The biological principles of swarm intelligence

The roots of swarm intelligence are deeply embedded in the biological study of self-organized behaviors in social insects. From the routing of traffic in telecommunication networks to the design of control algorithms for groups of autonomous robots, the collective behaviors of these animals have inspired many of the foundational works in this emerging research field. For the first issue of this journal dedicated to swarm intelligence, we review the main biological principles that underlie the organization of insects’ colonies. We begin with some reminders about the decentralized nature of such systems and we describe the underlying mechanisms of complex collective behaviors of social insects, from the concept of stigmergy to the theory of self-organization in biological systems. We emphasize in particular the role of interactions and the importance of bifurcations that appear in the collective output of the colony when some of the system’s parameters change. We then propose to categorize the collective behaviors displayed by insect colonies according to four functions that emerge at the level of the colony and that organize its global behavior. Finally, we address the role of modulations of individual behaviors by disturbances (either environmental or internal to the colony) in the overall flexibility of insect colonies. We conclude that future studies about self-organized biological behaviors should investigate such modulations to better understand how insect colonies adapt to uncertain worlds.     

Balancing organization and flexibility in foraging dynamics

Proper pattern organization and reorganization are central problems facing many biological networks which thrive in fluctuating environments. However, in many cases the mechanisms that organize system activity oppose those that support behavioral flexibility. Thus, a balance between pattern organization and pattern flexibility is critically important for overall biological fitness. We study this balance in the foraging strategies of ant colonies exploiting food in dynamic environments. We present discrete time and space simulations of colony activity that uses a pheromone-based recruitment strategy biasing foraging towards a food source. After food relocation, the pheromone must evaporate sufficiently before foraging can shift colony attention to a new food source. The amount of food consumed within the dynamic environment depends non-monotonically on the pheromone evaporation time constant—with maximal consumption occurring at a time constant which balances trail formation and trail flexibility. A deterministic, ‘mean field’ model of pheromone and foragers on trails mimics our colony simulations. This reduced framework captures the essence of the flexibility-organization balance, and relates optimal pheromone evaporation to the timescale of the dynamic environment. We expect that the principles exposed in our study will generalize and motivate novel analysis across a broad range systems biology.     

Orientation and foraging movements in a patchy environment by the ant Serrastruma lujae (formicidae-myrmicinae)

Serrastruma lujae ants individually search for collembolans in the leaf litter of humid tropical forests. During the dry season, collembolans aggregate in wet patches randomly scattered in the dry litter where numerous single workers come hunting from their nest. We simulated this situation in the laboratory, and observed that workers seem to be able to use the humidity gradient direction to efficiently orient themselves towards a wet patch. Once the patch has been reached, they exhibit area-concentrated searching,consisting, in particular, of adopting a high sinuosity and a low speed. After capturing a collembolan, the ants return to their nest along nearly straight paths. This ability may rely on a spatial memory of the nest location by means of a path-integration process. In the absence of prey, however, various behaviours were observed after an unsuccessful search. Comparisons between these data and the results obtained with a homogeneously wet environment simulating the rainy season situation showed that these ants do not simply respond to the humidity level but are also sensitive to the degree of patchiness of their environment. They can therefore be said to be able to adapt suitably to the considerable climatic changes they encounter during the year.     

Path formation in a robot swarm

We present two swarm intelligence control mechanisms used for distributed robot path formation. In the first, the robots form linear chains. We study three variants of robot chains, which vary in the degree of motion allowed to the chain structure. The second mechanism is called vectorfield. In this case, the robots form a pattern that globally indicates the direction towards a goal or home location. We test each controller on a task that consists in forming a path between two objects which an individual robot cannot perceive simultaneously. Our simulation experiments show promising results. All the controllers are able to form paths in complex obstacle environments and exhibit very good scalability, robustness, and fault tolerance characteristics. Additionally, we observe that chains perform better for small robot group sizes, while vectorfield performs better for large groups.     

