Jean-Louis Deneubourg

Publications of Jean-Louis Deneubourg

• Testing for collective choices in the two-spotted spider mite.

  Authors: Aina Astudillo Fernandez, Thierry Hance, Gwendoline Clotuche, Anne-Catherine Mailleux, Jean Louis Deneubourg

  Experimental & applied acarology. 04/2012;

  Silk is a vector for collective behaviour in many spinning arthropods, including social spiders, social caterpillars, and some spider mites. In this study, the potential for silk-mediated collectiveSilk is a vector for collective behaviour in many spinning arthropods, including social spiders, social caterpillars, and some spider mites. In this study, the potential for silk-mediated collective choices is evaluated for the two-spotted spider mite Tetranychus urticae. This subsocial mite lives in large colonies on plants, sheltered under a collectively spun silk web. The silk has an attractive and arresting effect. We test whether the silk trails left by the spider mites can give rise to the collective choice of a path. The experiment consists in offering two identical paths to a group of migrating mites. Our results show that the presence of a silk trail influences the mites, but not sufficiently to systematically provoke a collective choice. In order to determine the trail following potential of T. urticae, we parameterise a theoretical trail following model to fit our experiments and those found in the literature. Our prediction is that even after a large number of mites have passed (200), a systematic collective choice of path should not be expected under the tested conditions. Our results, combined with what is known from the literature, allow us to propose a general scenario for the dispersal behaviour of T. urticae.

• The locomotor activities on sites covered by silk produced by related and unrelated spider mites in Tetranychus urticae (Acari: Tetranychidae).

  Authors: Guillaume Jean Le Goff, Thierry Hance, Claire Detrain, Jean-Louis Deneubourg, Anne-Catherine Mailleux

  Comptes rendus biologies. 03/2012; 335(3):226-31.

  The two-spotted spider mite Tetranychus urticae is a silk producer known to live in groups with a common silk web that can cover entire plants and protect mites against predators, rain and wind. SilkThe two-spotted spider mite Tetranychus urticae is a silk producer known to live in groups with a common silk web that can cover entire plants and protect mites against predators, rain and wind. Silk also plays an important role during collective migration by aerial dispersal or by walking. In this context, we studied the locomotor activity i.e. time in movement, in resting, and in static exploration (probing behaviour) of an individual confronted to clean places and/or places covered by silk coming from its own population or from another. Two populations were tested, one of the red form (Tunisia) and another of the green form (Belgium). The experimental results showed that the presence or the absence of silk did not influence the relative proportion of each behavioural item either for the red or the green population. Individuals of the green form population spent more time moving and less time resting than individuals of the red form population and this, whatever the substrate (red/green silk, clean). Moreover, the silk from the red form population attracted individuals from both populations, whereas the silk produced by the green form population did not attract any individual either from the red or the green form. This surprising result might have been due to a difference in the quantity and/or quality of silk woven by the two forms. We discuss how the differences observed in the behaviour of these two populations may result from differences in their strategy to rapidly increase the population of the colony and reinforce the silk nest.

• The silk road of Tetranychus urticae: is it a single or a double lane?

  Authors: Gwendoline Clotuche, Anne-Catherine Mailleux, Jean-Louis Deneubourg, Claire Detrain, Thierry Hance

  Experimental & applied acarology. 01/2012;

  Tetranychus urticae (Acari: Tetranychidae) is a phytophagous mite that forms huge colonies. All active members of a colony (immatures and matures, females and males) spin silken threads. These mitesTetranychus urticae (Acari: Tetranychidae) is a phytophagous mite that forms huge colonies. All active members of a colony (immatures and matures, females and males) spin silken threads. These mites construct a common web that protects the colony from external aggression. The silk coverage is well-known to provide advantages to the colony but very little is known about the characteristics of the threads themselves. Here is the first quantification of the diameter of silken threads spun by two different stages (adult females and larvae) and its relationship with body size of the spinning individuals. Moreover, we observed how silk was deposited on the substrate through their two pedipalps. Threads were observed by means of transmission electron and fluorescence microscopy. Silken threads spun by larvae (0.055 ± 0.018 μm) were significantly thinner than threads spun by adult females (0.111 ± 0.038 μm). In the first step of the silk depositing behaviour, the mite attached the thread to the substrate by putting its pedipalps in contact with the surface (adhesion, double silken threads). When walking, silken threads became detached from the substrate and spitted up (silken threads were free). Finally, silken threads adhered to the surface. The presence of single and double threads makes thread diameter highly variable.

