Count Bacula, part 1

I wasn’t the first to own that nickname. I know Bruce Patterson got landed with it in grad school a decade before I did, and I wouldn’t be surprised if anyone studying mammalian genital bones had to have it at some point. If I ever write a book on them, that’ll be the title.

Bones, you say? Why yes. It is called a baculum, plural bacula, and here’s who has it:

  • All rodents
  • The majority of bats (and within them, all the megabats)
  • Nearly all carnivorans (the exception is the spotted hyena)
  • Nearly all primates (the exceptions are the spider monkey and the human)

How does that relate to mammals in general? Here’s a mammal group picture:



You can see that the above groups do not comprise a unified group, which would imply either (i) paralleiism or (ii) too funky a buried/invisible history even to contemplate. Ideas differ regarding the precise arrangement of mammalian suborders and superfamilies, but I submit that no current suggested version would imply a shared origin. There is apparently no single mammalian baculum, but several.

Oh, what am I waiting for? On to the porn. What you see here is the baculum of Syconycteris australis, a.k.a. the common blossom bat, a denizen of parts of Indonesia, New Guinea, and eastern Australia:

syco1syco2A couple of insights right away:

The bone is contained in the glans penis, the “head” for you vulgarians out there, and not in the length of the penile shaft. In other words, nothing to do with “stiffening the erection” as has been widely and erroneously said, and doesn’t replace or substitute for the corpora cavernosa (the spongy tissue that gets engorged with blood). Indeed the dorsal position (or as I like to say the saddle) is typical for bats; this is the case even when it’s a linear stick-like shape itself.

It’s not part of the skeleton; it’s embedded among complex non-muscular tissue. In rodents, it’s actually composed of three dramatically different types of bone, even in species with the simplest structure. When it’s structurally complicated, its various surfaces and flanges are associated with specific erectile units – the illustration doesn’t quite show it, but the rear end, the spread-out portion, actually sits on the end of the corpora cavernosa like a cap, and the distal (forward)extension part is surrounded by the erectile tissue of the glans.

Not to leave anyone out: in species with a baculum in males, typically if not universally female individuals have a homologous bone in the clitoris called the baubellum.

William G. Eberhard, Sexual Selection and Animal Genitalia

This is the first of several posts drawing from that tome which was my dissertation (link below; lucky you! Get ready for a real page-turner), but I should stress that my topic wasn’t “the baculum” but rather, “why and how genitals evolve,” a much bigger problem. It defies flip answers – no, creatures do not “need” this or that genital form, nor would speciating in the ecomorphological sense necessitate any particular shift in how they copulate, nor is there an ipso facto decree that species must be unable to copulate with one another, nor are we credibly to imagine that species run about copulating indiscriminately with whomever only to be prevented by incompatible genitals, nor should this feature necessarily be good nor bad for discerning species’ evolutionary history, nor is there a theoretically-grounded conflict of interest between male and female which suggests a selective pressure on this feature. Bill Eberhard got some perspectives on the discussion rolling in 1985 by suggesting that this has more to do with sexual stimulation within a species than with compatible-incompatible between species, and I came into that discussion more or less from a biomechanical perspective, soon to become phylogenetic.

So the baculum merely serves as a great case study for these questions:

  • Why are genitals diverse in shape among mammals?
  • Why do some mammal groups have a baculum and some not? In those groups with one, why do some species lose it?
  • What genital features evolve in tandem, or what changes are necessarily preceded by specific other changes?

Especially in cool bats. Especially the megabats, chock-full of subgroups and explosive speciation events and spread across so much of the world as they are, and every one of’em with a baculum.

Links: A phylogenetic approach to the evolution of mammalian genital form, with emphasis on the megachiropteran bats

Next: Count Bacula, part 2: Oh, the diversity!



5 thoughts on “Count Bacula, part 1

  1. Is the answer to “Why do some mammal groups have a baculum?” just “read the dissertation!”? As described here, and from my biased, baculum-free, human perspective, I’d guess the answer to be “no currently practical reason, just as a consequence of evolution,” where “consequence of evolution” needs a fair bit of explaining to avoid the kinds of “fitness misunderstood” traps you talk about in other posts. But if it’s in the dissertation, maybe you can help it be a page-turner by, um, turning me to the relevant page(s) .

    Which reminds me, I ran across an article recently (, from which I plucked a sentence as a replacement for “survival of the fittest.” The sentence is “About life, we know that it is not designed perfectly, but evolves under pressure to work well enough to survive.” I find bacula (I wanna say “baculae”, but that’s apparently Latin for “small berry”) far more comprehensible under “works well enough” than under “fittest.”


    • I am such a pain in the ass about this … that evolution, as a term, is not a cause. It’s an effect, and for that matter, including such a range of outcomes that as a category it’s barely understandable except as change. So “because evolution” is completely inadequate, that’s saying “it because it.” That bacula have come about in some mammals but not all (and as I say, probably separately among those groups), and that they have been lost in certain lineages within the “baculum” groups, is the evolution, and the question of what effects are these, and what constellation of causes were involved … that is a tough question. It’s tough to get at for any physical feature if one wants to avoid New Synthesis fairy-stories.

      Unrelenting in my ass pain, I go on to say that any and every attempt to “burst” the term evolution which includes the word “survival” is doomed. The blunt fact is that creatures do not survive. Not as individuals, not as populations, and not as species. It’s not even a meaningful component of any aspect of natural selection (the process most associated with that word, popularly) – reproduction is it, and only it.

      P.S. Etymology: baculum = wand/rod/staff (Latin), which as a role-player I’m sure you find as hilarious as I do; plural, bacula. It was assigned to this bone in 1915, and before that the only term was os penis (Greek).


    • Oh yeah – I don’t really recommend reading the dissertation as such. I’m trying to get the stuff people would like to know into these posts. The why-lose-it question is scheduled for May 2.


  2. I was perhaps more insightful than I realized when I left “… to survive” out of my “works well enough.” I think we talked about impact-on-actual-reproduction as the thing that matters before – maybe the lingering effects of that stripped away the “to survive”.

    “It because it” …effect not cause, OK. “Evolution does evolutiony things, duh” may be true, and it’s how I’d WANT to respond to a lot of “Why?” questions, but I wouldn’t want it to be an answer that blocks productive inquiry into meaningful specifics.


  3. Pingback: Count Bacula, Part 4: The whole chimichanga | Man nor Beast

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