Why do some animals swarm?

Birds of a feather flock together. But why exactly? It’s not just birds – mammals, reptiles, amphibians, fish, insects and even bacteria all do it. Virtually every corner of the animal kingdom has found that banding together is a great tactic for surviving in the wild. As with everything else in nature, it boils down to two things: finding enough food and avoiding becoming food for something else. Swarming can help with both.

Herring, for example, feed on copepods. These tiny crustaceans, one or two millimetres (0.04-0.08 inches) long can shoot suddenly sideways for a couple of centimetres to evade an approaching fish. Herring aren’t agile enough swimmers to react to this jump, so instead they swim in large schools with the gap between fish synchronised to the jump distance of the copepod. This increases each fish’s chance of a meal because if a copepod leaps out of the path of one fish, it lands directly in front of another.

At the other end of the food chain, another marine crustacean – the krill – swarms to avoid predation. A large swarm of randomly swirling krill makes it much harder for fish to pick out any single target. Ironically, krill swarms are so huge that it has become viable for a much larger kind of animal – the baleen whales – to evolve the specialised apparatus for straining out several tons of krill in a single gulp. Ironically the very behaviour that protects them from small fish and penguins makes them vulnerable to the biggest creatures of them all.

Swarming also helps because it increases the number of eyes and ears on the alert for danger. A herd of wildebeest or a flock of seagulls allows each member to borrow the senses of the other animals as a sort of long-range radar. There’s no need for direct communication; simply keeping up with the rest of their neighbours means that when one end of the group spots danger, the whole swarm wheels away from the threat as if it were a single organism. In fact, it’s possible that multicellular life itself began as nothing more than swarming behaviour. Dictyostelid amoebae (a form of slime mould) live the ordinary, solitary life of a single-celled organism when food is abundant. But when it runs out, they secrete a chemical signal called cyclic adenosine monophosphate (cAMP) that attracts other nearby Dictyostelids. At a certain critical mass, the amoebae form into a multicellular ‘slug’ up to four millimetres (0.16 inches) long and move off in search of new food. The ‘slug’ has a definite front and back end and moves towards heat, light and humidity. It acts like a simple multicellular animal, but it’s actually just an amoeba swarm.

Swarms have a dark side too though. Because the swarm moves and acts as one, it can quickly become unstable. At low population densities, locusts move about randomly or in small groups. This is controlled by the level of serotonin in each locust, which increases in response to stress. As the density rises they become more and more co-ordinated until, at about 74 locusts per square metre, they stop changing direction altogether and march like an army for hours at a time. Locust swarms begin in response to overcrowding, but because they all travel together, they just make the overcrowding worse, sweeping across farmland like a wildfire and destroying all plant life in their path. It’s precisely the co-ordination and synchronisation that can make the swarm so destructive.