Fire ant raft morphology and energetics under water wave forcing

Over the last decade, researchers have studied the rafting dynamics of the red imported fire ant (Solenopsis invicta). Fire ant colonies form living rafts of densely entangled ants [Mlot et al. 2011] when floodwaters strike. In nature, water ripples and waves perturb the rafts, yet the mechanism by which ants accommodate such perturbations and the energetic cost involved remain unknown. In laboratory experiments, we investigated the energetics and morphology of rafts of ~250-350 ants subjected to Faraday waves by vertically oscillating a closed fluid-filled container with 10-30 Hz and 0.6 g peak acceleration. On undisturbed water, the raft expands to a 2D mono-/bi-layer of ants, linked through mandibles and tarsal claws, and like Ko et al., 2022, the CO₂ emission rate is constant. However, minutes after the onset of container oscillation, the ant raft self-compacted in a roughly two-stage process: a rapid decrease in the projected area followed by a slower rearrangement into a 3D ovoid-like arrangement. We posited that this compaction could be beneficial in resisting perturbation but would result in increased CO₂ emission rate due to active contraction and rearrangement. Surprisingly, after 15 minutes (at 20 Hz and 0.6 g), the rate returned to the nonperturbed rate. We hypothesize that the animals achieve this collective reduction in energy expenditure via the discovery of suitable passive mechanical binding.