To cooperatively transport a large load, it is important that carriers conform in their efforts and align their forces. A downside of behavioural conformism is that it may decrease the group's responsiveness to external information. Combining experiment and theory, we show how ants optimize collective transport. On the single-ant scale, optimization stems from decision rules that balance individuality and compliance. Macroscopically, these rules poise the system at the transition between random walk and ballistic motion where the collective response to the steering of a single informed ant is maximized. We relate this peak in response to the divergence of susceptibility at a phase transition. Our theoretical models predict that the ant-load system can be transitioned through the critical point of this mesoscopic system by varying its size; we present experiments supporting these predictions. Our findings show that efficient group-level processes can arise from transient amplification of individual-based knowledge.
Any organism faces sensory and cognitive limitations which may result in maladaptive decisions. Such limitations are prominent in the context of groups where the relevant information at the individual level may not coincide with collective requirements. Here, we study the navigational decisions exhibited by Paratrechina longicornis ants as they cooperatively transport a large food item. These decisions hinge on the perception of individuals which often restricts them from providing the group with reliable directional information. We find that, to achieve efficient navigation despite partial and even misleading information, these ants employ a locally-blazed trail. This trail significantly deviates from the classical notion of an ant trail: First, instead of systematically marking the full path, ants mark short segments originating at the load. Second, the carrying team constantly loses the guiding trail. We experimentally and theoretically show that the locally-blazed trail optimally and robustly exploits useful knowledge while avoiding the pitfalls of misleading information.DOI: http://dx.doi.org/10.7554/eLife.20185.001
Collective motion by animal groups is affected by internal interactions, external constraints, and the influx of information. A quantitative understanding of how these different factors give rise to different modes of collective motion is, at present, lacking. Here, we study how ants that cooperatively transport a large food item react to an obstacle blocking their path. Combining experiments with a statistical physics model of mechanically coupled active agents, we show that the constraint induces a deterministic collective oscillatory mode that facilitates obstacle circumvention. We provide direct experimental evidence, backed by theory, that this motion is an emergent group effect that does not require any behavioral changes at the individual level. We trace these relaxation oscillations to the interplay between two forces; informed ants pull the load toward the nest whereas uninformed ants contribute to the motion's persistence along the tangential direction. The model's predictions that oscillations appear above a critical system size, that the group can spontaneously transition into its ordered phase, and that the system can exhibit complete rotations are all verified experimentally. We expect that similar oscillatory modes emerge in collective motion scenarios where the structure of the environment imposes conflicts between individually held information and the group's tendency for cohesiveness.collective motion | animal groups | emergent behavior | statistical physics | social insects T he collective motion of animal groups is affected by several factors. First, the tendencies for global alignment and cohesiveness (1-3) are often the result of near-neighbor interactions (2, 4-7). Quantitative variations in these local interactions can lead to qualitatively different global modes of collective motion (8). Second, the motion of a group is affected by influential leaders, which bring new knowledge into the system (9-15). Finally, the collective motion has to comply with environmental constraints such as boundaries or obstacles (16,17). When analyzing group motion, it is the interaction between these factors that must be considered (9,14,18). Observations suggest that such interactions could trigger transitions between different global modes of collective motion (17). A quantitative understanding of such collective behavioral shifts is still lacking.Here, we approach this question in the framework of cooperative transport (19-21) by Paratrechina longicornis ants (22,23). This behavior occurs as several ants coordinate their forces to collectively carry food items that are too big and heavy for any single ant.We have previously suggested that to efficiently move toward their nest, the ants balance between the well-coordinated pull of noninformed individuals and directional information brought in by informed leaders (15). However, these two forces are typically aligned and this obscures their relative effects. In this work, we use an external constraint to decouple the effect of informed and noninformed indivi...
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