You’d think that naming body parts would be about the simplest intellectual task around. If you put food in it, it’s a mouth, if you walk around on’em, they’re legs, right? … whereas, as it turns out, learning that this isn’t the case is one of the significant breaks from conventional thinking one makes to be educated in biology.
First there’s the homology issue. Creatures use different parts to do the same things, and they use the same parts to do different things, so for once, what “is” is, isn’t a a joke sentence. Which are we talking about? The entire meaning of “how did it evolve” completely metamorphoses depending on the answer. Is a bee a flowering plant’s penis?
Then there’s the overlapping-functions issue. Parts have a distressing tendency to combine multiple activities, and sticking-out parts are especially prone to all manner of multi-purposing. A snake’s hemipenis doesn’t urinate – does that make it less of “a real penis” than a mammal’s?
But come on, isn’t all this mere logic-chopping? The kind of nonsense to be cleared away with the master-wave of the common-sensical hand? Everyone knows that if the critter is to do a thing, it needs the working piece that does that thing, and that’s why it has it. … Annnnd, no. No is the answer to that.
Let’s get the thing being done defined anyway: it’s gamete transfer, which you might remember from Hermes and Aphrodite. But do you really need a penis for that? Nope! Lots of creatures – really a lot – transfer gametes externally. The easy presumption that “to get these reproductive cells together, you need an organ that inserts over here, obviously,” is simply false. The best I can tell you is that some creatures do use such an organ, and some don’t.
On to our own corner of Animalia, the Suborder Vertebrata, a group we like to think of as special for no good reason. Here’s a particularly excellent diagram because it shows how the traditional groupings (“reptile,” “bird,” “bony fish”) need to be junked. I’ll be using terms like tetrapod and amniote a lot in this post so check back to this when needed.
The term “fish” is also pretty useless in terms of actual evolutionary history, but with the present topic in mind, consider that most of the Chondrichthyes (sharks and rays) have internal fertilization and internal embryonic development, using extensions off the pelvic fins (misnamed “claspers”) to copulate with. But the Dipnoi, the fish that we’re related to, or better, are walking-around versions of, don’t have any such thing going on. All we, speaking collectively here as an tetrapod, inherited from them regarding this anatomy was the same-old vertebrate cloacal opening at the end of the body. The urinary tract, digestive tract, and gamete-transport ducts all go out there.
I’ll pass over the Amphibia most unfairly because although they certainly do many nifty things with sperm, apparently none of them have an intromittent organ and may be safely said to be Over There in terms of this issue. Let’s take a look at the amniotic vertebrates: that big complex clade at the top right of the diagram.
No one knows for sure, but there’s at least a case to be made that the basal or original genital arrangement for this class is a cloaca + penis. The latter lies inside the cloaca, on its ventral (belly-side) surface. You see this, or reasonably close versions thereof, in turtles, crocodilians, a few birds, and a few mammals.
That leaves Lapidosauria (snakes, lizards, tuatara), most birds, and most mammals as having done something interesting. Three totally different somethings, as it turns out.
Thing 1: to lose the penis while keeping everything else the same, which is what most birds have done. Here’s the deal with the ones who didn’t and the ones who did.
The few birds with a penis, among the group called ratites (ostriches and related), share tissue homology with the mammalian one (and I think the turtle and crocodilian ones), and it has a nifty erectile mechanism of its own, using lymphatic fluid instead of blood. Read about that in Ostrich penis clears up evolutionary mystery, allowing for the usual over-written and mildly-distorted version of evolution (“it because it”) found in Nature articles. It’s not even worth an eyebrow lift that the birds’ situation, historically speaking, uses the same parts as the other tetrapods with its own spin on exactly how they’re used. Differing erectile mechanisms are normal, not odd; among mammals alone, the differences are striking, including musculo-integumental as opposed to circulatory.
But as I was saying, most birds have no penis. And to make things more fun, they retain internal fertilization, meaning they copulate, with the relevant genital-to-genital activity gloriously and accurately dubbed the cloacal kiss. (Dibs on the rock band name. You heard me. I called it. Dibs.) Jokes aside – see how you don’t “need” a penis to copulate? Shoot, they even had one and now they don’t.
Thing 2: also an absent or rather “lost” penis, in the Lapidosauria, that is, the tuatara, snakes, and lizards. What differs here is the striking presence of an entirely new and non-cloacal intromittent organ, actually a paired feature, called the hemipenis (or hemipenes, plural). The spermatic ducts go there, whereas all the other tracts exit where they always did, the cloaca.
They ain’t small organs and can give you a bit of a fright if you’re not prepared; their detailed anatomy is a whole world of weirdness along the same lines as the mammalian penis/baculum stuff I’ve been posting about lately. In case you’re interested, only one hemipenis is employed during copulation, although I have no idea whether a given species or individual favors one side or the other for any reason.
Let’s stop a minute and think cladistically, though – go back up to the diagram. In the whole “sector” defined by 10-11-12, the Diapsida to us insiders, there seems to be a history of losing that penis. The crocodilians don’t lose it in the first place, the lapidosaurians do and then get the hemipenis, and the birds just lose it and carry on without.
Thing 3: which is a completely separate history, confined to the mammals and their kin, what we call Synapsida for reasons I will spare you. This set of events doesn’t lose anything; all the parts stay the same. What changes is a location, or perhaps segregation, placing the penis out of the cloaca and carrying the urinary tract with it, such that the anus is the only orifice left to the cloaca, which has effectively been divvied up to an extent that it’s gone. This history is also the only one to affect female anatomy of these parts, in that the vaginal and urinary orifices are now separated from the anal one just as in the male.
As with the birds, we have some fun evidence of the history, in that some mammals retain the cloacal arrangement. The culprits are a nice indicator of mammalian reproductive derivation: monotremes (egg-layers) have your basic cloaca + penis just like a turtle or crocodile or ratite bird, marsupials have a kind-of cloaca, and a few placental mammals like shrews do too.
Mammals do another weird thing too, regarding testes’ placement outside the body cavity, and lest anyone chime in with “keep them cool!” as the explanation, I shall merely groan and point to the notably hot-bodied birds with their internal testes right where they always were, and say yet again, quit mistaking recent detail with original evolution. Mammalian sperm vulnerability to heat is a result of being outside, not the cause of getting outside. What interests me more is that different mammals pop’em out in different spots, e.g. the rabbit scrotum is above (or in front of) the penis, not below (behind).
Not too easy a topic, right? So easy to think of our arrangement as being the “sensible” or “obvious” one. And way too easy to confound it with essential qualities of maleness and femaleness too … imagine, shall we, in our fun science fiction way, Star Trek type colorful-human aliens who happen to be like derived birds, with no penis and essentially identical genital anatomy between male and female, yet with fully copulatory behavior otherwise similar to humans.
Links: Do dragons have penises? (I wrote that in 2002); I feel it my duty as well to mention the Finnish band CMX’s first album Cloaca Maxima and the Dutch film Cloaca. Might as well list these too: Count Bacula, part 1; Count Bacula, part 2: Oh, the diversity!; Count Bacula, part 3: The case of the missing baculum.
Next: Count Bacula, Part 5: Indescribable geometries