The weird sex life of a flatworm

Remember tapeworms? The ones that you get from eating infected and improperly cooked beef or pork?

As it happens, they have lots of cousins. Lots. (Encyclopaedia Britannica states there have been 20,000 discovered— as of yet.) Some live in the soil, some in water, some latch on to the intestines of mammals and screw them over. The one we're concerned with lives in the warm seas and coral reefs— Pseudobiceros hancockanus. *sniggers childishly*

A little evolutionary background

Now, propagating the species is a much larger investment for females than males for almost every species. While most males have basically an unlimited amount of sperm cells to spread their genes onto the next generation, the ladies have to make (relatively) gigantic eggs which take up a lot of resources. Many mothers then have to carry their small ones inside them or find a safe place for the eggs to develop, sometimes providing constant protection. Depending on the species, the female also has to provide for her young for a considerable amount of time after it is born, while the male may or may not be involved.

Being a mom can be fairly exhausting, which is why females typically show much more discretion when choosing a mate. If you’re going to go through all of that, the guy (and his genes) had better be worth it. Females typically respond best to the most virile males as a means of ensuring that her babies will be in good health with a better chance for survival. It would be a shame if all of that effort and resources were all wasted.

What it means to Pseudobiceros and what they do about it

If you remember your high school biology, you'll remember that Platyhelminthes, or flatworms, are hermaphrodites. They are simultaneously male and female, producing both eggs and sperm at the same time. Funnily, or tragically (depending upon your sense of humor) insemination in hermaphrodites is quite the traumatic event. The male/father (who is giving the sperm) has to stab the mother's/female's (one receiving the sperm) body surface and release the sperm to fertilize the eggs.

Since flatworms are hermaphroditic, either partner can become either parent. All that matters is who penetrates who first. But neither wants to be the mom of its own volition, as it's infinitely more hassle. Unfortunately, unable to have a logical conversation about the matter, the flatworms solve the problem in the only way they know how: by trying to stab the other with their penis and the first to inseminate wins. Now, the idea of love or intimacy during copulation is hands down a complete rarity among animals, but it’s not exactly the epic battle that it is with flatworms.

When they approach one another and are ready to mate, it is instantly game on. They sit halfway up and get their double-headed penis (some of them have multiple penises) ready to strike. This also leaves them exposed to getting hit themselves. This can last an hour and result in the flatworms being stabbed multiple times. Ultimately, one deposits sperm into the other and emerges victorious, completing the paternal obligation. The flatworm that lost immediately begins to search for food to make up the resources required for making the eggs.

Know what they named this behavior?

Penis Fencing.

As if the name of the species wasn't a joke enough, they had to do this to the poor buggers. I almost feel sorr— nah I actually don't. Dick jokes never get old.

So, now you know about penis fencing. Go ahead and traumatize your friends with this new knowledge. Oh and also note: Male bonobos also "fence" with their penises when they find food to reduce social tension. Lame.