What are army ants?

Eciton burchellii return from a raid (click to enlarge)

Welcome to Army Ant Week!

Of the entire rich tradition of theme weeks, I do believe this is the first ever devoted to army ants. So stay tuned- we’re, um, making history here.

I’ll start with a brief introduction to the topic. What are army ants? Are they for real?

Yes! Army ants are real animals, and some are rather common, especially in tropical forests.

The key to understanding Army Ants is to recognize that they are high-energy insects. They require vast amounts of food, and they are willing to expend vast amounts of energy to obtain it. The unique traits that define army ants are thus intimately tied to a biology marked by large inputs and large expenditures.

What are the unique army ant traits?

  • Nomadism. Instead of permanent nests, army ants set up temporary encampments (“bivouacs”) constructed from their own bodies. Staying on the move prevents the voracious ants from depleting local food sources.
  • Group predation. You won’t find a lone army ant out scouting for food. Instead, they travel in columns or swarms. Food is located, killed, and processed by teams of ants working together.
  • Reproduction by colony fission. Because army ants must hunt in groups, colonies can’t follow the traditional ant life cycle starting from a single foundress queen. Rather, new colonies are formed when existing colonies divide. Young and old queens each go their separate ways with masses of workers. Related to this behavior, army ant queens never have to fly. Unlike most other ant queens they are wingless.

The army ant lifestyle is a successful one, and several evolutionarily distinct lineages of ants have taken it up. This means army ants are not a taxonomic category but a functional grouping. Still, a majority of species are related to each other, coming from a series of subfamilies collectively referred to as the “Doryline section” occurring in warmer regions around the world:

Distribution of the three largest lineages of army ants, borrowed from Kronauer (2009)

Army ants are predominantly tropical. This, too, is likely related to their constant need to feed. A winter dearth season is incompatible with a lifestyle of continuous movement.

All told, several hundred ant species are army ants. Most are small, subterranean predators of the brood of other ants, a prey specialization that likely reflects the ancestral condition. A few species have since evolved a more catholic above-ground diet, and it is these conspicuous generalist feeders- especially some Eciton (in the Americas) and Dorylus (in Africa)- that dominate the nature documentaries.

As you might imagine, any animal that stays on the move to avoid driving its food to extinction must put intense pressure on its environment. Indeed, army ants have profound effects on prey populations. The generalist species that eat just about anything are rightly considered top predators, equal in ecosystem impact to large mammals like jaguars or wolves.

All this week Myrmecos blog will feature photos, videos, and commentary exploring army ants. So stick around, and don’t be shy about joining the discussion!

A few resources:

25 thoughts on “What are army ants?”

      1. hmmm… Army Ants vs Grapefruits….

        I know !!! run it thru the Taxonomy Fail Index (TFI) = T/H

        That’s sure to pick a winner !!

  1. I really like ants. Martha and I were in the tropics once and photographing some army ants. What you do not want to do is kneel down to photograph them and put your knee on one of their soldiers. OUCH !!! It was my fault. They are hard to get loose once locked on. BTW, I was in short pants.

  2. As I understand it, the only ‘true’ nomadic ant is Dolichoderus cuspidatus and D. tubifer with Malaicoccus mealybugs. Nomads migrate and coordinate their lives with their livestock, so army ants, which hunt prey, are more like migratory large-scale hunter-gatherers than true nomads.

    Also, I read that dichthadiiform queens are also a unique aspect of the army ant syndrome, is that true?

    1. You’re right that dichthadiiform queens are part of the army ant syndrome, Jason.

      In my thinking that’s basically linked to colony reproduction, so I sort of subsumed that bit into the ‘colony fission’ example.

  3. I would have loved this theme even without the build-up (but it didn’t hurt).

    These are fabulous creatures, whose truth is stranger and far more beautiful than all the fiction about them. I look forward to all you can give us this week!

    For now, I’m eager to learn how you took the incredibly deeply focused, crisp photo of E. burchelli above, in the difficult lighting conditions of the forest.

  4. I am going to echo James, I would love to hear all the details about your photographs.

    Did you see any ant birds or ant butterflies?

    Can’t wait to hear more.

  5. An appetizing introduction!

    I’ve always been enamored with the word “bivouac” for some reason (and I’m not sure I can’t recall ever hearing it used in any other context).

  6. The most exciting “biological experience” I’ve ever had, was following these guys through a lot of undergrowth to suddenly stand in front of one of those amazing bivouacs.

    Looking at the two year old pictures from the “zoological excursion” of my university (Tübingen, Germany), lets me see how astonishing good your pictures actually are!

    Maybe you’d like to take a look too, especially on the bivouac:

    (Brasilian rainforest, near Alta Floresta)

    Looking forward to a great week!

  7. Those jaws always seem somewhat “unwieldy” to me. They look like they’d be really good for stabbing and pinching, but awkward for actually manipulating food. I’d love to see some shots of army ants eating if you’ve got some. Do all army ants have crazy pincer jaws?

    1. You’re quite right- army ants can’t do much with those jaws other than snare vertebrates.

      It’s only a small number of individuals in each colony, of some species in the genus Eciton, that bear those unwieldy defensive mandibles. Most army ants have standard triangular mandibles.

  8. What shall I say? Beautiful pictures, very interesting group of species and first hand information – something like heaven, but for real. Thankyou very much! And, sure, I´ll be reading´every word and watching every mov(i)e (without forgetting Roberta´s & Tschinkels Fire Ants (of) course)…

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