Several weeks ago I came across a curious highway of ants. They were streaming across the leaf litter in a column about a foot wide. The column started under an oak tree, traversed the leaf litter and hiking trail, then ended abruptly about forty feet away in an otherwise unremarkable patch of fallen leaves. Ants traveling away from the tree were carrying white, ant-sized objects in their jaws. Ants moving in the opposite direction were empty-mouthed. At the destination, a few smaller ants milled about, seemingly at ease among the larger ants that I was watching.
I suspected at the time that I was witnessing a raid by a so-called slave-making ant species. My skills as an ant taxonomist are limited and I turned to my colleagues for help. Thanks to James Trager and Ann Fraser, I’ve confirmed my suspicion and been able to tentatively identify the species in question as Formica subintegra and Formica subsericea (an aside: Ant Blog is a great place to seek answers about ants). The first species, the larger one, was attacking the nest of the other and carrying away eggs and larvae. These captured youngsters will be raised in the “den of thieves” and, when they emerge as adults, the newly pupated ants will have no idea that they do not belong. Because ants take their cues from the chemical milieu in which they grow up, the stolen ants consider themselves full members of the alien colony. This trickery buys the captors a work force to maintain the nest and rear more young. In some ant species, the captors are so dependent on the captured workers that they cannot survive without them, having lost the ability to feed themselves and take care of the brood.
In the biological literature this arrangement has, for many years, been called “slave-making.” This makes me deeply uncomfortable. Using a term — slavery — from a human institution that all (or nearly all) modern human societies have agreed is morally unacceptable seems unwise. Further, the “ant slavery” term implies a biological equivalence that does not exist. There is not a single biological parallel between the details of the situations in humans and ants (ants raid other species, ant societies and nervous systems differ radically from ours, etc). By using a term derived from human society, a term that comes with considerable moral heft, we blind ourselves to the otherness of the ants. So in addition to the moral argument (which is strong enough on its own, I think), there are scientific reasons for not using the term: our preconceptions may cause us to fail to understand ant biology.
In other areas of biology, we’ve thankfully tidied up our terminology a bit. Textbooks on animal behavior were formerly strewn with terms like divorce, rape, and prostitution. These days, textbooks generally leave these loaded terms at the door, although more popular media outlets and some scientists continue the unfortunate practice. For example: Wikipedia (of course), BBC, and The Independent (note how the coverage slips so easily into discussion of what is natural for humans; Hume shudders, as explained (of course) on Wikipedia). My point is not that conflict, coercion and suffering do not occur in nature (of course they do), but that the use of human categories to describe animal behaviors can lead us into trouble. This is especially true when those categories carry with them a strong emotional, intellectual or moral charge.
Back to the ants. I was particularly excited to see this process unfold because it has a place in the history of biological ideas. Darwin was fascinated by these ants and used them in Chapter Eight of On The Origin of Species as an example of how natural selection could mold behavior (or “instinct” as he called it). He writes:
We shall, perhaps, best understand how instincts in a state of nature have become modified by selection by considering a few cases. I will select only three, namely, the instinct which leads the cuckoo to lay her eggs in other birds’ nests; the slave-making instinct of certain ants; and the cell-making power of the hive-bee: these two latter instincts have generally and justly been ranked by naturalists as the most wonderful of all known instincts.
Darwin dug up and manipulated a number of nests in England, experimenting with the ants to better understand the nature of the “slaves” and “masters” as he termed them (Darwin was not shy here or elsewhere in his writing about linguistic cross-over from human behavior). He concludes that:
…natural selection might increase and modify the [parasitic] instinct—always supposing each modification to be of use to the species—until an ant was formed as abjectly dependent on its slaves as is the Formica rufescens.
The complete account is available in the many online copies of The Origin (or in the treasured copy of this volume on your bookshelf).
In the years since Darwin, hundreds of studies have been conducted on the socially parasitic ants, many of which are summarized in a short review by Buschinger. One recent study of particular note is the discovery of retaliation by a genus of ant that is frequently attacked by parasites. The host genus is Temnothorax — tiny ants that nest inside acorns (!) and hollow twigs — and the parasite is Protomognathus americanus. Unlike the larva-robbers that I observed, Protomognathus parasitic ants invade and take over the nest of the host. Temnothorax adults are killed and their young are co-opted to work for the parasite. It appears that these attacks are so common that natural selection has produced a counter-measure: genes in some of the host workers cause them to attack the parasite, killing the developing Protomognathus pupae.
The authors regrettably use the terms “slave rebellion” and “revolt against their oppressors” to describe the behaviors that they describe. Surely a human rebellion against slavery is biologically and morally different than a gene variant causing an ant to use chemical cues to bite a pupa? My grousing about language aside, this is a remarkable study. Darwin would have loved to add this co-evolutionary tale to his chapter on the evolution of animal behavior.
From now on, I’ll be examining acorns more closely.