The influence of the few: a stable 'oligarchy' controls information flow in house-hunting ants

Is collective nest site selection in ants influenced by the anchoring effect?

Evolutionary theory predicts that animals make decisions that maximize fitness. If so, they are expected to adhere to principles of rational choice, which a decision-maker must follow to reliably maximize net benefit. For example, evaluation of an option should not be influenced by the quality of other unchosen options. However, humans and other animals are known to evaluate a mediocre option more favorably after encountering poor options than after encountering no options, a phenomenon known as the 'anchoring effect'. Rationality is also expected in the consensus decisions of animal societies, but the anchoring effect has not previously been tested in that context. Here we show that colonies of the rock ant, Temnothorax rugatulus, demonstrate the anchoring effect during nest site selection - colonies moved more readily from a mediocre nest to a good nest when exposed to poor nests than when exposed to mediocre nests. This effect depended on both current conditions and past experience; movement probability was affected only when colonies were exposed to surrounding nests before and during the emigration. The effect was small, reaching statistical significance in only one of two experimental replicates. We discuss possible mechanisms and ultimate explanations for why colonies show this seemingly suboptimal behavior.

Two simple movement mechanisms for spatial division of labour in social insects

Many animal species divide space into a patchwork of home ranges, yet there is little consensus on the mechanisms individuals use to maintain fidelity to particular locations. Theory suggests that animal movement could be based upon simple behavioural rules that use local information such as olfactory deposits, or global strategies, such as long-range biases toward landmarks. However, empirical studies have rarely attempted to distinguish between these mechanisms. Here, we perform individual tracking experiments on four species of social insects, and find that colonies consist of different groups of workers that inhabit separate but partially-overlapping spatial zones. Our trajectory analysis and simulations suggest that worker movement is consistent with two local mechanisms: one in which workers increase movement diffusivity outside their primary zone, and another in which workers modulate turning behaviour when approaching zone boundaries. Parallels with other organisms suggest that local mechanisms might represent a universal method for spatial partitioning in animal populations.

An investigation of the role of leadership in consensus decision-making

Leadership is a widespread phenomena in social organisms and it is recognised to facilitate coordination between individuals. If the role of leadership in group foraging or swarm movement is well understood, it is not clear if leaders would also benefit more complex forms of coordination. In particular, a number of organisms coordinate by consensus decision-making, where individuals explicitly communicate their opinions until they converge toward a common decision. Taking inspiration from physical sciences, we extend a consensus formation model to integrate leaders, which we define by three traits: persuasiveness, talkativeness, and stubbornness. We use numerical simulations to investigate the effect of the number of leaders and their characteristics on the time a group spends to reach consensus, and the bias in final decision. We show that having a minority of influential individuals (leaders) and a majority of influenceable individuals (followers) reduces the time to reach consensus but biases the decision towards the preferences of the leaders. This effect emerges solely from the differences in individuals' personality traits, with the most determinant trait being the talkativeness of the individuals. Overall, we provide a comprehensive investigation of the effects of leaders and their traits on consensus decision-making.

Ants resort to majority concession to reach democratic consensus in the presence of a persistent minority

Social groups often need to overcome differences in individual interests and knowledge to reach consensus decisions. Here, we combine experiments and modeling to study conflict resolution in emigrating ant colonies during binary nest selection. We find that cohesive emigration, without fragmentation, is achieved only by intermediate-sized colonies. We then impose a conflict regarding the desired emigration target between colony subgroups. This is achieved using an automated selective gate system that manipulates the information accessible to each ant. Under this conflict, we find that individuals concede their potential benefit to promote social consensus. In particular, colonies resolve the conflict imposed by a persistent minority through "majority concession," wherein a majority of ants that hold first-hand knowledge regarding the superior quality nest choose to reside in the inferior one. This outcome is unlikely in social groups of selfish individuals and emphasizes the importance of group cohesion in eusocial societies.

No coordination required for resources allocation during colony fission in a social insect? An individual-based model reproduces empirical patterns

Social insects are classic examples of cooperation and coordination. For instance, laboratory studies of colony relocation, or house-hunting, have investigated how workers coordinate their efforts to swiftly move the colony to the best nesting site available while preserving colony integrity, i.e. avoiding a split. However, several studies have shown that, in some other contexts, individuals may use private rather than social information and may act solitarily rather than in a coordinated way. Here, we study resource allocation by a mature ant colony when it reproduces by fissioning into several colonies. This is a very different task than house hunting in that colony fission seeks the split of the colony. We develop a simple individual-based model to test if colony fission and resource allocation may be carried out by workers acting solitarily with no coordination. Our model reproduces well the pattern of allocation observed in nature (number and size of new colonies). This does not show that workers do not communicate nor coordinate. Rather, it suggests that independent decision making may be an important component of the process of resource allocation.

