Emergent oscillations assist obstacle negotiation during ant cooperative transport

Abstract

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...

Effects of load mass and size on cooperative transport in ants over multiple transport challenges

“…Obstacle navigation may pose substantially different challenges than initiating movement, in part because the group must decide on a new travel direction when the preferred direction is unavailable. Several other studies have been published on obstacle navigation during cooperative transport by P. longicornis (Fonio et al, 2016;Gelblum et al, 2016;McCreery et al, 2016b;Ron et al, 2018), focusing primarily on navigation dynamics and the mechanisms that allow groups to successfully move around complex obstacles. Rather than further examining these mechanisms, we instead focused on how object properties affect the performance of groups as they progress through the obstacle navigation process.…”

Behavioral diversity assisting obstacle navigation during group transportation in ant

“…Lastly, the simulation results revealed that stochastic force generation, whether 311 generated coincidentally by friction with the ground or intrinsically by the 312 system [11,12], can also assist in navigating more complicated obstacles. The success 313 rate could be further enhanced by using an informed-ant-attachment mechanism [3].…”

“…The speed is simply proportional 218 to group size in P. longicornis [11,12]. It is proportional at small group size and then 219 becomes saturated at large group size in the neotropical ants Pheidole oxyops [21] and 220 Aphaenogaster cockerelli [2].…”

“…Group transportation is a behavior in which two or more worker ants move a large 12 food item that not transportable by a single ant. The behavior was categorized into 13 three kinds or transportation by Czaczkes and Ratnieks: uncoordinated, encircling 14 coordinated, and forward-facing cooperative [1].…”

“…Furthermore, this species exhibits excellent performance during 33 obstacle navigation, switching its behavior intrinsically without the attachment of an 34 informed ant [11]. Moreover, under imposition of a constraint such as an obstacle 35 barrier, oscillatory group motion emerges which assists in navigating the obstacle [12].…”