independently, in factorial combination with motion path predictability. Density, rather than number, drove the confusion effect in our experiment and acted synergistically with the unpredictability of the direction of motion. The experimental paradigm we present offers the potential for isolating other factors affecting predation success on group-living prey, and forging links with the psychological literature on object tracking and visual search.
authors.affiliation: School of Experimental Psychology, University of Bristol, Bristol, UK.
authors.forename: Nicholas E
authors.initials: NE
authors.lastname: Scott-Samuel
authors.affiliation: School of Psychology, Cardiff University, Cardiff, UK.
authors.forename: Gavin
authors.initials: G
authors.lastname: Holmes
authors.affiliation: School of Experimental Psychology, University of Bristol, Bristol, UK.
authors.forename: Roland
authors.initials: R
authors.lastname: Baddeley
authors.affiliation: School of Biological Sciences, University of Bristol, Bristol, UK.
authors.forename: Innes C
authors.initials: IC
authors.lastname: Cuthill
keywords: Aggregation; Confusion effect; Group living; Object tracking; Predation risk; Visual search