Multi-trial Vector-based Whale Optimization Algorithm

The Whale Optimization Algorithm(WOA)is a swarm intelligence metaheuristic inspired by the bubble-net hunting tactic of humpback whales.In spite of its popularity due to simplicity,ease of implementation,and a limited number of param-eters,WOA's search strategy can adversely affect the convergence and equilibrium between exploration and exploitation in complex problems.To address this limitation,we propose a new algorithm called Multi-trial Vector-based Whale Opti-mization Algorithm(MTV-WOA)that incorporates a Balancing Strategy-based Trial-vector Producer(BS_TVP),a Local Strategy-based Trial-vector Producer(LS_TVP),and a Global Strategy-based Trial-vector Producer(GS_TVP)to address real-world optimization problems of varied degrees of difficulty.MTV-WOA has the potential to enhance exploitation and exploration,reduce the probability of being stranded in local optima,and preserve the equilibrium between exploration and exploitation.For the purpose of evaluating the proposed algorithm's performance,it is compared to eight metaheuristic algorithms utilizing CEC 2018 test functions.Moreover,MTV-WOA is compared with well-stablished,recent,and WOA variant algorithms.The experimental results demonstrate that MTV-WOA surpasses comparative algorithms in terms of the accuracy of the solutions and convergence rate.Additionally,we conducted the Friedman test to assess the gained results statistically and observed that MTV-WOA significantly outperforms comparative algorithms.Finally,we solved five engi-neering design problems to demonstrate the practicality of MTV-WOA.The results indicate that the proposed MTV-WOA can efficiently address the complexities of engineering challenges and provide superior solutions that are superior to those of other algorithms.     

Ant foraging on extrafloral nectaries of Qualea grandiflora (Vochysiaceae) in cerrado vegetation: ants as potential antiherbivore agents

Summary Qualea grandiflora is a typical tree of Brazilian cerrados (savanna-like vegetation) that bears paired extrafloral nectaries (EFNs) along its stems. Results show that possession of EFNs increases ant density on Q. grandiflora shrubs over that of neighbouring non-nectariferous plants. Frequency of ant occupancy and mean number of ants per plant were much higher on Qualea than on plants lacking EFNs. These differences resulted in many more live termitebaits being attacked by foraging ants on Qualea than on neighbours without EFNs. Termites were attacked in equal numbers and with equal speeds on different-aged leaves of Qualea. The greatest potential for herbivore deterrence was presented by Camponotus ants (C. crassus, C. rufipes and C. aff. blandus), which together attacked significantly more termites than nine other ant species grouped. EFNs are regarded as important promoters of ant activity on cerado plants.     

Individual complexity and self-organization in foraging by leaf-cutting ants

Leaf-cutting ants cut vegetation into small fragments that they transport to the nest, where a symbiotic fungus cultivated by the ants processes the material. Since the harvested leaf fragments are incorporated into the fungus garden and not directly consumed by the workers, it is expected that foraging workers select plants by responding to those physical or chemical traits that promote maximal fungal growth, irrespective of the potential direct effects of these leaf features on them. In this paper I summarize experimental work focusing on the decision-making processes that occur at the individual level, and discuss to what extent individual complexity contributes to the emergence of collective foraging patterns. Although some basic features of self-organizing systems, such as the existence of regulatory positive and negative feedback loops, are expected to be involved in the collective organization of leaf-cutting ant foraging, I contend that they are combined with complex individual responses that may result from the integration of local information during food collection with an assessment of colony conditions.     

Dispersed central place foraging in Australian meat ants

Summary Australian meat ants often inhabit colonies with widely dispersed nest holes, and this study examines how resource is harvested and distributed in a colony ofIridomyrmex sanguineus Smith (Formicidae: Dolichoderinae). The three principal types of foragers (tenders, honeydew transporters, scavengers) exhibited nest hole fidelity, where harvested resource was consistently delivered to the same nest hole by each foraging individual. Australian meat ants thus use a harvesting system based on dispersed central place foraging. Evidence of frequent larval transport among nest holes, age polyethism developing in the direction of foraging, and the tendency for nest-associated workers to accept new nest holes more readily than foragers, suggests that workers develop fidelity to the particular nest hole in which they eclose. Coupled with larval transport, nest hole fidelity may allow a colony with widely dispersed nest holes to adjust its structure to more efficiently harvest a resource distributed unevenly in space or time.     

