Ant Slaves

SLAVE-DRIVING ANTS ON A MISSION TO SURVIVE

In 1865, the Thirteenth Amendment to the U.S. Constitution was ratified, abolishing slavery in America, but the ant world took little notice.

Slavery among ants may be one of the most unusual forms of social behavior to have evolved, but workers of the species Polyergus breviceps-which are all female-live exclusively on slave labor and would die without workers of the Formica gnava species to take care of them. I spend my summers conducting field research in the Chiricahua Mountains of southeastern Arizona, where each afternoon I wait for an ant to emerge from her subterranean nest.

That's right, I'm waiting for one ant, but she's no ordinary creature. The colony's survival depends on this single antcalled a scout - who has one Herculean mission: to emerge from her nest, travel up to 150 yards, search under rocks and leaf litter for a disguised subterranean nest of F. gnava ants, find her way home, and then lead about 2,000 pumped-up P. breviceps workers on a slave

Raid back to the F. gnava nest. A slave raid? The workers of P. breviceps have lost the ability to forage for food, feed their brood or queen, or even clean their own nest. To compensate for these deficits, P. breviceps have become specialized at obtaining workers from the related genus F. gnava to do these chores for them. This is accomplished by the slave raid, in which P. breviceps penetrate the F. gnava nest and capture the resident's pupal brood. Back at the P. breviceps nest, the kidnapped brood is raised, and the emerging F. gnava workers then assume all responsibility for maintaining the permanent, mixed-species nest. They forage for food and regurgitate it to colony members of both species. To better understand the evolution of social parasitism in ants, I have focused on the bizarre way in which P. breviceps queens establish new colonies. For example, free-living (that is, nonparasitic) ants have a straightforward process of colony-founding by queens. After a mating flight, an insem-inated female excavates a chamber, lays a few eggs and then nourishes her larvae with stored nutrients. When the first brood matures into adult ants, these workers feed the queen and the larvae of her subsequent broods. But this sequence will not work for a parasitic queen ant such as P. breviceps, because she is, after all, a parasite. She can't feed herself, much less can she raise her own larvae. Her only recourse is to invade an F. gnava colony, kill the host queen and get the workers to accept her as their queen. The resident F. gnava workers will nurture the brood of the P. breviceps queen until her worker population is sufficiently large to supplement the slave force by staging raids on other F gnava colonies.

attempt to grab her. The P. breviceps queen's two main defensive adaptations are powerful mandibles for biting her attackers and a repellent chemical (pheromone) secreted from a gland in her abdomen.

With the worker opposition liquidated, the P. breviceps queen seizes and kills the F. gnava assembles any scattered F. gnava pupae into a neat pile and triumphantly stands on top of it. At this point, colony takeover is complete.

It has occurred to me that this abrupt appropriation of the colony could be accomplished by what I call a "chemical heist." The P. breviceps queen would acquire E. gnava queen chemicals during the very act of killing and licking her. To test this idea, I repeated the study, but with a twist: To each laboratory nest, I added a dead F. gnava queen that had been frozen for five minutes, and then defrosted. The results were exactly as I had predicted. Upon entering the nest, the P. breviceps queen ran past the attacking workers, pounced on the motionless F. gnava queen and proceeded to bite and lick her, just as if she were alive. After about a half hour of work-ing over the dead E gnava queen, the P. breviceps queen was again promptly groomed by the F gnava workers and permanently accepted as their new queen. Christine Johnson, one of my students, has since confirmed that during the act of killing the F. gnava queen, the P. breviceps queen does acquire a "chemical cocktail" from the vanquished female.

Because colonies of F. gnava and newly mated P. breviceps queens are very easy to collect, I have been able to make detailed observations of the takeover process in laboratory nests. Within seconds after being placed inside an F. gnava nest, the P. breviceps queen bolts for the F. gnava queen, literally A common question is why don't the F. gnava slaves run away, perhaps back to their own nest. The answer is that these ants are not "enslaved" in the human sense. Their situation is more like that of "adopting a baby." Because they are snatched as pupae and raised in a P. breviceps colony, the F. gnava workers become imprinted with the odors of the host ants. In the subterranean world of ants, where vision is virtually useless, a family-even one that's composed of different species-is defined by an arsenal of chemicals that is secreted by the ants and applied to all nest mates.

queen. Immediately after the host queen's death, the F. gnava nest undergoes a remark-able transformation. The F gnava workers approach the P. breviceps queen and start grooming her. The P. breviceps queen, in turn, In ants, you see, the family that sprays together, stays together. All