Few people can fail to marvel at a flock of birds swooping through the evening sky, homing in with certainty on its chosen resting place. The natural world abounds with other spectacular examples of animals moving in concert: a school of fish making a hairpin turn, an ant colony building giant highways, or locusts marching across the plains.
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Since ancient times, scientists and philosophers have pondered how animals coordinate their movements, often in the absence of any leader. Coordinated groups can range in scale from just a few individuals to billions, and they can consist of an intelligent species or one whose members have barely enough brainpower to recognize each other.