Takenori Tomaru

Emergent Runaway into an Avoidance Area in a Swarm of Soldier Crabs

Emergent behavior that arises from a mass effect is one of the most striking aspects of collective animal groups. Investigating such behavior would be important in order to understand how individuals interact with their neighbors. Although there are many experiments that have used collective animals to investigate social learning or conflict between individuals and society such as that between a fish and a school, reports on mass effects are rare. In this study, we show that a swarm of soldier crabs could spontaneously enter a water pool, which are usually avoided, by forming densely populated part of a swarm at the edge of the water pool. Moreover, we show that the observed behavior can be explained by the model of collective behavior based on inherent noise that is individuals’ different velocities in a directed group. Our results suggest that inherent noise, which is widely seen in collective animals, can contribute to formation and/or maintenance of a swarm and that the dense swarm can enter the pool by means of enhanced inherent noise.

Inherent noise appears as a Lévy walk in fish schools

Recent experimental and observational data have revealed that the internal structures of collective animal groups are not fixed in time. Rather, individuals can produce noise continuously within their group. These individuals’ movements on the inside of the group, which appear to collapse the global order and information transfer, can enable interactions with various neighbors. In this study, we show that noise generated inherently in a school of ayus (Plecoglossus altivelis) is characterized by various power-law behaviors. First, we show that individual fish move faster than Brownian walkers with respect to the center of the mass of the school as a super-diffusive behavior, as seen in starling flocks. Second, we assess neighbor shuffling by measuring the duration of pair-wise contact and find that this distribution obeys the power law. Finally, we show that an individual’s movement in the center of a mass reference frame displays a Lévy walk pattern. Our findings suggest that inherent noise (i.e., movements and changes in the relations between neighbors in a directed group) is dynamically self-organized in both time and space. In particular, Lévy walk in schools can be regarded as a well-balanced movement to facilitate dynamic collective motion and information transfer throughout the group.

Information transfer in a swarm of soldier crabs

Collective behavior is broadly observed in animal groups such as insect swarm, bird flock, and fish school. Both theoretical studies and field observations have investigated possible underlying principles based on local interaction among individuals in a group without global information via conductors or leaders. Information transferred among individuals would play a key role to understand it. In this study, to investigate how individual in a swarm uses information of its own past behavior or swarm mates’ behavior, we analyzed behavior of soldier crabs Mictyris guinotae in terms of local active information storage and local transfer entropy.

Exclusive shift from path integration to visual cues during the rapid escape run of fiddler crabs

Animals are equipped with sophisticated guidance systems that enable them to navigate efficiently. Previous studies suggest that foraging animals such as desert ants tend to combine guidance systems in a weighted manner, where the weight given to each cue gradually changes during the approach to the goal. However, when subjected to rapid enforced decision making (e.g. under predation risk), the animal may use alternative mechanisms, as recently suggested for ecologically relevant decisions involving time constraints in humans. We show here that fiddler crabs, Uca perplexa, scuttling to their burrows when threatened, responded to visual cues only if their path integration (PI) systems indicated nearness to their burrows. When homing errors were imposed by placing fake entrances (visual cues) along their homing paths and masking their true burrows (the goal of PI), the threatened crabs altered their behaviour towards visual cues according to the remaining PI vector length; if it was long, they continued running until they arrived at the masked true burrow, ignoring the visual cue, but if it was short, they suddenly stopped at the fake entrance. Our results suggest that, for fiddler crabs, PI and the view are mutually exclusive cues and that they instantly shift from one to the other if they approach the end of the home vector, instead of combining systems in a weighted manner. This could enable them to avoid entering the wrong burrow, where they would be ejected by the resident crab and be subject to predation.

Local perspectives of Plecoglossusaltivelis determine searching strategy

We argue for the notion of adaptability in collective behavior, treating real fish schools as an example. For evaluation, we use the inconsistent relation between local and global perspectives. We show that this inconsistency would affect whole group behaviors and lead them to use different strategies.We argue for the notion of adaptability in collective behavior, treating real fish schools as an example. For evaluation, we use the inconsistent relation between local and global perspectives. We show that this inconsistency would affect whole group behaviors and lead them to use different strategies.

Effects of departing individuals on collective behaviors

Utilizing living organisms’ abilities is an effective approach to realize flexible and unconventional computing. One possible bio-inspired computer might be developed from animal collective research by clarifying collective behaviors. Therefore, it is important to reveal how collective animal behaviors emerge. In many studies, individuals departing from the other individualsare generally ignored. Is it not possible that such departing individuals contribute to the organization of such collectives? To investigate the effects of individuals departing from a collective against collective behaviors, we observed and analyzed the behaviors of 40 soldier crabs in four types of experimental arenas. The recorded behaviors demonstrate a temporally changing pattern and the existence of departing individuals. We analyzed the relationship between global activity and cohesion levels and verified the features of departing individuals. The results imply that departing individuals contribute to collective behaviors.