There's a price for blindly following those in front of you. Take army ants for instance. These aggressive insects have a dangerous tendency to commit mass suicide just because they're following the leader.

This bizarre phenomenon — in which ants circle around and around until they all drop dead of exhaustion — is called an "ant mill." More colloquially, it's often referred to as an "ant death spiral." You can see it in action here:

So what's going on that causes these ants to seemingly go mad? It's all tied in to what makes them evolutionarily unique, where their advantage characteristics also serve to create at least one particular disadvantage.

Army ants — unlike most other ant species — are blind. They also lack permanent nesting sites. Instead of living at a single site, army ant colonies are constantly on the march en masse looking for food. As the first ant in line travels it leaves behind a pheromone trail that other ants sniff out and follow. When this system works well, it allows foraging parties lead larger groups back to food. When it doesn't work, the ants follow these pheromone trails as they flow back into each other, ending up in an endless loop that they follow to their doom. If the circle isn't broken for some reason, they will probably never escape.

Ant milling has probably been around for millennia, but it was first observed by science in 1936, when ant biologist T.C. Schneirla came across a mill of several hundred ants that lasted for an entire day. Even a heavy rain did not stop them. By the next day, most of them were dead, although a few continued to circle, weakly, close to death. He wrote about the mill and its aftermath in a 1944 paper describing the experience. "On the spot of yesterday's phenomenon little or no circling is to be seen. The entire area is strewn with the bodies of dead and dying Ecitons. A few of the survivors wander about slowly, while no more than three dozen of them form a small ... and rather irregular circular column in which they plod around slowly, counterclockwise." Interestingly, other nearby ant species made use of their fallen comrades: "various small myrmecine and dolichoderine ants of the neighborhood are busy carting away the dead."  

Although the largest ant mill ever observed was hundreds of feet across, most are just a few inches or feet across and comprise just a few dozen ants. Noted insect photographer Alex Wild wrote about the phenomenon on his blog a few years ago. "I used to see ant spirals all the time when I lived in Paraguay, and not just in the field. [Army ants have] no qualms about raiding through rural houses, and I'd come home to find circles of ants whirling about on top of my plates in the kitchen, or sometimes an intimate ring of 5-6 ants on a coffee mug. Unnaturally round objects, mostly." He writes that small swirls like this are deadly for individual ants but meaningless for the entire colony, which can comprise hundreds of thousands of ants.

Although there are more than 200 species of army ants living on both sides of the globe, genetic evidence indicates that they may all have common ancestors and have kept their evolutionary advantages and disadvantages for more than 100 million years. As Frédéric Delsuc wrote in PLOS Biology in 2003, all army ant species share the qualities of collective foraging, nomadic living and wingless queens that can produce vast quantities of young. These morphological and behavioral similarities enforce their collective behavior, with individual ants unable to survive well on their own. While evolution gave the ants a successful strategy to survive as a group, it may have also left them with remnant behavior, a "pathological" behavior that can be seen "as the footprints left by the evolutionary trajectory in which these ants have been trapped."

When that trap also traps them in a death spiral, it's the end of the line.

For more on ant death spirals, check out this video from the Discovery Channel:

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