• From Social Network (Centralized vs. Decentralized) to Collective Decision-Making (Unshared vs. Shared Consensus).

  Authors: Cédric Sueur, Jean-Louis Deneubourg, Odile Petit

  PloS one. 01/2012; 7(2):e32566.

  Relationships we have with our friends, family, or colleagues influence our personal decisions, as well as decisions we make together with others. As in human beings, despotism and egalitarianRelationships we have with our friends, family, or colleagues influence our personal decisions, as well as decisions we make together with others. As in human beings, despotism and egalitarian societies seem to also exist in animals. While studies have shown that social networks constrain many phenomena from amoebae to primates, we still do not know how consensus emerges from the properties of social networks in many biological systems. We created artificial social networks that represent the continuum from centralized to decentralized organization and used an agent-based model to make predictions about the patterns of consensus and collective movements we observed according to the social network. These theoretical results showed that different social networks and especially contrasted ones - star network vs. equal network - led to totally different patterns. Our model showed that, by moving from a centralized network to a decentralized one, the central individual seemed to lose its leadership in the collective movement's decisions. We, therefore, showed a link between the type of social network and the resulting consensus. By comparing our theoretical data with data on five groups of primates, we confirmed that this relationship between social network and consensus also appears to exist in animal societies.

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• Impact of starvation on the silk attractiveness in a weaving mite, Tetranychus urticae (Acari: Tetranychidae)

  Authors: Guillaume Jean Le Goff, Thierry Hance, Claire Detrain, Jean-Louis Deneubourg, Gwendoline Clotuche, Anne-Catherine Mailleux

  Journal of Ethology. 11/2011; 30:125-132.

  The two-spotted spider mite Tetranychus urticae is a silk producer known to live in groups. Its silk production plays an important role in protection against external aggressions (predators, rains,The two-spotted spider mite Tetranychus urticae is a silk producer known to live in groups. Its silk production plays an important role in protection against external aggressions (predators, rains, etc.). It is also used for group dispersal through the formation of silkballs or as a thread during individual migration by walking. Until now, the role of silk in enhancing migration has been poorly studied. In this paper, the influence of the silken thread presence on T. urticae's locomotor activity is studied. One virgin female is placed at the centre of a cover glass partially covered by silk. Moving and resting time are studied on the silk or the clean part of the cover glass as a function of the starvation level of mites (fed vs. starved) and the age of the silk (30 vs. 60 vs. 90 vs. 120 vs. 150 min). Results show that a fed female spends more time on the silk-covered part than on the clean part as long as the silk is fresh (less than 120 min). Thus, the age-related changes in chemical and/or physical properties of the silk influence the spatial distribution of fed mites. Whatever the age of the silk, starved mites spend more time on the clean part of the set-up. Indeed, the silk freshly laid by conspecifics is attractive only for fed mites; starved mites probably prefer sites without silk (and conspecifics). This study shows that that the silk influences the spatial distribution of T. urticae according to its level of starvation and that silk is probably is an indicator of the presence of conspecifics.

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• Impact of starvation on the silk attractiveness in a weaving mite, Tetranychus urticae (Acari: Tetranychidae)

  Authors: Guillaume Jean, Le Goff, bullet Thierry, Hance @bullet, Claire Detrain, Jean-Louis Deneubourg, bullet Gwendoline, Clotuche @bullet, Anne-Catherine Mailleux

  Ethology. 11/2011; 30:125-132.

  The two-spotted spider mite Tetranychus urticae is a silk producer known to live in groups. Its silk production plays an important role in protection against external aggressions (predators, rains,The two-spotted spider mite Tetranychus urticae is a silk producer known to live in groups. Its silk production plays an important role in protection against external aggressions (predators, rains, etc.). It is also used for group dispersal through the formation of silkballs or as a thread during individual migration by walking. Until now, the role of silk in enhancing migration has been poorly studied. In this paper, the influence of the silken thread presence on T. urticae's locomotor activity is studied. One virgin female is placed at the centre of a cover glass partially covered by silk. Moving and resting time are studied on the silk or the clean part of the cover glass as a function of the starvation level of mites (fed vs. starved) and the age of the silk (30 vs. 60 vs. 90 vs. 120 vs. 150 min). Results show that a fed female spends more time on the silk-covered part than on the clean part as long as the silk is fresh (less than 120 min). Thus, the age-related changes in chemical and/or physical properties of the silk influence the spatial distribution of fed mites. Whatever the age of the silk, starved mites spend more time on the clean part of the set-up. Indeed, the silk freshly laid by conspecifics is attractive only for fed mites; starved mites probably prefer sites without silk (and conspecifics). This study shows that that the silk influences the spatial distribution of T. urticae according to its level of starvation and that silk is probably is an indicator of the presence of conspecifics.

