The effect of changing topography on the coordinated marching of locust nymphs

Abstract

Collective motion has traditionally been studied in the lab in homogeneous, obstacle-free environments, with little work having been conducted with changing landscapes or topography. Here, the impact of spatial heterogeneity on the collective motion exhibited by marching desert locust nymphs was studied under controlled lab conditions. Our experimental circular arenas, incorporating a funnel-like narrowing followed by re-widening, did not constitute a major barrier to the locusts but, rather, mimicked a changing topography in the natural environment. We examined its effects on macroscopic features of the locust collective behavior, as well as the any changes in their marching kinematics. A major finding was that of the limited extent to which the changing topography affected system-level features of the marching locust group, such as the order parameter and the fraction of walking individuals, despite increased crowding at the funnel. Overall, marching kinematics was also very little affected, suggesting that locust marching bands adjust to the environment, with little effect on the overall dynamics of the group. These findings are in contrast to recent theoretical results predicting that environmental heterogeneities qualitatively alter the dynamics of collectively moving particles; and highlight the crucial role of rapid individual plasticity and adaptability in the dynamics of flocks and swarms. Our study has revealed other important features of the marching behavior of the desert locust in addition to its robustness: the locusts demonstrated both, clear thigmotaxis and a tendency to spread-out and fill the available space.

Intra- versus intergroup variance in collective behavior

“…On the basis of our previous work ( 4 , 34 ), we hypothesized that the number of individuals walking in the arena is a key stimulus, promoting marching. Accordingly, we calculated the conditional probability of each locust to walk as a function of the number of other walking locusts in the arena (see the example in Fig.…”

“…Locusts offer a quintessential example of animal coordinated collective behavior and are therefore exceptionally suited for study of the above questions: Swarms of marching locusts can comprise millions of individuals, aligning or synchronizing their movement across hundreds of square kilometers. Moreover, marching locusts will also demonstrate their distinctive collective behavior under controlled laboratory conditions ( 4 , 6 , 32 – 34 ) that can partially be reproduced in computer simulations. Although much studied, our knowledge of the complex dynamics and the mechanisms underlying the different aspects of locust collective behavior is far from complete [e.g., ( 6 , 35 )].…”

A Discrete Model of Collective Marching on Rings

“…The experiments inspiring our work are discussed in [5,3]. The mathematical modelling of the collective motion of natural organisms such as birds, locusts and ants, and the convergence of such systems of agents to stable formations, has been discussed in numerous works including [14,18,28,29].…”

“…In this work, we study the dynamics of "locust-like" agents moving on a discrete ringlike surface. The model we study is inspired by the following well-documented experiment [3]: place many locusts on a ringlike arena at random positions and orientations. They start to move around and bump into the walls and into each other, and as they do so, remarkably, over time, they begin to collectively march in the same direction; clockwise or counterclockwise (Figure 1).…”

“…As with much of the literature on swarm dynamics [10,7,4], our goal is not to study a mathematical model of locusts in particular (the precise mechanisms underlying locusts' behaviours are very complex and subject to intense ongoing research, e.g. [3,5]), but to study the kinds of algorithmic local interactions that lead to collective marching and similar phenomena. As such, we are interested in simple "algorithmic" principles that yield collective marching or that approximate the biologically emerging behaviours.…”