Pyramica vs Strumigenys: why does it matter?

rostrata2
Pyramica (or is it Strumigenys?) rostrata, Illinois

I’ve been thinking today about the Wikipedia edits to the Pyramica page, and my curiosity about the controversy prodded me to attempt a quick phylogenetic analysis.  Before I get to the analysis, though, here is some background.

The Ants.  Forests in warmer regions around the world hold a great number of tiny, sluggish ants covered with bizarre hairs of unknown function.  These oddly ornate little insects are predators of other arthropods.  Mites, springtails, and the like.  Because of their size, their preference for below-ground prey, and their habit of freezing when disturbed, these are not easy ants to find even where they are abundant.  Most belong to the genera Pyramica and Strumigenys.

While all species are stealth predators, approaching prey by edging almost imperceptibly slowly towards it, species show either of two distinct ways to subdue prey once they get within striking range.  Pyramica use their mandibles like pliers, holding fast while they swing their abdomen around to sting.  They are slow but grippy.  Strumigenys are trap-jaw ants, stunning their prey with a sudden blow when their mandibular trap slams shut.  They are fast but lack a powerful grip.

The photos below show the two mandible types.  However different their jaws may be, the ants are otherwise quite similar.

Pyramica reflexa
Pyramica reflexa
Strumigenys louisianae
Strumigenys louisianae

I’ll also point out that while mandible length alone often indicates the hunting strategy a particular ant uses, the trait isn’t absolute.  Some Pyramica slow-grip with long mandibles, rather more like needle-nose pliers, while a few Strumigenys sport surprisingly stubby trap-jaws.  If you wish to peruse the diversity, Antweb hosts extensive specimen galleries of Pyramica here and Strumigenys here.

The Taxonomy. The taxonomic issue is more substantive than the usual nomenclatural squabbling that systematists frequently engage in.  It’s not just about what to call the ants, but a deeper disagreement about how they evolved.  Because the ants have different mandible shapes and hunting strategies, the phylogenetic arrangement of the various species will have profound implications for how we infer traits to evolve.

For instance, if Pyramica turns out to be an ancestral radiation from which Strumigenys later emerged, then the trap-jaw must have arisen as a special case of the slow-grip mandibles, and the evolutionary trend over time in some lineages is towards longer mandibles and prey capture by shock-and-awe.  In the converse situation, Strumigenys birthing the slow-grip Pyramica, then the trend is instead towards shorter mandibles, and with the slow-grip being a subsequent modification of a trap-jaw.  Finally, it could also be that both groups are independent, each having created its specialized form by modifying the generalized mouthparts of a common ancestor.

At the moment, myrmecologists hold conflicting views on which scenario they prefer.  E. O. Wilson and Bill Brown famously raised what I call the Incredible Shrinking Mandible hypothesis, preferring trap-jaws to be ancestral to the slow-grippers.  This view is shared by Barry Bolton, who revised the larger tribe Dacetini in 2003.  Cesare Baroni-Urbani prefers the opposite, postulating that trap-jaws emerged second, and that the groups are so blurred in places that they ought to all be placed in a single genus, Strumigenys.  Apparently, adherents of both views have taken to alternating edits of Wikipedia.

My Half-Baked Analysis. This morning, my interest piqued by controversy, I visited Genbank to see if I could find species with enough overlapping gene fragments to generate a phylogeny.  Turns out that 5 species from each genus had sequences posted from the same 2 genes: the 28S ribosomal gene and the COI mitochondrial gene.  Considering the hundreds of species distributed across the two genera, this is certainly a paltry representation.  But it’s enough for a preliminary look.

I took about an hour to align the sequences and prepare a file for analysis, and another hour for my computer to run it.  (If you’d like the details, email me).  Here’s the result:

phylogeny1
A phylogeny of Pyramica (red) and Strumigenys (blue) based on DNA sequence data from the genes 28S & COI. Generated using the freeware program MrBayes; numbers are the Bayesian posterior probabilities supporting the indicated relationships.

What does this tell us?