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• On following behaviour as a mechanism for collective movement.

  Authors: Aina Astudillo Fernandez, Jean Louis Deneubourg

  Journal of theoretical biology. 06/2011; 284(1):7-15.

  During collective movement, animals display a wide variety of mechanisms to maintain cohesion. In some species, individuals rely mainly on following their direct predecessor, thereby formingDuring collective movement, animals display a wide variety of mechanisms to maintain cohesion. In some species, individuals rely mainly on following their direct predecessor, thereby forming spectacular processions of individuals in single file. Despite being the simplest case of following behaviour, it is largely absent from the theoretical literature on collective migrations. The objective of this study is to quantify the efficiency of following the predecessor, in terms of ensuring cohesion. The situation we consider is a sequence of individuals facing a bifurcation. The choice between left and right is influenced by the choice of the predecessor. First, we model this situation with a two-state Markov chain with a symmetric transition matrix. Cohesion is quantified as the expected number of individuals on either side, and the expected number of consecutive followers. Although cohesion increases with the probability of following, it remains surprisingly low unless the probability is almost equal to one. Furthermore, cohesion decreases with group size regardless of the probability of following. Then, we generalise our model to situations in which individuals have a preference for one of the two choices (asymmetric transition matrix). For some parameter sets, the tendency to follow each other leads a large fraction of the individuals to the non preferred side. Moreover, this fraction increases with the total population size. Finally, we include the possibility to follow N individuals. This provides the link between our model and other collective migration models. If enough individuals are perceived, the results shift from symmetrical (low cohesion) to asymmetrical (high cohesion) distribution of the individuals. All in all, our results suggest that following the direct predecessor must be complemented with other cohesive behaviours (involving the perception of more individuals or a navigation system) to guarantee its efficiency. We discuss our findings in the context of the different following behaviours covered in the literature.

• Group size, grooming and fission in primates: a modeling approach based on group structure.

  Authors: Cédric Sueur, Jean-Louis Deneubourg, Odile Petit, Iain D Couzin

  Journal of theoretical biology. 03/2011; 273(1):156-66.

  In social animals, fission is a common mode of group proliferation and dispersion and may be affected by genetic or other social factors. Sociality implies preserving relationships between groupIn social animals, fission is a common mode of group proliferation and dispersion and may be affected by genetic or other social factors. Sociality implies preserving relationships between group members. An increase in group size and/or in competition for food within the group can result in decrease certain social interactions between members, and the group may split irreversibly as a consequence. One individual may try to maintain bonds with a maximum of group members in order to keep group cohesion, i.e. proximity and stable relationships. However, this strategy needs time and time is often limited. In addition, previous studies have shown that whatever the group size, an individual interacts only with certain grooming partners. There, we develop a computational model to assess how dynamics of group cohesion are related to group size and to the structure of grooming relationships. Groups' sizes after simulated fission are compared to observed sizes of 40 groups of primates. Results showed that the relationship between grooming time and group size is dependent on how each individual attributes grooming time to its social partners, i.e. grooming a few number of preferred partners or grooming equally or not all partners. The number of partners seemed to be more important for the group cohesion than the grooming time itself. This structural constraint has important consequences on group sociality, as it gives the possibility of competition for grooming partners, attraction for high-ranking individuals as found in primates' groups. It could, however, also have implications when considering the cognitive capacities of primates.

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• Scalable rules for coherent group motion in a gregarious vertebrate.

  Authors: Marie-Hélène Pillot, Jacques Gautrais, Patrick Arrufat, Iain D Couzin, Richard Bon, Jean-Louis Deneubourg

  PloS one. 01/2011; 6(1):e14487.

