Biology Of Slave Making Ants

Invasion Of The Booty Snatchers

An Introduction To The Behavior of The Slave-Making Ant Polyergus topoffi

In 1865, the Thirteenth Amendment to the United States 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 speciesPolyergus topoffi- which are all female - live exclusively on slave labor and would die without workers ofFormicato take care of them. That's what brings me to 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 ant- called a scout - who has one Herculean mission: emerge from her nest, travel up to 150 yards, search under rocks and leaf litter for a disguised subterranean nest of the ant speciesFormicagnava, find her way home, and then lead about 2,000 pumped-upPolyergusworkers on a slave raid back to the Formica nest. A slave raid?

The workers ofPolyergushave lost the ability to forage for food, feed their brood or queen, or even clean their own nest. To compensate for these deficits,Polyergushas become specialized at obtaining workers from the related genusFormicato do these chores for them. This is accomplished by a slave raid, in whichPolyerguspenetrate aFormicanest, and capture the resident's pupal brood.

Back at thePolyergusnest, the raided brood is reared through development, and the emergingFormicaworkers then assume all responsibility for maintaining the permanent, mixed-species nest. They forage for food and regurgitate it to colony members of both species.

Of all behavioral adaptations necessary for the evolution of social parasitism, I have focused on the bizarre way in whichPolyergusqueens establish new colonies. To clarify this point, consider the straightforward process of colony founding by queens of free-living ants. After a mating flight, an inseminated female excavates a chamber, lays a few eggs, and 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 simply will not work for a parasitic ant such asPolyergus, because she is, after all, a parasite: she can't feed herself, much less rear her own larvae. Her only recourse is to invade aFormicacolony, kill the host queen, and somehow get the workers to accept her as their queen. If successful, these residentFormicaworkers will rear the brood of thePolyergusqueen until her worker population is sufficiently large to supplement the slave force by staging raids on otherFormicacolonies.

Because colonies ofFormicaand newly-matedPolyergusqueens are are very easy to collect, I was able to make detailed observations in laboratory nests of the takeover process. Within seconds after being placed inside aFormicanest, thePolyergusqueen bolts for theFormicaqueen, literally pushing aside anyFormicaworkers that attempted to grab her. ThePolyergusqueen's two main defensive adaptations are powerful mandibles for biting her attackers, and a repellant chemical (pheromone) secreted from the Dufour's gland in her abdomen. With the worker opposition liquidated, thePolyergusqueen seizes and kills theFormicaqueen. Immediately after the death of the host queen, theFormicanest undergoes a most remarkable transformation. TheFormicaworkers approach thePolyergusqueen, and start grooming her. ThePolyergusqueen, in turn, assembles any scatteredFormicapupae into a neat pile, and "triumphantly" stands on top of it. At this point, colony takeover is complete!

It occurred to me that this abrupt appropriation of the colony could be accomplished by what I call a "chemical heist." Accordingly, thePolyergusqueen would acquireFormicaqueen chemicals during the very act of killing and licking her. To test this idea, I repeated the study, but with an interesting twist: to each laboratory nest, I added a deadFormicaqueen that had been frozen for 5 minutes, and then defrosted. The results were exactly as I had predicted. Upon entering the nest, thePolyergusqueen ran past the attacking workers, pounced on the motionlessFormicaqueen, and proceeded to bite and lick her just as if she were alive. And after about 30 minutes of working over the deadFormicaqueen, thePolyergusqueen was again promptly groomed by theFormicaworkers and permanently accepted as their new queen. Recently, one of my students, Christine Johnson, demonstrated that during the act ofFormicaqueen killing, thePolyergusqueen does indeed incorporate a veritable cocktail of chemicals from the vanquished female.

A common question is why don't theFormicaslaves run away, perhaps back to their own nest. The answer is that these ants are not enslaved in the human sense. Indeed, it's more like adopting a baby. Because they are snatched as pupae and reared in aPolyerguscolony, theFormicaworkers become imprinted with the odors of the host ants. In a subterranean world where vision is virtually useless, a family - even one composed of different species - is defined by an arsenal of chemicals secreted by the ants and applied to all nest mates. In ants, the family that sprays together stays together!

Howard Topoff 2011