To be honest, not as much as I’d hoped.  Support values are low throughout the tree, undercutting the confidence we may have in the depicted relationships.  But it does show that, for these species and for these genes, there is no immediately clear-cut distinction between Strumigenys and Pyramica. Contra Baroni-Urbani, we do get a hint that the trap-jaw came first.  But contra Bolton, the two groups may be intermingled to an uncomfortable degree. It might be that everyone is wrong!

A proper resolution to the Pyramica problem will require sampling hundreds more species than I did here, using more genes and a more exhaustive set of analyses.  More that just a couple hours’ work on a Saturday.  But this is exactly the kind of approach we need.  In an ideal world, the genetic research would be accompanied by behavioral assays to determine what each species eats and how each uses its mandibles, as well as a careful biomechanical assessment of the mouthparts themselves.  Only then will a clear picture emerge about how these ants came to acquire such strange jaws.


footnote:  The DNA data I used is publicly available, and the software programs I used (Mesquite and MrBayes) are both freeware.  A surprising amount of evolutionary biology can be done without spending a dime.

22 thoughts on “Pyramica vs Strumigenys: why does it matter?”

  1. Great post. I’ve been pondering how one might get a handle on the ecology-morphology aspect of this story for a while. It’s hard not to think about such gorgeous ants!

  2. Well, I’m not very good in phylogenetic things but, if your tree has some meaning, two things result clearly from it:
    1. Separation of Pyramica from Strumigenys is wrong.
    2. Wilson’s, Brown’s and Bolton’s idea, apparently supported also in your blog, for which ‘Pyramica turns out to be an ancestral radiation from which Strumigenys later emerged’ is also wrong.

    The taxonomy underlining this conclusion was already published 150 years ago by Roger, the discoverer of Pyramica.

  3. Is it necessary that they form two separate lineages? Could not different species switched from one strategy to another over time as they evolved in response to changing environments?

  4. Alex, I’m not sure whether to applaud or show concern for the amount of scientific ponderings on your blog. I think it’s good for science, but not the proprietor of the blog.

    I wouldn’t be so bold as to publish so many evaluations of ideas without the backing of formal peer review. I wouldn’t be as concerned about the validity of my criticisms, but rather the perceived validity. Perhaps I’m hypersensitive to alienating other scientists. I just wouldn’t want to be responsible for airing other people’s dirty laundry.

    I’m not saying that you’re unfair. But I think most people whose work is being reviewed on your site feel that it won’t stand up to rigorous review.

    1. Colleague:

      If you or somebody else thinks that the logic or the information of a comment is wrong, he should simply point to and explain the weak points of the comment in the same blog.
      I fear understanding that comments that you dislike and you are unable to contradict should be prevented from publication in the blog. Please reassure me, possibly with facts and not with dialectic exercises.

      In this spirit, thank you Pdiff.

  5. James C. Trager

    Colleague:

    Alex and any other scientist reader knows it won’t stand up to rigorous review. That’s why it’s in a blog, not an online journal or other formal outlet. Pondering of this sort, followed by discussion with colleagues leads to good research. This post clearly points to the need for further scientific research and eventual peer-reviewed publication of the results, and is not a substitute for these.

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  7. Blogging on published research is part of the process that used to be carried on in hallways, on the telephone, and in the bar at conferences. It’s actually part of the post-publication peer review process in which professionals noodle on published research, thinking about its meaning, teasing out implications, wondering about extensions.

    The issue is not whether it will stand up to “rigorous review,” it’s whether it’s an interesting take on the research. Admittedly, some professionals don’t appreciate those takes being published for the world to see, but then, that’s the way the world is nowadays.

  8. From a research point of view, I’d agree with RBH – blogs can be the new tea room/pubs (well maybe not the latter – I’m too familiar with how I spout off after a few beers to let myself blog then, but Coopers & Cladistics does come to mind).

    From an ex-lecturer’s point of view, I’d say that blogs can be like classrooms too. I think that one of the better ways to teach science is to present (blog) two or more contrasting explanations for a hypothesis and let the class (readers) ponder their interpretations (0 comments) or disagree. I know blogs (and professors) are infamous for their political ranting, but there is no reason why every blog or classroom should be a political pulpit. Presenting an alternative to an established scientific hypothesis is science and political in only the most abstract sense.

