Swarming behavior has always fascinated physicists, biologists, and behavioral scientists alike—as well as anyone who’s seen a sky-darkening flock of starlings twist into its mesmerizing shapes. It’s hard not to wonder how such elegantly concerted behavior arises on the fly, or how on earth the birds keep from running into one another. But birds aren’t the only things that swarm like this, and while the idea of The Birds acting as a collective is scary enough to merit a Hitchcock film, this might just be a psychological sublimation of the instinctive fear of a very real and far-more-threatening swarm: Locusts. Now, research from the University of Bath gives us some understanding of how this swarming behavior happens in insects, and how we might disrupt it.
To those in the developed world, a plague of locusts sounds like a comically archaic anxiety—when was the last time THAT happened, the old testament? But the terrifying reality is that five times in the last century, locust populations have reached official “plague” status, sometimes covering as much as 20% of the Earth’s landmass. Locusts, a sub-type of grasshopper, exhibit swarming behavior in all stages of life, forming “bands” as hoppers, and swarms when they take to the skies. However, unlike flocks of birds, the relationship between locusts in a swarm is an uneasy one—they’ll eat each other given the chance.
The main insight from the study, which sought to mathematically model locusts’ tendency to move in groups, arises from this cannibalistic behavior. A swarm begins, scientists discovered, with as few as three locusts—as the one in the middle tries to eat one neighbor without getting eaten by the other, the three start to move in synchronized fashion. The more locusts there are in the group, the less often the group changes directions: a sharp turn could put one bug’s flank right into the jaws of the bug behind it. This is, ultimately, bad for our prospects of disrupting locust plagues with air currents, one of the more feasible current proposals for swarm dispersal experiments.
This result rang a bell with me because I recently played Agar, the massively-multiplayer version of the classic blob-eating game that took the internet by — ahem — swarm a few weeks ago. The rules are simple: eat smaller blobs (called cells) to grow in size, and don’t get eaten by larger ones. Smaller cells move faster, so if one cell is just a bit larger than another, the larger one can’t catch the smaller one as long as it’s moving in a straight line. There’s a “splitting” mechanic as well, which larger cells can use to catch ones that are less than half their size, but if the larger cell isn’t big enough to “split-eat” the smaller one, swarming patterns much like the ones described in this study emerge. When you’re being chased by a slightly larger cell, the best strategy is to move in as straight a line as possible, to gain ground. If the cell trying to prey on you responds quickly, turning in any direction costs you ground, as the predator can cut the hypotenuse between your travel vectors. In the same way, locust swarms get a kind of inertia—while locusts with no neighbors move randomly, hoping to find vegetation or an unsuspecting fellow locust to eat, ones that are surrounded are forced to move as fast as they can, each chasing the bug in front of it as much as it’s fleeing the swarm behind, until they discover new territory with uneaten vegetation. I’ve cautioned in the past against trying to apply physics results to larger systems and human behavior, but in the simplified world of Agar, the parallels abound.
Study author Dr. Kit Yates wrote a popular summary of the article for academic news outlet The Conversation, which can be found here.