Information gathering prior to emigration in house-hunting ants

Dans les sociétés animales, les décisions collectives résultent souvent de processus auto-organisés : des choix collectifs complexes émergent à partir d'interactions locales entre individus suivant des règles comportementales simples. Complexité cognitive et diversité des individus ne sont en général pas considérées nécessaires à l'ajustement fin des choix collectifs. La sélection d'un nouveau nid chez les abeilles et les fourmis du genre Temnothorax constitue un exemple classique de décision collective. Lors d'une émigration (déplacement de la colonie vers un nouveau site), les colonies de fourmis sont capables de choisir collectivement le meilleur site disponible par le biais de processus décentralisés. Ma thèse porte sur le rôle de la mémoire et de l'expérience des individus sur la sélection de nid par des colonies de fourmis Temnothorax albipennis. La première partie de ma thèse décrit l'impact sur la performance collective des colonies d'une familiarisation préalable avec certains sites. La deuxième partie décrit les mécanismes permettant l'exploitation collective des informations récoltées par les ouvrières avant l'émigration. D'après les données expérimentales, la collecte préalable d'informations sur des sites de bonne qualité permet aux colonies d'améliorer leurs performances collectives lors d'émigrations ultérieures (vitesse d'émigration, cohésion, et/ou précision du choix). De plus, les fourmis ajustent collectivement leurs critères de préférence en fonction de la qualité de leur propre nid et de celle des sites disponibles dans les environs. Cela permet aux colonies de prendre des décisions adaptées aux conditions environnementales. Une analyse détaillée révèle, en outre, que les ouvrières qui ont visité un site de bonne qualité mémorisent la position et la qualité de ce site et réutilisent ultérieurement les informations mémorisées, ce qui leur confère un rôle particulièrement important. Un transfert d'informations a également lieu au sein des sites familiers au cours de l'émigration, mettant en jeu phéromones et interactions sociales entre ouvrières. Dans l'ensemble, cette étude montre que les fourmis T. albipennis sont capables de réaliser des tâches cognitives complexes, puisqu'elles peuvent mémoriser certaines informations et les réutiliser au moment opportun. Enfin, il semble que certains individus ont une influence particulièrement importante sur les décisions du groupe. Ceci montre que les décisions collectives auto-organisées peuvent grandement bénéficier à la fois de la complexité cognitive des individus et d'un certain degré de diversité au sein des membres d'un même groupe.In animal societies, collective decisions are often self-organised: complex collective choices simply emerge from local interactions between group members following simple and relatively fixed behavioural rules. House-hunting by honeybees and ants of the genus Temnothorax is a classical example of collective decision-making. During an emigration (i.e. the relocation of a colony to a new nest site), Temnothorax colonies are able select the best available nest site through decentralised, self-organised processes. My PhD has the aim of investigating the role of individual memories and previous experience in nest site selection by the rock ant Temnothorax albipennis. In the first part, I present data on the influence of prior familiarisation with available nest sites on collective performance in emigrations. In the second part, I investigate the mechanisms underlying the collective exploitation of information previously gathered by individual workers. Experimental results show that familiarisation with high-quality nest sites leads to increased speed, higher cohesion and/or improved choice accuracy in later emigrations. Additionally, ants collectively adjust their preference and choice criteria according to the respective qualities of their home nest and of surrounding available nest sites. This allows colonies to tune collective decisions according to their environment. A detailed analysis of the underlying mechanisms reveals that informed individuals memorise the position and suitability of high-quality, available sites, and later retrieve and use that memorised information. Well informed individuals therefore play a key role in emigrations to good, familiar nest sites. Additionally, information transfer between individuals takes place inside familiar nest sites during emigrations: chemical cues (pheromones) and social interactions allow naïve individuals also to exploit the information previously gathered by their nestmates. The present study therefore indicates that T. albipennis ants have high cognitive abilities, as they are able to memorise information about available nest sites and retrieve that memory when required. Finally, it appears that some individuals have a disproportionate influence on collective choices. This suggests that self-organised collective decisions may actually greatly benefit from both individual cognitive complexity and inter-individual variability

Information Certainty Determines Social and Private Information Use in Ants

Decision-making in uncertain environments requires animals to evaluate, contrast and integrate various information sources to choose appropriate actions. In consensus-making groups, quorum responses are commonly used to combine private and social information, leading to both robust and flexible decisions. Here we show that in house-hunting ant colonies, individuals fine-tune the parameters of their quorum responses depending on their private knowledge: informed ants evaluating a familiar new nest rely relatively more on social than private information when the certainty of their private information is low, and vice versa. This indicates that the ants follow a highly sophisticated `copywhen-uncertain' social learning strategy similar to that observed in a few vertebrate species. Using simulations, we further show that this strategy improves colony performance during emigrations and confers well-informed individuals more weight in the decision process, thus representing a novel mechanism for the emergence of leadership in collective decision-making

Polydomy enhances foraging performance in ant colonies.

