Penis Fencing in Flatworms

Penis Fencing Flatworms

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Two flatworms engage in a mating session...or a fight? (flickr)

"When a man and a woman love each other very much..."

Well, then the birds and the bees come and do their business. But what happens when both individuals have a penis? And also ovaries?

Naturally some competition will occur.

Marine flatworms are hermaphrodites, meaning they have both male and female reproductive structures. And in mating, there is a strong advantage to being the male. Flatworms will fight for this advantage, literally.

Mating is a complex process that begins when one flatworm slides up to the other and rears up, beginning an unusual behavior known as penis fencing. The flatworms wrestle each other in a race to be the first to stab the other flatworm with its penis. The flatworm who stabs the other first becomes the father, while the flatworm who bears the wounds becomes the mother. Fathers are rewarded with direct access to the eggs while the mother must bear deep wounds and loses any control over the sperms access to the eggs (Michiels and Newman 1998).

This website explores this behavior in detail. Using Tinbergen's four questions we will look at all sides of this behavior and attempt to answer the following questions.

  • How does the behavior work?
  • How does it develop in an individual?
  • How could it have evolved in the species?
  • Why it is beneficial?



Four Questions