Colony size affects collective decision-making in the ant Temnothorax albipennis

Social insects are well-known for their ability to achieve robust collective behaviours even when individuals have limited information. It is often assumed that such behaviours rely on very large group sizes, but many insect colonies start out with only a few workers. Here we investigate the influence of colony size on collective decision-making in the house-hunting of the ant Temnothorax albipennis. In experiments where colony size was manipulated by splitting colonies, we show that worker number has an influence on the speed with which colonies discover new nest sites, but not on the time needed to make a decision (achieve a quorum threshold) or total emigration time. This occurred because split colonies adopted a lower quorum threshold, in fact they adopted the same threshold in proportion to their size as full-size colonies. This indicates that ants may be measuring relative quorum, i.e. population in the new nest relative to that of the old nest, rather than the absolute number. Experimentally reduced colonies also seemed to gain more from experience through repeated emigrations, as they could then reduce nest discovery times to those of larger colonies. In colonies of different sizes collected from the field, total emigration time was also not correlated with colony size. However, quorum threshold was not correlated with colony size, meaning that individuals in larger colonies adopted relatively lower quorum thresholds. Since this is a different result to that from size-manipulated colonies, it strongly suggests that the differences between natural small and large colonies were not caused by worker number alone. Individual ants may have adjusted their behaviour to their colony’s size, or other factors may correlate with colony size in the field. Our study thus shows the importance of experimentally manipulating colony size if the effect of worker number on the emergence of collective behaviour is to be studied. Received 13 December 2005; revised 9 May 2006; accepted 15 May 2006.     

Composite collective decision-making

Individual animals are adept at making decisions and have cognitive abilities, such as memory, which allow them to hone their decisions. Social animals can also share information. This allows social animals to make adaptive group-level decisions. Both individual and collective decision-making systems also have drawbacks and limitations, and while both are well studied, the interaction between them is still poorly understood. Here, we study how individual and collective decision-making interact during ant foraging. We first gathered empirical data on memory-based foraging persistence in the ant Lasius niger. We used these data to create an agent-based model where ants may use social information (trail pheromones), private information (memories) or both to make foraging decisions. The combined use of social and private information by individuals results in greater efficiency at the group level than when either information source was used alone. The modelled ants couple consensus decision-making, allowing them to quickly exploit high-quality food sources, and combined decision-making, allowing different individuals to specialize in exploiting different resource patches. Such a composite collective decision-making system reaps the benefits of both its constituent parts. Exploiting such insights into composite collective decision-making may lead to improved decision-making algorithms.     

Group personality during collective decision-making: a multi-level approach

Collective decision-making processes emerge from social feedback networks within a group. Many studies on collective behaviour underestimate the role of individual personality and, as a result, personality is rarely analysed in the context of collective dynamics. Here, we show evidence of sheltering behaviour personality in a gregarious insect (Periplaneta americana), which is characterized by a collective personality at the group level. We also highlight that the individuals within groups exhibited consistent personality traits in their probability of sheltering and total time sheltered during the three trials over one week. Moreover, the group personality, which arises from the synergy between the distribution of behaviour profiles in the group and social amplifications, affected the sheltering dynamics. However, owing to its robustness, personality did not affect the group probability of reaching a consensus. Finally, to prove social interactions, we developed a new statistical method that will be helpful for future research on personality traits and group behaviour. This approach will help to identify the circumstances under which particular group compositions may improve the fitness of individuals in gregarious species.     

Economic investment by ant colonies in searches for better homes

Organisms should invest more in gathering information when the pay-off from finding a profitable resource is likely to be greater. Here, we ask whether animal societies put more effort in scouting for a new nest when their current one is of low quality. We measured the scouting behaviour of Temnothorax albipennis ant colonies when they inhabit nest-sites with different combinations of desirable attributes. We show that the average probability of an ant scouting decreases significantly with an increase in the quality of the nest in which the colony currently resides. This means that the greater the potential gain from finding a new nest, the more effort a colony puts into gathering information regarding new nest-sites. Our results show, for the first time to our knowledge, the ability of animal societies to respond collectively to the quality of a resource they currently have at their disposal (e.g. current nest-site) and regulate appropriately their information gathering efforts for finding an alternative (e.g. a potentially better nest-site).     

