The Evolution of Swarm-Raiding Army Ants

Today is not only the final day of Army Ant Week, but Charles Darwin‘s 202nd birthday.

So I close Army Ant Week with a bit of speculation about evolution, and what army ants suggest about the nature of the evolutionary process.

Neivamyrmex californicus workers attack a pavement ant (California)

The 300 or so army ant species vary in behavior, but most are specialized ant predators that hunt in soil or leaf litter. Their limbs are short and stubby, ideal for subterranean maneuvering, and the workers are largely to completely blind.

The “classic” army ants, the surface-foraging swarm raiders familiar to most people, are in fact a minority of species (Eciton burchellii, Labidus praedator, and several Dorylus). Where did these spectacular swarm raiders come from?

You might think the swarm raiders are ancient, and that the specialist ant predators evolved recently. After all, a generalized surface predator should be able to birth all manner of more specialized descendants. But the available evidence suggests the opposite.

Here’s a figure from Kronauer et al (2007) showing key transitions in the African genus Dorylus:

The early lineages of Dorylus were underground ant specialists, and it is these that gave rise to the later swarm raiders. The evolutionary relationships of the new world army ants (not shown) indicate the same pattern.  Swarm raiders emerged from below ground stock.

All told, swarm raiding has evolved at least three times, each advent independent of the others, and each from similarly ant-eating ancestors.

In my opinion, this threefold replicate of leggy swarm-raiders emerging from stubby underground ant predators cannot be coincidence. Nor do I think it chance that swarm raiders don’t evolve from non-ant predators (except perhaps here).  The swarm raiding lifestyle is obviously a successful one- so why has it only been achieved via detours through underground ant predation?

Ultimately, the answer may not be knowable. But, the convoluted path to swarm raiding is a reminder that the evolutionary process does not draw direct lines progressing from ancestral to descendant lifestyles. Rather, natural selection works only in the moment, unable to see what might be advantageous generations ahead. Swarm raiding may simply be easier to evolve by taking the long road underground.

Eciton burchellii

21 thoughts on “The Evolution of Swarm-Raiding Army Ants”

    1. The mandibles on Dorylus have a cutting edge thus facilitating more prey that are available to consume.This coincides with much larger colonies than neotropical army ants who ,generally speaking lack the same cutting ability and profit from added stings.

  1. Michael Suttkus, II

    My speculation would be that small surface swarms aren’t a very successful strategy. Perhaps a small surface group is more noticeable than individual foraging ants, but not proportionately better at gathering food. The “ant nest raiding” behavior can start small, and become more effective with increasing numbers. This “preadapts” the species towards large raiding parties, which can be successful above the leaf litter.

    Again, nothing but speculation, and the perverse joy in hearing myself talk.

    1. I don’t know about individual foragers being less noticeable. If the ants make any kind of trunk trail, like the Pogonomyrmex ants, they are quite noticeable. You find one and there’s the trail to the nest and lunch, if you eat ants. This is what horned lizards (Phrynosoma) do: they find a trail of Pogonomyrmex and just pick of lunch as it comes by, with just a flick of the tongue.

      If you are thinking of individual in terms of diffuse foraging, perhaps that is the case.

  2. what do the sister groups to other army ant syndrome lineages do? for example what do the sister species to raiding Leptogynes do – is it a similar situation to ecitonines and dorylonines?

  3. Marc "Teleutotje" Van der Stappen

    I have read already a few times that swarm raiders evolved for one reason, the expansion of the diet so that the colonies can grow bigger…. It are mostly the biggest colonies that are swarm-feeders, like Dorylus (Anomma) species….

  4. Marc "Teleutotje" Van der Stappen

    This year it is 100 years ago that W. M. Wheeler introduced the concept of the Superorganism in Myrmecology. Like H. & W. once said, army ants are one of the big examples of a superorganism, along e.g. leafcutter ants (Atta and Acromyrmex), weaver ants (Oecophylla) and honeybees (Apis).

    Among the army ants the above-ground (or epigaeic) swarm-raiders are the best known, e.g Eciton burchellii, Dorylus (Anomma) wilverthi and D. (A.) nigricans. A few above-ground column raiders are also studied, like E. hamatum. But tese are all big species with big colonies living above-ground.

    The smaller and hidden (or hypogaeic) species are less known, most are only described with a few remarks. Only recently, some studies are shedding some ligth on these species, especially the work of Berghoff. She published already a few articles but her thesis you can find here:

    Hope more of the smaller and hidden species are studied in the near future so that army and driver ants are better understood behaviorally AND phylogenetically!!!

  5. Interesting post, and timely for me: I’m about 2 chapters shy of completing Gould’s “Wonderful Life”…which addresses at length many of the same themes you presented here. A nice wrap-up for a thoroughly enjoyable, informative and entertaining week, Alex!

  6. Army Ant week draws to a close.

    Prove to be a big hit with your readers/followers. Will we see more of Ant of the Week or some other Bug, Alex

    1. Not for a while, I’m afraid. Army Ant Week was fun, but the preparation took more time than I normally can devote to the blog. Maybe in another few months I’ll put another topic together.

      1. I can see how it must have taken a ton of time to put together this block of posts. Rest assured that it was great fun to read (even if I was a bit late to reading some of them)!

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