How Ants Get Trapped in a Deadly Circle

Ants can accidentally form a nonstop circle called an ant mill, following one another until they become exhausted and die.

Biology

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Ants normally rely on chemical scent trails to keep a group moving toward food or back toward safety. That system is remarkably effective, but it can fail in an unusual way. When a group becomes separated from its normal trail, ants may begin following the scent and movement of other ants instead. If the path loops back into itself, the group can form a continuously rotating circle known as an ant mill or death spiral. Each ant keeps following the one ahead, while the repeated movement can reinforce the circular route and make it difficult for the group to escape. The behavior has been observed in army ants and is an example of how a simple rule followed by many individuals can create an unexpected group pattern. Similar self-following patterns have also been observed in other animals, including processionary caterpillars and some fish.

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