Ants moving in a circle (Niamh, Costa Rica)

I am currently living in Costa Rica; I did a quick search after witnessing the following but could find no information. On our back porch we noticed a group of ants running around in a circle for hours. There were quite a few of them and there was a bunch of live ants stuck in the center of the circle. They were going that way for over 5 hours with the circle getting bigger than a large donut. I think they were going clockwise. What's up?

Dear Niamh,

Since you are asking about a very interesting and odd behavior in some ants from Costa Rica, we called in our colleague Dr. John Longino who is an expert on ants from Costa Rica. You can check out his really great website here. Here is what Dr. Longino said:

"Interesting observation! I have never seen anything exactly like that. Army ants can stream across a porch for hours, but usually in one or multiple directions. They are blind and tend to follow each other, so theoretically they could get caught in a closed loop by accident, like Fabre's famous processionary caterpillars (there is filmed version of this in the movie Microcosmos), but I have never seen it actually happen. Another possibility is a colony of some other ant that has been flushed from its nest by army ants or somehow disturbed, resulting in an evacuation of the nest. I have seen this, with a large number of workers carrying brood waiting in a group on a big leaf or on a wall (could be a patio). There may be workers on the periphery of the group that are more active, patrolling the border. But what I have seen is just irregular motion, not coordinated movement in a circle. A third possibility is ants recruiting to food (e.g. a dropped bit of jelly). Workers in the center can be motionless at the food, licking up fluids, while other workers patrol around the periphery. But again, these ants around the border are usually moving irregularly, not in a circle, and some ants are coming and going."

Also, see this video for a potential explanation:

John Longino & the AntAsk Team

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