Today's lesson is on the reproductive anatomy of the domestic pig (Sus scrofa domestica), which probably applies to the wild species in the pig family as well. Although we may reflexively think about invertebrates when pondering diversity of copulatory organs, mammals are not too bad in that department either. After all, the sperm is delivered in some species into the vagina (e.g., dog), in others into the cervix (e.g., pig) and in yet others into the uterus (e.g., horse), so different strategies are needed for different goals.
The cervix is approximately one inch in diameter and about 6-8 inches in length, and connects the vagina and the uterus. It is made of tough connective tissue and contains limited amounts of glandular and muscular tissue. It contains a series of five interdigitating pads (Figure 1) which provide pressure points for locking of the penis (or AI catheters). Its primary functions are to serve as a locking mechanism for the penis. The cervix is also a flexible structure and can open and close under the influence of hormones. The cervix is important for protecting the fetuses and will remain tightly closed except at estrus and at farrowing, when it will dilate to accommodate the boar's penis and to allow passage of the piglets through the birth canal. The cervix is also the primary source of mucus. Under estrogen stimulation, such as that which occurs at estrus, the mucus becomes watery and can sometimes be seen seeping from the vulva. This mucus serves as a lubricant for the penis of the boar. Under progesterone stimulation during pregnancy, the cervical mucus will thicken and form a plug to prevent any contaminants from entering the sterile uterine environment. This cervical plug will dissolve just prior to farrowing.
And here is an even shorter excerpt of the boar's penis (you CAN click on the link above for additional information, it's OK):
The extension of the penis causes tension in the fibroelastic tissues of the boar penis and causes twisting of the free end of the penis to form a corkscrew shape. The corkscrew shape is perfectly formed to match the patterns of pads inside the female's cervix. Upon erection and intromission of the penis into the cervix, the locking of the penis into the cervical pads is associated with pressure. This pressure stimulus is needed to induce the ejaculation reflex in the boar.
So, to translate....boar has a corkscrew penis and the sow has a corkscrew cervix. The two are a perfect fit. If you take the two organs preserved in formaldehyde (thus stiffened), you can actually screw the penis into the cervix (we did it in vet school way back when...). Now you know where the slang term for sex comes from...
Of course, nobody has taken an MRI of two pigs during copulation (doing it with humans is difficult enough) to make sure what is really happening, but something here does not make sense. We know that the penis does not make several rotations during copulation. We also know that the boar does not spin like a propeller around the sow's hind end either. Thus, there is no way that the penis actually penetrates in a corkscrew fashion. What it likely does is go straight in, crossing each of the cervical ridges one at a time until it assumes the position in which it completely fits inside the groove.
Now, my dirty mind starts working and I remember an old limerick about Clarence Cool (...who was born with a spiral tool, etc...it is even less safe for work than the rest of this post, but I can e-mail you if you are interested) and his sad predicament, wondering if handedness may be important. What if the boar's penis has a right-handed twist and the sow's cervix has a left-handed thread?
dextral and sinistral snails have a hard time mating with each other. In some species with low-spired shells, it may be impossible to have cross-chiral matings.
In those snails, the difference is under the control of a single gene.
Something like chirality is likely to have a simple genetic control in the early development of the pig as well, allowing for a small mutation to completely change the handedness of the reproductive system. Will it be possible for the two pigs of opposite handedness to mate? And if not, would that be a pre-fertilization reproductive barrier leading to instant speciation?
In the process of hardening during copulation the penis probably corkscrews into place as it takes it's shape to fit into the cervix.
I can vouch for it. I had a degree in animal science and I took a whole semester of farm animal reproductive physiology and I learned ALL about that. The class even dissected a sow's reproductive system. Amazing.
The best part of that semester, my professor trying to collect sperm from a boar with an old veggie can. No luck.