Don't step on those ants. They may be little farmers, and more like us than you thought.

Scientists say more than 200 species of ants have evolved agricultural practices much like ours, using composting, fertilizers and herbicides to produce the mushroom and yeast crops they grow in dark, sponge-like subterranean galleries.

New research by University of Maryland biologist Ulrich Mueller has revealed that the ants have also survived food crises by adopting fungal crops grown by other ant species, or even by domesticating new varieties from the "wild."

"That may be the message of the ants," said Mueller. "If you want to make it through millions of years of evolution, you'd better make sure you preserve that biodiversity out there."

The work by Mueller, Stephen Rehner of the University of Puerto Rico, and Ted Shultz of the Smithsonian Institution, was reported last week in the journal Science.

A native of Germany, Mueller, 38, came to America as an undergraduate, stayed, and became an evolutionary biologist. He confessed to "an inordinate fondness for social insects," and admitted it is "sometimes hard to share that. People look at you funny."

Supported by the Smithsonian Institution, he and his colleagues spent a year and a half tramping through forests from Texas to Brazil, gathering ants and their fungal gardens and analyzing their DNA at the Smithsonian Tropical Research Institute in Panama.

"You have to have an eye for where the ants are. Then you have to know how to excavate without trashing their gardens," he said. Their collection at College Park now contains 200 colonies.

Biologists have identified 10,000 species of ants, but only 200 belong to the farming "attine" tribe, Mueller said. All of them live in the Americas. (A few beetles and Old World termites also have learned to cultivate fungi, examples of parallel evolution, he said.)

From DNA analyses, scientists believe all the attine ants evolved from a single species that probably lived in the Amazon forest more than 50 million years ago.

The attines marched into Central America when the Isthmus of Panama formed a million years ago. One species is now found as far north as New York. Mueller has found some at the Patuxent Wildlife Research Center in Maryland.

The very first attine ants -- like the first people -- probably were hunter-gatherers, as most ants still are today.

"What must have happened was the same sort of transition that humans went through independently 10,000 years ago," he said. A shortage of wild game, an increase in the population or environmental change must have created a food crisis, forcing the hungry to find new foods or perish.

People turned to grains -- early, wild varieties of wheat, rice or corn -- and found ways to grow them.

Finding a 'good substitute'

Likewise, Mueller said, "at some point it became more profitable for these ants not to hunt down prey -- maybe to evade competition with other ant species -- and to start cultivating a fungus that proved a good substitute for whatever dietary needs they had."

The favored fungi are mushrooms and yeasts that live by decomposing plant matter into sugars and proteins, on which the ants learned to subsist. Today, they can't survive without them.

Mueller said the first crop may have been a wild soil fungus the ants already ate as part of a varied diet. Or, they may have routinely nibbled it to keep its underground fibers out of their nests. From there, it would have been a short step to consuming it as food.

"Once you have some digestive capability to take advantage of fungi, you just have to learn cultivative skills," Mueller said.

The ants learned well. Today, they create a sort of potting soil from plant litter gathered from the forest floor. They compost it and form it into three-dimensional, sponge-like garden beds. A few species -- the famous leaf-cutter ants -- carve up 20 percent of the foliage in tropical forests every year to build their gardens.

Composting allows any toxic compounds to degrade. Any fungicidal waxes on the leaves are licked off. The ants manure the mix with their own feces, and they excrete fungicides that suppress the growth of unwanted fungi.

New queens carry fungi like seed packets from their parents' nests, and plant them in the walls and floors of their new beds. There they reproduce.

Gazing back 50 million years

For a century, Mueller said, scientists assumed that fungus cultivation was a "freak accident 50 million years ago," in which one species of ants domesticated one fungus and passed that single crop species down. "We proved that's not true," he said.

The study found the seven most primitive families of attine ants grow 57 fungal varieties, or "cultivars." DNA analyses of all 57 showed they are descended from fungi domesticated in at least five separate events. Two of the crop fungi are genetically indistinguishable from wild varieties, persuading Mueller that they represent very recent domestications.

"At least some ants retained the ability to occasionally go back to the wild varieties and bring them into an agricultural existence," Mueller said. Humans today maintain seed banks for the same purpose. "Our second finding is that they pass on their fungal cultivars laterally between different species," he said. Faced with the ant equivalent of an Irish potato famine, they will share or steal new crop species to live.

Identical ant species were found growing different fungi. Conversely, identical fungi were found in the gardens of genetically different ants in Panama and Guyana. A species introduced into Florida in this century has adopted a fungus cultivated by native species. Crop sharing, Mueller said, "has been incredibly important in human evolution."

In an accompanying Science article, University of California physiologist Jared Diamond noted that, even before East met West in 1492, maize cultivation had spread across human cultures throughout the Americas, and wheat farming and horse domestication were widespread in Old World societies, bringing with them dramatic cultural change.

Today, he said, "a typical McDonalds restaurant meal includes foods based on beef, chicken, potatoes and kola nut [Coca-Cola], originating from the Fertile Crescent, China, Andes and tropical West Africa, respectively."

Pub Date: 9/29/98