Nobody wants to be your mama! Flatworms Battle in Sex

Flatworms, Phylum platyhelminthes, are hermaphrodites, therefore having both male and female reproductive organs, they can inseminate and be inseminated (Jaro, 2010). As a result of this sexual conflict, sex can turn into an ugly battle where each wants to be the one to inseminate, indicating their preference to be the male partner. This is the case with the marine flatworm, Pseudoceros bifurcus, where their copulation has been termed ‘penis fencing’ (Munson & Sankaran, 2010).

Penis fencing is a mating behaviour that begins when two sexually aroused marine flatworms, Pseudoceros bifurcus, glide toward each other and make physical contact. As each flatworm approaches the other, they rear up, raising the front portion of their bodies and a pair of everted penises protrude out from the underside of the mid section of their bodies, as depicted in the photo below.

Flatworms Penis Fencing
Source: Jensen, D. (2012)

These penises act as daggers as they are used to jab the opponent. This process is seen in the video below (TheAwkblog, 2009) which clearly illustrates the protrusion of penises followed by the battle they fight to inseminate the other.

Flatworms Penis Fencing

While each of them continuously tries to jab the other whilst also avoiding from being jabbed, eventually one, usually the first to jab, will be successful in injecting its sperm into the skin of the other where it is then absorbed. Penis fencing is not a quick process as it can last from 7.7 to 32.8 minutes (Michiels & Newman, 1998).

The losing flatworm then takes up the role of motherhood as its egg gets fertilised by the injected sperm (Munson & Sankaran, 2010). Michiels & Newman (1998) explains that penis fencing is an attempt to increase the benefit of sperm donation over the cost of sperm receipt. The flatworm that receives the sperm bears the costs of wound healing from all the jabbing and it loses control over fertilisation, hence has to raise the young and care for the developing eggs (Bicknell, 2010), while the father shoulders no responsibility.

After bearing and releasing the offspring, the mother flatworm then takes on another round of brutal penis fencing, in hopes of not becoming a mother again.




Jaro, N., 2010. Top 10 Strangest Animal Behaviours. TopTenz, 24 Nov 2010. URL: (accessed on 7 April 2013).

Michiels, N. & Newman, L. J., 1998. Sex and violence in a hermaphrodites. Nature, 391: 647.

Munson, A. & Sankaran, M., 2010. Penis Fencing in Flatworms. Reed College. URL: (accessed on 7 April 2013).

“Penis Fencing Flatworms,” by Jensen, D. Mr Jensen’s Virtual Zoo, 24 May 2012. URL: (accessed on 8 April 2013).

“Really weird animal behavior,” by Bicknell, E. Squidoo, 2010. URL: (accessed on 8 April 2013).

“Flatworms Penis Fencing!!!,” by TheAwkblog Youtube Channel, 18 March 2009. URL: (accessed on 7 April 2013).