  Individuals of gregarious species that initiate collective movement require mechanisms of cohesion in order to maintain advantages of group living. One fundamental question in the study of collectiveIndividuals of gregarious species that initiate collective movement require mechanisms of cohesion in order to maintain advantages of group living. One fundamental question in the study of collective movement is what individual rules are employed when making movement decisions. Previous studies have revealed that group movements often depend on social interactions among individual members and specifically that collective decisions to move often follow a quorum-like response. However, these studies either did not quantify the response function at the individual scale (but rather tested hypotheses based on group-level behaviours), or they used a single group size and did not demonstrate which social stimuli influence the individual decision-making process. One challenge in the study of collective movement has been to discriminate between a common response to an external stimulus and the synchronization of behaviours resulting from social interactions. Here we discriminate between these two mechanisms by triggering the departure of one trained Merino sheep (Ovis aries) from groups containing one, three, five and seven naïve individuals. Each individual was thus exposed to various combinations of already-departed and non-departed individuals, depending on its rank of departure. To investigate which individual mechanisms are involved in maintaining group cohesion under conditions of leadership, we quantified the temporal dynamic of response at the individual scale. We found that individuals' decisions to move do not follow a quorum response but rather follow a rule based on a double mimetic effect: attraction to already-departed individuals and attraction to non-departed individuals. This rule is shown to be in agreement with an adaptive strategy that is inherently scalable as a function of group size.

• How group size affects vigilance dynamics and time allocation patterns: the key role of imitation and tempo.

  Authors: Pablo Michelena, Jean-Louis Deneubourg

  PloS one. 01/2011; 6(4):e18631.

  In the context of social foraging, predator detection has been the subject of numerous studies, which acknowledge the adaptive response of the individual to the trade-off between feeding andIn the context of social foraging, predator detection has been the subject of numerous studies, which acknowledge the adaptive response of the individual to the trade-off between feeding and vigilance. Typically, animals gain energy by increasing their feeding time and decreasing their vigilance effort with increasing group size, without increasing their risk of predation ('group size effect'). Research on the biological utility of vigilance has prevailed over considerations of the mechanistic rules that link individual decisions to group behavior. With sheep as a model species, we identified how the behaviors of conspecifics affect the individual decisions to switch activity. We highlight a simple mechanism whereby the group size effect on collective vigilance dynamics is shaped by two key features: the magnitude of social amplification and intrinsic differences between foraging and scanning bout durations. Our results highlight a positive correlation between the duration of scanning and foraging bouts at the level of the group. This finding reveals the existence of groups with high and low rates of transition between activities, suggesting individual variations in the transition rate, or 'tempo'. We present a mathematical model based on behavioral rules derived from experiments. Our theoretical predictions show that the system is robust in respect to variations in the propensity to imitate scanning and foraging, yet flexible in respect to differences in the duration of activity bouts. The model shows how individual decisions contribute to collective behavior patterns and how the group, in turn, facilitates individual-level adaptive responses.

• The formation of collective silk balls in the spider mite Tetranychus urticae Koch.

  Authors: Gwendoline Clotuche, Anne-Catherine Mailleux, Aina Astudillo Fernández, Jean-Louis Deneubourg, Claire Detrain, Thierry Hance

  PloS one. 01/2011; 6(4):e18854.

  Tetranychus urticae is a phytophagous mite that forms colonies of several thousand individuals. These mites construct a common web to protect the colony. When plants become overcrowded and foodTetranychus urticae is a phytophagous mite that forms colonies of several thousand individuals. These mites construct a common web to protect the colony. When plants become overcrowded and food resources become scarce, individuals gather at the plant apex to form a ball composed of mites and their silk threads. This ball is a structure facilitating group dispersal by wind or animal transport. Until now, no quantitative study had been done on this collective form of migration. This is the first attempt to understand the mechanisms that underlie the emergence and growth of the ball. We studied this collective behaviour under laboratory conditions on standardized infested plants. Our results show that the collective displacement and the formation of balls result from a recruitment process: by depositing silk threads on their way up to the plant apex, mites favour and amplify the recruitment toward the balls. A critical threshold (quorum response) in the cumulative flow of mites must be reached to observe the emergence of a ball. At the beginning of the balls formation, mites form an aggregate. After 24 hours, the aggregated mites are trapped inside the silk balls by the complex network of silk threads and finally die, except for recently arrived individuals. The balls are mainly composed of immature stages. Our study reconstructs the key events that lead to the formation of silk balls. They suggest that the interplay between mites' density, plant morphology and plant density lead to different modes of dispersions (individual or collective) and under what conditions populations might adopt a collective strategy rather than one that is individually oriented. Moreover, our results lead to discuss two aspects of the cooperation and altruism: the importance of Allee effects during colonization of new plants and the importance of the size of a founding group.

