This story is adapted from Phallacy: Life Lessons from the Animal Penis, by Emily Willingham.
For humans and other vertebrates, the literal response to the question of what makes a penis is that it consists, to varying degrees, of connective tissue, spongy swellable tissue, muscles, and a blood supply. And it’s pretty straightforward to see many vertebrate organs that look like penises, call them that (“and that’s a penis, and that’s a penis!”), and be right. But not always.
If you were on Mars and found an animal with something you suspected was a penis, what features would you use to rule your hypothesis in—or out? How about “something that inserts into a partner’s genitalia during copulation and transmits gametes”? Seems reasonable, although it’s even possible to argue about what copulation is. Some other time. Let’s look at some of the things that animals insert into their partner’s genitalia during copulation and see if they fit this vision of a “penis.”
Millipedes are probably best known for all those legs, although it’s not the thousand that the name implies. The millipede record holder for most legs has only 750, and most have far fewer. You can distinguish a member of the eighty thousand or so species of millipedes from its less speciose centipede doppelgängers by how many legs they have per segment, if you want to get that close. Millipedes have two pairs of legs per segment, while centipedes have one.
The millipede leg pairs of interest for our purposes are the eighth ones, which these animals use as intromitta, or gonopods (which basically means “feet that copulate”). These arthropods are not alone among their arthropodish kind in coopting an appendage this way. The genetics of limb building also might contribute to phallus formation in vertebrates. These realities give fresh relevance to tired, aspirational penis-related jokes about “third legs,” although as noted, it’s the eighth pair of legs in millipedes.
The eighth pair of legs isn’t the entire copulatory story for these multipoded animals, at least for the well-studied members of the genus Parafontaria. These species also pull in the services of the second pair of legs, which is where their genital openings are. Rather than getting involved in intromission, though, as you might think genitalia would, this pair of legs near the genitalia simply source sperm for the eighth pair.
An amorous millipede begins courting by first trying to insert his uncharged eighth-leg-pair intromitta into his partner of choice. If she does not reject this trial poke and he’s successful, his second pair of legs will charge up the old eighth pair with sperm. Now fully loaded, he’ll intromit again, this time with the goods. The pair will become immobile and stay coupled for 29 to 215 minutes.
Why would a millipede have any need to do a dry run with its legs-slash-intromitta when life is short, especially for millipedes, and dangers probably lurk all around while he stalls with a test poke? If you’ll recall, millipede species are copious in number. Millipedes are only millipedes, and they can make mistakes about which species is which. In this genus, at least, the test thrust is one way for the suitor to confirm that his target partner is, in fact, the right fit for his intromittent legs. This quick test saves him from wasting a bolus of precious (seriously) sperm on a millipede inamorata from the wrong species. Given how long that second bout can last, it’s probably also a way to make sure the male is investing all of that precious time on the right mating partner.
You probably never pictured yourself viewing millipedes as a pattern of normal sex behavior, but now’s the time. Although employing four pairs of legs to inseminate a female seems outside the bounds of human experience (and it is), the millipede at least inserts the intromittum into the partner’s genitalia. The first time, there is no sperm, but the second time, if there is a second time, the sperm cometh. For millipedes, we’ll have to split the difference on whether they meet our test of “something that inserts into a partner’s genitalia during copulation and transmits gametes.”
The Well-Armatured Intromittum
We can’t say that much for the many species of insect that skip that bit completely and just inject sperm any old place on a mate’s body. Some flatworms also have no choice but to do this because of a lack of a “receptive female aperture,” as one embryologist put it. There are no genitalia to serve as the insertee, so the animal is forced to use a “stylet” at the tip of its “protrusible sperm duct” to pierce the partner’s body just about anywhere. Following the piercing, the injected sperm undergo a great migration through the partner to the eggs.
Although the lack of an obvious opening for sperm deposition explains this sloppy target practice on the part of flatworms, it doesn’t explain all cases of hypodermic sperm delivery. Some species of spiders and insects, which are not especially closely related groups, have evolved this adaptation and use quite similar forms to achieve it. Whatever the pressures are that shape the hypodermic intromitta, they seem to converge again and again on the same structure: something stabby but with a hollow tube to deliver sperm. But it doesn’t deliver the sperm into the genitalia, definitely not meeting the criterion of “something that inserts into a partner’s genitalia during copulation and transmits gametes.” For now, let’s just call these “intromitta” and not get any more specific.
How about this adaptation in many insects: the aedeagus?
This structure is penislike and comes in all kinds of shapes and sizes. It’s an outward extension of rigid plates covering the insect abdomen, connected by ducts to the testes so that sperm can be delivered on demand. It’s essentially well-armored abdominal intromitta.
And well armatured, too. These intromitta can be long and spiraling and have hooks and flaps and valves or claspers for grabbing the female. They can look quite alarming from a human perspective, or “gaze,” as the sociologists call it. Humans have spent a lot of time gazing at these structures, in fact, because it’s one of the primary ways they sort out species from each other. We even make videos of them in the act. The genre of “arthropod (and invertebrate) sex films” is small but mighty.
About 99 million years ago, a spider-ish animal—a member of a group of arachnids called the harvestmen and more familiarly known to some as “daddy longlegs”—was skittering around in a tropical forest when it suddenly met its end inside a dollop of sap. Unluckily for it, but better for us, the sap preserved what is currently the world’s oldest fossilized erection. The harvestman not-spider clearly had a penis—erect, tubelike genitalia, probably inserted into a female (or would have been had it not been for that damned sap), ready to deposit sperm. So sure, you think, harvestmen are a safe bet for “yes, that is absolutely, unequivocally a penis.”
