Ant march in northern Illinois

On August 18, 2013 while visiting a naturally restored area of Orland Park, my wife and I witnessed an ant "march" from under the base of a tree, across a parking lot, into another spot, east of the tree, filled with bushes and plants, which is near a pond. We approximated the observable distance at 120 ft. with ants covering an area about 12 inches wide. By counting the ants in the photographs we took, then doing some calculations, we estimated there were about 4,700 ants.
We observed this process for an hour and a half, as they went to wherever their "end place" beyond our field of vision in the underbrush was, to all ants returning to their original hole under the tree. They were not carrying anything visible in either direction, but we did observe them stop periodically to touch antennae. The ants on the "march" were red, with three yellow-gold lines, over a whitish area on their abdomens, and not larger than 1/2 inch. Around the "nest site" were numerous small holes and a couple larger entrances which were guarded by black ants that looked identical in size and also had similar, but less pronounced color on their abdomens. In addition, there were smaller, black, winged ants crawling around the nest perimeter. During the return to the main entry hole, black and red ants periodically touched antennae
This behavior was fascinating to observe, but puzzled us as there did not appear to be a "hostile takeover" by another species and no food was brought back to the nest.
What explanations could be offered for this expenditure of energy?


Dear Anthony:

Thanks for sending the detailed account, and pictures, of your observations of these ants' behavior. You saw an ant species that has long been known as Polyergus breviceps. A little bit of nomenclature housekeeping - It turns out that this name has been applied to several different, closely related species, and the ants you observed used to be, and will soon again be officially know by the name Polyergus mexicanus, a widely distributed western North American species originally named over 100 years ago, on the basis of specimens collected in the mountains of Mexico. This ant is near the northeastern edge of its geographic range in Illinois.
What you have observed is a (failed) brood-robbing raid of these so-called slave-maker ants. Most of the time, when such a large party of Polyergus heads out as you saw, they end up at a nest of the black ant species, Formica subsericea, enter, and steal hundreds of their brood. Each Polyergus worker returns to its own nest with a pupa, or less often, a full-grown larva. Back home, the black ants already there in the nest mound, raise the young, along with the Polyergus brood. Eventually, the stolen brood emerges as adults having no knowledge of their species identity, and thus incorporated seamlessly into the work force of the mixed species colony.
The mixed species nest is that of the Polyergus, i.e., their queen and brood are there, and only workers of the "slave" species live there. Polyergus workers are highly adapted to a parasitic lifestyle, experts in acquiring host species workers, but incapable of other, normal ant behaviors such as feeding themselves, building the nest, or raising their own brood. All of these duties are performed by the Formica workers. When not out raiding with the energy and determination that you observed, the Polyergus workers remain inside or near the entrance of the nest, and in the words of the great American myrmecologist of the early 1900s, William Morton Wheeler, "sit about in stolid idleness ... begging food or ... burnishing their ruddy armor".
The pictures you sent, posted below are the Polyergus workers out on a raid, a Polyergus and Formica worker at the nest entrance, and a bonus picture of the little black, white-winged males of the Polyergus, about to head out on a mating flight in quest of virgin Polyergus queens.




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