Curr. Biol. doi: 10.1016/j.cub.2008.10.070 (2008)

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.