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: