Physical processes rely on the transmission of energy and information across scales. In the last century, theoretical tools have been developed in the field of statistical physics to infer macroscopic properties starting from a microscopic description of the system. However, less attention has been devoted to the remodelling of microscopic degrees of freedom by macroscopic feedback. In recent years, ideas from non-equilibrium physics have been applied to characterise biological and artificial intelligence systems. These systems share in common their structure in discrete scales of organisation that perform specialised functions. To correctly regulate these functions, the accurate transmission of information across scales is crucial. In this thesis we study the role of macroscopic feedback in the remodelling of microscopic degrees of freedom in two paradigmatic examples, one taken from the field of biology, the self-organisation of specialisation and plasticity in a social wasp, and one from artificial intelligence, the remodelling of deep neural networks in a stochastic many-particle system. In the first part of this thesis we study how the primitively social wasp Polistes canadensis simultaneously achieves robust specialization and rapid plasticity. Combining a unique experimental strategy correlating time-resolved measurements across vastly different scales with a theoretical approach, we characterise the re-establishment of the social steady state after queen removal. We show that Polistes integrates antagonistic processes on multiple scales to distinguish between extrinsic and intrinsic perturbations and thereby achieve both robust specialisation and rapid plasticity. Furthermore, we show that the long-term stability of the social structure relies on the regulation of transcriptional noise by dynamic DNA methylation. In the second part of this thesis, we ask whether emergent collective interactions can be used to remodel deep neural networks. To this end, we study a paradigmatic stochastic manyparticle model where the dynamics are defined by the reaction rates of single particles, given by the output of distinct deep neural networks. The neural networks are in turn dynamically remodelled using deep reinforcement learning depending on the previous history of the system. In particular, we implement this model as a one dimensional stochastic lattice gas. Our results show the formation of two groups of particles that move in opposite directions, diffusively at early times and ballistically over longer time-scales, with the transition between these regimes corresponding to the time-scale of left/right symmetry breaking at the level of individual particles. Over a hierarchy of characteristic time-scales these particles develop emergent, increasingly complex interactions characterised by short-range repulsion and long-range attraction. As a result, the system asymptotically converges to a regime characterised by the presence of anti-ferromagnetic particle clusters. To conclude, we characterise the impact of memory effects and demographic disorder on the dynamics. Together, our results shed light on how non-equilibrium systems can employ macroscopic feedback to regulate the propagation of fluctuations across scales.

Social behaviour is shaped by complex relationships between evolutionary and ecological processes interacting at different scales. Benefits gained from social associations can range from predator dilution to collective sensing, but little is known about how these can be influenced by social structure and phenotypic composition. In this thesis, I investigated how individual decision-making affects phenotypic social structure, and how this mediates social behaviour through emergent properties of collective group behaviour. First, using mixed-species flocks as a model system, I showed individual tits (Paridae, chapter 2) and thornbills (Acanthizae, chapter 3) varied significantly in their social positions. Within-species variation in network position was as large as between-species variation, sug- gesting that prescribing functional roles at the species level may not sufficiently account for potential differences in fitness operating at the individual level. Rather, this suggested that structure may be driven by phenotypic traits, underpinning network structure (chapter 4). Next, I used an extensive data set of foraging records to explore factors determining the composition, of flocks of great tits (Parus major, chapter 5). For example, assortment by dispersal phenotype (immigration status) was the result of spatial disaggregation, and I showed that this may facilitate social selection for breeding territories (chapter 6). Finally, I investigated how decision-making shaped mixed-species social structure. I found that tits used a common strategy for managing pressures of predation and starvation by shifting from exploration to exploitation at different times of the day (chapter 7). I then found that a very simple interaction rule successfully replicated mixed-species group structure (chapter 8). Strikingly, the same rule was applied to both conspecifics and het- erospecifics, potentially playing an important role in the maintenance of flock structure. Through experimental manipulation of ecological conditions, I found that heightened per- ceived predation resulted in stronger social attraction overall, whereas increased competition led to a reduction in attraction to conspecifics (chapter 9). Simulations suggested this could be one potential mechanism underpinning fission-fusion dynamics in these species. Together, the results in this thesis form a framework linking social behaviour to individ- ual fitness where natural selection is shaped by the social environment. This approach may prove useful for testing whether following common social rules reduces variance in benefits accrued by individuals, and how within-species variation in social behaviour can impact emergent properties of groups.

