A roving hoard marches across the forest floor, millions strong, crushing their prey between powerful, pincer-like mandibles. It’s a raid, and with millions of bodies to feed this swarm moves with startling efficiency. Thousands of the raid’s victims will likely be dead by day’s end. These are the army ants, one of Earth’s most aggressive predators and also one of Earth’s most impressive examples of social cooperation.
Dorylus sp. army ant raid shortly after the rains.
It is hard to describe the behavior of these ants perfectly, however, because we use the term army ant to refer to over two hundred species of ants. These species are actually incredibly diverse and many species have not shared a common ancestor for about one hundred million years1. That means African army ants (sometimes also called safari ants or driver ants) have been evolving independently from South American army ants about fifteen times longer than humans and chimpanzees! Amazingly, several fundamental characteristics of army ants have persisted in all of these species: obligate social foraging, nomadism, and highly modified queens.
What does all this mean?
Obligate social foraging is probably the most complicated and the most fascinating of these traits. Simply put, it means that a single army ant never forages alone. All foraging is done with the colony and for the colony. Simple as this may sound, it is biologically very difficult to achieve! An army ant raid, for example, has multiple lanes of traffic2. Ants searching for food and moving away from the bulk of the colony occupy the two outside lanes of a raid column, fanning out and leaving a light pheromone trail so that other foraging ants don’t follow their path exactly. Ants returning to the main colony—those carrying captured prey for example—occupy the center lane of the raid and lay down a much thicker pheromone trail. Many army ants also have different castes like the workers and soldiers described in the opening paragraph, though this morphological division of labor is not true of all army ant species. Finally, like other ant species, most army ants in a colony come from the same queen. This means they are highly related to each other, and reduces the genetic cost of sharing resources.
Traffic flows seamlessly around an unlucky caterpillar that has fallen into a stream of Dorylus army ants.
You might be wondering how a colony of millions could all come from a single queen, which brings us to another characteristic of army ants: highly modified queens. The abdomens of army ant queens are capable of significant expansion when producing eggs, allowing queens of some species to lay three to four million eggs in one month1. The name for this modification is one of my favorite words: dichthadiigyny. If you ever find yourself playing charades with entomologists, do me a favor and bust that one out. Army ant queens are also wingless, possessing more developed legs instead.
These queen modifications facilitate the third characteristic of army ants, nomadism. Army ant colonies consume far too much food to stay in one place and as such never develop permanent nests. Instead, army ants form bivouacs, living balls of ants that temporarily shelter the queen and her larvae3. Like the raid columns, these bivouacs seem fairly chaotic at first, but are actually highly organized. Larger workers tend to be found on the outside of the bivouac, which will become covered in soldiers at any sign of threat. In the morning, this ant bolus dissolves into a raid column, with larger workers called porters carrying the larvae to their next destination. And thus the ants go marching . . .
Army ants in the genus Dorylus, from eastern D.R. Congo [Credit: Neetha Iyer]
While the individual traits of army ants are interesting, they add up to something incredible. Army ant colonies have a huge impact on their environment, and are often central to entire ecosystems. The small invertebrates a raid doesn’t kill are often still flushed from their shelters, making them easy prey for other animals. Many larger animals, birds in particular, will often follow army ant colonies, eating the insects that flee from the raids. One species of army ant in South America has over three hundred known associated animal species! If you ever cross paths with an army ant raid, try to avoid this situation, but then look around. You never know what you will see.
[By: Alexander Vining]
1Brady, S. G. (2003). Evolution of the army ant syndrome: the origin and long-term evolutionary stasis of a complex of behavioral and reproductive adaptations. Proceedings of the National Academy of Sciences, 100(11), 6575-6579.
2Gotwald, W. H. (1995). Army ants: the biology of social predation. Ithaca, NY: Cornell University Press.
3Schneirla, T. C., Brown, R. Z., & Brown, F. C. (1954). The bivouac or temporary nest as an adaptive factor in certain terrestrial species of army ants. Ecological Monographs, 24(3), 269-296.
[Featured images and video credit: Neetha Iyer]