Speed versus accuracy in decision-making ants: expediting politics and policy implementation

Compromises between speed and accuracy are seemingly inevitable in decision-making when accuracy depends on time-consuming information gathering. In collective decision-making, such compromises are especially likely because information is shared to determine corporate policy. This political process will also take time. Speed–accuracy trade-offs occur among house-hunting rock ants, Temnothorax albipennis. A key aspect of their decision-making is quorum sensing in a potential new nest. Finding a sufficient number of nest-mates, i.e. a quorum threshold (QT), in a potential nest site indicates that many ants find it suitable. Quorum sensing collates information. However, the QT is also used as a switch, from recruitment of nest-mates to their new home by slow tandem running, to recruitment by carrying, which is three times faster. Although tandem running is slow, it effectively enables one successful ant to lead and teach another the route between the nests. Tandem running creates positive feedback; more and more ants are shown the way, as tandem followers become, in turn, tandem leaders. The resulting corps of trained ants can then quickly carry their nest-mates; but carried ants do not learn the route. Therefore, the QT seems to set both the amount of information gathered and the speed of the emigration. Low QTs might cause more errors and a slower emigration—the worst possible outcome. This possible paradox of quick decisions leading to slow implementation might be resolved if the ants could deploy another positive-feedback recruitment process when they have used a low QT. Reverse tandem runs occur after carrying has begun and lead ants back from the new nest to the old one. Here we show experimentally that reverse tandem runs can bring lost scouts into an active role in emigrations and can help to maintain high-speed emigrations. Thus, in rock ants, although quick decision-making and rapid implementation of choices are initially in opposition, a third recruitment method can restore rapid implementation after a snap decision. This work reveals a principle of widespread importance: the dynamics of collective decision-making (i.e. the politics) and the dynamics of policy implementation are sometimes intertwined, and only by analysing the mechanisms of both can we understand certain forms of adaptive organization.     

Noise improves collective decision-making by ants in dynamic environments

Recruitment via pheromone trails by ants is arguably one of the best-studied examples of self-organization in animal societies. Yet it is still unclear if and how trail recruitment allows a colony to adapt to changes in its foraging environment. We study foraging decisions by colonies of the ant Pheidole megacephala under dynamic conditions. Our experiments show that P. megacephala, unlike many other mass recruiting species, can make a collective decision for the better of two food sources even when the environment changes dynamically. We developed a stochastic differential equation model that explains our data qualitatively and quantitatively. Analysing this model reveals that both deterministic and stochastic effects (noise) work together to allow colonies to efficiently track changes in the environment. Our study thus suggests that a certain level of noise is not a disturbance in self-organized decision-making but rather serves an important functional role.     

Individual discrimination capability and collective decision-making

Amplification is the main component of many collective phenomena in social and gregarious insects. In a society, individuals face a mixed palette of odours coming from different groups (lines, strains) and individuals present discrimination capabilities. However, often at the collective level, different groups may cooperate and act together. To understand this apparent contradiction, we use a model of food recruitment where each group of foragers have its own blend of pheromone trail that is partly recognized by the others groups. The model shows that a low level of recognition between signals is sufficient to produce a collaborative pattern between groups and that beyond a critical value of recognition, only the aggregation of all the groups around the same food source is observed. The comparison between this model and one describing the site selection by gregarious insects (e.g. cockroach) suggests that such collective response is a generic property of social phenomena governed by amplification processes.     

Social prophylaxis: group interaction promotes collective immunity in ant colonies

Life in a social group increases the risk of disease transmission. To counteract this threat, social insects have evolved manifold antiparasite defenses, ranging from social exclusion of infected group members to intensive care. It is generally assumed that individuals performing hygienic behaviors risk infecting themselves, suggesting a high direct cost of helping. Our work instead indicates the opposite for garden ants. Social contact with individual workers, which were experimentally exposed to a fungal parasite, provided a clear survival benefit to nontreated, naive group members upon later challenge with the same parasite. This first demonstration of contact immunity in Social Hymenoptera and complementary results from other animal groups and plants suggest its general importance in both antiparasite and antiherbivore defense. In addition to this physiological prophylaxis of adult ants, infection of the brood was prevented in our experiment by behavioral changes of treated and naive workers. Parasite-treated ants stayed away from the brood chamber, whereas their naive nestmates increased brood-care activities. Our findings reveal a direct benefit for individuals to perform hygienic behaviors toward others, and this might explain the widely observed maintenance of social cohesion under parasite attack in insect societies.     

