An Ant Mystery: Why the Annual Move?

Here's something you may have not known about ants: Once a year, for no discernible reason, some colonies just pack it up and move. Now, a new study attempts to investigate the "why" of this mystery by fleshing out some of the "how."

The study, recently published in the journal PLOS One, details how nests of harvester ants in a pine forest on the Florida panhandle are semi-nomadic - moving the entire colony about once a year.

Study author and entomologist Walter Tschinkel spent three years of his life tracking more than 400 of these mysteriously nomadic colonies, trying to understand why they didn't just stay in one place.

The researcher had first theorized that these movements occurred because the colony had grown too big and needed more room to expand. However, this proved not to be the case soon after he discovered that all the colonies surveyed moved about once annually, regardless of their size or health.

"Almost all moves occurred between May and November peaking in July when more than one percent of the colonies moved per day," he wrote in the paper. "Move directions were random, and... distance moved was not related to colony size."

Tschinkel suggests that annually moving a colony may actually be intrinsic to ant behavior, and is not a real-time adaptation to any specific set of circumstances. However, why these ants developed this behavior in the first place remains a complete mystery.

And while the expert has done little to pull back the curtain on that mystery, his research at least heavily contributes to understanding how these impressive moves occur.

Tschinkel found that moves typically took four to six days, depending on the size of the colony, and started with just the eldest of the workers contributing to the cause. Soon, however, younger ants started filing into a train of movers, bearing seed and even the queen's larvae while the older ants continued their work excavating their new home.              

"Moves were diurnal, peaking in the mornings and afternoons dipping during mid-day, and ceasing before sundown," the researcher wrote, adding that, interestingly, the shape of the new home resembled that of the old, even if its location seemed utterly different.

These dramatic moves, Tschinkel wrote, appear to be important if not utterly necessary in harvester ant life, even if the cause remains unclear. If anything, that just goes to show how little we actually know of the world beneath our feet.