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1. Treadmilling and dynamic protrusions in fire ant rafts

   https://doi.org/10.1098/rsif.2021.0213  

   Wagner, Robert J. ; Such, Kristen ; Hobbs, Ethan ; Vernerey, Franck J. ( June 2021 , Journal of The Royal Society Interface)

   null (Ed.)

   Fire ants ( Solenopsis invicta ) are exemplary for their formation of cohered, buoyant and dynamic structures composed entirely of their own bodies when exposed to flooded environments. Here, we observe tether-like protrusions that emerge from aggregated fire ant rafts when docked to stationary, vertical rods. Ant rafts comprise a floating, structural network of interconnected ants on which a layer of freely active ants walk. We show here that sustained shape evolution is permitted by the competing mechanisms of perpetual raft contraction aided by the transition of bulk structural ants to the free active layer and outward raft expansion owing to the deposition of free ants into the structural network at the edges, culminating in global treadmilling. Furthermore, we see that protrusions emerge as a result of asymmetries in the edge deposition rate of free ants. Employing both experimental characterization and a model for self-propelled particles in strong confinement, we interpret that these asymmetries are likely to occur stochastically owing to wall accumulation effects and directional motion of active ants when strongly confined by the protrusions' relatively narrow boundaries. Together, these effects may realize the cooperative, yet spontaneous formation of protrusions that fire ants sometimes use for functional exploration and to escape flooded environments. 

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2. Agitated ants: regulation and self-organization of incipient nest excavation via collisional cues

   https://doi.org/10.1098/rsif.2022.0597  

   Avinery, Ram ; Aina, Kehinde O. ; Dyson, Carl J. ; Kuan, Hui-Shun ; Betterton, Meredith D. ; Goodisman, Michael A. ; Goldman, Daniel I. ( May 2023 , Journal of The Royal Society Interface)

   Ants are millimetres in scale yet collectively create metre-scale nests in diverse substrates. To discover principles by which ant collectives self-organize to excavate crowded, narrow tunnels, we studied incipient excavation in small groups of fire ants in quasi-two-dimensional arenas. Excavation rates displayed three stages: initially excavation occurred at a constant rate, followed by a rapid decay, and finally a slower decay scaling in time as t −1/2 . We used a cellular automata model to understand such scaling and motivate how rate modulation emerges without global control. In the model, ants estimated their collision frequency with other ants, but otherwise did not communicate. To capture early excavation rates, we introduced the concept of ‘agitation’—a tendency of individuals to avoid rest if collisions are frequent. The model reproduced the observed multi-stage excavation dynamics; analysis revealed how parameters affected features of multi-stage progression. Moreover, a scaling argument without ant–ant interactions captures tunnel growth power-law at long times. Our study demonstrates how individual ants may use local collisional cues to achieve functional global self-organization. Such contact-based decisions could be leveraged by other living and non-living collectives to perform tasks in confined and crowded environments. 

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3. Better tired than lost: Turtle ant trail networks favor coherence over short edges

   https://doi.org/10.1371/journal.pcbi.1009523  

   Chandrasekhar, Arjun ; Marshall, James A. ; Austin, Cortnea ; Navlakha, Saket ; Gordon, Deborah M ( October 2021 , PLoS computational biology)

   Creating a routing backbone is a fundamental problem in both biology and engineering. The routing backbone of the trail networks of arboreal turtle ants (Cephalotes goniodontus) connects many nests and food sources using trail pheromone deposited by ants as they walk. Unlike species that forage on the ground, the trail networks of arboreal ants are constrained by the vegetation. We examined what objectives the trail networks meet by comparing the observed ant trail networks with networks of random, hypothetical trail networks in the same surrounding vegetation and with trails optimized for four objectives: minimizing path length, minimizing average edge length, minimizing number of nodes, and minimizing opportunities to get lost. The ants’ trails minimized path length by minimizing the number of nodes traversed rather than choosing short edges. In addition, the ants’ trails reduced the opportunity for ants to get lost at each node, favoring nodes with 3D configurations most likely to be reinforced by pheromone. Thus, rather than finding the shortest edges, turtle ant trail networks take advantage of natural variation in the environment to favor coherence, keeping the ants together on the trails. 

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4. Molecular interactions of melatonin with lipid rafts

   https://doi.org/10.32794/mr112500123  

   Bolmatov, Dima ; Lavrentovich, Maxim ; Reiter, Russel J. ( June 2022 , Melatonin Research)

   Biological membranes are composed of a lipid bilayer with a heterogeneous structure and complex dynamics, both of which can be modulated by the presence of melatonin. The lateral heterogeneities in lipid bilayers, also known as lipid rafts, have unique molecular interactions with melatonin, which we review here. Specifically, we discuss the molecular-level, physicochemical influences of melatonin on dynamics of lipid rafts and their structural properties, including melatonin’s propensity to preserve the structural integrity of lipid rafts at different length scales, as revealed through a range of experimental techniques and theoretical approaches. 

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5. A Model for Ant Trail Formation and its Convergence Properties

   https://doi.org/10.4230/LIPIcs.ITCS.2021.85  

   Charikar, Moses ; Garg, Shivam ; Gordon, Deborah M. ; Shiragur, Kirankumar ( January 2021 , 12th Innovations in Theoretical Computer Science Conference (ITCS 2021))

   null (Ed.)

   We introduce a model for ant trail formation, building upon previous work on biologically feasible local algorithms that plausibly describe how ants maintain trail networks. The model is a variant of a reinforced random walk on a directed graph, where ants lay pheromone on edges as they traverse them and the next edge to traverse is chosen based on the level of pheromone; this pheromone decays with time. There is a bidirectional flow of ants in the network: the forward flow proceeds along forward edges from source (e.g. the nest) to sink (e.g. a food source), and the backward flow in the opposite direction. Some fraction of ants are lost as they pass through each node (modeling the loss of ants due to exploration observed in the field). We initiate a theoretical study of this model. We note that ant navigation has inspired the field of ant colony optimization, heuristics that have been applied to several combinatorial optimization problems; however the algorithms developed there are considerably more complex and not constrained to being biologically feasible. We first consider the linear decision rule, where the flow divides itself among the next set of edges in proportion to their pheromone level. Here, we show that the process converges to the path with minimum leakage when the forward and backward flows do not change over time. On the other hand, when the forward and backward flows increase over time (caused by positive reinforcement from the discovery of a food source, for example), we show that the process converges to the shortest path. These results are for graphs consisting of two parallel paths (a case that has been investigated before in experiments). Through simulations, we show that these results hold for more general graphs drawn from various random graph models; proving this convergence in the general case is an interesting open problem. Further, to understand the behaviour of other decision rules beyond the linear rule, we consider a general family of decision rules. For this family, we show that there is no advantage of using a non-linear decision rule, if the goal is to find the shortest or the minimum leakage path. We also show that bidirectional flow is necessary for convergence to such paths. Our results provide a plausible explanation for field observations, and open up new avenues for further theoretical and experimental investigation. 

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