The Secret Life of Ants

A myrmecologist captures the delicate subterranean mansions of the insect world's master architects

By Brent Humphreys, Jack McClintock|Friday, November 07, 2003
RELATED TAGS: UNUSUAL ORGANISMS

This is perfect soil for this kind of work,” says Walter Tschinkel, as sweat runs down his face. “You can dig a six-foot hole in an hour here.” His voice is muffled because that’s just what he has done—and then some. He’s crouched in the bottom of an eight-foot-deep hole in the ground, in a hot, dry, open field between the Tallahassee, Florida, airport and a sewage-treatment plant. Clay-infused sand comes flying out of the hole as he shovels. Tschinkel, 62, rests a moment, then picks up a trowel and pokes sand away from one wall of the pit. He glances up. “I think we can start taking it out,” he says.

As he prods with the trowel tip, a fantastic sculpture begins to emerge from the earth. He scrapes away more sand, revealing tubes and elliptical lobes that a moment ago were completely buried. The thing is so delicate that, as Tschinkel and his graduate students remove it from the ground, it breaks into dozens of pieces. An ant nest, perfectly cast in three dimensions, it will be reassembled later in the laboratory. Tschinkel has spent this fine April morning mixing dental plaster to the consistency of eggnog, adding glass fibers, propping a cupped leaf against the nest’s entrance as a funnel, pouring the plaster, and letting it set. That’s when he dug the pit beside the nest and liberated the cast.

JH PeteCarmichael

Pogonomyrmex badius

Range: coastal plains of the southeastern United States, from Georgia to Louisiana

Average nest depth: 7 feet

Average number of chambers: 100

Average colony size: 5,000

A mature nest of the pogo, or Florida harvester ant, can reach 10 feet and contain 200 chambers. Typically, the colony survives for 15 years, until the death of the queen.

“It’s a typical ant nest—a vertical tunnel with horizontal chambers,” he says, carefully laying out bits of it on the grass. For more than a decade, Tschinkel, a myrmecologist, or ant specialist, at Florida State University in Tallahassee, has studied the behavior and social organization of ants. His curiosity was aroused by their nests—mysterious underground caverns never clearly seen by scientists. “It’s hard to visualize what’s underground,” he says. What did the nests look like, really? What could they reveal about ants and how they structured their lives? A few sketches had appeared in scientific journals but rarely to scale and with little detail. Tschinkel tried excavating nests and making his own sketches but found he couldn’t see their three-dimensional structure clearly. Then, 15 years ago, he got an idea. He mixed plaster and poured it into a fire-ant nest. When he dug up the casting and painstakingly glued the pieces back together, “it was a revelation.”  

Now, he says, we can describe ant-nest architecture much more precisely, leading to a better understanding of the insects and the mysterious principle known to science as self-organization—simple units of nature forming larger patterns through interactions with one another. An ant colony develops when each individual does its job in response to outside cues. The rules for this behavior, Tschinkel says, are “somehow internally programmed; they result from the way the nervous system is organized.” Each of thousands of earth-nesting ant species has a specific nest design, and each builds from a particular set of rules. “What is that set of rules? How do they come by them? How do they execute them?” Tschinkel wonders. “How does a group of individuals with no leader, no plan, create such complex structures in the dark?”

Myrmecologist Walter Tschinkel of the University of Florida holds up a partial zinc cast of a seven-foot-deep Florida harvester-ant nest. The flat chambers are living quarters as well as storerooms for seeds, which the foraging harvesters collect, shuck, and deliver to the workers underground for stocking and sorting.