In everyday situations individuals make decisions. For example, a tourist usually chooses a crowded or recommended restaurant to have dinner. Perhaps it is an individual decision, but the observed pattern of decision-making is a collective phenomenon. Collective behaviour emerges from the local interactions that give rise to a complex pattern at the group level. In our example, the recommendations or simple copying the choices of others make a crowded restaurant even more crowded. The rules of interaction between individuals are important to study. Such studies should be complemented by biological experiments. Recent studies of collective phenomena in animal groups help us to understand these rules and develop mathematical models of collective behaviour. The most important communication mechanism is positive feedback between group members, which we observe in our example. In this thesis, we use a generic experimentally validated model of positive feedback to study collective decision-making. The first part of the thesis is based on the modelling of decision-making associated to the selection of feeding sites. This has been extensively studied for ants and slime moulds. The main contribution of our research is to demonstrate how such aspects as "irrationality", speed and quality of decisions can be modelled using differential equations. We study bifurcation phenomena and describe collective patterns above critical values of a bifurcation points in mathematical and biological terms. In the second part, we demonstrate how the primitive unicellular slime mould Physarum Polycephalum provides an easy test-bed for theoretical assumptions and model predictions about decision-making. We study its searching strategies and model decision-making associated to the selection of food options. We also consider the aggregation model to investigate the fractal structure of Physarum Polycephalum plasmodia. / <p>Fel serie i tryckt bok /Wrong series in the printed book</p>

Eukaryotic organisms rely on the coordinated beating of motile cilia for a multitude of fundamental reasons. In smaller organisms, such as Paramecium and the single cell alga Chlamydomonas reinhardtii, it is a matter of propulsion, to swim towards a higher concentration of nutrients or away from damaging environments. Larger organisms use instead the coordinated motion of cilia to push fluid along an epithelium: examples common to mammals are the circulation of cerebrospinal fluid in the brain, the transport of ovules in the fallopian tubes, and breaking the left/right symmetry in the embryo. Another notable example, and one that is central to this thesis, is mucociliary clearance in human airways: A carpet of motile cilia helps keeping the cell surface free from pathogens and foreign particles by constantly evacuating from lungs, bronchi, and trachea a barrier of mucus. The question of how motile cilia interact with one another to beat in a coordinated fashion is an open and pressing one, with immediate implications for the medical community. In order for the fluid propulsion to be effective, the motion of cilia needs to be phase-locked across significant distances, in the form of travelling waves (``metachronal waves''). It is still not known how this long-range coordination emerges from local rules, as there is no central node regulating the coordination among cilia. In the first part of this thesis I will focus on studying the coordination in carpets of cilia with a top-down approach, by proposing, implementing, and applying a new method of analysing microscope videos of ciliated epithelia. Chapter 1 provides the reader with an introduction on motile cilia and flagella, treating their structure and motion and reporting the different open questions currently tackled by the scientific community, with particular interest in the coordination mechanisms of cilia and the mucociliary clearance apparatus. Chapter 2 introduces Differential Dynamic Microscopy (DDM), a powerful and versatile image analysis tool that bridges the gap between spectroscopy and microscopy by allowing to perform scattering experiments on a microscope. The most interesting aspects of DDM for this work are that it can be applied to microscope videos where it is not possible to resolve individual objects in the field of view, and it requires no user input. These two characteristics make DDM a perfect candidate for analysing several hundred microscope videos of weakly scattering filaments such as cilia. In Chapter 3 I will present how it is possible to employ DDM to extract a wealth of often-overlooked information from videos of ciliated epithelia: DDM can successfully probe the ciliary beat frequency (CBF) in a sample, measure the direction of beating of the cilia, and detect metachronal waves and read their direction and wavelength. In vitro ciliated epithelia however often do not show perfect coordination or alignment among cilia. For the analysis of these samples, where the metachronal coordination might not be evident, we developed a new approach, called multiscale DDM (multiDDM), to measure a coordination length scale, a characteristic length of the system over which the coordination between cilia is lost. The new technique of multiDDM is employed