Locusts live two lives. Usually solitary, something in them stirs when the density of their fellows reaches a certain threshold. They then change their markings and exhibit an intense desire to swarm together.
Using a mathematical model called percolation theory, Andy Reynolds of Rothamsted Research in Harpenden, UK, and three colleagues explain why this behaviour might have evolved. If a landscape is divided into patches — clumps of plants, say — then each patch has a density of locusts below which it is not worth a predator expending the effort to look for them. So at low densities, locusts are relatively safe. With increasing density, they would become worth hunting, a smorgasbord of crunchy treats allowing well-fed predators to move around the landscape. Before this happens, locusts flip into a highly bunched distribution in which most patches are locust-free and only a few patches are full, so fewer predators are on hand to menace the entire swarm.
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Mathematical biology: Sensible swarming. Nature 457, 132 (2009). https://doi.org/10.1038/457132e
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DOI: https://doi.org/10.1038/457132e