Eciton burchellii (Ecuador)

In yesterday’s open thread many of you raised questions about army ants. Below, I do my best to field answers. You’d do well to keep in mind, though, that I’m not an army ant researcher myself.

(Andrew): How long does a queen live?

M: This is one of those simple questions that turns out to be nearly impossible to answer. Army ants don’t keep well in captivity, and it’s rather tiresome to follow a single queen in the wild waiting for her to die, especially since ant queens can live for many years. Army ant guru Carl Rettenmeyer once recaptured an Eciton queen he’d marked 5 years earlier, so they can live at least that long. Beyond that? I don’t think anyone knows.

(Mike B): Given the pervasive nature of army ants in the tropics, what sort of defenses have their prey evolved to avoid being eaten?

M: One of the more spectacular natural ant phenomena is the mass evacuation response. If prey ants can get their brood and queens out of the nest and into the safety of low vegetation they stand a good shot at at avoiding the worst of the raid.

See, for example, Adrian Smith’s video of harvester ants bailing out after smelling a single army ant worker:

A few ants with large colonies and specialized soldiers fight back. One of my favorite ant photos of all time is one by Scott Powell showing leafcutter majors forming a line to block an intruding Nomamyrmex raiding party:

Nomamyrmex vs. Atta

You can read about the behavior here, and a similar encounter between Neivamyrmex and Pheidole here.

Other ants use stealth to avoid detection. Some tropical colonies mature at surprisingly small sizes, much smaller than for temperate ants. Thaumatomyrmex nests, for example, may hold fewer than a dozen workers, making their colonies difficult to detect and probably not worth the army ants’ efforts. Tropical species are also smaller, on average, than temperate species. The reasons behind the miniaturization of tropical ants are unknown, but escape from the incessant pressure of army ants might be one factor.

(MrILoveTheAnts): Somehow I got it in my head that not all army ants migrate long and vast distances. Is it true that some of the smaller or lesser known species, that are more subterranean perhaps, actually don’t migrate more than say a common Formica rufa colony budding?

M: Your question brings attention to the unfortunate fact that nothing is known about the natural history of a majority of army ant species, especially the small subterranean ones.

In my limited experience, I’ve never seen even small species in migratory phase remain in an area for more than a few days. An individual migration event might be less than a standard Formica nest emigration, but I suspect over time the army ants will move much farther across the landscape.

Neivamyrmex bohlsi, Argentina

(Andrew): How do army ants start new colonies? Do they do so when a new queen buds off from an already existing colony(I think I read that somewhere)? And if this is true, what happens? Do they just go separate ways(how do they disperse?)?

M: Large, reproductively mature colonies reproduce by division. In Eciton burchellii, new queens are reared about once a year. The colony divides very nearly in two, and the split that inherits the daughter queen will receive males that fly in from other nests.

Myrmecologist Nigel Franks has a 1985 paper that summarizes the reproduction of Eciton burchellii.

(Troy Bartlett): I was wondering how bivouac sites are selected. You know, like how honeybees have a sort of voting process until a tipping point is reached.

M: I would love to know this, too! Deciphering the honey bee quorum-sensing behavior took decades of research, and honey bees are a commercially important insect adapted to captivity. Army ants are much more problematic to work with. They don’t keep well in the lab, and studying them in the field is logistically difficult because they’re always moving around. So the short answer is, we don’t know.

(Teleutotje): Although I know it is mostly about New World species, I have two questions: 1) Do you have more info about Cheliomyrmex? 2) How about the subgenera of Dorylus?

M: Cheliomyrmex is a phylogenetically interesting ant. It has a number of morphologically primitive characteristics. It is, for instance, the only new world army ant with a single petiolar segment. But Cheliomyrmex is also rare. I spent time looking for raiding columns during my Ecuador trip with the same result as most hopeful Cheliomyrmex hunters: nothing.

Cheliomyrmex, via Antweb.org

As to Dorylus, this is an ant I have only seen alive once. I haven’t spent more than a couple weeks in Africa, and I’m not all that familiar with the technical literature. I will refer you instead to Kronauer (2007), a paper on foraging evolution I consider to be among the best of recent Dorylus work.

(JasonC): Are Eciton also facultative frugivores like Labidus coecus?

M: Not that I’ve noticed.

(Alice Zee): I was wondering if there are any examples of army ants evolving to take advantage of human-altered ecosystems.

M: Some species seem to tolerate mild levels of habitat disturbance. Labidus coecus is one that seems to persist. One recent study concluded that surface-foraging species like Eciton are more vulnerable to landscape changes than subterranean species.

The overall effect of humans on army ants is devastating, though, as it is for many wild animals. I don’t know of any examples of army ants evolving to take advantage of human-altered ecosystems.

(Matt): Where does the name Eciton come from?

M: The name was created by French entomologist Latreille in 1804, but he provides no further explanation.

(Julio Chaul): I’d like to know if there is any morphological trait that army ants’ males have in comon. Couldn’t the sausage-like style be considered one of the characteristics of the army ant syndrome? How looks like an Eciton male?

Carl Rettenmeyer's classic photo of Eciton burchellii mating

M: You’re absolutely right that most army ant males have a distinct look. Males of Ecitoninae, Dorylinae, and Aenictinae in particular are large and elongate, like little flying sausages, and they often have bizarre mandibles for gripping the queen during mating (above).

Whole suites of army ant traits are related to reproduction, and since these seem to travel together through evolution, using all of them to define an army ant syndrome is redundant, in my opinion. That’s why I left male traits off my description of army ants.

Neglecting males entirely is probably a bad idea, though, as they are integral to ant biology and are historically under-studied.

Neivamyrmex swainsonii, male (Arizona)