Why would an ant walk in an endless circle until it dies of starvation and exhaustion? Known as an ant death spiral, or circular milling, this deadly phenomenon can cause hundreds of ants to follow each other, driven by instinct, in a never-ending circular pattern that lasts for hours, only ending when the last circling ant has died.
Army ants are blind and must follow pheromone trails to pick up the paths laid by others in their colony.
©Klaus Mohr/Shutterstock.com
It’s All About the Pheromones
The strange phenomenon was first noticed by scientists when Theodore C. Schneirla of the American Museum of Natural History published a study detailing the behavior occurring in a colony of army ants in 1944. Schneirla observed the ants were stuck walking in a circle, which he defined it as “circular milling.”
Why would ants exhibit this strange behavior, ultimately leading to their deaths? It all has to do with a chemical trail of scents called pheromones. Army ants (Labidus praedator) are blind. Since they can’t see, they rely on pheromone trails to know where to go next. As the ants follow the trail, they leave their own trail signaling for others to follow them.
The pheromone trail works like a charm for leading the ants in a straight line to a food source, that is, unless something goes wrong.
What Happens When An Ant Trail Goes Wrong?
You can watch the behavior in action in a YouTube video posted by Real Science. In the video, the ants are seen circling around themselves, stuck in a loop. As the video explains, the more ants that continue to join the spiral, the stronger the pheromone trail becomes. And the stronger the trail, the more motivated the ants are to keep going.
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Army ants can get caught in an endless loop following a scent trail and doubling back on themselves, never stopping until they die of exhaustion.
©Real Science / YouTube – Original
The spiral of death can begin with just one ant. If one ant is being followed by a group of ants, but she somehow ends up doubling back, the loop cycle is created.
This might occur when the lead ant encounters an obstacle and has to turn around. The lead ant, not knowing they are in the lead, now begins following the trail of the others. This can cause the entire group to get caught in a loop cycle, marching endlessly in a circle. The ants don’t know they are circling, but will continue on, faithfully following the trail until they die.
Why Did the Ants Not Evolve Out of This Dangerous Behavior?
Typically, a species will evolve out of behaviors that do not lead to its survival. We might think that nature would have selected ants that aren’t likely to get caught up in a death spiral. An article on NPR discusses a possible reason explained by Alex Wild, a photographer and biologist.
Wild suggests, “Army ant colonies are huge, their daily intake immense, their fecundity explosive. A few hundred spinning workers lost around the margins may not make all that much difference.” In other words, a few hundred ants won’t make or break the survival of a colony of millions, explaining why the behavior continues and has not been removed through natural selection.
What Types of Ants Are Known to Get Caught in Death Spirals?
There are tens of thousands of species of ants in the world. Only two species have been seen and recorded as engaging in circular milling. These are driver ants (the Dorylus species from Africa) and army ants (the Eciton species from South America).
Ants follow scent trails to forage for food and return home. As they go, they lay their own scent trail to help reinforce the path.
©dhtgip/Shutterstock.com
These particular species are both blind and follow scent trails to forage. Their colonies may contain 20 million individuals. They send batches of workers out to forage, and these workers rely heavily on scent markers. With the colony numbering in the millions, it doesn’t affect the group as a whole when a few hundred get caught up marching around in a circle.
What Is the Largest Known Ant Mill?
Even before Schneirla published his paper on ant milling, the phenomenon was described in 1921 by William Beebe. Beebee described an ant mill with a circumference of 1,200 feet in the “Edge of the Jungle“. To date, this is the largest ant mill ever recorded. They are typically much smaller, containing only a few hundred ants.
Beebe wrote, “Careful measurement of the great circle showed a circumference of twelve hundred feet. We timed the laden Ecitons and found…a given individual would complete the round in about two hours and a half.”
Beebe described the death march poetically, saying, “Through sun and cloud, day and night, hour after hour there was found no Eciton with individual initiative enough to turn aside an ant’s breadth from the circle which he had traversed perhaps fifteen times: the masters of the jungle had become their own mental prey.”
The post This Deadly Behavior Is Perfectly Natural for Ants. It’s Also Their Doom. appeared first on A-Z Animals.
