Flatworm mating behaviour

"Flatworms have both male and female sex organs... If you think about it, that's a great way to be. If you're rare and you're [alone] looking for a mate, you don't have to meet another female or another male. You just have to meet another one" - Leslie Newman

The world is continually surprising and delightful to me. I was ever one of those children who could be found lying on my belly watching a line of ants running across a path, bees probing flowers in their never-ending quest for nectar or tadpoles darting around in ponds (or occasionally, to my parents' dismay, jam jars). My sense of childlike wonder is still very much alive - lately I've taken to being fascinated by hummingbirds, which are still in some ways impossible, mythological creatures; rather like seeing dragons flying over the hills.

That same sense of wonder was stunned earlier today when I came across a reference to flatworm mating behaviour. So, here for your education, and possibly entertainment, is a description of one of the odder sex rituals. It turns out that all species in the Turbellaria class (Platyhelminthes phylum) are hermaphrodites, each bearing a full complement of male and female sexual parts, and each therefore capable of being either a father or a mother. Some of these species may co-operate in mating, but others turn sex into a battle, each wishing to impregnate the other.

The Penis Mightier Than The Sword

It is hard to think of them as mates, rather they are combatants in an engagement that ends when the winner becomes a father and the loser has to raise the kids. Now as if fighting to impregnate your mate weren't odd enough, you should also be aware that many of these species have two penises. Yes, you read that correctly. Each individual has two male organs, and during their courtship (if I can call it that) they endeavour to use their pointy, stabby penises to pierce the skin of their, um, opponent and inject sperm through their skin and into their body. Yes, you heard that right, as well - there is no equivalent of a vagina or even cloaca, and once the winner has done the dread deed, the fight is over. Apparently, the mating-fight can continue for an hour, after which the winner can swim away to fight again. The loser has to retire hurt, often pierced by several stabs from its opponent's penis. Then it has to gestate the young internally before giving birth to either tiny versions of itself, or as a larval form which continues to develop.

It seems like odd behaviour, but the world is an odd place. The reason for it in this case seems to be that it takes a lot more of a creature's resources to bring the young to birth, and there is some individual advantage to winning the fight. I cannot begin to imagine how this would translate into the human world, though I imagine that many mothers would be delighted to turn the tables on their husbands and say "You can carry the next baby!"

And Finally...

Bonobos, it turns out, also engage in behaviour known as penis fencing, although it is not for the purposes of mating. There are occasions when captive male bonobos have been observed rubbing their genitals together. This form of frottage takes place with two males hanging by their arms, face-to-face, rubbing their penises together. Scientists are unable to state with any certainty that this is sexual - it may be part of bonding behaviour. Bonobos also exhibit other genital-to-genital contact, especially after a physical confrontation, when they stand back-to-back and rub their scrotums together. Presumably this is the bonobo equivalent of shaking hands like gentlemen after a duel.

So, with the sex-fights of the flatworm and the bonobos' frotting, suddenly the world once more becomes a little more fascinating to me. Next week, cock fighting? Or, as rootbeer277 pointed out, there's always slug sex.

Penis fencing video