Parasitic ants, unwise language, and a little glimpse of Darwin

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.

15 thoughts on “Parasitic ants, unwise language, and a little glimpse of Darwin

  1. Robley Hood

    Thank you, David. I really enjoyed reading this post. As a long-time English teacher, I appreciate language refinement.

  2. Ann Fraser

    My first year seminar group has been reading about social bee-ings and the co-construction of human and other animal societies. One book we are reading is by a sociologist, Diane Rodgers, titled “Debugging the link between social theory and social insects.” Dense reading, but enlightening at the same time! The terminology misuse is one thing addressed, something we often overlook. Great to see you draw attention to it here David.

    1. David George Haskell Post author

      Thank you, Ann. Sounds like a book I need to look up. I’m using Wilson’s Social Conquest book this semester which makes much of the parallels among the processes underlying social evolution in social insects and humans.

      Thank you for the help with ID!

  3. manayunkia

    A rose by any other name…
    Regardless of our terminology, it is an incredibly fascinating behavior and ritual of these ants. And that some actually retaliate is even more fascinating. I often read a note or see a fungus or an insect wing and wonder, “How in the world did that happen?” And here I am again.

    Another example of humans imposing morality on animals through language comes from what I read when I was a kid, “Only ants and humans wage war.” Depending on what you consider war or battles, I now know there are other examples. Ant ‘war’, however, doesn’t stir my stomach as does this example of our closest relative conducting raids: . At least for me, questions of morality and disturbing behavior seem to be linked to how many base pairs I share with you.

    1. David George Haskell Post author

      Whoa. Chimps can sure play on our empathy buttons. Maybe closeness of the evolutionary tree has some moral bearing — increased probability that neural organization is similar? Which is not a bad place to start when we’re getting into empathy?

      “How in the world did that happen?” is a good thought. Let’s make it the mantra for Intro Bio.

    1. David George Haskell Post author

      Wow! I had no idea that the overlords had such a storied cultural history.

      I for one will fight back. (I do every time my hands hits a fire ant nest in the garden…)

      Thanks for this link — very funny.

  4. James C. Trager

    Hello David —
    One little thing about the Formica on Formica raids: The robbed brood nearly always consists of only pupae, as in your pictures. This makes good energetic/evolutionary sense, since pupae do not require feeding, and some of the pupae can even be processed and easily digested as food for the raiders’ mixed colony at home, so they are the most cost-effective brood form for which to go through the effort of discovering and carrying home.
    Yeah, the terminology thing is difficult. I got interested in the attitudes about, and use of, the word “slavery” for ants, and a few years ago I did an informal survey of myrmecologists (ant scientists) on it. The result, roughly: Young Americans were most likely to dislike the term and to avoid it, while older Europeans were the most at ease with use of this metaphor, even defended it as too entrenched to dispense with now, after over 200 years of use in the myrmecological literature. The term slavery, and derivative terminology, are still with us in recent myrmecological literature, as you note. My preference is to refer to this behavior as work-, or labor parasitism. On the other hand, there are also the terms dulosis (noun), and dulotic (adjective), derived from the Greek for slave. They are new coinings, which literally translated would mean something like “slaveyness”, and “performing slaveyness”. For most people, except for a few Classics and Greek scholars, these terms are not even remotely laden in the way that “slavery” is, but even so, have met with objection. What is a myrmecologist to do?

    1. David George Haskell Post author

      Hi James, Thank you *very* much for your thoughtful and informative reply. Good to know about the pupae — I wish my camera was better able to work in low light. A lot of the shots were too blurred. But most did indeed look like pupae.

      The language question is a tough one. One cultural factor affecting Northern Europeans (where I came from) is an absence of any recent experience of slavery’s evils in human society. That is not something that is true here, perhaps creating a more acute sense of the moral problems with using the words for ants. In other words, in Europe slavery is mostly encountered as a distant idea; here it is encountered as a strong echo of a forceful reality (here in TN, how many of our tunnels and roads were built by slaves? A lot.).

      Hard questions. I find the generational shift that you mention very interesting. Language evolves…

  5. allencraig

    As a watcher and wonderer of nature coming from a different background (photographer and casual environmentalist) I am left wondering about the invaded ant colony. What habits have they embraced to allow them to generationally survive these attacks? Sheer numbers? (Normally distant) colony location? Hidden eggs?

    No species is always a victim, so I’m just curious to know more about this situation, and the fuller story of balance and interconnectedness.

    Glad to have come across your blog. Wonderful stuff.

    1. David George Haskell Post author

      In general the prey ant species do try to fight back when invaded, but are often overpowered. Previous studies have shown that these prey species may evolve a number of counter-measures, incl the ability to better recognize invaders, the ability to fight back, and the ability to evacuate and leave quickly. But these species are still subject to much lost productivity because of parasitism. The paper that I cited states: “Field studies have shown a significant reduction in productivity and average life expectancy of host colonies in parasitized populations” So, like many other animals that are preyed on or parasitized, countermeasures only go so far.

      Thanks for reading the blog!

  6. kayaerbil

    Dear David,

    Thank you for this fascinating article. I am a chemist (Ph.D., UC Berkeley, 2009) who is currently in seminary at Princeton Theological Seminary. I ran across the article about you in the science section of the New York Times recently. I am taking a class called Darwin and Theology. As part of this class, I read the Origin of Species and encountered Darwin’s writings on slave making ants. This blog entry is fascinating to me to see that these ants also live in the Southeastern US (I am from Georgia). I would like to try to find a copy of your book to read when classes are over. In the mean time, I will try to read some entries in your blog.

    I also have a blog where I post various thoughts on topics of religion, science, and humanity ( I have an interest in trying to use heart rate monitors to analyze how the heart and mind interact during prayer, meditation, and other types of religious behaviors. For example, last night I recorded my heart rate as a function of time while reciting poetry and tried to compare it to sitting meditation ( I hope to gain a greater understanding on how my heart responds to different kinds of religious practices. It seems your writing in “The Forest Unseen” may provide a model for my own introspective work. I would like to write about my inner experiences and relate them to my heart rate to see how language may influence physiology.


    1. David George Haskell Post author

      Thank you for this thoughtful comment. It sounds like you are living out the connection among different ways of knowing — science, religion, and literature. I was fascinated by your experience with the heart rate monitor. A good reminder of the effects of our mental states on our bodies. Or, put another way, that our mental states is part of our body and vice versa. The Darwin and Theology course sounds great. To me, one of the great unresolved questions is what do we do with the knowledge that the separation that was assumed between “humans” and “all other life forms” does not exist, at least on a biological level.


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