Moving as a group imposes constraints on the energetic efficiency of movement.

Abstract

Movement is a key part of life for both solitary and group-living animals. In solitary animals, the energetic costs of making large displacements can be mitigated by energetically efficient strategies--specifically faster, straighter movements. However, little is known about whether moving as part of a collective enhances or limits the ability for individual group members to express such strategies. Drawing on 6 years of population-level high-resolution (1Hz) GPS tracking of group-living vulturine guineafowl (Acryllium vulturinum), we detected 886 events from 94 tagged individuals where their groups made large displacements, shifting their home ranges in response to changing environmental conditions. We contrasted these movements with data with 94 large displacement events by 19 lone, dispersing individuals. Our results suggest that individuals moving as part of group can significantly reduce their energetic cost of transport when making large displacements (15.5% reduction relative to their normal daily ranging behaviours) by increasing the speed and straightness of their movements. However, even during their most-efficient movements, individuals in groups could not achieve or maintain the same increases in speed that others could when moving alone, resulting in significantly limited efficiency gains (solitary individuals were 34.9% more efficient than individuals in groups). Overall, this study provides evidence for a substantial, and previously hidden, energetic cost arising from collective movement.