Eciton burchellii, the swarm raider

Eciton burchellii is, according to Wikipedia, “the archetypal species of army ant“. Insofar as this is the most-studied species, and the ant that dominates the nature documentaries, I suppose the moniker is true.

Yet, the biology of E. burchellii is not terribly representative of army ants. It is an outlier, an ant whose behavior has diverged in significant ways from its relatives, even from its congeners.

media and minor workers stream towards the front

The primary difference is gustatory. Most army ants boast a fine-tuned pallate, favoring the brood of particular ant genera. E. burchellii is vulgar by comparison. It’ll eat just about any sort of animal protein. Spiders? Katydids? Lizards? Termites? Ants? They’re all good.

a submajor worker carries a spider
media workers pull a termite from a rotting log

Corresponding to the broad diet, Eciton burchellii‘s foraging behavior is a radical departure from the army ant norm. For being an “archetypal” army ant, its raids ironically lack the military precision of other species.

An overhead depiction of an Eciton burchellii swarm raid (from Rettenmeyer 1963)

Instead of tight, focused columns that concentrate the ants’ efforts at a single point at the raid front, E. burchellii’s raids are messy, diffuse affairs. Foragers spread out in a swarm, casting a vicious stinging, biting net for animals too slow to escape. The strategy isn’t great for capturing concentrated food sources like another ant colony, but it works well for a generalist’s lunch.

Raids set out in the morning with a swarm front near the overnight bivuoac. As the day progresses, the swarm moves through the forest for a distance about that of a football field.  The back end of the swarm organizes into a series of converging trails to funnel captured prey back to the bivouac, so that by mid-day the swarm from above looks rather like the figure at left. Last night’s video featured one such trail. By nightfall the ants are back in the bivuoac, where, depending on the development of their larvae, they either regroup to march to a new bivouac site by morning, or they stay put and launch a new raid in a different direction the following day.

Eciton burchellii raids are fascinating to watch, but they aren’t indicative of how most army ants behave. The fame of this one species is partly an artifact of the degree to which its unique biology intersects with the cognitive quirks of our own species.

These ants are large, they forage above ground, and their sprawling raids cover extensive areas. And they raid primarily during the day, when we humans tend to walk about in the forest. That they catch our attention may be due as much to our biology as to theirs.

[photos 1,2,4 taken at Jatun Sacha; 3,5 at Maquipucuna]

17 thoughts on “Eciton burchellii, the swarm raider”

  1. I’ve always found the “ant specialist” army ants to be so much more captivating, in terms of their foraging biology. Kind of like Chimps eating only baboons, or thinking more specifically about predators, lions that only prey upon jaguars. Or perhaps the best vertebrate analogy: king snakes (specialist predators on other, usually venomous, snakes). These kinds of top predators are usually of enormous conservation interest, but unfortunately not ant specialists. My experience with Neivamyrmex in Florida suggests that this genus is a top predator of the most abundant ants in natural areas (e.g. Pheidole morrisi), yet we know very little about their true ecological impacts. Cool stuff Alex!

      1. Alex,
        Yes, I’m hoping to get something going on Florida army ants in the next couple of years. I’m trying to get a Pheidole morrisi natural history monograph published this year (with Corrie, Andy Yang, Stef, and Walter) and I’m hoping to move on to Neivamyrmex and Formica natural history projects next. That is, after Walter and I figure out whether or not queens select habitat (our current NSF grant).

        1. Josh – “These kinds of top predators are usually of enormous conservation interest, but unfortunately not ant specialists.”
          Do I take you to mean the ants that are ant specialists have not received the interest of conservationists, and that they ought to? I can think of a couple of examples of such interest:
          — It has been noted, in the forest fragmentation studies in Amazonia, that Eciton (also including the swarm raiders, plus the Labidus ones) drop out of smaller fragments pretty quickly.
          — Also, CITES recognizes quite all or most dulotic ants, all specialized predators of a sort, as being of conservation concern, corroborating your concern. Distribution and size of host species nests have here and there in the literature been reported to be sparser and smaller near, say, a healthy Polyergus colony.

          And, also relevant to this thread and your response, an anecdote: Last September, in my yard, I found Neivamyrmex nigrescens around dusk, trailing in the direction of a Formica pallidefulva colony I had been observing and photographing since spring. Next morning, I came out to find the Formica straggling back to their nest with small brood, and they were completely gone the next day. I didn’t get to see the action, but it seems very likely they were raided by the army ants, and it had a big impact on this formerly busy, healthy, and spatially stable prey colony.

        2. Michael Suttkus, II

          I was excited yesterday when the map of army ants had Florida colored in. I couldn’t believe Florida had army ants and I was somehow unaware of this. This article makes it a bit more clear, they aren’t the “popular” sort of army ant, meaning Hollywood doesn’t care.

          Are there any layman resources on them? I’d love to know what to look for when I’m prowling the woods.

        3. James,
          Yes, I was aware of the dulotic species’ listing, especially Polyergus – recall our JIS article makes mention of this particular issue in Florida’s pine flatwoods. I was thinking of Neivamyrmex as a species of conservation concern more along the lines of a “keystone predator.” I think they and perhaps Diplos may be extremely important biotic “forces” in structuring the ant communities of FL. Anyhow, thanks for the Formica mention – I’ve not yet seen raids on Formica. Primarily Aphaenogaster and Pheidole (N. texanus), P. dentata (N. opacithorax), and P. floridana (N. carolinensis) – note the size correlation between prey and predator.

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