Adventures Among Ants - Mark W. Moffett [103]
An acorn from Ohio containing a colony of dark brown Protomognathus americanus slavemaker ants and their orange Temnothorax slaves.
Glancing around, I was struck by the unreality of the situation: all this time I had been so caught up in the action that I had forgotten I was in a laboratory, surrounded by Petri dishes and Bunsen burners. To learn about slavery in a nutshell, I had come to the Mecca of acorn-ant research: Ohio State University, base of Joan Herbers. Joan specializes in Protomognathus americanus, which enslaves three species of Temnothorax in the temperate deciduous forests and yards of eastern North America.6 It is difficult to observe raids of these pygmies in nature. Before I flew to Columbus, Joan had been kind enough to sort out some colonies for me, collected by her students and encased in Ziploc bags. Each bag contained either a slavemaker colony or a colony of free-living Temnothorax, housed in an acorn. All I had to do was put a mix of these acorns in a plastic arena, settle down in front of it, and wait.
Fifteen hours later, I had finished documenting my first Protomognathus slave raid. Over the course of an hour, the slavemakers had taken part of the Temnothorax brood to their old acorn, while expanding their colony into the new one as well. Having multiple nests like this is called polydomy, and it is common among ants that live in acorns and other small, convenient places. Meanwhile, the Temnothorax adults were still scattered over the ground, having lost both progeny and home.
To bring troops to a Temnothorax colony, these slavemakers employ a variation on something called tandem running, a follow-the-leader approach to recruitment in which an ant tracks the successful scout to a site by touching her repeatedly or, if they lose contact, by orienting to a short-range pheromone released by the first ant. Because the leader is responsive to the follower, stopping at intervals to wait for her touch, the relation has been likened to that between teacher and student.7 With Protomognathus, the “teacher” brings along a whole class, for a conga line of several nestmates follows the successful scout.8
The Amazon ants, Polyergus, belong to the Formicinae, a group of ants that includes the carpenter ants and their relations. The Formicinae evolved slavemaking several times independently in different species in different locations. Protomognathus belongs to the Myrmicinae, a second large group in which slavemaking is common.9 The Temnothorax species it enslaves commonly reside in fallen acorns that have been opened up by one of two acorn specialists, the acorn moth or the acorn weevil. The adult females of both these insects lay eggs on or in the nuts; the larva then eats part of the meat before chewing a hole in the nutshell, from which it emerges. The exit hole becomes the entryway for a succession of motley residents, often culminating in Temnothorax.
I described this array of relationships for National Geographic magazine while I was a graduate student looking for cool projects in my neighborhood.10 In researching that article, I spent a lot of time gathering acorns and dropping them in water. Those with residents float because of the eaten-out cavities. Cracking them open—and exercising patience—I eventually uncovered a whole society: several dozen Temnothorax with a queen and pale brood, occupying hollows carved in the nut. After hundreds more acorns, I came upon a mixed colony with two forms of worker—much scarcer. One kind (with a bigger head, stronger jaws, and a groove along each side of the head into which she withdraws her antennae for battle) was the Protomognathus; the other was a Temnothorax worker—in this circumstance, a slave.
Slavemakers like the Amazon