Leadership - not followership - determines performance in ant teams

Economic theory predicts that organisations achieve higher levels of productivity when tasks are divided among different subsets of workers. This prediction is based upon the expectation that individuals should perform best when they specialise upon a few tasks. However, in colonies of social insects evidence for a causal link between division of labour and performance is equivocal. To address this issue, we performed a targeted worker removal experiment to disrupt the normal allocation of workers to a cooperative team task - tandem running. During a tandem run a knowledgeable leader communicates the location of a new nest to a follower by physically guiding her there. The targeted removal of prominent leaders significantly reduced tandem performance, whereas removal of prominent followers had no effect. Furthermore, analyses of the experience of both participants in each tandem run revealed that tandem performance was influenced primarily by how consistently the leader acted as a leader when the need arose, but not by the consistency of the follower. Our study shows that performance in ant teams depends largely on whether or not a key role is filled by an experienced individual, and suggests that in animal teams, not all roles are equally important.

Adaptive Foraging in Dynamic Environments Using Scale-Free Interaction Networks

Group interactions are widely observed in nature to optimize a set of critical collective behaviors, most notably sensing and decision making in uncertain environments. Nevertheless, these interactions are commonly modeled using local (proximity) networks, in which individuals interact within a certain spatial range. Recently, other interaction topologies have been revealed to support the emergence of higher levels of scalability and rapid information exchange. One prominent example is scale-free networks. In this study, we aim to examine the impact of scale-free communication when implemented for a swarm foraging task in dynamic environments. We model dynamic (uncertain) environments in terms of changes in food density and analyze the collective response of a simulated swarm with communication topology given by either proximity or scale-free networks. Our results suggest that scale-free networks accelerate the process of building up a rapid collective response to cope with the environment changes. However, this comes at the cost of lower coherence of the collective decision. Moreover, our findings suggest that the use of scale-free networks can improve swarm performance due to two side-effects introduced by using long-range interactions and frequent network regeneration. The former is a topological consequence, while the latter is a necessity due to robot motion. These two effects lead to reduced spatial correlations of a robot's behavior with its neighborhood and to an enhanced opinion mixing, i.e., more diversified information sampling. These insights were obtained by comparing the swarm performance in presence of scale-free networks to scenarios with alternative network topologies, and proximity networks with and without packet loss.

Division of labour promotes the spread of information in colony emigrations by the ant Temnothorax rugatulus

The fitness of group-living animals often depends on how well members share information needed for collective decision-making. Theoretical studies have shown that collective choices can emerge in a homogeneous group of individuals following identical rules, but real animals show much evidence for heterogeneity in the degree and nature of their contribution to group decisions. In social insects, for example, the transmission and processing of information is influenced by a well-organized division of labour. Studies that accurately quantify how this behavioural heterogeneity affects the spread of information among group members are still lacking. In this paper, we look at nest choices during colony emigrations of the ant Temnothorax rugatulus and quantify the degree of behavioural heterogeneity of workers. Using clustering methods and network analysis, we identify and characterize four behavioural castes of workers-primary, secondary, passive and wandering-covering distinct roles in the spread of information during an emigration. This detailed characterization of the contribution of each worker can improve models of collective decision-making in this species and promises a deeper understanding of behavioural variation at the colony level.

Nest Site Selection during Colony Relocation in Yucatan Peninsula Populations of the Ponerine Ants Neoponera villosa (Hymenoptera: Formicidae)

In the Yucatan Peninsula, the ponerine ant Neoponera villosa nests almost exclusively in tank bromeliads, Aechmea bracteata. In this study, we aimed to determine the factors influencing nest site selection during nest relocation which is regularly promoted by hurricanes in this area. Using ants with and without previous experience of Ae. bracteata, we tested their preference for refuges consisting of Ae. bracteata leaves over two other bromeliads, Ae. bromeliifolia and Ananas comosus. We further evaluated bromeliad-associated traits that could influence nest site selection (form and size). Workers with and without previous contact with Ae. bracteata significantly preferred this species over others, suggesting the existence of an innate attraction to this bromeliad. However, preference was not influenced by previous contact with Ae. bracteata. Workers easily discriminated between shelters of Ae. bracteata and A. comosus, but not those of the closely related Ae. bromeliifolia. In marked contrast, ants discriminated between similar sized Ae. bracteata and Ae. bromeliifolia plants, suggesting that chemical cues and plant structure play an important role. Size was also significant as they selected the largest plant when provided two dissimilar Ae. bracteata plants. Nest site selection by N. villosa workers seems to depend on innate preferences but familiarization with plant stimuli is not excluded.