Most ant colonies begin when one newly mated queen digs a single-chambered nest, seals herself in, and rears a first brood of workers. Queen ants need be fertilized only once: They store a lifetime supply of sperm in a sac, and in mature colonies, if the ambient temperature is warm enough—72 degrees Fahrenheit—some queens can lay 1,000 eggs a day for many years. The brood hatches in a week and, feeding on reserves in the queen’s body, grows to maturity in a month. Then the workers begin foraging—in the case of Florida harvester ants, for insects and seeds—to feed the next brood of eggs. And so the colony expands. Workers live about a year, but a colony can survive 10 or 20 years, until the queen dies. The colonies of most ant species, including the harvester, are social, cooperative, seamless organisms, differing from what we think of as an individual organism only in that “they’re not stuck together,” as Tschinkel puts it. The colony is a kind of creature—a superorganism.

Tschinkel made his first cast in 1985—a nest of fire ants, known as Solenopsis invicta, meaning “the unvanquished.” They create huge, long-lived colonies with a quarter of a million individuals, and queens that live for seven years. While most ants defend only their nests, fire ants ferociously defend surrounding territory, too, often over 1,000 square feet, and their stings are memorable even to mammals. Tschinkel had completed groundbreaking studies revealing “the behavioral rules governing the flow of food” in their colonies. He had explored their nests—first chloroforming the inhabitants, partly for his own safety but mostly “to knock them down where they stood so I could see how they were distributed in the nest”—and thought he had a good idea of the nests’ geometry. But when he poured dental plaster into one and then dug it out, the picture was much clearer. “The nests of fire ants are a lot more patterned and less randomly arranged than I had thought,” he says. “They were obviously organized, regular, predictable—so interesting. I got into the architecture.”

  Graphic by Don Foley

Building From the Ground Down

Floor plans for ant nests vary by species, but Tschinkel’s research has revealed that many colonies follow a common sequence in carving out their underground homes. A colony starts when a newly mated queen digs a tunnel and rears her first few dozen larvae from nutrients stored in her body. Within a month, the adult workers sort themselves by age, with the oldest workers taking on tasks specific to the upper nest and the younger ones doing the deeper work. As chambers and corridors are added, the queen retreats into the nether nest and begins laying as many as 1,000 eggs to keep up with the construction crews. In three to five years, the colony completes the final version of the nest, and the queen slows down, effectively ending the baby boom and shifting the makeup of the mature colony from 50 percent youngsters to 25 percent. —Jocelyn Selim

A few years later, he cast the nest of Odontomachus brunneus, the trap-jaw ant, named for its unusual facial structure. The trap-jaw’s gigantic mandibles protrude to the sides, giving it the look of a hammerhead shark. The jaws are remarkably strong: If the ant clamps something too smooth and round to hold on to and its jaws slip off, they snap shut with enough force to shoot the ant three inches backward. In this cast, Tschinkel recognized the same construction he’d seen in the fire-ant nest, “only here the internal nest consisted of a single unit—the shish-kebab unit.” That is Tschinkel’s description of chambers strung one after another along a single vertical tunnel, giving the cast itself a lumps-along-a-stick appearance. “So I got the idea of a basic, widespread architectural unit that might be fundamental to many ant nests.” 

Fire-ant nests are shallow; most of the chambers are linked closely to the core near the surface and come out of the ground pretty much intact. But the trap-jaw ants had built more of a sprawling nest, one that would lose real character—and data—if it was incomplete. Tschinkel had to retrieve all the pieces, and to see it whole, he had to devise a way of gluing and supporting an entire reassembled cast.

Trachymyrmex septentrionalis

Range: Long Island to Illinois; south to the Gulf Coast and Florida

Average nest depth: 3 feet

Average number of chambers: 2

Average colony size: 500 to 1,500

Each egg-shaped chamber in the nest of this agriculturally inclined ant is a garden packed with composted caterpillar droppings, which fertilize the fungus the colony feeds on.

