"I’m a Stormtrooper. Like all of them, I was taken from a family I’ll never know and raised to do one thing." These words were spoken by Finn, hero of the new Star Wars movie, but they might as well have been spoken by a young Formica ant. Individuals of this species are regularly stolen and enslaved by another type of ant: the Polyergus breviceps "slave-makers."
Just like the fearsome First Order from Star Wars, Polyergus kidnaps and enslaves pupae of the ant species Formica atipetens. The slave-makers enact a strategy that is uniquely tailored to suppress rebellion and enforce obedience by manipulating a basic feature of ant biology: the ability to differentiate between nestmate and foe. By stealing and enslaving pupae from many colonies, then exposing them to a battery of chemical "propaganda," the Polyergus slave-making ants turn their slaves into meek workers who are unlikely to rebel. They are basically making Storm Troopers.
Here's how it works. Polyergus ants conduct regular raids of neighboring colonies of Formica. Despite the brave defensive maneuvers of Formica, the slave-makers often succeed in stealing pupae from their colonies and carrying them home, where they are raised to perform nest maintenance, foraging, and brood care for their captors. Further, the slave-makers kidnap from a diverse array of colonies in each raid; in other words, they purposely avoid selecting many pupae from one colony -- a strategy that is creepily akin to the human custom of keeping apart slaves who spoke the same language to prevent an uprising.
This is exceedingly clever, because ants have developed a special system of friend-versus-foe recognition that can be circumvented by this exact strategy. Ants tell friend from foe due to chemical "labels" on their exterior called "cuticular hydrocarbons." These waxy compounds are recognized by others who share the same type of compounds; in this manner, young ants newly emerged from the pupal stage "imprint" on familiar chemicals and learn to differentiate between familiars and outsiders.
But when enslaved ants are captured from a variety of genetically diverse colonies, and raised in a chemically and genetically diverse environment,
(i) ants fail to imprint on on their own colony- and species- specific signature
(ii) enslaved ants become familiar with the chemical signal of their captors
(iii) enslaved ants, exposed to a variety of chemical signals and genetic makeups of other slaves and their captors, have a lower chemical "acceptance threshold" and are less aggressive towards outsiders and more compliant.
The original authors write:
"Slave-making ants appear to take advantage of the fact that their kidnapped host workers, after eclosion, imprint on the chemical cues of individuals they encounter. They then use this information as a reference for nestmate-specific odor cues later in life, resulting in the integration of enslaved workers into the slave-maker colony [54, 55]. Additionally, all colony members may share and recognize a common odor or set of cues, known as a “gestalt” odor . Such a phenomenon may also explain how Polyergus colonies maintain cohesiveness among a worker population that originates from different genera and colonies. ...In our study system, enslaved Formica may shift their acceptance threshold such that the rules of matching the template and the label are less stringent."
Indeed, experimental evidence confirms that enslaved adult ants are less aggressive to non-nestmates than free-living ants. Relatedly, separate research found that less genetically diverse colonies more often act as the aggressors in inter-colony war than high-diversity colonies.
Ants are a strange parallel to humans-- they, like us, have slavery, war, and agriculture. And now we know that ant societies with low diversity are more openly aggressive.
Might there be a lesson in there somewhere for us?
During the summer, Polyergus conducts raids on Formica colonies, explains Tsutsui. During the raids, workers from the slave-making species leave the colony as a group and attempt to steal Formica pupae. The attacked adults do their best to defend themselves and the ant babies. .,. Once stolen, the victim babies are brought back to the Polyergus colony. The attackers then continue to raid different Formica colonies, creating a highly diverse group of victims. As they grow older, the victim ants perform nest maintenance, brood care and foraging for their slaveholders, not even realizing their predicament.