Novel interactions of non-pollinating ants with pollinators and fruit consumers in a tropical forest

The tropical ants Ectatomma ruidum and E. tuberculatum (Formicidae) regularly patrol leaves, flowers, and fruits of the understory shrub, Psychotria limonensis (Rubiaceae), on Barro Colorado Island, Panama. Ant and pollinator exclusion experiments elucidated both positive and negative effects of ant attendance on plant reproductive success, including pollination, fruit set, fruit loss, and fruit removal. Ants did not pollinate flowers but did contribute to higher pollination success, probably by increasing the relocation frequency of winged pollinators and thus the rate of flower visitation. Ants also prevented fruit loss to herbivorous insects which were common during the early stages of fruit development. Thus, ant attendance strongly improved both pollination and fruit set whereby plants with ants set more fruit per flower and also lost fewer fruits during fruit maturation. In contrast, ants had a negative effect on the removal of ripe fruits by avian frugivores. Thus, ant attendance has a non-trivial influence on plant reproduction, this interaction being beneficial at some stages of the plant reproductive cycle and carrying costs at another stage. A tight ecological or co-evolved relationship between these Ectatomma spp. and P. limonensis is unlikely given that ant attendance of plants is detrimental to fruit removal. Received: 18 May 1998 / Accepted: 1 March 1999     

The short-term regulation of foraging in harvester ants

In the seed-eating ant Pogonomyrmex barbatus, the return ofsuccessful foragers stimulates inactive foragers to leave thenest. The rate at which successful foragers return to the nestdepends on food availability; the more food available, the morequickly foragers will find it and bring it back. Field experimentsexamined how quickly a colony can adjust to a decline in therate of forager return, and thus to a decline in food availability,by slowing down foraging activity. In response to a brief, 3-to 5-min reduction in the forager return rate, foraging activityusually decreased within 2–3 min and then recovered within5 min. This indicates that whether an inactive forager leavesthe nest on its next trip depends on its very recent experienceof the rate of forager return. On some days, colonies respondedmore to a change in forager return rate. The rapid colony responseto fluctuations in forager return rate, enabling colonies toact as risk-averse foragers, may arise from the limited intervalover which an ant can track its encounters with returning foragers.     

Interaction between flowers, ants and pollinators: additional evidence for floral repellence against ants

It has been commonly suggested that ants negatively affect plant pollination, particularly in the tropics. We studied ant–flower–pollinator interactions in a lowland rainforest in Borneo. Frequency and duration of pollinator visits were compared between flowers attended by ants and flowers from which ants were excluded. In all four plant species studied, the activity of ants decreased the rate and/or duration of the pollinators’ floral visits. For this and other reasons it is expected that plants repel ants from flowers during anthesis. We tested this prediction for a different set of plant species in which we observed the behaviour of Dolichoderus thoracicus ants when encountering flowers. In eight out of 18 plant species studied, ants showed a significantly higher rejection rate when they encountered flowers than when they encountered controls. Our results are thus consistent with the hypothesis that ants may negatively affect plant fitness by reduced intensity of pollinator visits and that ants are repelled from flowers of many tropical plant species, although this repellence is clearly not ubiquitous.     

Secondary pollen carryover by ants in a natural population of Scleranthus perennis (Caryophyllaceae)

Summary Secondary pollen carryover is defined as the process whereby a pollinator receives previously deposited pollen grains when visiting a flower and transfers them into a new (secondary) carryover sequence. The secondary pollen carryover in a system of ants, Formica rufibarbis, visiting Scleranthus perennis (Caryophyllaceae) was studied using fluorescent dyes as pollen analogues. The mean secondary carryover was found to be 1.2 flowers compared with 4.5 flowers for the primary carryover. The number of dye grains deposited per flower visited is lower and the frequency of zero deposition is higher in the secondary carryover sequence than in the primary.     

