Slave-making ants get rough in New York

The slavery racket in the ant world is more violent in New York than in West Virginia, even though the same species are involved.

Small Leptothorax slaves tend a big slave-making ant queen. Hare

That’s the conclusion of the first laboratory analyses of how slavers and their slaves might be driving each other’s evolution, explains Susanne Foitzik of the University of Regensburg in Germany. In the June 7 Proceedings of the Royal Society of London B, she and her colleagues report finding more built-in defensiveness in the enslaved species where slavers strike more fiercely.

Vickie L. Backus of Middlebury College in Vermont, another ecologist who has studied ants, calls the report “the first . . . I’m aware

of to look at coevolution between slave makers and slaves.”

Some 20 ant species take slaves by sweeping into another species’ nest and carrying away the brood. These youngsters mature in the slavers’ nest and labor in it as if it were their own. They even join raids against their own species.

The idea that slaves can fight back has gotten short shrift in the ant community, Backus points out. An influential 1981 paper asked why there were no ant slave rebellions and answered that the slavers’ raids were too occasional to drive the evolution of defenses in the raided species.

Foitzik attributes part of this attitude to researchers’ focus on showy cases of slave making. In a European species, for instance, one tiny queen subdues a nest of huskier ants by catching a ride underneath the rightful queen and slowly choking her. “She strangles in about 3 weeks, and her workers do nothing to save her,” Foitzik says.

For a new look at ant slavery, Foitzik and her colleagues collected some 200 geographically diverse colonies of the slaver species Protomognathus americanus and a species it raids, Leptothorax longispinosus. In the lab, the researchers videotaped slaving raids.

The slave-making ants from New York did more damage than their West Virginia counterparts, capturing a greater percentage of each invaded brood and killing more queens. Much of their success comes from their greater likelihood to post an attacker at the invaded nest’s entrance, says coauthor Christopher DeHeer at North Carolina State University in Raleigh.

“The guard is like a bouncer–no one in, no one out without her okay,” she says. The guard lets besieged adults escape only if they relinquish any youngster they may be trying to carry to safety.

Likewise, New York slaves fight back more than their West Virginia kin. The New Yorkers sting slaver scouts more often than West Virginians do.

The differences between New York and West Virginia ants may have something to do with the density of species that can be enslaved in these two regions, Foitzik speculates. In New York, she found several 25-square-meter plots bristling with as many as 50 colonies of potential slaves. There, slavers can strike hard and often, kill queens, and wreak havoc, without destroying their potential slave-labor force, she notes.

West Virginia didn’t offer such easy pickings, and Foitzik muses that perhaps slavers there prosper best by husbanding their resources, doing less damage, and moving on frequently. This scenario echoes the slave system of the same species in Michigan (SN: 8/19/00, p. 116).

Comparing more and less violent slave systems, Foitzik hesitates to say which came first. Perhaps ants that were more benign got rough when crowded into New York densities, or perhaps vicious New York–style ants mellowed when they arrived in live-and-let-live West Virginia.

Susan Milius is the life sciences writer, covering organismal biology and evolution, and has a special passion for plants, fungi and invertebrates. She studied biology and English literature.