Penis fencing, yes it is a thing

Standing in a defensive stance ready for battle, two individuals face one another, each with erect phalluses. The battle is not for territory or supremacy, it is for the privilege of fatherhood, with the loser becoming the mother. Neither of the brightly coloured hermaphroditic flatworms wants to bear the burden of pregnancy. Using their dual, pointed dagger-like penises the mating ritual is a battle for fatherhood.

Photo source –

This bizarre mating behaviour known as “penis fencing” is unique to marine flatworm genus Pseudobiceros, the loser is injected with sperm that travels through skin pores to fertilise the mother’s eggs. When fertilization occurs the duel is over and the father swims off triumphantly, leaving his biological legacy in the now pregnant-burden mother.

This mating act, while arguably far less enjoyable that what us humans do makes perfect sense. While the joys of motherhood are well known, in reality how many people out there if given the choice would actually want to bear this burden of pregnancy and still produce a child?

While passing on our genes is an almost compulsory biological behaviour that ensures the future survival of a species. Actually being pregnant and giving birth is… well I will let the image below from a Google search autocomplete capture the spirit.

While because of my gender I will never personally experience the burden of pregnancy and the immense pain of childbirth. However, I have had to listen to many friends and family members describe it, sometime in too much detail. From my understanding I’m stoked that I will never have to experience pregnancy and childbirth.

Flatworms might not go through the same biological changes that humans do. However, when pregnant they still have to eat more, and for flatworms that means hunting more. Also while they are pregnant they cannot mate until they give bearth.

The father also has another genetic advantage, being good at penis fencing enables the father to pass on more genes as they spend less time pregnant. Therefore the genes of the more efficient penis fences get a greater representation in the population, evolving into populations of first class penis fencers.

These hermaphroditic flatworms really have the pregnancy debate figured it out, fighting for the privilege of fatherhood, not simply being burden with pregnancy because of our genes.

7 Responses to “Penis fencing, yes it is a thing”

  1. ajqlee says:

    Sir This post offends me greatly. You have challenged my honour and as such I will challenge you to a duel…….. just not with penises. hahaha but in all honesty great post!

  2. kpenrose says:

    Peel back the layers and the Animal Kingdom just keeps getting weirder and weirder..

  3. tessae says:

    Haha I know the QI one you mean! This is great, I love the google search.

  4. pdeleo says:

    Penis fencers? ahahah not really good images came into my mind! ahahah
    Good article, thanks! Except for scaring me with the childbirth thing! 😉

  5. farns says:

    Classic Rob 😀 More men should fight for the privilege of fatherhood with their m manlihood…

  6. Cameron P says:

    Ooh, I remember this being mentioned on QI a few years ago. IIRC it came up because the longest animal in the world is a type of flatworm…

  7. Karen says:

    Whoever named this behaviour sure is a genius. Anyway, great article, I certainly learned something fascinating.