in Chapter 4 to study how the coordination among cilia changes as a response to changes in the rheology of the mucous layer. In particular, we show that cilia beating under a thick, gel-like mucus layer show a larger coordination length scale, as if the mucus acted as an elastic raft effectively coupling cilia over long distances. This is corroborated by the coordination length scale being larger in samples from patients affected by Cystic Fibrosis than in healthy samples, and much shorter when the mucus layer is washed and cilia therefore beat in a near-Newtonian fluid. We then show how it is possible to employ multiDDM to measure the effectiveness of drugs in recovering, in CF samples, a coordination length scale typical of a healthy phenotype. In the second part I will focus instead on the single cilium scale, showing how we can attempt to link the beating pattern of cilia to numerical simulations studying synchronisation in a model system. In particular in Chapter 5 I will describe our approach to quantitatively describe the beating pattern of single cilia obtained from human airway cells of either healthy individuals or patients affected by Primary Ciliary Dyskinesia. Our description of the beating pattern, and the selection of a few meaningful, summary parameters, are then shown to be accurate enough to discriminate between different mutations within Primary Ciliary Dyskinesia. In Chapter 6 instead I report the results obtained by coarse-graining the ciliary beat pattern into a model system consisting of two ``rotors''. The rotors are simulated colloidal particles driven along closed trajectories while leaving their phase free. In my study, the trajectories followed by the rotors are analytical fits of experimental trajectories of the centre of drag of real cilia. The rotors, that are coupled only via hydrodynamics interactions, are seen to phase-lock, and the shape of the trajectory they are driven along is seen to influence the steady state of the system.

Au cours de cette thèse nous avons étudié expérimentalement les phases denses de disques polaires vibrés, un bon modèle de liquide actif.L'expérience consiste à mettre en vibration des disques durs designés avec deux pattes avant et arrière différentes. Cela leur confère une polarité et sous l'effet de la vibration les disques avancent avec persistance dans la direction de leur polarité.Il a été montré que ce disque polaire est un bon modèle de particule active. Par ailleurs, il a été observé que ces disques forment un liquide polaire ordonnée pour des fractions surfaciques autour de 0.40.Au cours de cette thèse nous avons travaillé à plus haute densité pour étudier la cristallisation de ce liquide actif. Nous avons observé une dynamique fortement intermittente au cours de laquelle des agrégats denses et ordonnés se forment mais se fragmentent sans cesse. Ce régime perdure jusqu'à la fraction surfacique de 0.83, la plus élevée que nous ayons pu atteindre dans un premier temps.Pour étudier l'existence éventuelle d'une phase cristalline stable, nous avons réalisé des expériences dans une enceinte hexagonale, où il est possible d'imposer des fractions surfaciques de l'ordre de 0.89 proche de celle de l'empilement hexagonal compact. Nous observons un cristal dans le cas où l'enceinte est totalement remplie, en revanche lorsque que l'on retire quelques disques le cristal se met à tourner de manière spectaculaire tout en conservant un ordre positionnel.Enfin, nous avons réalisé des expériences de mesures de pression mécanique. Nous avons observé que la pression mécanique exercée par les disques polaires dépend de la nature des murs de l'enceinte. On conclu que la pression mécanique n'est pas une variable d'état pour ce système. / During this thesis, we studied experimentally the dense phases of vibrated polar disks, a good model of active liquid.The experiment involves vibrating hard discs with two different front and rear legs. This provides them with a polarity such that they perform persistent directed motion when they are shaken vertically.These polar discs has been shown to be a good active particle model. Moreover, it has been observed that these disks form an ordered polar liquid for surface fractions around 0.40.During this thesis we worked at a higher density to study the crystallization of this active liquid. We observed a highly intermittent dynamics during which dense and ordered aggregates form but are constantly fragmented. This regime lasts until the surface fraction of 0.83, the highest that we have been able to reach at first.In order to study the possible existence of a stable crystal phase, we carried out experiments in a hexagonal arena, where it is possible to impose surface fractions of the order of 0.89 close to that of the compact hexagonal packing. We observe a crystal in the case where the arena is completely filled, however when removing a few discs the crystal flows and rotates spectacularly while maintaining a positional order.Finally, we carried out mechanical pressure measurement experiments. We have observed that the mechanical pressure exerted by the polar disks depends on the nature of the walls of the arena. We conclude that mechanical pressure is not a state variable for this system.

