How blood-red ants became slave snatchers

Every summer, blood-red ants of the species Formica sanguinea go on a mission to capture slaves. They infiltrate the nest of another ant species, like the peaceful F. fusca, assassinate the queen, and kidnap the pupae to raise as the next generation of slaves. Once the slaves hatch in their new nest, they appear none the wiser to their abduction, dutifully gathering food and defending the colony as if it were their own.

Scientists have long wondered how such slavemaking behavior evolved. Now, new evidence suggests that today’s slave snatchers started out as temporary parasites—ants that laid their eggs in the nests of other species and then used those workers as part-time caregivers for their own offspring.

The evolution of enslavement in Formica ants has long eluded scientists, largely because they didn’t know how species in the genus were related. So Jonathan Romiguier, a molecular biologist at the University of Lausanne in Switzerland, and colleagues sequenced and meticulously mapped the genetic relationships of 15 Formica species to create the most robust family tree to date. The tree includes major branches for slavemakers, species without slaves, and parasitic species that exploit foreign workers on a temporary basis.

The order of those branches tells the story of how enslavement evolved. By tracing their way down to the base of the tree, the researchers gleaned that the ancestors of all Formica ants formed colonies without recruiting slaves. Parasitic ant species soon arose, in which queens laid their eggs in neighboring nests and enlisted the resident workers to care for their broods. Then another branch diverged, and it was there that the full-blown master-slave relationship was born, the researchers reported late last month in BMC Evolutionary Biology . Because the slavemakers are lumped together with the parasites in their own distinct section of the Formica family tree, Romiguier says he suspects temporary parasitism was a “preadaption” to slavemaking behavior.

Not everyone is convinced. Christian Rabeling, an evolutionary biologist at Arizona State University in Tempe, says that although the study furthers our understanding the evolutionary history of the Formica genus, the family tree included less than 10% of the 175 known species, a major limitation. “The picture is just more complex than they outline in the paper,” he says.

To address that issue, the team redid their analyses with a bigger data set—one that included more species, but lower-quality genetic data. Romiguier says their results still held, but further research is needed to be certain.

Another outstanding question is more basic: How could the ants’ genes have enabled slavery to evolve? Susanne Foitzik, an evolutionary biologist at Johannes Gutenberg University Mainz in Germany who was not involved in the new research, has discovered a few candidate genes in another slavemaking group of ants—the Myrmicinae subfamily. The genes that she has found are involved in creating a chemical disguise that tricks neighboring ants into welcoming slavemakers into their nests. Romiguier is also on the hunt for similar genetic adaptations that have helped some ants foray into the ruthless world of slavemaking.