Who’s Your Daddy?

For marine flatworms, Pseudoceros bifurcus, “Who’s your daddy?” is a loaded question.

As hermaphrodites, marine flatworms have both male and female sexual organs (Nelson, 2010). They engage in a battle of “penis fencing” to determine who will be a dad, and who will be a mum (Bicknell, 2010). It is undesirable to be a mum because pregnancy exhausts their body (Nelson, 2010).


The flatworms have two white and pointed penises (Nelson, 2010). Source: Ever So Strange


During mating, the goal is for one flatworm to stab its partner with one of its penises (Nelson, 2010), and inject sperm under the partner’s skin (Michiels & Newman, 1998). Michiels and Newman (1998: 647) observed that “individuals try to stab one another, but show strong avoidance behavior when struck by their partner.” The injector is considered the winner as the battle is to avoid being hit by a penis (Nelson, 2010). It will become the dad, while its partner will become the mum. The battle can last for an hour and results in gouged holes on the body.



You may wonder what drives these seemingly docile creatures to engage in physically damaging sex (Michiels & Newman, 1998). “Penis fencing” can be explained using a cost-benefit analysis. For dads, injecting sperm offers direct access to eggs without any responsibility, ability to father more eggs in more partners and fewer wounds to heal; whereas mums bear the costs of wound healing, lose control over fertilization (Michiels & Newman, 1998), raise the young and care for the developing eggs (Bicknell, 2010). Sperm injections by better “stabbers” produce more successful offspring too (Michiels & Newman, 1998).

The benefits of stabbing greatly outweigh the costs of being stabbed, which explains why marine flatworms favor the role of a dad and make love in a warlike manner.


Literature Cited

“Flatworms and penis fencing,” by Nelson, B. Scienceray, 15 Apr 2010. URL: http://scienceray.com/biology/flatworms-and-penis-fencing/ (accessed on 21 Mar 2013).

“Flatworm penis-fencing,” by Ever So Strange. Ever So Strange, 6 Feb 2011. URL: http://www.eversostrange.com/2011/02/06/flatworm-penis-fencing/ (accessed on 21 Mar 2013).

Michiels, N. K. & Newman, L. J., 1998. Sex and violence in hermaphrodites. Nature, 391(6668): 647.

“Really weird animal behavior,” by Bicknell, E. Squidoo, 2010. URL: http://www.squidoo.com/animal-behavior (accessed on 21 Mar 2013).

“World’s Weirdest – Flatworm Penis Fencing – YouTube,” by National Geographic. NatGeoWild YouTube Channel, 4 Jun 2012. URL: http://www.youtube.com/watch?v=wn3xluIRh1Y (accessed on 21 Mar 2013).