Studies of ants have the ring of medieval epics: there are queens, castes, warring soldiers and scouts. Certain ant species go on violent raids of neighboring colonies, picking up the young in their jaws and carrying them back to their own homes. The stolen ants become workers that live to serve these "slave-makers." A newly discovered slave-making ant species, though, sometimes accomplishes the same kind of coup without the need for violence.
Slave-making ants live all over, but scientists have so far found the new species in just three spots. In the early 2000s, the ants turned up in a state park in Vermont and a nature preserve in Rensselaerville, New York. In recent years, they've been found at Sleeping Bear Dunes National Lakeshore in Michigan.
If you stand on the Sleeping Bear dunes, you might never guess that this postcard-perfect vista hides a small-scale war. But in the forest, tucked inside holes in acorns, sticks, and hickory nuts, Temnothorax pilagens—the "pillage ant"—is at work. Bernhard Seifert of the Senckenberg Museum for Natural History in Goerlitz, Germany, and his colleagues say pillage ants have two preferred host species: Temnothorax longispinosus and Temnothorax ambiguous. Yes, all three share a name because they're relatives. "Ninety percent of slave-maker ants are related to their slave-species," Seifert says. The pillage ant is "much closer than usual" to T. longispinosus, though; comparing the ants' DNA revealed that the two are sister species. The slave-makers don't treat these sisters well. Scouts from a slave-maker colony go out looking for nests to raid, either alone or with up to four worker ants following. When they find a target and charge inside, they may meet resistance. In this case the slave-makers make "frequent and effective use of the stinger," the authors write: a sting by a pillage ant quickly paralyzes and kills a host ant. In the raids that the scientists observed, casualties in the invaded ant colony could reach as high as 100 percent. Yet the takeovers could also be peaceful. In some cases, especially in nests without a queen, the invaded ants barely objected to their pillaging guests. Slave-makers carried off not just larvae and pupae from the colony, but adult ants too. "Occasionally, host workers [tried] to drag slavemakers out of the nest," the authors write, but at other times they didn't resist. The whole raid could go off with hardly any violence. The scientists think pillage ants somehow disguise themselves, so that ants in the invaded nests surrender without objecting. Seifert says "different forms of camouflage" could be responsible. The slave-makers might mimic the coloring, shape, or movement of their hosts well enough that the hosts don't notice when the raid begins. Or the mimicry might be chemical—pheromones given off by the slave-makers might convince their victims everything's fine. Back at the pillage ants' home nest, a typical colony includes about four workers and a dozen slaves. Compared to other slave-makers, the pillage ant is unusual for its tiny raiding parties. Seifert says this species is also unique in that a "good number [of] raids can be undertaken without experiencing resistance." Surrounded by the beauty of the dunes, the slave-makers' victims are lulled into submission. By the time their young grow into hostage adults in their enemies' nests, it's too late to fight. Images: (top) Miriam Papenhagen; (bottom) me.
Bernhard Seifert, Isabelle Kleeberg, Barbara Feldmeyer, Tobias Pamminger, Evelien Jongepier, & Susanne Foitzik (2014). Temnothorax pilagens sp. n. – a new slave-making species of the tribe Formicoxenini from North America (Hymenoptera, Formicidae) ZooKeys : 10.3897/zookeys.368.6423
Note: It's not always too late to fight. In what scientists call a "slave rebellion," some enslaved ants have been seen tearing their captors' young to pieces or abandoning them outside the nest.