La prise de décision collective est un des mécanismes usités par les espèces sociales lors d’événements de collectes de nourriture, d’optimisation de chemins, de mouvements collectifs, de fuites face à un prédateur ou bien même de sélections d’habitat. Les zebrafish (Danio rerio), animaux grégaires de référence en biologie, n’ont jamais été étudiés dans des contextes de choix collectifs binaires et ils nous semblait légitime d’analyser leurs comportements décisionnels. L’objectif de cette thèse est de répondre aux attentes du projet ASSISI|bf de création d’un système expérimental modulable capable d’accueillir robots comme poissons, de mise au point d’une société mixte adaptative auto-organisée, et de caractérisation des comportements individuels et collectifs des zebrafish implémentables dans les robots. Nos observations montrent que la prise collective de décision et la cohésion des groupes en environnement ouvert, et présentant deux points de repère identiques, sont des processus dynamiques qui varient selon la souche de Danio rerio (AB ou TL) et sont liées à la densité des populations. Constamment en mouvement, les poissons oscillent entre les deux points de repère identiques installés dans leur environnement. La souche AB est toujours plus attirée par ces hétérogénéités et est beaucoup moins cohésive que la souche TL. En environnement contraint, de type deux chambres reliées par un couloir, nous réitérons le même type d’expérience chez la souche la plus à même de prendre des décisions collectivement (AB) et en faisant varier la densité des groupes de 1 à 20 individus. Il ressort que les Danio rerio de type AB restent essentiellement en groupe mais que l’augmentation de leur densité tend à les diviser. Nous remarquons aussi que la densité influe partiellement sur la topologie du groupe : il existe, lors des sorties collectives, une corrélation entre l’ordre de sortie des zebrafish (les suiveurs) et l’ordre de distance de chaque individu par rapport à l’initiateur desdites sorties. Cette corrélation est d’autant plus forte quand les suiveurs sont soit très proches, soit les plus éloignés de l’initiateur. Le dispositif expérimental précédent nous permet d’autre part de nous concentrer sur la notion de leadership chez des groupes de 2 à 10 Danio rerio AB. Nous mettons en évidence que chaque poisson initie au moins une fois un départ collectif, le nombre d’initiations effectives est proportionnel au nombre de tentatives d’initiations, que tous les poissons présentent le même taux de succès d’initiation après une tentative et qu’il existe une corrélation positive entre initiations et vitesse moyenne de nage.Une analyse poussée des mouvements collectifs nous fait constater que les zebrafish se déplacent essentiellement en groupe et transitent sans interruption ni périodicité entre les chambres. Nous sommes actuellement en train d’améliorer cette étude en apportant de nouveaux points de comparaison afin de vérifier si la non-périodicité des transitions entre les chambres est liée à la densité des groupes testés. Enfin, nous nous inspirons du précédent système expérimental – les deux chambres reliées par un couloir – pour créer une version actualisée d’un labyrinthe en Y. Ce labyrinthe en Y est dit perpétuel car il combine tout le savoir faire que nous avons développé dans l’élaboration des précédents systèmes expérimentaux : une faible habituation des zebrafish pour leur milieu, une réduction des interactions entre l’expérimentateur et les animaux et la possibilité pour les poissons de circuler dans un labyrinthe à symétrie centrale, sans zone de départ ni zone de fin formalisées (typiques des labyrinthes en Y) / The collective decision-making is one of the mecanisms used by social species during foraging, path optimisation, collective movements, predator avoidance or habitat selection. Zebrafish (Danio rerio), gregarious animal models in biology, have never been studied in the context of collective binary choices. We found logical to analyse their decision behaviours. The goal of this thesis is to meet the expectations of the ASSISI|bf project of creation of an andjustable experimental setup able to sustain robots and fish, of development of an adaptive and self-organised mixed society and of the caracterisation of individual and collective behaviours of the zebrafish, implementable in the robots. We show that the collective decision making and the cohesion of groups of zebrafish in an open environment, composed of two identical landmarks, are dynamical processes that vary in function