For those arriving recently, this series of posts is dragging readers through the muck and mire of my twenty-year-old dissertation concerning the evolution of genitals in mammals. (room clears) This time it’s about the attention-catching detail that within the mammal groups that do have a baculum (the penis bone), there are subgroups which don’t, and that primates are a good example, with ourselves as the odd man out, and woman too, as there’s a female homologue called the baubellum.
Ah ha, why? Why? What’s the baculum do? Why don’t we do that? The questions flap about in excitement. I have riveted scientific seminars, classrooms, bars, and unsuspecting cocktail parties with this very topic.
Sadly for pop science, the whole context of “for” and “didn’t need it” must be junked. It’s nonsense not only for the baculum (which when all is said and done is merely an element within the glans penis), but for intromittent genitals, which is to say, a penis, at all. I’ll post later about the diversity across vertebrates as far as that is concerned.
But that doesn’t mean we can’t ask real questions. Obviously, never mind the groups of mammals who clearly never had it. Never mind the rodents: bacula bacula everywhere in those, no absence. OK.
Take a look at that diagram of the Carnivora, which is very simplified into a split between arcturoids (dogs, bears, seals) vs. aileuroids (cats, hyenas, mongoose). The circle at the bottom is presumably the basal or ancestral description of these creatures’ baculum and associated erectile tissue, which the arcturoids still have: big and complex, respectively. The aileuroids have a much “reduced” situation, small and simple, respectively. And one of them, the spotted hyena, has both a big penis, compared to its fellow aileuroids, and no baculum.
H’mm. Funny, if you look at the catarrhine primates, there’s the same basal arrangement with a big baculum and complex erective tissue, and look … a certain group has a simplified erectile anatomy and a very small baculum. And one species in this group has a distinctively big penis, compared to its fellow apes, and no baculum. (Something similar is happening with the spider monkey in the platyrrhine primates too; in fact, it’s very similar to the hyena.)
Double h’mm! Any other mammals lying around with bacula, hopefully with one or more instances of absence in the midst? Why look, the microchiropterans, or microbats. Here’s the mother lode of derived bacular absence, because:
- There are four superfamilies in the Suborder, and one entire superfamily, the Phyllostomoidea, has no baculum.
- Within another superfamily, Vespertilionidea
- In one family, the Vespertilionidae
- One subfamily, the Miniopterinae, lacks the baculum entirely
- In another family, the Molossidae
- Two genera lack the baculum among a host of them which have it
- In one family, the Vespertilionidae
Awesome … if only there were as thorough a skeletal dataset for these animals as for carnivorans and primate, but there isn’t … I can relate, though, as we are talking about very small and light creatures; it’s not like the baculum was exactly a priority in preserving specimens. There’s just enough known to get some hint of whether the pattern holds, and it might. The erectile simplification + reduced-size baculum step apparently does.
Constructive speculation goggles: on. Perhaps in the creatures with reduced complexity in the glans penis, and a much simplified and smaller baculum … if the penis then evolves into a much bigger size, it does so without including the corresponding genetic info (i.e., the developmental steps) regarding the baculum. Therefore in raw developmental terms of volume and tissue interaction, the baculum’s now-miniscule instructions don’t “hold” in the ongoing steps. You don’t have to silence or lose those instructions, they just don’t have the heft to make the thing in this larger context of tissue events. (Take a look at Evo Devo thoughts behind this in We are, in fact, Devo).
Consider the megabats for a useful contrast, as they typically have a big penis, and like primates lack a sheath to keep the penis neatly held against the belly, so they go flop-flop like us too. But they have a baculum, and often a pretty elaborate one. Therefore, apparently none of them underwent the simplification step, and if you get big without doing that, then the baculum gets big with you
So! Or provisionally, If! The question is therefore not a missing baculum at all, but rather, two steps: simplification and big penis. That latter is obviously an interesting question considering the case of the spotted hyena, with its famous pseudophallus in females, meaning, there’s probably an interesting history in every instance with no reason to think they’re similar in selective terms. But our characteristic fascination with porn could blind us to the obvious importance of the first step – the reduction of baculum complexity and its complex interaction with the erectile tissues of the glans penis.
At the very least, we can put an end to the nonsense prated about the human no-baculum, much of it the usual handwaving about “because we got bipedal!” or something.
Next: Count Bacula, part 4: The whole chimichanga