Ant colonies maintain social homeostasis in the face of decreased density

Interactions lie at the heart of social organization, particularly in ant societies. Interaction rates are presumed to increase with density, but there is little empirical evidence for this. We manipulated density within carpenter ant colonies of the species Camponotus pennsylvanicus by quadrupling nest space and by manually tracking 6.9 million ant locations and over 3200 interactions to study the relationship between density, spatial organization and interaction rates. Colonies divided into distinct spatial regions on the basis of their underlying spatial organization and changed their movement patterns accordingly. Despite a reduction in both overall and local density, we did not find the expected concomitant reduction in interaction rates across all colonies. Instead, we found divergent effects across colonies. Our results highlight the remarkable organizational resilience of ant colonies to changes in density, which allows them to sustain two key basic colony life functions, that is food and information exchange, during environmental change.

Tandem-running and scouting behaviour are characterized by up-regulation of learning and memory formation genes within the ant brain

Tandem-running is a recruitment behaviour in ants that has been described as a form of teaching, where spatial information possessed by a leader is conveyed to following nestmates. Within Temnothorax ants, tandem-running is used within a variety of contexts, from foraging and nest relocation to-in the case of slavemaking species-slave raiding. Here, we elucidate the transcriptomic basis of scouting, tandem-leading and tandem-following behaviours across two species with divergent lifestyles: the slavemaking Temnothorax americanus and its primary, nonparasitic host T. longispinosus. Analysis of gene expression data from brains revealed that only a small number of unique differentially expressed genes are responsible for scouting and tandem-running. Comparison of orthologous genes between T. americanus and T. longispinosus suggests that tandem-running is characterized by species-specific patterns of gene usage. However, within both species, tandem-leaders showed gene expression patterns median to those of scouts and tandem-followers, which was expected, as leaders can be recruited from either of the other two behavioural states. Most importantly, a number of differentially expressed behavioural genes were found, with functions relating to learning and memory formation in other social and nonsocial insects. This includes a number of up-regulated receptor genes such as a glutamate and dopamine receptor, as well as serine/threonine-protein phosphatases and kinases. Learning and memory genes were specifically up-regulated within scouts and tandem-followers, not only reinforcing previous behavioural studies into how Temnothorax navigate novel environments and share information, but also providing insight into the molecular underpinnings of teaching and learning within social insects.

The Neglected Pieces of Designing Collective Decision-Making Processes

Autonomous decision-making is a fundamental requirement for the intelligent behavior of individual agents and systems. For artificial systems, one of the key design prerequisites is providing the system with the ability to make proper decisions. Current literature on collective artificial systems designs decision-making mechanisms inspired mostly by the successful natural systems. Nevertheless, most of the approaches focus on voting mechanisms and miss other fundamental aspects. In this paper, we aim to draw attention to the missed pieces for the design of efficient collective decision-making, mainly information processes in its two types of stimuli and options set.

Symmetry breaking and pivotal individuals during the reunification of ant colonies

Maintenance of a social group requires the ability to reach consensus when faced with divisive choices. Thus, when migrating colonies of the ant Temnothorax rugatulus split among multiple sites, they can later reunify on the basis of queen location or differences in site quality. In this study, we found that colonies can reunify even without obvious cues to break the symmetry between sites. To learn how they do so, we observed both symmetric reunifications (between identical nests) and asymmetric reunifications (between nests of unequal quality) by colonies of individually marked ants. Both reunification types were accomplished by a tiny minority that carried nestmates from the 'losing' to the 'winning' site. Reunification effort was highly skewed in asymmetric splits, where the majority of the work was done by the first ant to transport, which nearly always came from the winning site. This contrasted with symmetric splits, where the initiator did not play an outsize role and was just as likely to come from the losing site. Symmetric reunifications were also characterized by high transporter attrition, which may help to prevent deadlocks. Tandem runs were abundant in both types and were typically led by transporters as they returned to the losing site to fetch another nestmate. Few tandem followers joined the transport effort, suggesting that tandem runs do not serve to recruit transporters but may have another, as yet unidentified role. Our results underscore the potentially large contribution of highly active individuals to group behaviour, even in decentralized societies such as ant colonies.

What's going on at the entrance? A characterisation of the social interface in ant nests

Nest entrances are key locations where information about environmental opportunities and constraints are shared between foragers and inner-nest workers. However, despite its functional value, we still lack a detailed characterisation of the interface between the nest and the environment. Here, we identified the social interface in the ant Myrmica rubra as being the population of ants that faced the nest entrance and that received significantly more contacts from returning foragers than other nearby ants. We also spatially delineated the entrance area that hosted the social interface, a 2-centimetre radius area from the nest openings, which influences the position, orientation, and behaviour of ants. Then, we studied the impact of additional entrances on this social interface as well as on the flow of foragers. The size of the social interface increased according to the number of open entrances through the progressive reorientation of the ants toward new openings. We also observed a significant, although less than proportional, increase in the flows of ants that were progressively distributed homogeneously between all open entrances. Thus, our work highlights the flexibility of both the social interface and the flow of foragers to changes in the numbers of passageways between the nest and the environment.