The type of ants in this study – harvester ants – are one of the largest insect societies in the western United States, with ranges covering hundreds of miles and nests so large they're visible from airplanes.
"This is the ant that runs the west – it's everywhere," Rissing said.
The researchers had noticed that in certain areas – mainly southeastern Arizona and New Mexico – some of the male harvester ants looked different. So they collected several dozen pairs of queens and males and brought these pairs back to the laboratory for genetic testing, with surprising outcomes.
"The DNA of some of these ants was just weird – we certainly didn't expect to get the results that we did," Rissing said. "It seems that the queens in these colonies mate with males from two different genetic lineages. And when a queen and male with the same lineage usually mated, it usually produced a reproductive female – another queen. But when a queen and male from different genetic lineages mated, that pairing overwhelmingly produced a sterile worker.
"This kind of reproductive behavior is very different from what we expect to see in ant societies," he continued. "We'd expect to see the same DNA sequence from all ants in a given colony. But that's not what happened here."