    However, as RBH notes, blogs are really different from watercoolers and classrooms in one critical way – they are potentially in everyone’s face. I’m not sure how I’m going to respond if and when someone starts rubbishing one of my favourite hypotheses, but I imagine waking up at 3 am with the perfect response and then heading for the computer in my pajamas and without my usual collegial facade.

  9. I would applaud Alex for doing so. Why should he, or anyone else, be shy about proposing and debating ideas? If someone gets their toes or favorite theory stepped on and doesn’t like it, then they are either in the wrong business or should come up with a reply or defense. They are free to comment here or elsewhere. The “Blog” is simply another way for the scientific method to poke and prod hypotheses. That’s what the whole game is about. This new medium is no different in spirit from the numerous public letters, journals, and even layman’s newspapers of past years. Some of the best and most animated debates on evolution, for example, were carried out this way.
    I say carry on the ideas, rebuttals and debate.

  10. I don’t know Alex. First you dare to post your expert opinion on some parts of Hölldobler and Wilson’s latest book. Now you provide a succinct review on a topic of your specialty while actually doing your homework and throwing in some original results. What’s next, an insightful view on soon-to-be-published high end research?

    Let me tell you something. This “critical scientist” attitude of yours, using this novel digital medium, won’t let you anywhere young man.

    1. When we thought that the earth was the center of the universe nobody could dare doubting the ideas of the Vatican. Are we confronted with an ant Vatican?

  11. (channeling Uguthecrow) – Aren’t Alex’s opinions/hypotheses/etc. being peer reviewed right here, right now, in the comments? And doesn’t Alex refer to this as a “half-baked analysis” that requires more resources “to do the job right”? I agree that Alex would get more academic “credit” (i.e., a notch on his CV) with a more detailed, peer-reviewed critique, but that might not be the best use of his time and resources right now (in this particular case).

    At least he provoked, perhaps, someone else to take up the reigns to solve this taxonomic and evolutionary conundrum. My postdoc and I are posterchildren for how the free flow of scientific ideas can motivate research on important topics. One of you science bloggers (not Alex) made a statement recently on your blog about something that sent me and my postdoc on a wild and crazy mission. Now we have a MS to be submitted in August-ish that I think a certain group of you will find quite compelling. This research never would have happened without your science blogging – THIS “just throwing that out there” kind of science blogging. We simply wouldn’t have thought of addressing this question.

    Keep up the good work, bro.

  12. Great blog!!!

    This question has been on my mind quite a bit lately because I have some species lists up on my site of southern US ants and am wondering when or if I should be changing some names. Also, we have a new species of Pyramica from Mississippi, but should I call it Strumigenys or Pyramica? I have corresponded about this matter with both Bolton and Baroni-Urbani, both of whom I greatly respect for their obvious dedication and love of myrmecology and their many contributions to the ant world. But, obviously, they have radically different viewpoints about this issue. I suppose for me, at least right now, the question is are people following Baroni-Urbani’s recent treatment of the group? I noticed that the names on Antbase.org have been changed to reflect his synonymy. Are most people following this now?

    I went through a few weeks ago and found the same species that you used in your phylogeny on the Barcode of Life Data Systems site, probably from the same projects. Although I don’t really know much about phylogenies, it looked to me (from the small sample anyway) that it would be hard to separate these species from one another. I certainly don’t have any problem calling them all Strumigenys, although the ones here in North America are easy enough to separate.

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  14. Great post, and the comments about blogging are just as interesting. I’ve written a short note about this edit war on iPhylo, partly as an excuse to show another history flow visualisation of the edits to a Wikipedia page. It would be interesting to see whether other taxon pages in Wikipedia pages that have been the subject of edit wars will also generate analyses of the kind you’ve undertaken here.

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  17. This is a great blog post, and gets all the more interesting, at times amusing, from the entailing discussion. I love this blog.

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