Collective foraging confers benefits in terms of reduced predation risk and access to social information, but it heightens local competition when resources are limited. In social insects, resource limitation has been suggested as a possible cause for the typical decrease in per capita productivity observed with increasing colony size, a phenomenon known as Michener's paradox. Polydomy (distribution of a colony's brood and workers across multiple nests) is believed to help circumvent this paradox through its positive effect on foraging efficiency, but there is still little supporting evidence for this hypothesis. Here, we show experimentally that polydomy enhances the foraging performance of food-deprived Temnothorax nylanderi ant colonies via several mechanisms. First, polydomy influences task allocation within colonies, resulting in faster retrieval of protein resources. Second, communication between sister nests reduces search times for far away resources. Third, colonies move queens, brood and workers across available nest sites in response to spatial heterogeneities in protein and carbohydrate resources. This suggests that polydomy represents a flexible mechanism for space occupancy, helping ant colonies adjust to the environment

Improving Decision Speed, Accuracy and Group Cohesion through Early Information Gathering in House-Hunting Ants

BACKGROUND: Successful collective decision-making depends on groups of animals being able to make accurate choices while maintaining group cohesion. However, increasing accuracy and/or cohesion usually decreases decision speed and vice-versa. Such trade-offs are widespread in animal decision-making and result in various decision-making strategies that emphasize either speed or accuracy, depending on the context. Speed-accuracy trade-offs have been the object of many theoretical investigations, but these studies did not consider the possible effects of previous experience and/or knowledge of individuals on such trade-offs. In this study, we investigated how previous knowledge of their environment may affect emigration speed, nest choice and colony cohesion in emigrations of the house-hunting ant Temnothorax albipennis, a collective decision-making process subject to a classical speed-accuracy trade-off. METHODOLOGY/PRINCIPAL FINDINGS: Colonies allowed to explore a high quality nest site for one week before they were forced to emigrate found that nest and accepted it faster than emigrating naïve colonies. This resulted in increased speed in single choice emigrations and higher colony cohesion in binary choice emigrations. Additionally, colonies allowed to explore both high and low quality nest sites for one week prior to emigration remained more cohesive, made more accurate decisions and emigrated faster than emigrating naïve colonies. CONCLUSIONS/SIGNIFICANCE: These results show that colonies gather and store information about available nest sites while their nest is still intact, and later retrieve and use this information when they need to emigrate. This improves colony performance. Early gathering of information for later use is therefore an effective strategy allowing T. albipennis colonies to improve simultaneously all aspects of the decision-making process--i.e. speed, accuracy and cohesion--and partly circumvent the speed-accuracy trade-off classically observed during emigrations. These findings should be taken into account in future studies on speed-accuracy trade-offs

Short-term activity cycles impede information transmission in ant colonies.

Rhythmical activity patterns are ubiquitous in nature. We study an oscillatory biological system: collective activity cycles in ant colonies. Ant colonies have become model systems for research on biological networks because the interactions between the component parts are visible to the naked eye, and because the time-ordered contact network formed by these interactions serves as the substrate for the distribution of information and other resources throughout the colony. To understand how the collective activity cycles influence the contact network transport properties, we used an automated tracking system to record the movement of all the individuals within nine different ant colonies. From these trajectories we extracted over two million ant-to-ant interactions. Time-series analysis of the temporal fluctuations of the overall colony interaction and movement rates revealed that both the period and amplitude of the activity cycles exhibit a diurnal cycle, in which daytime cycles are faster and of greater amplitude than night cycles. Using epidemiology-derived models of transmission over networks, we compared the transmission properties of the observed periodic contact networks with those of synthetic aperiodic networks. These simulations revealed that contrary to some predictions, regularly-oscillating contact networks should impede information transmission. Further, we provide a mechanistic explanation for this effect, and present evidence in support of it

Consistency of Leadership in Shoals of Mosquitofish (Gambusia holbrooki) in Novel and in Familiar Environments

In social animal groups, an individual's spatial position is a major determinant of both predation risk and foraging rewards. Additionally, the occupation of positions in the front of moving groups is generally assumed to correlate with the initiation of group movements. However, whether some individuals are predisposed to consistently occupy certain positions and, in some instances, to consistently lead groups over time is as yet unresolved in many species. Using the mosquitofish (Gambusia holbrooki), we examined the consistency of individuals' spatial positions within a moving group over successive trials. We found that certain individuals consistently occupied front positions in moving groups and also that it was typically these individuals that initiated group decisions. The number of individuals involved in leading the group varied according to the amount of information held by group members, with a greater number of changes in leadership in a novel compared to a relatively familiar environment. Finally, our results show that the occupation of lead positions in moving groups was not explained by characteristics such as dominance, size or sex, suggesting that certain individuals are predisposed to leadership roles. This suggests that being a leader or a follower may to some extent be an intrinsic property of the individual