Ant Death Spiral

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A while back this video was making the rounds:

It’s a self-reinforcing circular mill composed of- now here’s a change of pace! – army ants. I thought I’d reintroduce it in honor of this week’s festivities.

Labidus praedator

The species is Labidus praedator, a swarm-raiding army ant from Central and South America, and these circular mills are a common byproduct of army ant navigation. Labidus is completely blind, so ants in this genus get about by following the insect in front and laying down a chemical trail. The system works well enough in a straight line.

The trouble begins when the ants loop around and intersect their own path. The poor insects end up on a mobius strip of their own making, circling around and around until some either chance to leave the mill and the circle is broken, or they run of steam and perish. Thus, the Ant Death Spiral.

The phenomenon was first noted by pioneering army ant biologist Schneirla, who published a paper on it in 1944 under the less sensational term “circular milling”.  Schneirla’s detailed analysis is worth a read, not just for the natural history and a surprising amount of physics, but for a remarkable concluding sentence in which he asserts that people are clearly better than ants.*

In any case, youtube user l314kimo recently created a clever computer simulation showing how the mill arises. A simple set of behavioral rules given to digital ants is enough to recreate the phenomenon:

I used to see ant spirals all the time when I lived in Paraguay, and not just in the field. Labidus has 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.

You’d think spiral-induced mortality would be selected against, that ants would have evolved a counter-measure to such obviously maladaptive behavior. But 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.


*”It may be observed that while army ants are constitutionally susceptible to the predominance of circular-column behavior and can be freed from it only by the incidental fact of environmental variation, man is by no means susceptible in the same sense, with his cortical basis for versatile corrective patterns which under encouragement may reduce milling to the minor role of an occasional subway rush.”

18 thoughts on “Ant Death Spiral”

  1. I feel like someone should make a PSA video about the dangers of circling in ants.

    Also do the ants seriously die? Any idea if they eventually run out of the chemical pheromone? Is it correct to say the chemical lasts longer than the ant itself?

    1. Ha! I’d love to see a PSA video for this. Complete with a catchy, three-word action plan to prevent death spiraling. (Stop! Turn! Escape!).

      The ants do seriously die. In the mill that Schneirla documented, it was dessication that killed them. I would just end up with some dead ants in my coffee mugs.

  2. And still any questions who made all those crop circles?

    Okay, crap, but aren’t other ants prone to display the same antics? How do other ant species cope with the same problem? I give that they don’t have that large columns, and thus probably are indeed less likely to end up in a not that strange loop, but… Couldn’t, say, some Pheidole sp. get into similar difficulties?

    Not to forget: Mr. Wild, you are just what makes this cold season in Germany just that much better to endure – thanx, thanx, thanx.

    1. Thanks for your kind words, Ossein.

      I’ve only seen this in Labidus, but there are accounts of other species running into the same trap. Schneirla talks about some of these in the 1944 paper.

  3. Hey, I lived in Paraguay for a while, too! So what kind of ants where those huge ones with the horns on their heads, that made FULLY AUDIBLE clicking sounds? *shudder*

    1. Nde! Mba’epa nde rejaporaka’e Paraguaype?

      Those evil clicking toothy ants are the infamous Odontomachus trap-jaw ants, known for the immense speed with which they close their mandibles.

      Relevant to army ant week, they are also the preferred prey of the beautiful giant army ant Eciton rapax.

      And, the death-spiraling Labidus praedator army ants featured here are what the Paraguayans call Tahyi nê, or “stink ants.” They smell pretty awful.

      1. I was in Paraguay for a mission–and I’m still SO in love with the people! I’m so glad most of them are on the internet now…when I first left, I was so sad because most didn’t have phones or access to computers. Many have looked me up since then, years later on my email or on facebook. 🙂 I learned to understand some Guarani, but I didn’t spend enough time in the campo to learn to speak it well. But I’m still fluent in spanish at least, thank goodness!

        I remember asking the locals what they do for an exterminator when those big ants move in under their house. They said, “we move!” That is crazy that there are ants that prey on those huge ants. It almost sounds like those ‘huge mosquitos in the South’ jokes.

        Thanks for the fascinating blog!!

        1. Maybe Joe is right…I looked up the photos for the Odontomachus, and it doesn’t look like the ant I have in mind. I actually have an ant carcass in my souvenirs (normally I would say some explanation here to try to not feel silly, but heck! I’m talking to ant guys!), and it is quite different. I did a search on Atta in the database, and it looks like my culprit. Dem suckahs was beeg!

        2. Hello Mary, good to see about your nice experiences with the Guarani. The west did a lot unrighteousness to them and it is good to see people try to help them. We met in Asuncion Christians of the hutterian brotherhood working with them. We are here in Southern Ecuador working on His Kingdom…

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