Giant number fluctuations in fire-ant collectives

Many active systems comprised of self-propelled particles, including living systems, are known to undergo collective motion and exhibit giant number fluctuations (GNFs). Fire ants at sufficiently high density exhibit activity cycles consistent in time-instances where nearly all ants in the system move and time-instances where a large fraction of the ants assemble in non-moving clusters. We here quantify number fluctuations and demonstrate that fire ants in their actively moving state exhibit GNFs, suggesting that this state could correspond to the collective-motion state associated to the activity waves found in vertical columns. The corresponding scaling exponent is, in fact, consistent with that found for other polar active systems. Interestingly, similar behavior is found in the cluster phase, but in this case due to the presence of density heterogeneities. Our experimental results underscore that the underlying reason for GNFs depends on both structural as well as dynamical properties.