You probably saw this coming: not all harvestmen meet these criteria. One group of these animals, a suborder called Cyphophthalmi but more commonly known as mite harvestmen, falls a little short. This ticklike group of not-exactly spiders are tiny, moss-dwelling little jewels that are only a few millimeters long. They don’t seem to share the obvious penis of their harvestbrethren and have genitalia that don’t intromit—instead, they evert, or flip it out.
These tiny animals use this eversible structure to poke their spermatophores—packets of sperm on a stick—into their partner’s genitalia without inserting the structure itself. When an animal uses this kind of tube to deposit eggs, the tube is called an “ovipositor.” So I guess that means the genitalia on these spiders are not so much an intromittum as they are a “spermopositor.”
Harvestmen (besides these mite harvestmen) stand out with their true penises because most arachnids don’t have a special structure devoted solely to intromission. If a spermatophore is a sperm packet on a stick, then the structures spiders favor is like a pair of elaborately wrapped sperm packets on a pair of sticks (actually leglike appendages known as pedipalps.) Spider intromitta are called palpal organs, and each is tipped with a hard structure called an embolus. This structure releases the sperm bolus into the female1 once the spider’s “arm,” or pedipalp, thrusts it into her. The average palpal organ looks very much like a mitten at the tip of the spider’s arm, but the embellishments of the mitten (intromitten?) vary from species to species. Some are quite hairy and large, with folds, extensions, and pointy bits, while others are simpler and less daunting.2
The way spiders use their pedipalps can illuminate the question of “what makes a penis.” The male has a pore where sperm emerges. He captures the sperm on silk he’s prepared for the purpose and draws it up like fluid into a turkey baster, pulling it into his palpal bulb at the end of his pedipalp and stuffs the bulb into the female, releasing the sperm. The two steps of insertion and ejaculation can take no more than a five-count in some species.
Is the pedipalp–palpal bulb combination a penis? It involves a tube, intromission, and ejaculation. It seems to fulfill all of the elements of “something that inserts into a partner’s genitalia during copulation and transmits gametes” and then some.
But the spider also uses its pedipalps to taste and smell, definitely not familiar uses of a penis to us. Some spiders even use part of the pedipalp, a bit just under the palpal bulb at the tip, to make music (stridulate) as part of a courtship ritual. That’s not very penislike from our perspective (I’ve yet to hear of a human penis making music), either, but perhaps we should start viewing these organs as what they are: penis capable, sure, but also able to do so much more in the world of sensory communication and courtship.
The Unequivocal Penis
Barnacles have long attracted the attention of naturalists, while also leaving some of them deeply confused. Carl Linnaeus, the Swedish botanist, is best known for his publication Systema Naturae, in which he laid out the hierarchy for classifying organisms. Barnacles seem to have mystified him, and he consigned them to the “Animalia Paradoxa” (paradoxical animals) section of his first iteration of Systema Naturae, sandwiched in between Draco (dragons) and the Phoenix (the Phoenix). He also seems to have thought, along with others before him, that barnacles originated from rotting plant life on beaches.
His misapprehension in that regard had deep roots. For centuries in Europe, at least, the origin of barnacles had left many a protoscientist baffled. As late as 1661, the first president of the newly founded, highly respected scientific body the Royal Society posited the most preposterous of the barnacle origin stories. Sir Robert Moray, who is not responsible for the name of the eel,3 stood before the august body and read, with a sober face, a paper of his own describing a “bird-like creature” within the shell of a barnacle on a ship and making the argument that the barnacle goose, a resident of the British Isles, had developed by metamorphosis from this odd-looking, ship-adherent creature. Yes, he proposed that a bird had started life as a barnacle. Science is at its heart the process of changing conclusions with new information. After sufficient investigation, we can say with 100 percent certainty that barnacles do not morph into birds.4
Perhaps you have heard of Charles Darwin (1809–1882) and could even name what one group of researchers has described as his “monumental work.” If you’re thinking of On the Origin of Species4 as the tome that high praise references, you’re wrong. The phrase describes Darwin’s runaway best seller5 on barnacle taxonomy, or classification, which he wrote following more than seven years of study. Darwin, secure of his audience, one presumes, published his work in a series of four monographs, no doubt leaving readers panting for more after completing each installment. All of which is to say, naturalists are bonkers for barnacles.6
Apart from wild speculation about barnacles and birds, the other attraction of these odd crustaceans is their genitalia. The general barnacle intromittum is a long cylinder of stacked, folded rings, like a sperm-delivering Slinky with a membrane around it. These organs have bristly extensions called setae, which the barnacles can fan out to sense chemicals in their environment that signal the presence of a potential mate.
None of this can start, though, until a hermaphroditic barnacle’s “female function” is in place. This function involves sending out “female” chemical signals to neighbors, activating their “male functions,” and leading them to unroll their long penises from among their cirri to feel around for that chemically alluring “female.” If the barnacle locates a welcoming mantle cavity with its penis, it pokes its intromittum into it, ejects some semen, and then, if events warrant, does it a few times more.
Depending on the species, the penises can grow for the mating season and then be shed. If wave action is heavy, barnacle penises can thicken in both the muscles and the surrounding tissue, and the length can increase through the addition of more rings to the penile Slinky.
The barnacle definitely meets the criterion of “something that inserts into a partner’s genitalia during copulation and transmits gametes,” at least during the seasons the penis is intact. The barnacle thus takes its place in this section on the unequivocal penis and earns some extra credit for occurring in an animal that also has reciprocal genitalia with a lot more bells and whistles.
Adapted from Phallacy: Life Lessons from the Animal Penis by arrangement with Avery, an imprint of Penguin Publishing Group, a division of Penguin Random House LLC. Copyright © 2020, Emily Willingham, PhD.