Creatures of the earth engage in various strange mating behaviors. Male giraffes drink the urine of the females; red-sided garter snakes annually form mating balls of up to 30,000 strong and the reproductive organs of male wasp spiders snap off in the female during reproduction, but ‘penis fencing’? What in the world?



‘Penis fencing’ is an unusual reproductive activity commonly practiced by the marine flatworm (Pseudobiceros hancockanus), which can grow up to about 4-6cm long. Basically, ‘penis fencing’ is an unassuming phrase that biologists use to describe the vicious battle between two hermaphrodite flatworms consisting of each of them trying to stab the skin of the other using one of its two penises. This dual may last an hour and result in gaping wounds on the body of the loser. The ritual is done to facilitate the delivery of sperm from one flatworm to another to ensure reproduction, where the first successful ‘stabber’ becomes the de-facto male and the other, the de-facto female. The de-facto female then has bear the burden of motherhood while healing her wounds.

One might wonder what motivates this seemingly placid creature to engage in such ferocious behavior against its own kind even in the absence of any kind of sex drive. Scientists suggest that it is the result of a cost-benefit analysis, where members of the hermaphrodite population weigh the benefits of stabbing, against the costs of being stabbed. The benefits of stabbing include holding control of the fertilization process and gaining direct access to the eggs under the skin of the de-facto female. In addition, the de-facto males would have the valuable opportunity to pass on their genes to even more offspring while having fewer wounds to heal. On the other end, the costs of being stabbed include expanding copious amounts of energy caring and developing the eggs whilst healing gaping wounds.

Evidently, the tremendous benefits of stabbing another vastly outweigh the costs of being stabbed, which might serve to explain why marine flatworms approach this supposed act of ‘love’ in such a primal, warlike fashion.


“Fighting to mate: Flatworm penis fencing,” by Leslie Newman. PBS, n.d. URL: (accessed on 5 Apr 2010)

N. K. Michiels & L. J. Newman, 1998. Sex and Violence in Hermaphrodites. Nature, 391: 647.

Flatworms penis fencing by TheAwkblog Youtube Channel, 18 March 2009. URL: (assessed on 5 April 2010)

“Spiders sacrifice genitals to ensure paternity,” by Sarah Bartlett. Cosmos Magazine, 9 Mar 2007. URL: (assessed on 5 April 2010)

“Giraffes,” by Animal Corner, n.d. URL: (assessed on 5 April 2010)

“The Flatworms” by Lost In Arizona. Scienceray: The Great Barrier Reef: Jewels of the Sea. URL: (assessed on 5 April 2010)