• Group living enhances individual resources discrimination: the use of public information by cockroaches to assess shelter quality.

  Authors: Stéphane Canonge, Jean-Louis Deneubourg, Grégory Sempo

  PloS one. 01/2011; 6(6):e19748.

  In group-living organisms, consensual decision of site selection results from the interplay between individual responses to site characteristics and to group-members. Individuals independently gatherIn group-living organisms, consensual decision of site selection results from the interplay between individual responses to site characteristics and to group-members. Individuals independently gather personal information by exploring their environment. Through social interaction, the presence of others provides public information that could be used by individuals and modulates the individual probability of joining/leaving a site. The way that individual's information processing and the network of interactions influence the dynamics of public information (depending on population size) that in turn affect discrimination in site quality is a central question. Using binary choice between sheltering sites of different quality, we demonstrate that cockroaches in group dramatically outperform the problem-solving ability of single individual. Such use of public information allows animals to discriminate between alternatives whereas isolated individuals are ineffective (i.e. the personal discrimination efficiency is weak). Our theoretical results, obtained from a mathematical model based on behavioral rules derived from experiments, highlight that the collective discrimination emerges from competing amplification processes relying on the modulation of the individual sheltering time without shelters comparison and communication modulation. Finally, we well demonstrated here the adaptive value of such decision algorithm. Without any behavioral change, the system is able to shift to a more effective strategy when alternatives are present: the modification of the spatio-temporal distributions of individuals leading to the collective selection of the best resource. This collective discrimination implying such parsimonious and widespread mechanism must be shared by many group living-species.

• Excavated substrate modulates growth instability during nest building in ants.

  Authors: Etienne Toffin, Jonathan Kindekens, Jean-Louis Deneubourg

  Proceedings. Biological sciences / The Royal Society. 09/2010; 277(1694):2617-25.

  In social insects, the nests of the same species can show a large difference in size and shape. Despite these large variations, the nests share the same substructures, some appearing during nestIn social insects, the nests of the same species can show a large difference in size and shape. Despite these large variations, the nests share the same substructures, some appearing during nest growth. In ants, the interplay between nest size and digging activity leads to two successive morphological transitions from circular to branched shapes (budding along the perimeter of the circular cavity and tunnelling of the galleries). Like several other self-organized collective behaviours, this phenomenon, as well as the entire nest-digging process, is thought to be modulated by environmental properties. The present study investigates the effect of excavated substrate on the nest morphogenesis and the morphological transitions by using two materials with different cohesions. Here, we show that the two morphological transitions occur more frequently with a cohesive substrate than with a granular one: 96 per cent of cohesive experiments showed both transitions, whereas only 50 per cent did in granular experiments. We found that transitions and excavation cessation follow area-response thresholds: the shape transitions take place and the digging activity stops when the dug area reaches the corresponding threshold values. The shape transition thresholds are lower with the cohesive substrate and that of stopping digging is independent of nest shape and material. According to simulations, the experimental frequencies of transitions found their origin in the competition between transitions and activity cessation and in the difference between the transition threshold values of each substrate. Our results demonstrate how the substrate properties modulate the collective response and lead to various patterns. Considering the non-specific mechanisms at work, such effects of substrate coarseness have their counterparts in various collective behaviours, generating alternative patterns to colonize and exploit the environment.

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• Effect of retentive markers on the dynamics of settlement: The case of arthropod silk.

  Authors: Aina Astudillo Fernandez, Anne-Catherine Mailleux, Fabrice Saffre, Bertrand Krafft, Jean Louis Deneubourg

  Journal of theoretical biology. 08/2010; 265(3):411-21.

  During a settlement decision, the presence of conspecifics is crucial to species subject to Allee effects, for which the number of founders affects the subsequent growth of the colony. Marking theDuring a settlement decision, the presence of conspecifics is crucial to species subject to Allee effects, for which the number of founders affects the subsequent growth of the colony. Marking the area (physically or chemically) conveys information about the number of conspecifics present in a new patch. Here, we study how an individual affinity for the marker affects the dynamics of a foundation process. A generic population model is presented, in which marking and affinity for the marker are at stake. Our results show that population size thresholds can appear, below which settlement is not possible. This model is then used to study the dynamics of migration and aggregation in a set of interconnected populations. We show that affinity for the marker can induce asymmetries in the population distribution. Anelosimus eximius is a social spider subject to Allee effects, for which silk potentially acts as a marker. We test our predictions with field experiments involving two populations of A. eximius in a Y-shaped setup. The agreement between our experimental and theoretical results strongly supports the validity of the model. This allows us to use the model to estimate a realistic set of parameters of biological significance to this social spider.