Optimization, conflict, and nonoverlapping foraging ranges in ants

An organism's foraging range depends on the behavior of neighbors, the dynamics of resources, and the availability of information. We use a well-studied population of the red harvester ant Pogonomyrmex barbatus to develop and independently parameterize models that include these three factors. The models solve for an allocation of foraging ants in the area around the nest in response to other colonies. We compare formulations that optimize at the colony or individual level and those that do or do not include costs of conflict. Model predictions were compared with data collected on ant time budgets and ant density. The strategy that optimizes at the colony level but neglects costs of conflict predicts unrealistic levels of overlap. In contrast, the strategy that optimizes at the individual level predicts realistic foraging ranges with or without inclusion of conflict costs. Both the individual model and the colony model that includes conflict costs show good quantitative agreement with data. Thus, an optimal foraging response to a combination of exploitation and interference competition can largely explain how individual foraging behavior creates the foraging range of a colony. Deviations between model predictions and data indicate that colonies might allocate a larger than optimal number of foragers to areas near boundaries between foraging ranges.     

High surface temperatures select for individual foraging in ants

Natural selection favors signals, receptors, and signaling behaviorthat maximize the received signal relative to background noiseand that minimize signal degradation. The physical propertiesof the environment affect rates of attenuation and degradationof the signal, and thus temperature may influence the evolutionand maintenance of volatile chemical signals. We tested this hypothesis in ants, where nest mate recruitment to a food sourceby laying trail pheromones on a surface is a common phenomenon.We collected data on maximal soil surface temperatures duringthe ants' activity and mode of foraging (recruitment or solitary).By using two different comparative methodologies, we demonstrateda relationship between maximal soil temperature at which speciesare active and recruitment behavior (which is hypothesized to be related to the presence or absence of chemical signals).The species that were active at lower temperatures proved tobe those that used chemical signals to recruit nest mates duringforaging. This is also the case when comparing sympatric speciesand thereby controlling for other environmental factors. Moreover,all seven nonrecruiter species developed from recruiter ancestries,which is consistent with our hypothesis because ample evidence suggests a forest and tropical origin for ants. Thus, contraryto previous hypotheses, species that forage individually cannotbe categorically considered primitive, but rather appear tobe derived from recruiter species. Therefore, we conclude thattemperature influences the evolution and/or stability of chemicalsignals in ants by determining the recruitment of nest mates.     

Self-organisation and communication in groups of simulated and physical robots

In social insects, both self-organisation and communication play a crucial role for the accomplishment of many tasks at a collective level. Communication is performed with different modalities, which can be roughly classified into three classes: indirect (stigmergic) communication, direct interactions and direct communication. The use of stigmergic communication is predominant in social insects (e.g. the pheromone trails in ants), where, however, direct interactions (e.g. antennation in ants) and direct communication (e.g. the waggle dance in honey bees) can also be observed. Taking inspiration from insect societies, we present an experimental study of self-organising behaviours for a group of robots, which exploit communication to coordinate their activities. In particular, the robots are placed in an arena presenting holes and open borders, which they should avoid while moving coordinately. Artificial evolution is responsible for the synthesis in a simulated environment of the robot’s neural controllers, which are subsequently tested on physical robots. We study different communication strategies among the robots: no direct communication, handcrafted signalling and a completely evolved approach. We show that the latter is the most efficient, suggesting that artificial evolution can produce behaviours that are more adaptive than those obtained with conventional design methodologies. Moreover, we show that the evolved controllers produce a self-organising system that is robust enough to be tested on physical robots, notwithstanding the huge gap between simulation and reality.Electronic Supplementary Material Supplementary material is available for this article at     

Experimental study of pollination by ants in Mediterranean high mountain and arid habitats

