Christian Jost

Self-organized aggregation in cockroaches

Aggregation is widespread in invertebrate societies and can appear in response to environmental heterogeneities or by attraction between individuals. We performed experiments with cockroach, Blattella germanica, larvae in a homogeneous environment to investigate the influence of interactions between individuals on aggregations. Different densities were tested. A first phase led to radial dispersion of larvae in relation to wall-following behaviours; the consequence of this process was a homogeneous distribution of larvae around the periphery of the arena. A second phase corresponded to angular reorganization of larvae leading to the formation of aggregates. The phenomenon was analysed both at the individual and collective levels. Individual cockroaches modulated their behaviour depending on the presence of other larvae in their vicinity: probabilities of stopping and resting times were both higher when the numbers of larvae were greater. We then developed an agent-based model implementing individual behavioural rules, all derived from experiments, to explain the aggregation dynamics at the collective level. This study supports evidence that aggregation relies on mechanisms of amplification, supported by interactions between individuals that follow simple rules based on local information and without knowledge of the global structure.

Self-Organized Aggregation Triggers Collective Decision Making in a Group of Cockroach-Like Robots

Self-amplification processes are at the origin of several collective decision phenomena in insect societies. Understanding these processes requires linking individual behavioral rules of insects to a choice dynamics at the colony level. In a homogeneous environment, the German cockroach Blattella germanica displays self-amplified aggregation behavior. In a heterogeneous environment where several shelters are present, groups of cockroaches collectively select one of them. In this article, we demonstrate that the restriction of the self-amplified aggregation behavior to distinct zones in the environment can explain the emergence of a collective decision at the level of the group. This hypothesis is tested with robotics experiments and dedicated computer simulations. We show that the collective decision is influenced by the available spaces to explore and to aggregate in, by the size of the population involved in the aggregation process and by the probability of encounter zones while the robots explore the environment. We finally discuss these results from both a biological and a robotics point of view.

The embodiment of cockroach aggregation behavior in a group of micro-robots

We report the faithful reproduction of the self-organized aggregation behavior of the German cockroach Blattella germanica with a group of robots. We describe the implementation of the biological model provided by Jeanson et al. in Alice robots, and we compare the behaviors of the cockroaches and the robots using the same experimental and analytical methodology. We show that the aggregation behavior of the German cockroach was successfully transferred to the Alice robot despite strong differences between robots and animals at the perceptual, actuatorial, and computational levels. This article highlights some of the major constraints one may encounter during such a work and proposes general principles to ensure that the behavioral model is accurately transferred to the artificial agents.

The structure of gallery networks in the nests of termite Cubitermes spp. revealed by X-ray tomography

Recent studies have introduced computer tomography (CT) as a tool for the visualisation and characterisation of insect architectures. Here, we use CT to map the three-dimensional networks of galleries inside Cubitermes nests in order to analyse them with tools from graph theory. The structure of these networks indicates that connections inside the nest are rearranged during the whole nest life. The functional analysis reveals that the final network topology represents an excellent compromise between efficient connectivity inside the nest and defence against attacking predators. We further discuss and illustrate the usefulness of CT to disentangle environmental and specific influences on nest architecture.

Aggregation behaviour as a source of collective decision in a group of cockroach-like-robots

In group-living animals, aggregation favours interactions and information exchanges between individuals, and thus allows the emergence of complex collective behaviors. In previous works, a model of a self-enhanced aggregation was deduced from experiments with the cockroach Blattella germanica. In the present work, this model was implemented in micro-robots Alice and successfully reproduced the agregation dynamics observed in a group of cockroaches. We showed that this aggregation process, based on a small set of simple behavioral rules of interaction, can be used by the group of robots to select collectively an aggregation site among two identical or different shelters. Moreover, we showed that the aggregation mechanism allows the robots as a group to “estimate” the size of each shelter during the collective decision-making process, a capacity which is not explicitly coded at the individual level.

