Energy Management in Self-Compacting Mechanically Oscillated Fire Ant Clusters

While the rafting (self-aggregation) mechanism of fire ants (S. Invicta) has been well documented, how such clusters withstand mechanical perturbations (e.g., water waves and rainfall) remains underexplored. Here, we oscillated small (~100-1000) groups in a water-filled container at 20Hz with ~0.6g peak acceleration; CO₂ emission rate spiked initially but returned to baseline within 15 minutes. To investigate the importance of clustering in the presence of greater stresses we oscillated ants on solid surfaces at 30 Hz with peak accelerations of 2-6g, and saw that CO₂ emissions rose continuously, never returning to baseline emission rate. When clusters were confined vertically (limiting movement and attachment) ants showed an 82% reduction in CO₂ emissions compared to unconfined conditions. However, applying baby powder to unconfined ants compromised anchoring and attachment, causing a 48% increase in CO₂ emissions. These results indicate that decreased passive attachment via tarsal claws increases energy expenditure. To elucidate the roles of anchoring and attachment in energy use in collective clustering, we developed a Discrete Element Method (DEM) model. In the model, each ant was represented as a particle, employed with various strategies to adhere to its neighbors and container base. The simulation predicts that ants can optimize energy without reducing the cohesiveness of a cluster by transitioning between active grabbing and passive anchoring based on local proximity to other ants.