In this paper, we report the results of an experimental study on ant pollination of three plant species inhabiting the Mediterranean high mountains (Alyssum purpureum, Arenaria tetraquetra and Sedum anglicum) and four species inhabiting the aridlands (Lepidium subulatum, Gypsophyla struthium, Frankenia thymifolia and Retama sphaerocarpa) of South-eastern Spain. We determined several plant and ant traits, as well as the composition and abundance of the pollinator assemblage. Insects belonging to 29 families and five orders visited the flowers of the plant species studied. In all but two, L. subulatum and G. struthium, the ants comprised 70–100% of the flower visitors. The results clearly show that five out of seven of these plant species were pollinated by ants. The role of the ants as pollinators seems to depend heavily on the relative abundance of the ants with respect to the other species of the pollinator assemblage, ant pollination becoming evident when ants outnumber other floral visitors. The ant-pollination systems analysed in this study may be the result of prevailing ecological conditions more than an evolutionary result of a specialized interaction.     

Sucking pump activity in feeding behaviour regulation in carpenter ants

Modulation of liquid feeding-rate would allow insects to ingest more food in the same time when this was required. Ants can vary nectar intake rate by increasing sucking pump frequency according to colony requirements. We analysed electrical signals generated by sucking pump activity of ants during drinking solutions of different sucrose concentrations and under different carbohydrate-deprivation levels. Our aim was to define parameters that characterize the recordings and analyse their relationship with feeding behaviour.Signals showed that the initial and final frequencies of sucking pump activity, as well as the difference between them were higher in sugar-deprived ants. However, these parameters were not influenced by sucrose solution concentration, which affected the number of pump contractions and the volume per contraction. Unexpectedly, we found two different responses in feeding behaviour of starved and non-starved ants depending on concentration. Starved ants drank dilute solutions for the same length of time as non-starved ants but ingested higher volumes. While drinking the concentrated solutions, starved ants drank the same volume, but did so in a shorter time than the non-starved ones. Despite these differences, for each analysed concentration the total number of pump contractions remained constant independently of sugar-deprivation level. These results are discussed in the frame of feeding regulation and decision making in ant foraging behaviour.     

Synergy between social and private information increases foraging efficiency in ants

Insect societies integrate many information sources to organize collective activities such as foraging. Many ants use trail pheromones to guide foragers to food sources, but foragers can also use memories to find familiar locations of stable food sources. Route memories are often more accurate than trail pheromones in guiding ants, and are often followed in preference to trail pheromones when the two conflict. Why then does the system expend effort in producing and acquiring seemingly redundant and low-quality information, such as trail pheromones, when route memory is available? Here we show that, in the ant Lasius niger, trail pheromones and route memory act synergistically during foraging; increasing walking speed and straightness by 25 and 30 per cent, respectively, and maintaining trail pheromone deposition, but only when used together. Our results demonstrate a previously undescribed major role of trail pheromones: to complement memory by allowing higher confidence in route memory. This highlights the importance of multiple interacting information sources in the efficient running of complex adaptive systems.     

Endophytic fungi alter foraging and dispersal by desert seed-harvesting ants

Endophytic fungi are thought to interact mutualistically with host plants by producing alkaloid metabolites that deter herbivory. Since such fungi are transmitted via seed in some grasses, the presence of endophytes may also protect plants from seed predators. We conducted seed choice experiments for two dominant seed harvesting ants, Pogonomyrmex rugosus in the Sonoran desert and Pogonomyrmex occidentalis at a higher elevation, riparian zone in Arizona, USA. Non-infected fescue (Festuca arundinacea) seeds and seeds infected with the endophytic fungus, Acremonium coenophialum, were presented to ant colonies in three different populations. Infected seeds were harvested less frequently than non-infected seed for the two populations of Pogonomyrmex rugosus but not for the population of Pogonomyrmex occidentalis. We also a conducted seed dispersal experiment for one population of Pogonomyrmex rugosus. Of the seeds that were harvested, most of the colonies discarded more infected seeds into refuse piles than expected by chance. Seeds discarded into refuse piles have greater germination success than surrounding areas. The most important interaction of endophytes and grasses may be deterrence of seed predation and enhancing the probability of germinating in favorable sites, since these processes directly increase plant fitness.