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Is a bigger ant a better ant? FSU biologist tests 150-year-old evolutionary question

The small tepee-like tents draped over 130 fire ant mounds surrounding a picturesque country pond may resemble more a re-creation of a community of pint-sized natives than scientific research.

But science it is—in fact a manipulation of Mother Nature. Florida State University biologist Walter Tschinkel wants to know if there are benefits to nature’s practice of producing large and small workers in a colony of fire ants. He’s testing a 150-year-old theory espoused by Charles Darwin to explain how natural selection shapes traits in the
sterile workers of social insect colonies.

The answer may tell us how natural selection can shape traits in animals that do not themselves reproduce, namely, worker ants.

Darwin suggested that workers’ traits are selected through the reproductive effectiveness of the colony as a whole. The largest fire ant workers in a colony are four to five times bigger than the smallest workers, a condition called polymorphism. While the workers provide the foundation of the colony’s existence, it is the winged male and female ants that determine a colony’s fitness—that is, its ability to produce more ant colonies—and that is the ultimate measure of whether a colony’s genes will spread in a population or will wane.

Tschinkel wants to find out if the production of male and female ants is affected by changing the proportion of large and small workers in his experimental colonies. Some scientists have tested the theory in the lab using indirect measures of fitness, such as colony growth rates, but none of these studies have yielded a clear answer. Tschinkel is the first to test it under actual conditions using a direct measure of fitness.

"Nobody yet has clearly demonstrated that there are benefits of being polymorphic, that is, of having a broad range of worker sizes," Tschinkel said. "It is the colony that starts the most new colonies that has the highest fitness. That’s the object of the game. The question is, does polymorphism play a role in attaining this?"

One of the nation’s leading ant experts, Tschinkel spent last summer trapping male and female ants in the tops of the mesh tepee-like tents to find out how many each colony produced under natural conditions. This fall, he began manipulating those colonies to alter the proportion of large and small workers. This is done by removing about 30,000 ants at a time from a mound using a small vacuum then replacing them with a like number of pupae of either small ants, large ants or a mixture of both.

Over the course of the three-year project, which is being funded through a National Science Foundation grant, Tschinkel will determine whether colonies that are populated with primarily large worker ants produce more, fewer or the same number of male and female ants as colonies comprised mostly of small worker ants or unmanipulated colonies.

"A possible finding will be that the natural proportion of worker sizes in fire ants is optimum," Tschinkel said. "In that case, large-and small-worker colonies will produce fewer males and females. It is also possible that one or the other will produce more males and females than the unmanipulated colonies, or that there might be no effect at all. I really don’t know how it will come out."

The project is not only testing Tschinkel’s scientific prowess, but certain domestic abilities as well. Realizing that no manufacturer is into the commercial production of ant-trapping tents, he purchased 200 yards of wedding veil netting from a local fabric store and sewed pieces together at home to make the approximately four-foot-high tents. The tents are held upright with a small PVC pipe.

Within the first few months of the project, Tschinkel discovered that the veil netting weathers and tears easily. He’s now sewn new tents for all of the colonies in his experiment using polyester pointe d’esprit, which he hopes will weather less rapidly.

"Brides rarely spend enough time outdoors for weathering to be an issue, so I had no idea the fabric would fall apart so quickly," Tschinkel quipped. "Maybe that’s why wedding veils are only worn once."