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We examined the overlap in the genes associated with daily rhythms and with behavioral plasticity in ants. We first investigated the daily rhythms of gene expression in the harvester ant,Pogonomyrmex barbatus, and how the rhythmic genes overlap with others previously shown to be associated with plasticity of foraging behavior. Then, to consider whether the overlap is conserved across ant species, we compared rhythms of gene expression in the diurnal, desert harvester ants with those previously reported for a distantly related nocturnal, subtropical carpenter ant,Camponotus floridanus. First, daily transcriptomes inP. barbatusshowed that most genes were expressed in light-dark (LD) and constantly dark (DD) conditions at about the same levels; only 11 genes showed at least a two-fold change in expression. Network analysis identified eleven modules ofP. barbatusgenes under LD conditions. Of these 11 clusters, modules C1 and C2 seem to be central nodes of the gene expression network, because they are the most highly connected in LD, and show the strongest preservation in DD vs. LD, and contain core clock genePeriod. Only one module, C2, showed significant overlap withP. barbatusgenes that have 24h-rhythmic expression in both LD and DD. There was significant overlap between modules C1, C2, C10, C11, andP. barbatusgenes found previously to be associated with plasticity in the regulation of foraging activity to manage water loss. A comparison of the daily transcriptome ofP. barbatuswith that ofC. floridanusshowed significant overlap of 24h-rhythmic genes in LD. Modules C1 and C2 of P. barbatus also overlap withC. floridanusgenes previously shown to differ in expression rhythms in nurses and foragers. In combination, these results indicate that genes linking plasticity of the circadian clock and of behavior may be broadly conserved in ants. 

ABSTRACT The architecture of an ant colony's nest entrance modulates the regulation of activity in and out of the nest. This study considers how the architecture of nests of the desert harvester antPogonomyrex barbatusfacilitates the regulation of foraging activity in an arid environment. Colonies must spend water, in water lost to evaporation when outside the nest, to obtain food and water. Previous work shows that encounters in the chamber just inside the entrance function as the valve to manage this tradeoff by regulating whether foragers decide to leave the nest on another trip. Here both complete and partial excavations, and observations inside active nests, were made in a long‐term study population in New Mexico, US. Both the overall nest architecture and the set of chambers around the nest entrance are structured as a minimum spanning tree with the entrance chamber as hub. The entrance chamber is surrounded by 1–6 adjacent chambers not linked to each other, with 2–3 tunnels that lead to strings of widely spaced chambers descending 1–2 m. Observations with a videoscope inside active nests show that exterior workers of different tasks move up to the entrance chamber each day during foraging activity and descend below it afterwards, and that interior workers quickly carry food down from the entrance chamber to the deeper nest. Larger, older colonies have more nest entrances with tunnels leading to a single entrance chamber than younger, smaller colonies; this may reduce variability in encounter rates. The nest entrance architecture facilitates rapid adjustment of activity outside the nest. However, in current, deepening drought conditions, it is susceptible to damage when the upper clay layer of the calichi soil dries out, disrupting encounters in the entrance chamber and inhibiting the colony's capacity to manage water loss. 

Abstract With the accelerating pace of climate change, we urgently need to understand how physiological traits shape behavioural plasticity in response to environmental stress. In social insects, collective behaviour operates without central control but through interactions among individual participants. In the aggregate, this produces a collective response to environmental conditions.Here we consider how variation among desert ant colonies in the cuticular hydrocarbons (CHCs) that prevent water loss is associated with variation among colonies in the collective behaviour that manages water stress. Colonies of the desert ant,Pogonomyrmex barbatus, differ in the collective regulation of foraging activity to manage water loss to evaporation while foraging. Foraging is regulated through olfactory interactions between outgoing and returning foragers, which determine a forager's decision whether to leave the nest on the next trip. Some colonies are risk‐averse, with foragers less likely to make foraging trips in dry conditions, while others are risk‐tolerant, with foragers who do not reduce foraging trips in dry conditions.We found that behavioural differences among colonies are associated with the capacity of their foragers' CHCs to prevent water loss. In risk‐averse colonies whose foragers make fewer trips in dry conditions, the abundance of alkenes was significantly higher. High abundance of alkenes, with a low melting point, makes the CHC layer more permeable, increasing susceptibility to water loss. In one of 2 years of this study, we found that workers in risk‐averse colonies also had significantly shortern‐alkanes, which further enhance water permeability of the CHC layer and thus desiccation risk.To our knowledge, this is the first report of variation among conspecific colonies in CHC profile that is linked to colony differences in collective behaviour. Read the freePlain Language Summaryfor this article on the Journal blog. 

