FAQs: What is the Ants 'circle of death' ?

- October 28, 2023

Ant Mill: Ants Spiral of Death

By Ivan Miyingo Quintus

The ant-mill, also known as the circle of death or the death spiral is a phenomenon where a group of ants gets trapped in a never-ending loop or circle, seemingly unable to escape. This behavior is observed in certain ant species, such as the Argentine ant (Linepithema humile), when they encounter a chemical barrier, like a circle of a repellent substance, which disrupts their foraging trail.

A group of army ants, separated from the main foraging party, lose the pheromone track and begin to follow one another, forming a continuously rotating circle. This circle is commonly known as a "death spiral" because the ants might eventually die of exhaustion. The phenomenon has been reproduced in laboratories and in ant colony simulations.

The phenomenon is a side effect of the self-organizing structure of ant colonies. Each ant follows the ant in front of it, which works until a slight deviation begins to occur, typically by an environmental trigger, and an ant mill forms. An ant mill was first described in 1921 by William Beebe, who observed a mill 1200 ft (~370 m) in circumference. It took each ant two and a half hours to make one revolution. Similar phenomena have been noted in processionary caterpillars and fish.

Here's how the "circle of death" of ants typically occurs:

Foraging Trail: Ants often follow a chemical trail laid down by scout ants to locate food sources. When a foraging ant finds food, it marks the path with pheromones, creating a trail for other ants to follow.

Repellent Substance: If, at some point, a circle of repellent substance is introduced into the path of the ant trail, such as a repellent chemical or even a natural substance like chalk, the ants will avoid crossing it.

Endless Loop: As the foraging ants approach the circle of repellent, they will turn away and try to find an alternative path. However, as they continue to follow the pheromone trail, they keep encountering the repellent circle, causing them to continually change direction, forming a loop.

Stalemate: This loop can continue indefinitely, with the ants seemingly trapped in a circular pattern. They are unable to break free from the loop because they are instinctively following the chemical cues left by their fellow ants.

The "circle of death" can be a puzzling and somewhat amusing behavior to observe in ants. It highlights the strong reliance on pheromone communication in ant colonies and how they can be momentarily confused by the presence of a chemical barrier. To break the loop, one of the following actions can be taken:

Physically disrupting the circle to allow the ants to continue on their way.

Removing the repellent substance to let the ants pass freely.

Providing an alternative path for the ants to follow.

It's important to note that while the term "circle of death" is commonly used to describe this behavior, it doesn't imply that the ants are actually in any danger; they are simply following their natural instincts and communication cues.