of the strain of Danio rerio (AB or TL) and are related to the density of the populations. Always on the move, the fish oscillate between the two landmarks. The AB zebrafish are more attracted by the landmarks and are less cohesive than the TL zebrafish. In a contrained environment, two rooms connected by a corridor, we do the same type of experiments with only AB zebrafish and vary the density of the groups from 1 to 20 individuals. AB zebrafish swim together. The increase of the density of the groups make them split. We noticed that the density of the groups has an effect on the topology of the groups : during collective departures from a room to the other one, there is a correlation between the rank of exit of the zebrafish (followers) and the rank of the distances of each individual from the initiator of the exits. This correlation is higher when the followers are either the closest or the farest from the initiator.Using the previous experimental setup, we focused also on the topics related to leadership for groups of 2 to 10 individuals. We show that each fish intiate at least once a collective departure and that the number of initiations is proportional to the number of initiation attempts. Also, we found that all the fish have the same success rate in the initiations after an initiation attempt and that there is a positive correlation between the intiations and the average speed. A deeper analysis of the collective movements shows that the zebrafish swim in group and transit without any interruption or even without periodicity between the rooms. We are currently improving this study and will bring new points of comparison to check if the non-periodicity of the transitions could be related to the density of the groups. Finally, we developed a new version of the Y-maze based on the previous observations.This perpetual Y-maze shows many usefull features for the study of the behaviour : a low habituation rate, a reduction of the interactions between the animals and the experimentators, and allows the fish to move in a Y-maze without starting nor ending zones as in typical Y-maze

Thesis (M. A. (Sociology)) --University of Limpopo, 2017 / The purpose of this study was to explain the events surrounding the Marikana miners’ strike in 2012, using Marxism’s concept of class consciousness. The labour disputes witnessed at Marikana in 2012 represent one of the major labour movements that South Africa has witnessed since the inception of democracy. This study adopted qualitative research methods to inquire into the events of the Marikana 2012 labour disputes. Methods used in this study include qualitative research, descriptive research design, Marxism critical inquiry, purposive sampling and critical discourse analysis. Moreover, the study investigated employee relations in the mining sector. In addition, this study also examined the Marikana miner’s working and living conditions and probed the role of social control agencies, including, trade unions, bargaining councils and the police, during the protests. This study proposes a fair distribution of wealth in the mining sector and the removal of the Migrant Labour System. Additionally, this study recommends transparency in the mining sector, the transformation of the education system and the restructuring of trade unions. / University of Limpopo Research Office

Das Zusammenleben in Gruppen ist im Tierreich ein weit verbreitetes Phänomen. Einer der Vorteile des Gruppenlebens könnte die sogenannte „Schwarmintelligenz“ sein, das heißt die Fähigkeit von Gruppen kognitive Probleme zu lösen, die die Problemlösekompetenz einzelner Individuen übersteigt. In der vorliegenden Dissertation untersuchte ich, ob die Gruppengröße beim Menschen und bei Fischen mit einer verbesserten Entscheidungsfindung einhergeht. Beim Menschen analysierte ich zunächst das Abschneiden von Einzelpersonen, die später als Teil einer Gruppe getestet wurden, in einfachen Einschätzungsaufgaben sowie komplizierteren Satz-Rekonstruktionstests. Meine Frage war, ob es Individuen in Gruppen gelingt bessere Entscheidungen zutreffen als das einem durchschnittlichen Individuum der Gruppe alleine möglich wäre und ob Gruppen sogar die Leistung ihres besten Mitglieds in den individuellen Tests überbieten könnten. Tatsächlich konnte ich zeigen, dass Gruppen die Leistung des besten Mitglieds übertreffen, wenn die Problemstellung für Einzelpersonen zu komplex ist oder sich häufig wiederholt. Weiterhin gelang mir zu zeigen, dass Gruppen von Menschen bei einer simulierten Prädationssituation, ähnlich