Group structure in locust migratory bands
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Locust swarms are spectacular and damaging manifestations of animal collective movement. Here, we capture fundamental features of locust mass movement in the field, including a strongly non-linear relationship between collective alignment and density known only from earlier theoretical models and laboratory experiments. Migratory bands had a distinct structure, with a single high-density peak at the front, where collective alignment was high, followed by an exponential decay in density. As predicted by theory, alignment decreased with decreasing density, and fluctuations of movement direction became large until order amongst group members at the back of the band was totally lost. Remarkably, we found that the coordinated movement of migratory bands, which can be several kilometres wide and contain many millions of individuals, results from interactions occurring at a scale of 13.5 cm or less. Our results indicate that locust band structure and dynamics differ markedly from what is known (or assumed) about other large moving groups such as fish schools or bird flocks, yet they still conform to key general predictions made by collective movement models that explain how billions of individuals can align using local interactions.
KeywordsCollective behaviour Collective movement Locusts Migration
We thank Mark Logue and Alex Stewart from the WA Department of Agriculture and Food, Brad Hazell from the NSW Department of Primary Industries, Ted Deveson, Rob Graham, and Peter Spurgin from the Australian Plague Locust Commission, and the landowners of the surveyed properties. We thank Julie-Anne Popple, Marie-Pierre Chapuis, Karine Berthier, Gabriel Miller, and Matthew Collett for their assistance in the field. We also thank two anonymous referees for their invaluable comments and suggestions. This work was funded by the Australian Research Council (ARC) Linkage and Discovery programmes, and SJS was supported by ARC Federation and Laureate Fellowships.
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