• Ant traffic rules.

  Authors: Vincent Fourcassié, Audrey Dussutour, Jean-Louis Deneubourg

  The Journal of experimental biology. 07/2010; 213(Pt 14):2357-63.

  Many animals take part in flow-like collective movements. In most species, however, the flow is unidirectional. Ants are one of the rare group of organisms in which flow-like movements areMany animals take part in flow-like collective movements. In most species, however, the flow is unidirectional. Ants are one of the rare group of organisms in which flow-like movements are predominantly bidirectional. This adds to the difficulty of the task of maintaining a smooth, efficient movement. Yet, ants seem to fare well at this task. Do they really? And if so, how do such simple organisms succeed in maintaining a smooth traffic flow, when even humans experience trouble with this task? How does traffic in ants compare with that in human pedestrians or vehicles? The experimental study of ant traffic is only a few years old but it has already provided interesting insights into traffic organization and regulation in animals, showing in particular that an ant colony as a whole can be considered as a typical self-organized adaptive system. In this review we will show that the study of ant traffic can not only uncover basic principles of behavioral ecology and evolution in social insects but also provide new insights into the study of traffic systems in general.

• Group effect on fertility, survival and silk production in the web spinner Tetranychus urticae (Acari: Tetranychidae) during colony foundation

  Authors: Guillaume Jean Le Goff, Anne-Catherine Mailleux, Claire Detrain, Jean-Louis Deneubourg, Gwendoline Clotuche, Thierry Hance

  Behaviour. 05/2010; 147(147):1169-1184.

  In many vertebrates and invertebrates, individuals reared in isolation show biological modi-fications compared with those reared in groups of two of more. The spider mite Tetranychus urticae isIn many vertebrates and invertebrates, individuals reared in isolation show biological modi-fications compared with those reared in groups of two of more. The spider mite Tetranychus urticae is characterised by a communal organization and displays some forms of cooperative behaviour (aggregation and common web spinning). To evaluate the potential fitness cost or gain of group living, we investigated the effect of being in group on life history parameters (silk production, fecundity, death rate, feeding rate). In this respect, virgin single females and grouped females (two to six individuals) were compared every day for 5 days. Grouped mites produced significantly more web/mite per day (since the second day of experiment) and more eggs/mite per day (since the fourth day of experiment) than single mites. Moreover, single mites had a higher death rate than mites living in groups. However, no difference was found concerning feeding rate. We assume that mites could benefit from the web production of other individuals and invested resources in other activities such as egg production. We showed that these group effects already exist in small groups. In natural conditions with a higher group size, the impact of group living might be stronger for the dynamics of T. urticae populations and the colony foundation.

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• Differences in nutrient requirements imply a non-linear emergence of leaders in animal groups.

  Authors: Cédric Sueur, Jean-Louis Deneubourg, Odile Petit, Iain D Couzin

  PLoS computational biology. 01/2010; 6(9):e1000917.

  Collective decision making and especially leadership in groups are among the most studied topics in natural, social, and political sciences. Previous studies have shown that some individuals are moreCollective decision making and especially leadership in groups are among the most studied topics in natural, social, and political sciences. Previous studies have shown that some individuals are more likely to be leaders because of their social power or the pertinent information they possess. One challenge for all group members, however, is to satisfy their needs. In many situations, we do not yet know how individuals within groups distribute leadership decisions between themselves in order to satisfy time-varying individual requirements. To gain insight into this problem, we build a dynamic model where group members have to satisfy different needs but are not aware of each other's needs. Data about needs of animals come from real data observed in macaques. Several studies showed that a collective movement may be initiated by a single individual. This individual may be the dominant one, the oldest one, but also the one having the highest physiological needs. In our model, the individual with the lowest reserve initiates movements and decides for all its conspecifics. This simple rule leads to a viable decision-making system where all individuals may lead the group at one moment and thus suit their requirements. However, a single individual becomes the leader in 38% to 95% of cases and the leadership is unequally (according to an exponential law) distributed according to the heterogeneity of needs in the group. The results showed that this non-linearity emerges when one group member reaches physiological requirements, mainly the nutrient ones - protein, energy and water depending on weight - superior to those of its conspecifics. This amplification may explain why some leaders could appear in animal groups without any despotism, complex signalling, or developed cognitive ability.