wie es bereits für andere Tierarten beschrieben wurde, anhand von so genannten „Quorum“-Regeln durch non-verbale Kommunikation entscheiden, ob sie bleiben oder flüchten. Dabei dienen einfache Bewegungsmuster als Schlüsselreiz. Individuen einer Gruppe erhöhen durch diesen Mechanismus gleichzeitig ihre echt positiven und verringern ihre falsch positiven Entscheidungen. Beim Guppy, einem Süßwasserfisch aus Trinidad, untersuchte ich in deren natürlichem Habitat, ob die Fähigkeit einzelner Individuen zwischen einer genießbaren und einer ungenießbaren Futterquelle zu unterscheiden, mit der Gruppengröße ansteigt. Meine Ergebnisse zeigen, dass Guppys mit größerer Wahrscheinlichkeit eine genießbare Futterquelle identifizierten, sobald sie Teil einer größeren Gruppe waren. / Group living is a widespread phenomenon. One of its assumed advantages is collective cognition, the ability of groups to solve cognitive problems that are beyond single individuals’ abilities. In this thesis, I investigated whether decision-making improves with group size in both humans and fish, thus using the strengths of each system. In humans, I tested individual performance in simple quantity estimation tasks and a more difficult sentence reconstruction task first alone and then as part of a group. My question was whether groups were able to improve not only on average individual decisions, but also to beat their best members. Indeed, when a given problem is recurrent or too complex for individuals, groups were able to outperform their best members in different contexts. Furthermore, I showed that in a simulated predation experiment, groups of humans decided to stay or to escape using quorum thresholds based on movement behaviour without verbal communication, as has been shown in other animals. This simple movement mechanism allowed individuals in groups to simultaneously increase true positives and decrease false positives. In the guppy, a freshwater fish from Trinidad, I tested in their natural environment whether individuals’ ability to distinguish between an edible and a non-edible food item increases with group size. My results indicate that guppies had better chances to identify the edible food item when part of bigger groups. By investigating several populations with different ecological backgrounds, in particular differing in predation levels, I found that, despite a lower sampling activity in high predation habitats, predation did not affect the improvement of decisions in groups.

La construction du nid chez les fourmis génère des structures assumant de nombreuses fonctions, qui sont en grande partie dépendantes de l’architecture produite. L’omniprésence de ces fonctionnalités contraste avec la forte diversité intra et inter-spécifique de la morphologie du nid. Ce travail a pour objectifs de déterminer d’une part la morphogenèse du nid durant son excavation par les fourmis (Chapitre 3), et d’autre part de quantifier l’impact de la taille du groupe (Chapitre 3) et de la qualité de l’environnement (qualité du matériau, gravité; Chapitres 4 & 5) sur cette séquence. Pour répondre à ces questions, nous avons utilisé un dispositif de creusement en deux dimensions (2D) afin de suivre la dynamique d’excavation et l’évolution de la morphologie du nid au cours du temps. Nous avons tout d’abord mis en évidence (Chapitre 3) un changement brutal de la morphologie du nid au cours de sa croissance. Durant une première période d’excavation homogène, le nid est constitué d’une seule cavité de forme circulaire et au contour régulier. Par la suite, le pourtour de la cavité devient plus irrégulier, l’apparition de ‘‘bourgeons’’ lui donnant une apparence plus plissée. Enfin, la cavité centrale cesse de croître lorsque des ramifications se déploient depuis certains des ‘‘bourgeons’’. Nous avons qualifié de transitions morphologiques ces brusques changements de forme, dont la fréquence d’apparition augmente avec la taille de la population. Notre analyse et le recours à la simulation semblent indiquer que ce phénomène soit basé non pas sur un changement de comportement des ouvrières excavatrices mais sur la densité d’activité au front de creusement. Lorsque celle-ci est importante, des phénomènes d’encombrement se manifestent et le nid est excavé de manière homogène. À l’inverse, lorsque l’encombrement diminue les ouvrières peuvent focaliser leur travail et excavent des galeries. Un modèle analytique (Chapitre 6) a permis d’étudier les conditions d’apparition de cette transition morphologique. Nos autres résultats