Collective strategy for obstacle navigation during cooperative transport by ants

Abstract

Group cohesion and consensus have primarily been studied in the context of discrete decisions, but some group tasks require making serial decisions that build on one another. We examine such collective problem solving by studying obstacle navigation during cooperative transport in ants. In cooperative transport, ants work together to move a large object back to their nest. We blocked cooperative transport groups of Paratrechina longicornis with obstacles of varying complexity, analyzing groups' trajectories to infer what kind of strategy the ants employed. Simple strategies require little information, but more challenging, robust strategies succeed with a wider range of obstacles. We found that transport groups use a stochastic strategy that leads to efficient navigation around simple obstacles, and still succeeds at difficult obstacles. While groups navigating obstacles preferentially move directly toward the nest, they change their behavior over time; the longer the ants are obstructed, the more likely they are to move away from the nest. This increases the chance of finding a path around the obstacle. Groups rapidly changed directions and rarely stalled during navigation, indicating that these ants maintain consensus even when the nest direction is blocked. Although some decisions were aided by the arrival of new ants, at many key points, direction changes were initiated within the group, with no apparent external cause. This ant species is highly effective at navigating complex environments, and implements a flexible strategy that works for both simple and more complex obstacles.

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

“…Just after initiating movement, P. longicornis groups perform well in both of these measures; while their sinuosity was affected by challenging objects, it remained fairly low (maximum 2.7), and groups almost never stalled. Paratrechina longicornis groups excel at maintaining coordination during initial movement, which agrees with earlier results testing easy-to-carry objects (Gelblum et al, 2015;McCreery et al, 2016b). Our results add that this is true even for heavy, unwieldy objects.…”

“…To make baits attractive, we applied a uniform volume of tuna-permeated oilfrom tuna packed in olive oilto the circumference of each bait. Tuna is a commonly used ant bait that elicits group transport behaviour in P. longicornis (McCreery et al, 2016b). Baits were allowed to dry before use, with oil absorbing into the object, and always elicited vigorous transport attempts from P. longicornis workers.…”

“…We have only a limited understanding of how transport mechanisms and behaviours interact with different cooperative transport phases. Previous mechanistic studies have looked primarily at a particular phase; for example, at the recruitment phase (Czaczkes and Ratnieks, 2012;Daly-Schveitzer et al, 2007;Robson and Traniello, 1998), or at how groups actively move toward the nest (Buffin and Pratt, 2016;Fonio et al, 2016;Gelblum et al, 2015;McCreery et al, 2016b). Even for behaviours within a particular phase, our understanding of how object properties affect performance is relatively poor.…”

“…Army ants are among the species for which we understand the most about how object mass affects transport dynamics (Franks, 1986;Franks et al, 1999Franks et al, , 2001), yet the cooperative transport of army ants differs substantially from cooperative transport in most ants, as they use 'forward-facing' rather than 'encircling' transport (Czaczkes and Ratnieks, 2013). A number of more recent experiments have provided detailed, mechanistic analyses of cooperative transport dynamics, in species with encircling transport, but have typically tried to remove the potential influence of object properties by providing groups with essentially uniform baits (Berman et al, 2011;Buffin and Pratt, 2016;Czaczkes et al, 2011;Fonio et al, 2016;Kumar et al, 2013;McCreery et al, 2016b;Robson and Traniello, 2002). We still lack a clear understanding of how object properties independently affect cooperative transport across the differing challenges that it entails.…”

Miniaturisation decreases visual navigational competence in ants

“…Obstacle avoidance behaviour has implications for navigation by both walking insects such as ants (Collett et al, 1992;McLeman et al, 2002;Schmidt et al, 1992;Thorpe, 1949) and flying insects such as bees (Dittmar et al, 2010), and even for collective food transport (McCreery et al, 2016) and animal cognition (Kabadayi et al, 2017). Miniaturisation should affect this behaviour as miniature ants tend to have reduced sensory capabilities.…”