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• Personality and collective decision-making in foraging herbivores.

  Authors: Pablo Michelena, Raphaël Jeanson, Jean-Louis Deneubourg, Angela M Sibbald

  Proceedings. Biological sciences / The Royal Society. 12/2009;

  The mechanisms by which group-living animals collectively exploit resources, and the role of individuals in group decisions, are central issues for understanding animal distribution patterns. WeThe mechanisms by which group-living animals collectively exploit resources, and the role of individuals in group decisions, are central issues for understanding animal distribution patterns. We investigated the extent to which boldness and shyness affect the distribution of social herbivores across vegetation patches, using sheep as a model species. Using an experimental and a theoretical approach, we show that collective choices emerge through the nonlinear dynamics of interactions between individuals, at both short and long distances. Within a range of parameter values derived from the observation of homogeneous groups of each behavioural type, we propose a simple mechanism whereby the same interaction rules can result in different patterns of distribution across patches for bold and shy individuals. We present a mathematical model based on behavioural rules derived from experiments, in which crowding and conspecific attraction affect the probability of entering or leaving patches. Variation in the strength of social attraction is sufficient to account for differences in spatial distribution across patches. The model predicts that resource fragmentation more strongly affects the distribution patterns of shy groups, and suggests that the presence of both bold and shy individuals within groups would result in more flexible behaviour at the population level.

• Shape transition during nest digging in ants.

  Authors: Etienne Toffin, David Di Paolo, Alexandre Campo, Claire Detrain, Jean-Louis Deneubourg

  Proceedings of the National Academy of Sciences of the United States of America. 11/2009; 106(44):18616-20.

  Nest building in social insects is among the collective processes that show highly conservative features such as basic modules (chambers and galleries) or homeostatic properties. Although ant nestsNest building in social insects is among the collective processes that show highly conservative features such as basic modules (chambers and galleries) or homeostatic properties. Although ant nests share common characteristics, they exhibit a high structural variability, of which morphogenesis and underlying mechanisms remain largely unknown. We conducted two-dimensional nest-digging experiments under homogeneous laboratory conditions to investigate the shape diversity that emerges only from digging dynamics and without the influence of any environmental heterogeneity. These experiments revealed that, during the excavation, a morphological transition occurs because the primary circular cavity evolves into a ramified structure through a branching process. Such a transition is observed, whatever the number of ants involved, but occurs more frequently for a larger number of workers. A stochastic model highlights the central role of density effects in shape transition. These results indicate that nest digging shares similar properties with various physical, chemical, and biological systems. Moreover, our model of morphogenesis provides an explanatory framework for shape transitions in decentralized growing structures in group-living animals.

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• Spatial distribution and inbreeding in Tetranychus urticae.

  Authors: Guillaume Le Goff, Anne-Catherine Mailleux, Claire Detrain, Jean-Louis Deneubourg, Gwendoline Clotuche, Thierry Hance

  Comptes rendus biologies. 10/2009; 332(10):927-33.

  In group living, species spatial distribution results from responses to environmental heterogeneity and/or mutual interactions between individuals. These mutual interactions can be regulated byIn group living, species spatial distribution results from responses to environmental heterogeneity and/or mutual interactions between individuals. These mutual interactions can be regulated by genetic and/or epigenetic factors. In this study, we focus on genetic factors and investigate how the spatial distribution of some individuals colonizing a new environment is influenced by inbreeding. Our biological model is Tetranychus urticae, a phytophagous mite considered as a major pest of many cultivated plants. Groups of T. urticae were composed of individuals from successive inbreeding (sister-brother sib-mating). Our results show that the inter-individual distances increase with inbreeding. Indeed, inbreeding level seems to be an important factor affecting the intra-plant spatial distribution of mites. These results confirm that mites have the capability to discriminate their kin and, moreover, that they are able to accurately perceive differences between close relatives from sib-mating lines. To cite this article: G. Le Goff et al., C. R. Biologies 332 (2009).