indiquent que le matériau de construction influence très fortement la morphogenèse du nid (Chapitre 4) : les nids excavés dans un milieu cohésif sont plus fréquemment ramifiés et la transition morphologique apparaît à des surfaces de nid plus petites que dans un milieu granulaire. Il semblerait qu’à nouveau, il n’y ait aucune variation comportementale impliquée, mais que l’environnement joue un rôle de ‘‘médiateur’’ des interactions entre les fourmis: le milieu change le temps d’extraction et donc la probabilité individuelle d’excavation, cette variation est exacerbée par les mécanismes d’amplification à l’œuvre, modifiant alors sensiblement la dynamique collective de creusement et la structure du nid. Enfin, une série d’expériences (Chapitre 5) nous a permis de déterminer l’influence de la gravité sur la morphogenèse. Il apparait que la gravité sert de gabarit à la construction, puisque les nids excavés dans un dispositif à l’orientation verticale sont tous dirigés vers le bas. Aussi, si le nid prend directement une forme de long puit vertical sans jamais présenter de chambre, le mécanisme de transition morphologique reste visible, sous forme de bifurcation des galeries à leur extrémités. Ce phénomène de tip-splitting semble aussi apparaître sous la contrainte de la densité d’ouvrières excavant au fond des galeries. Notre travail a donc mis en évidence un phénomène de transition morphologique capable de produire les modules de base de tout nid - chambres et galeries -, et qui semble découler non pas de modifications comportementales, mais de la seule interaction de l’amplification de l’activité et de l’encombrement au front d’excavation. Ce phénomène ubiquiste propose une explication à la grande variété de structures observées sur le terrain, en lien notamment avec la diversité et l’hétérogénéité des milieux, et la dynamique des colonies. Enfin, les similitudes de la transition morphologique avec les instabilités de croissance observées dans de nombreux systèmes biologiques et physico-chimique invitent à considérer ces derniers phénomènes à la lumière de nos résultats.

Throughout the animal kingdom, individuals often differ consistently from one another in how they cope with their environment. In particular, consistent behavioural variation, known as animal personality, is a substantial driver of a range of important ecological and evolutionary processes. As most animal species are social for at least part of their lives and group living is common, a crucial link between personality and the social context may be expected. In this thesis I systematically investigate this link, using three-spined stickleback fish (Gasterosteus aculeatus) as my model system. I begin by showing that fish vary consistently in their boldness and sociability, with only boldness being positively linked to food intake for fish at rest. This finding provides support for evolutionary theory that links personality variation to life-history strategies, and lays the basis for work related to the social context. I continue by investigating how the social context may modulate personality variation and show that short contact with a social group may have carry-over effects and obfuscate personality expression when individuals are alone. Next, I observed fish in different pairs over time and found that social experience from both the current as well as previous social contexts are integrated in the risk-taking and leadership decisions of individuals but also depends on their boldness type. This result provides support for the importance of social feedback in the expression of personality differences. I go on to demonstrate that, in a pair, bolder fish have lower social attraction, with positive effects on individual’s leadership but negative effects on social coordination. Finally, by detailed tracking of the collective movements and group foraging of free-swimming shoals, I reveal boldness and sociability have complementary driving effects of on social structure, collective behaviour, and group functioning. Furthermore, I show that in turn the group composition determines the performance of individual personality types, providing a potential adaptive explanation for the maintenance of personality variation. Taken together, these studies provide an integrated account of animal personality and the social context and highlight the presence of a feedback loop between them, with personality variation being a key driver of collective behaviour and group functioning but also strongly affected and potentially maintained by it.