Courtesy John Moser, USDA Forest Service, Southern Research Station

He was still pondering the problem when he got interested in the Florida harvester ant—Pogonomyrmex badius, casually known as the pogo. One of the more impressive ant species, the harvester constructs an elaborate, seven-foot-deep nest in less than a week, moving pounds of sand in the process. Then foragers search their territory for seeds, which are stored—as many as 300,000 of them—in subterranean chambers. Workers crush the seeds into pulp and feed it to the larvae. In turn, Tschinkel thinks, the larvae probably return a nourishing liquid to the workers, supplementing their diet of sweet plant exudates, aphid honeydew, and juices sucked from prey insects. Tschinkel’s early attempts to clearly describe the areas in the nests where all this happens were unsuccessful. But in the early 1990s, he found a freshly abandoned pogo nest, and he filled the entire thing with a single five-gallon pour of dental plaster. Once the plaster hardened, the cast came out of the ground—in 180 pieces.

“I cleaned them up, and they sat on my lab bench for three or four years,” he says. “Assembling it seemed daunting.” But Tschinkel, a hobby woodworker whose house is filled with elegant handmade furniture of his own design, devised a method of gluing the broken casting together with epoxy and mounting the cast in front of a tall plywood backboard, supporting it with projecting steel welding rods so that it would hang in space in the same orientation it occupied in the ground. “I started assembling subunits on the lab table,” he says, and over months—many times longer than it took the ants to build the nest—“I reassembled the cast into perhaps a dozen subunits and then figured out how these went together.” The nest of the harvester colony has 130 chambers connected by about 30 feet of vertical tunnels.

He did the same with other species, including Aphaenogaster ashmeadi and Pheidole morrisii, and some of those mounted casts occupy Plexiglas cases outside his office on the Florida State campus. They are, as Tschinkel describes them, “physically, intellectually, and aesthetically pleasing.”

Tschinkel believes that an ant colony grows just as a single organism does, by rules that guide interactions among its cells and between it and its environment, a process called embryogenesis. A colony is “produced from the single, mated queen through the rules and interactions of sociogenesis”—the process by which a society grows and changes according to its internal rules. “And just as mature organisms differ, reflecting the rules of embryogeny,” he says, mature ant colonies differ as well, reflecting variations in the rules of sociogenesis.

Tschinkel is trying to describe those rules. He studies, for example, how worker size, distribution, and labor patterns change as an ant colony grows, and how labor division by worker size and age helps shape the colony’s structure and habits. Such factors appear to organize the workforce the way a factory floor plan organizes personnel. Young workers start out low in the nest, looking after the brood and the queen, and then move upward as they age, taking on more-responsible jobs—“general nest maintenance, food preparation, seed storage. Finally, they move even higher to become guards and trash collectors and, at last, foragers.”

Courtesy April Noble/Antweb 

Solenopsis invicta

Range: southeastern United States        

Average nest depth: 5 feet

Average number of chambers: 100       

Average colony size: 220,000

The nest of the highly territorial fire ant, an invasive species from Brazil and Argentina, ismade up of many identical tunnel-and-chamber arrays, which Tschinkel describes as shish kebabs, densely packed together. 

He is also documenting how new ant colonies begin, including some unusual variations on the model in which the queen digs a hole and starts things rolling. Although newly mated fire-ant queens usually found new colonies alone, sometimes they do it in cooperation with other newly mated queens that arrive on the scene simultaneously. That’s a puzzle because it would seem risky: Worker ants tend to kill all but one such queen. Sometimes a mated queen will settle in an orphaned, queenless colony, although she’s unrelated to the workers there, and take over as a kind of royal parasite. Tschinkel has no idea why the workers are willing to serve such a usurper. In addition, a new colony’s workers often steal a brood from other new colonies, whose workers steal it back, and so on, until one colony wins. Then all the workers go and live in the winning nest, thus abandoning a mother.

 

Formica pallidafulva

Range: southeastern United States

Average nest depth: 1 to 2 feet

Average number of chambers: 15 to 20

Average colony size: 500 to 3,000

Common, but little studied, this inconspicuous ant carries away the excavated dirt from its nest, making a colony difficult to find. The colony is efficient as well as evasive, building a nest that closely reflects the number of ants within.

