The Best Ant for the Job

Twenty years ago, an unsuspecting ant crossed the floor of a basement laboratory at the University of Oregon’s Volcanology Building and laid a trail for Robert Schofield, PhD ’90. The senior research associate was then studying physics under Harlan Lefevre, now professor emeritus. Schofield was working on a new type of microscope, one that uses protons rather than electrons or light to generate an image.

Once the machine was built, the two men set out to discover something with it. Schofield saw the ant, placed it under the microscope, and was amazed. The proton microscope showed the ant’s mandibles and the striations of the muscles that moved them. And because it also identified chemical elements, Schofield saw “these weird little zinc teeth” on the tips of the mandibles. “That’s how I got started,” he says. “It’s continued to enchant me to this day.”

Scofield went on to study other creatures: spiders, scorpions, even fruit flies. Their mandibular teeth, tarsal claws, legs, and stingers possessed biomaterial of heavy elements: zinc, manganese, bromine. This biomaterial is present in the sharp, translucent tips of the walking legs of Dungeness crabs, for example. It can bend six times farther before breaking than material found elsewhere on the crab. Schofield’s discovery was ground-breaking at the time. “I found this stuff everywhere,” he explains, “and biologists didn’t know about it.”

Fifty million years before humans developed agriculture, leafcutter ants had already begun the practice. Their crop, fungus; their soil, a mix of plant materials brought to the nest, often after being cut to size. Their agricultural skills support colonies populated with millions of ants organized by intricate communication and caste systems and living in gigantic, elaborate subterranean nests, each made up of hundreds of interconnected fungus garden chambers. They are, according to noted entomologists Bert Hölldobler and E. O. Wilson, a “superorganism”—composed not of cells and tissues but of closely cooperating individuals.

The leafcutter forager caste—the ants whose job it is to cut leaves—were the ideal insects to help answer one of Schofield’s research questions: does wear matter for such tiny creatures with such short lives (an average female worker’s lifespan is one to three years)? He suspected that the large amounts of zinc found in the teeth of their mandibles, like those of the ant in his lab, made them stronger. His hypothesis was that wear-resistant mandibles were essential to the well-being of the entire colony. His plan was to set up a colony in the lab, and then compare results with a colony in the wild, to prove it.

The ant colony in Schofield’s lab began with a single queen from Costa Rica. In the wild, she would have dug a shallow hole in the soil and begun to lay eggs. In the laboratory, her nest was constructed for her, as was an entire artificial landscape consisting of lengths of tubing, or trails, leading to separate glass and plastic chambers.

In the lab, Schofield, tall and thin with gray-blue eyes and dark brown hair pulled back from an angular face, drops leaves of the Northwest’s infamous Himalayan blackberry into a large aquarium. The tiny red ants, Atta cephalotes, diligently cut away at the greenery, travel the tubing trails, and tend to the fungal gardens. They are everywhere. Schofield estimates there are several thousand now. He carefully picks up a writhing ant between his index finger and thumb. It doesn’t take a microscope to see that her mandibles could inflict some damage.

Photograph CC Alejandro Soffia by-NC-ND-3.0 This particular ant is a major worker, or a defender—the largest of four morphological castes, each with its particular tasks. “I like to call them the bulldozers,” says Schofield. “On a normal day when they’re not being attacked by a large animal like me, these ants maintain the trails.” Schofield points to examples of the other castes: the gardening and nursing ants, the in-nest generalists, and the foragers. The foragers are easy to spot: they are the ants cutting and carrying the leaves and the subjects of Schofield’s experiment, which began in 2004.

Schofield describes how coresearcher Kristen Emmett timed how long it took an ant to cut out a leaf disc, then measured the disc, collected the ant that cut it, and photographed the ant’s mandibles under a microscope. She did this over and over. From the photographs, Schofield and his team measured the length of the teeth of mandibles. Schofield then replicated the experiment with a colony in the rainforest of Soberania National Park near Gamboa, Panama.

“We could say that this ant cuts one millimeter a second, and this one cuts half a millimeter. And it turned out that the ants with the more worn mandibles cut slower,” Schofield explains. Was cutting also more difficult for these ants? Schofield built another machine to answer this second question—basically a tiny saw into which ant mandibles can be fitted like blades. The apparatus then moves the mandibles across the leaf, registering the force required to do so.

The results confirmed the team’s suspicions. Older, slower-cutting ants use twice as much energy to cut through a leaf as newbies with their razor-sharp teeth.

The answer to the question, does wear matter? “Is yes,” says Schofield. “It matters in a big way.”

The team’s findings, published last winter in the journal Behavioral Ecology and Sociobiology, received a tremendous amount of attention. National Public Radio’s Science Friday discussed the work. The BBC ran a story as did U.S. News and World Report. But articles about Schofield’s research focus on more than just its scientific implications. Their titles are variations on the riff: “Leaf-cutter ants have their own form of Social Security.” Schofield’s team had guessed that the colony, based so fundamentally on cooperation, might find another job for aging ants. They discovered that out of all the ants they collected, the 10 percent with the most-worn mandibles were not cutters but almost exclusively carriers. “It makes a lot of sense,” says Schofield. “It’s what you’d do if, for example, you were no longer a good basketball player because you’re getting a bit older. You might become a lawyer!”

Can an ant really recognize the fact that she’s no longer cutting so well and then make a conscious decision to change jobs? Schofield is quick to clarify that this hypothesis has not been tested. He offers two possibilities. One, as the ants reach a certain age, genetic programming instructs them to stop cutting and start carrying. Or two, they are capable of self-evaluation. Schofield suspects the latter. “I see it as I watch them cut. If it seems way too hard they’ll give up or go find a different place and start cutting. Well, maybe if they can’t find a place they’ll just carry a leaf back instead,” he says.

Schofield also believes that the leafcutters might have something more to teach us than a good fable. As humans continue to build smaller and smaller high-tech machines and tools, how might we learn from tiny creatures that have adapted to wear for millions of years? Is there something within the biomaterial of the ants’ teeth that might be useful to us?

In retrospect, it might not seem like such a leap for a physicist dealing in tiny things like protons to become interested in creatures like ants. In fact, Schofield still spends half his time studying astrophysics. He is part of a group of scientists that works at two Laser Interferometer Gravitational-Wave Observatories, the nearest located in Richland, Washington. These instruments were built to detect vibrations set off across the universe when cataclysmic events occur, like stars exploding into supernovas or black holes colliding.

Yet, if detected, such events would create a wave whose crest would only rise a tiny fraction of the width of the nucleus of an atom. At the heart of the project “is a very fancy measuring device,” Schofield told a reporter. “All we do is measure distance.” Here is one way to consider how something as grand as an exploding star and as tiny as a leafcutter ant can be connected: through measurement.

By Tara Rae Miner ’96