Distributed leadership and adaptive decision-making in the ant Tetramorium caespitum

In the ant species Tetramorium caespitum, communication and foraging patterns rely on group-mass recruitment. Scouts having discovered food recruit nestmates and behave as leaders by guiding groups of recruits to the food location. After a while, a mass recruitment takes place in which foragers follow a chemical trail. Since group recruitment is crucial to the whole foraging process, we investigated whether food characteristics induce a tuning of recruiting stimuli by leaders that act upon the dynamics and size of recruited groups. High sucrose concentration triggers the exit of a higher number of groups that contain twice as many ants and reach the food source twice as fast than towards a weakly concentrated one. Similar trends were found depending on food accessibility: for a cut mealworm, accessibility to haemolymph results in a faster formation of larger groups than for an entire mealworm. These data provide the background for developing a stochastic model accounting for exploitation patterns by group-mass recruiting species. This model demonstrates how the modulations performed by leaders drive the colony to select the most profitable food source among several ones. Our results highlight how a minority of individuals can influence collective decisions in societies based on a distributed leadership.     

Behavioral mechanisms of collective nest-site choice by the ant Temnothorax curvispinosus

This paper examines the individual behavior underlying collective choice among nest sites by the ant Temnothorax (formerly Leptothorax) curvispinosus. Colonies can actively compare options, rejecting a mediocre site when it is paired with a good one, but accepting the same mediocre design if it is instead paired with a worse site. This ability emerges from the behavior of an active minority of workers who organize emigrations. When one of these finds a promising site, she recruits nest mates to it, but only after a delay that varies inversely with site quality. Ants first recruit fellow active ants via slow tandem runs, but eventually switch to speedier transports of the colony’s passive majority. Later transports grow faster still, as ants improve their speed with experience. An ant’s choice of recruitment type is governed by a quorum rule, such that her likelihood of starting to transport increases with the population of the new site. The size of the quorum depends on experience, with ants demanding a larger population to launch immediately into transport than they do to switch to transport after first leading a few tandem runs. Perception of quorum attainment requires direct contact between ants. The ants’ behavior qualitatively matches that of T. albipennis, where models have shown that decentralized choice of the best site depends on quality-dependent recruitment delays, amplified by a quorum rule for initiating transport. Parameter estimates for an agent-based model show significant quantitative differences between the species, and suggest that T. albipennis may place relatively greater emphasis on emigration speed. Received 11 February 2005; revised 10 May 2005; accepted 20 May 2005.     

Leaders benefit followers in the collective movement of a social sawfly

The challenges of maintaining cohesion while making collective decisions in social or aggregating insects can result in the emergence of a leader or leaders. Larval aggregations of the steel-blue sawfly Perga affinis forage nocturnally, and some larvae lead the aggregation on foraging trips more often than expected by chance. We investigated the relationship between these leader and follower roles by comparing the weight and growth of individual larvae with different roles. Our observations reveal no significant difference between the growth of leaders and followers, suggesting that the role of leadership may not provide direct foraging benefits. However, by experimentally manipulating the social structure of larval aggregations, we found that individuals within aggregations that comprise a mixture of leaders and followers enjoy higher growth rates than those in aggregations comprising a single behavioural type. These data demonstrate, for the first time, individual benefits to maintaining a balance of leader and follower roles within larval aggregations, and highlight the importance of considering the perspectives of both leaders and followers when investigating the evolutionary significance of this behavioural variation within animal groups.     

Follow the straggler: zebrafish use a simple heuristic for collective decision-making

Animal groups often make decisions sequentially, from the front to the back of the group. In such cases, individuals can use the choices made by earlier ranks, a form of social information, to inform their own choice. The optimal strategy for such decisions has been explored in models which differ on, for example, whether or not agents take into account the sequence of observed choices. The models demonstrate that choices made later in a sequence are more informative, but it is not clear if animals use this information or rely instead on simpler heuristics, such as quorum rules. We show that a simple rule ‘copy the last observed choice'', gives similar predictions to those of optimal models for most likely sequences. We trained groups of zebrafish to choose one arm of a Y-maze and used them to demonstrate various sequences to naive fish. We show that the naive fish appear to use a simple rule, most often copying the choice of the last demonstrator, which results in near-optimal choices at a fraction of the computational cost.     