Key behavioural factors in a self-organised fish school model

The dynamics of a self-organised model of shoaling fish are explored within a Lagrangian (or individual based) framework in order to identify the key behavioural factors that shape its dynamic landscape. By exploring systematically all possible initial states we identify the transitions to and between the different possible stationary states (schooling vs. swarming or milling). The route to these stationary states is explained from an individual perspective. On the behavioural level we discuss in particular the decisive impact of two traits, the perception angle and the manoeuvrability of the fish. A key result of this study is that the fish density in certain stationary states reaches values where each fish perceives each other; local interactions actually become global interactions. We further discuss the specific value of such Lagrangian studies in comparison to analytical approaches, in particular the freedom to include any important biological trait and the importance of an exhaustive numerical investigation.

The interplay between a self-organized process and an environmental template: corpse clustering under the influence of air currents in ants

Many spatial patterns observed in nature emerge from local processes and their interactions with the local environment. The clustering of objects by social insects represents such a pattern formation process that can be observed at both the individual and the collective level. In this paper, we study the interaction between air currents and clustering behaviour in order to address the coordinating mechanisms at the individual level that underlie the spatial pattern formation process in a heterogeneous environment. We choose the corpse clustering behaviour of the ant Messor sanctus as an experimental paradigm. In a specifically designed experimental set-up with a well-controlled laminar air flow (approx. 1 cm s−1), we first quantify the modulation of the individual corpse aggregation behaviour as a function of corpse density, air flow intensity and the ants position with respect to corpse piles and air flow direction. We then explore by numerical simulation how the forming corpse piles modify the laminar air flow around them and link this result with the individual behaviour modulation. Finally, we demonstrate on the collective level that this laminar air flow leads to an elongation and a slow displacement of the formed corpse piles in the direction of the air current. Both the individual behaviour modulated by air flow and the air flow modulated by the forming corpse piles can explain the pile patterns observed on the collective level as a stigmergic process. We discuss the generality of this coordinating mechanism to explain the clustering phenomena in heterogeneous environments reported in the literature.

How temperature influences displacements and corpse aggregation behaviors in the ant Messor sancta

Abstract.We studied the effects of temperature on movement and individual corpse aggregation behaviors (picking up or dropping a corpse) in the ant Messor sancta. Dispersion of ants in space was higher at 30°C than at 16°C. Concerning aggregation behaviors, we observed at both temperatures that the probability of picking up a corpse from a pile was negatively correlated with pile size, while the probability of dropping a corpse was positively correlated with pile size. The combined picking up and dropping behaviors represent a local amplification of corpse aggregation that is stronger at 30°C than at 16°C. Overall, this study shows clearly that the transport of corpses by ants, as well as the amplification process involved in corpse aggregation, is modulated by temperature. The implications of these results on the dynamics of ant cemetery formation (and other aggregation behaviors) are discussed. We propose that the modulation of a local amplification process by an environmental factor could be a general mechanism involved in the coordination of ant activity to adapt the external and internal shape and organization of the nest to environmental conditions.

A new test of random walks in heterogeneous environments

Environmental heterogeneities can change animal movement in two different manners. First, they can modify movement characteristics (move lengths or turning angles), in which case the movement remains of the diffusive kind. Second, they can bias displacement towards a particular direction in which case it becomes non-diffusive. We propose in this paper a simple method that only requires computing the mean length of a sample of trajectories in some bounded area to distinguish between these two kinds of movement. We show through simulations that the method allows to detect the presence of heterogeneities that orient animal movement. We apply it to experimental trajectories of Messor sancta ants engaged in corpse aggregation to show that their displacement is oriented at the contact of the formed corpse piles and that their trajectories become non-diffusive.

Self-Organized Linear and Helicoidal Ramps in Insect Nests

The nests built by termites of the genus Apicotermes present a regular succession of floors interconnected by vertical passages. By scanning these nests with X-ray tomography we observed that two different configurations of vertical passages coexist: ramps and helices. Based on our current knowledge of the mechanisms of nest building behaviour in different groups of social insects we formulate hypotheses about the mechanisms that could lead to the formation of these structures. In particular, we show that a 3D model of nest building in Lasius niger ants (Khuong et al, 2016) is capable of producing layered structures with vertical helices similar to the structures built by Apicotermes by simply running it with parameters different from those empirically measured for ants. It is possible that similar self-organised building mechanisms underlie the construction of the different nest structures produced by different groups of social insects.