Colonies of the arboreal turtle ant create networks of trails that link nests and food sources on the graph formed by branches and vines in the canopy of the tropical forest. Ants put down a volatile pheromone on the edges as they traverse them. At each vertex, the next edge to traverse is chosen using a decision rule based on the current pheromone level. There is a bidirectional flow of ants around the network. In a previous field study, it was observed that the trail networks approximately minimize the number of vertices, thus solving a variant of the popular shortest path problem without any central control and with minimal computational resources. We propose a biologically plausible model, based on a variant of the reinforced random walk on a graph, which explains this observation and suggests surprising algorithms for the shortest path problem and its variants. Through simulations and analysis, we show that when the rate of flow of ants does not change, the dynamics converges to the path with the minimum number of vertices, as observed in the field. The dynamics converges to the shortest path when the rate of flow increases with time, so the colony can solve the shortest path problem merely by increasing the flow rate. We also show that to guarantee convergence to the shortest path, bidirectional flow and a decision rule dividing the flow in proportion to the pheromone level are necessary, but convergence to approximately short paths is possible with other decision rules. 

Changing climatic conditions are shaping how density mediates resource competition. Colonies of the seed-eating red harvester ant, Pogonomyrmex barbatus, live for about 30 years in desert grassland. They compete with con- specific neighbors for foraging area in which to search for seeds. This study draws on a long-term census of a population of about 300 colonies from 1988 to 2019 at a site near Rodeo, New Mexico, USA. Rainfall was high in the first decade of the study, and then declined as a severe drought began in about 2001–2003. We examine the effects on colony survival and recruitment of the spatial configuration of the local neighborhood of conspecific neighbors, using Voronoi polygons as a measure of a colony’s foraging area, and consider how changing rainfall influences the effects of local neighborhoods. The results show that a colony’s chances of surviving to the next year depend on its age and on the foraging area available in its local neighborhood. Recruitment, measured as a founding colony’s chance of surviving to be 1 year old, depends on rainfall. In the earlier years of the study, when rainfall was high, colony numbers increased, and then began to decline after about 1997–1999, appar- ently due to crowding. As rainfall decreased, beginning in about 2001–2003, recruitment declined, and so did colony survival, leading to a trend toward earlier colony death which was most pronounced in 2016. As rainfall declined, apparently decreasing food availability, more foraging area was needed to sus- tain a colony: although the number of colonies declined, the impact of crowding by intraspecific neighbors increased. These processes maintain over- dispersion on the scale of about 8 m, with transient clustering at larger spatial scales. In addition, other factors besides crowding, such as the colony’s regula- tion of foraging activity to manage water loss, appear to contribute to a col- ony’s survival. The adaptive capacity for selection on the collective behavior that regulates foraging activity may determine how the population responds to ongoing climate change and drought. 

Differences among groups in collective behavior may arise from responses that all group members share, or instead from differences in the distribution of individuals of particular types. We examined whether the collective regulation of foraging behavior in colonies of the desert red harvester ant ( Pogonomyrmex barbatus ) depends on individual differences among foragers. Foragers lose water while searching for seeds in hot, dry conditions, so colonies regulate foraging activity in response to humidity. In the summer, foraging activity begins in the early morning when humidity is high, and ends at midday when humidity is low. We investigated whether individual foragers within a colony differ in the decision whether to leave the nest on their next foraging trip as humidity decreases, by tracking the foraging trips of marked individuals. We found that individuals did not differ in response to current humidity. No ants were consistently more likely than others to stop foraging when humidity is low. Each day there is a skewed distribution of trip number: only a few individuals make many trips, but most individuals make few trips. We found that from one day to the next, individual foragers do not show any consistent tendency to make a similar number of trips. These results suggest that the differences among colonies in response to humidity, found in previous work, are due to behavioral responses to current humidity that all workers in a colony share, rather than to the distribution within a colony of foragers that differ in response. 

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. 

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. 

The collective intelligence of online communities often depends on implicit forms of coordination, given the fluidity of membership and the lack of traditional hierarchies and associated incentive structures. This coordination drives knowledge production. Studying temporal dynamics may help elucidate how coordination happens. Specifically, the rate of interaction with an artifact such as a Wikipedia page can function as a signal that affects future interactions. Many activities can be characterized as bursty, meaning activity is not evenly spread or random, but is instead concentrated. This study analyzes 3,260 Wikipedia articles and shows that the coordination pattern in the Wikipedia community is mostly bursty. More importantly, the extent of burstiness affects article quality. This work highlights the important role temporal dynamics can play in the coordination process in online communities, and how it can affect the quality of knowledge production.