Published: August 2006
Army Ants: Inside the Ranks
At the pinnacle of social cooperation, army ants overwhelm their prey through their sheer force of numbers.
Text and Photograph by Mark W. Moffett

In Japanese the word "ant" is intricately written by linking two characters: one meaning "insect," the other meaning "loyalty." Altruistic and cooperative toward one another, nestmates readily go to war to preserve their colony. Renowned biologist and lifelong ant observer Edward O. Wilson introduces our new occasional series on these highly social creatures.

Ants are our co-rulers of the land. An estimated ten thousand trillion strong worldwide, they weigh very roughly the same as all of humanity. They abound everywhere except on icy mountain peaks and around the Poles. From underground to treetops, they serve as the chief predators of insects and other invertebrates and the principal scavengers of small dead bodies. Although their 12,000 known species compose only about 1.4 percent of the world's insect species, their share of the collective body weight is easily ten times greater.

I was ﬁrst drawn to these remarkable creatures almost 70 years ago as a boy in Washington, D.C. Inspired by the tales of Frank Buck and other wildlife adventurers,I launched my own expeditions from our family apartment into the "jungles" of Rock Creek Park. Ants especially intrigued me because of an article by William M. Mann in the August 1934 National Geographic: "Stalking Ants, Savage and Civilized." Mann was also director of the National Zoo, hence doubly my hero. The myrmecological lineage continued decades later with Mark Moffett, who earned a Ph.D. under my direction at Harvard and whose groundbreaking photography of ants focuses in this issue on army ants.

Ants are important for more than their ubiquity and environmental impact. They also exhibit social behavior as exotic as any we may ever hope to ﬁnd on another planet. For most of each year colonies consist only of females: queens that reproduce for the colony and infertile workers that conduct all the labor. Males are bred and kept for short periods, exclusively for the insemination of virgin queens. The communication systems of ants are radically nonhuman. Where we use sound and sight, they depend primarily on pheromones, chemicals secreted by individuals and smelled or tasted by nestmates. Since the brain of an ant weighs less than one-millionth as much as a human brain, it is not surprising that a given species produces just ten to twenty signals. Unlike human language, these messages are entirely instinctual.

These marvelous little creatures have been on Earth for more than 140 million years. The most complex social organizations among them, such as those of the army ants and leafcutter ants, rank with Earth's greatest wildlife spectacles. Ants easily outlasted the dinosaurs, and they will easily outlast humanity should we stumble.

Forget lions, tigers, and bears. Forget even our own famously aggressive species. When it comes to the art of war, it's army ants that will make you break into a cold sweat. Armored tough, with machete jaws, these masterful ﬁghters hack and dice prey vastly larger than themselves by acting in numbers beyond easy comprehension. Imagine hordes of spear-wielding humans at a wooly mammoth's feet. That’s the scale of army ant operations when they're attacking a tarantula or scorpion. Army ant colonies succeed at making tens of thousands such kills each day. Folklore to the contrary, their prowess does have limits. Their dragnets don't take down livestock or people (though some African species occasionally live up to that image).

I went to one of the best places to observe army ants in action, Barro Colorado, a six-square-mile island in a lake created by the Panama Canal and home to perhaps 50 colonies of Eciton burchellii, the most studied army ant in the world.

The ways of E. burchellii helped give rise to the name army ant. Their colonies are huge, ranging from 300,000 to 700,000. They are mobile, moving from nest site to nest site. Though not all army ant species share these characteristics, there's one hallmark they have in common: a shock-and-awe hunting strategy. Other ants search for food individually, sometimes using scouts. Army ants set out en masse. Being blind, they can't see what's ahead of them, but moving in such numbers they easily overwhelm their prey. For E. burchellii that's usually non-army ants and large arthropods. They can also kill, but don't eat, lizards, snakes, and frogs that fail to get out of the way. Their attack formation is called a swarm raid. As many as 200,000 ants leave the nest in a swarm that broadens into a fan as wide as 15 yards. Specialized birds follow the raiders, picking off prey as it scatters in vain attempt to survive.