M


illions of tiny footprints in the sand tell


Iain Couzin he’s in the right place. Each is as small as a period. Together, though, they warn of what’s to come. Soon, he’ll hear a buzzing in the distance. He’ll see a black cloud forming. As this cloud gets closer, he’ll see it’s millions of flying grasshoppers called locusts. Flying locusts will bump into his face, tangle


in his hair, and cling to his clothes. Some will march over him. T en they’ll take off flying again in the same direction. At least, that’s what Couzin hopes he’ll see


here in Africa. T is biologist studies swarm behavior. He wants to learn why large numbers of animals of the same species move together.


Come Together Locusts aren’t the only animals that swarm. For example, it’s what birds do when they flock. It’s what fish do when they school. T ese animals swarm for diff erent reasons.


Yet for each, the behavior starts the same way. It starts with their sensory receptors. Tucked in skin, eyes, and even on antennae,


these receptors are part of an animal’s nervous system. T ey pick up stimuli, or signals, from the environment. T e signals could be smells, light, or even pressure. T e nerve cells send messages to the brain. T en the animal reacts. Couzin studies the link between sensory


inputs and how animals react. He’s traveled around the world to learn about swarms.


An Army of Ants Couzin is deep in a rain forest in Panama. T e ground around him ripples like it’s alive. It’s a swarm of army ants. More than a million nearly blind ants make up this swarm. Yet they march with the precision of an army. T ey zip back and forth without bumping into one another. At a stream, the ants build a bridge with their bodies, ants crawling over other ants. T e ants use receptors in their antennae to


stay organized. Some receptors pick up smells, or chemical inputs. Other receptors respond to pressure, or mechanical inputs. T ese inputs tell them where to go. Here’s how. A scouting team of ants finds


a cricket. As they return to the nest, they rub their abdomens on the ground. T is makes a chemical trail that other ants can smell. It tells them where to go to find food. Each ant sweeps the air with its antennae.


Its receptors pick up the chemical scent. Nerve cells carry the smell message to the ant’s brains. Soon, thousands of ants swarm, following the trail to the food. T ey find and kill the cricket. T e ants also use touch receptors in their


antennae to feel which way to go. When one ant’s antenna bumps another ant, the first ant knows to move aside. Swarm behavior keeps these ants alive.


Sometimes, an ant wanders away from the swarm and loses its way. It doesn’t know what to do. It has no trail to smell, no ant to touch. It walks in circles until it dies from exhaustion.


A desert locust like this can swarm. It joins millions of other locusts, hopping and fl ying in the same direction.


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