How chimpanzees solve collective action problems

We presented small groups of chimpanzees with two collective action situations, in which action was necessary for reward but there was a disincentive for individuals to act owing to the possibility of free-riding on the efforts of others. We found that in simpler scenarios (experiment 1) in which group size was small, there was a positive relationship between rank and action with more dominant individuals volunteering to act more often, particularly when the reward was less dispersed. Social tolerance also seemed to mediate action whereby higher tolerance levels within a group resulted in individuals of lower ranks sometimes acting and appropriating more of the reward. In more complex scenarios, when group size was larger and cooperation was necessary (experiment 2), overcoming the problem was more challenging. There was highly significant variability in the action rates of different individuals as well as between dyads, suggesting success was more greatly influenced by the individual personalities and personal relationships present in the group.     

How collective comparisons emerge without individual comparisons of the options

Collective decisions in animal groups emerge from the actions of individuals who are unlikely to have global information. Comparative assessment of options can be valuable in decision-making. Ant colonies are excellent collective decision-makers, for example when selecting a new nest-site. Here, we test the dependency of this cooperative process on comparisons conducted by individual ants. We presented ant colonies with a choice between new nests: one good and one poor. Using individually radio-tagged ants and an automated system of doors, we manipulated individual-level access to information: ants visiting the good nest were barred from visiting the poor one and vice versa. Thus, no ant could individually compare the available options. Despite this, colonies still emigrated quickly and accurately when comparisons were prevented. Individual-level rules facilitated this behavioural robustness: ants allowed to experience only the poor nest subsequently searched more. Intriguingly, some ants appeared particularly discriminating across emigrations under both treatments, suggesting they had stable, high nest acceptance thresholds. Overall, our results show how a colony of ants, as a cognitive entity, can compare two options that are not both accessible by any individual ant. Our findings illustrate a collective decision process that is robust to differences in individual access to information.     

Queen-queen competition by precocious male production in multiqueen ant colonies

Arriving earlier in the breeding area than his rivals may be beneficial for a male when females mate only once or during a short time span. The timing of a male's entrance is usually determined by the male himself, e.g., through returning early from his winter quarters or through accelerated larval development . Here, we document a surprisingly simple way of "first come, first served" in a species with local mate competition. In multiqueen colonies of a Cardiocondyla ant, mother queens make sure that their own sons are the first to monopolize mating with a large harem of female sexuals by producing extremely long-lived males early in colony life. Whereas queens in newly founded single-queen colonies started to produce male and female sexuals only several weeks after the eclosion of their first worker offspring, queens in multiqueen colonies precociously reared sons long before the first female sexuals and even before the emergence of their first workers. These early males killed all later emerging males in the nest and mated with all female sexuals subsequently produced. Our data document that the patterns of growth and productivity of insect colonies are surprisingly flexible and can be turned upside down under appropriate selection pressures.     

Nest raiding by the invasive Argentine ant on colonies of the harvester ant, Pogonomyrmex subnitidus

Invasive ants often displace native ants, and published studies that focus on these interactions usually emphasize interspecific competition for food resources as a key mechanism responsible for the demise of native ants. Although less well documented, nest raiding by invasive ants may also contribute to the extirpation of native ants. In coastal southern California, for example, invasive Argentine ants (Linepithema humile) commonly raid colonies of the harvester ant, Pogonomyrmex subnitidus. On a seasonal basis the frequency and intensity of raids vary, but raids occur only when abiotic conditions are suitable for both species. In the short term these organized attacks cause harvester ants to cease foraging and to plug their nest entrances. In unstaged, one-on-one interactions between P. subnitidus and L. humile workers, Argentine ants behaved aggressively in over two thirds of all pair-wise interactions, despite the much larger size of P. subnitidus. The short-term introduction of experimental Argentine ant colonies outside of P. subnitidus nest entrances stimulated behaviors similar to those observed in raids: P. subnitidus decreased its foraging activity and increased the number of nest entrance workers (many of which labored to plug their nest entrances). Raids are not likely to be the result of competition for food. As expected, P. subnitidus foraged primarily on plant material (85% of food items obtained from returning foragers), but also collected some dead insects (7% of food items). In buffet-style choice tests in which we offered Argentine ants food items obtained from P. subnitidus, L. humile only showed interest in dead insects. In other feeding trials L. humile consistently moved harvester ant brood into their nests (where they were presumably consumed) but showed little interest in freshly dead workers. The raiding behavior described here obscures the distinction between interspecific competition and predation, and may well play an important role in the displacement of native ants, especially those that are ecologically dissimilar to L. humile with respect to diet. Received 15 July 2005; revised 19 October 2005; accepted 26 October 2005.