Slavery is widespread among certain ants in the United States and has even altered the genetic and chemical diversity of enslaved ant victims, a new study finds.
Some bees, wasps, beetles, crickets and other creatures also either enslave or trick others to do their work, showing the behavior persists in nature. As for slave-making Polyergus breviceps ants, they have been benefiting from the arrangement for years.
Co-author Neil Tsutsui explained to Discovery News that these ants "are successful. They produce more ants who do this and the behavior spreads. Evolution proceeds in basically the same way as for any other trait."
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"It's no more cruel than consuming other organisms as food, competing with another organism for resources, or the myriad other ways that organisms come into conflict in the natural world," added Tsutsui, who is a professor in the Department of Environmental Science, Policy & Management at the University of California, Berkeley.
He and co-author Candice Torres compared the chemical, genetic, and behavioral characteristics of enslaved and free-living colonies of the Formica altipetens ant, which is the target of Polyergus' slave-making.
They found the slaves were less aggressive toward non-nest mate ants than were their free-living counterparts. The findings, published in the journal PLOS ONE, suggest that slavery can dramatically change both the chemical and genetic context in which the kidnapped victims develop.
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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.
"There are cases of enslaved ants putting up a vigorous fight to try and repel the slave-makers when their colonies are being raided," he said.
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.
"The way that ants normally develop a sense of colony identity makes them vulnerable to this type of slave-making behavior," Tsutsui said. "Normally, young ants emerge from pupation and imprint on the odors around them. They then use the memory of these odors to figure out who is a member of their colony versus who is an ant from a different colony."
The victim ants have no idea that they are enslaved. They appear to believe that they are in their home colony.
The scientists suspect the system in ants first evolved many years ago, after an ancestor of Polyergus might have occasionally raided other ant colonies, retrieving the larvae and pupae for food. This could have led to some of the raided pupae emerging as adult worker ants that, believing nothing was amiss, toiled away for the attackers. This would have then enhanced the survival and reproduction of Polyergus ants that practiced the raiding, making the behavior more common over time.
There seems to be no benefit for the enslaved ants, but the slave-makers do experience some risks. Some are killed during the raids.
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"There's also some evidence of the enslaved ants doing a bad job of caring for the slave-maker babies, and of laying their own eggs while enslaved," Tsutsui said.
James Trager, a myrmecologist (ant expert) and naturalist at the Shaw Nature Reserve in Missouri, says the ant study, "digs deeper into our understanding of the phenomena that are involved in making the system work."