Courtesy Alex Wild

Ant-nest design has a basic theme, Tschinkel says: Vertical tunnels for movement and transport, and horizontal chambers for work, storage, and housing the brood. But nests differ in shape, number, size of chambers, and how they’re connected, depending on the species. With the Florida harvester-ant nest, for instance, the largest chambers are near the surface and closely spaced, becoming smaller and farther apart deeper in the ground. Small chambers are oval in shape; larger ones are multilobed and more complex.

But exactly how the workers “know” to generate these shapes is not so obvious. “As they’re doing the work, each worker responds to what needs to be done,” he says. “What are the properties of individual ant workers so that once each has made her contribution, the sum is a particular outcome?”

One of Tschinkel’s graduate students, Sasha Mikheyev, analyzed 17 nest casts of Formica pallidafulva. She consistently found that when the descending tunnels are vertical, the adjoining chambers are round, and when the tunnels are inclined, the chambers are oval or teardrop-shaped and lined up along the tunnel’s axis. In a simple way, this observation illustrates one of the rules for how nests are built, Tschinkel says: If a tunnel is vertical, the ants doing the digging tend to distribute themselves evenly as they work, and if it is sloped, they tend to collect in the lower end.

That’s a start, but it’s still unknown which workers do the digging, whether they have this directional bias individually or as a group, or how the number of ants may influence nest size and shape. “I can imagine if there are only a few, they might dig only a tunnel, because they wouldn’t be crowded. But if there are more, they might dig chambers too,” Tschinkel says.

Months later, on an August morning, Tschinkel is deep in the Apalachicola National Forest with a whole new idea packed into the bed of a pickup truck. Over the years, Tschinkel has cast ant nests with latex, plaster of paris, and dental plaster enhanced with glass fibers. Each has advantages, but none is perfect. So today he’ll be trying something new: molten metal. He has spent months fabricating a clever foundry based on a kiln of fireclay in a galvanized garbage can and an air blower made from an auto heater fan.

Courtesy of the President and Fellows of Harvard University

Aphaenogaster ashmeadi

Range: southeastern United States

Average nest depth: 1 to 2 feet

Average number of chambers: 5 to 8

Average colony size: 100 to 200

This scavenger ant, commonly found in woodlands, has an elongated body and forms small colonies. The chamber design is shallow and uncomplicated.

Tschinkel sets up the works, piles in charcoal, lights it, and then waits an hour for 30 pounds of scrap zinc to melt. Meanwhile, he builds a mud dam around the entrance of a pogo nest and blows away loose sand through a plastic tube. Finally, he pours in the molten zinc. It flows so smoothly that Tschinkel worries it might be disappearing down a subterranean rat hole. After waiting 10 minutes for it to cool and harden, he starts digging beside the nest with his favorite shovel.

“It’s like buried treasure,” says Kevin Haight, a graduate student, as gleaming metal emerges from the ground. Bristling from some of the tunnels are hairlike projections, perfectly captured—the tunnels of another ant species, the tiny, sneaky thief ant Monomorium viridum, which survives by raiding the broods of other ant species. Haight ties a rope to the heavy cast and helps haul it out of the ground. It emerges in just eight pieces. “Fabulous,” Tschinkel says.

But later, when he has time to think about it, he concludes that zinc is too dense. The metal cools and sets up before it reaches the bottom of the nest. Next time, he says, he’ll make a first pour with molten aluminum and a second pour in zinc.

He has many opportunities to perfect his technique for making 3-D casts. There are 50 ground-nesting ant species in the area alone, and roughly 5,000 worldwide, each with its own unique way of life and shape of nest. For instance, there’s the genus Atta, the leaf cutter, which builds the world’s largest nests, up to 35 feet deep and covering as much surface as a small house. “I’d love to do an Atta nest,” Tschinkel says, smiling, “but I’d need several tons of plaster.”

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