Penis fencing is amatingbehavior engaged in by many species offlatworm, such asPseudobiceros hancockanus. Species which engage in the practice arehermaphroditic; each individual has both egg-producingovaries and sperm-producingtestes.[1]
The flatworms "fence" using extendable two-headeddagger-like stylets. These stylets are pointed (and in some species hooked) in order to pierce their mate's epidermis and inject sperm into thehaemocoel in an act known as intradermal hypodermic insemination, ortraumatic insemination. Pairs can either compete, with only one individual transferring sperm to the other, or the pair can transfer sperm bilaterally. Both forms of sperm transfer can occur in the same species, depending on various factors.[2]
One organism will inseminate the other, with the inseminating individual being the father. The sperm is absorbed through pores or sometimes wounds in the skin from the partner's stylet, causing fertilization in the other, who becomes the mother.[3][4][5] The battle may last for up to an hour in some species.[6]
Parturition, while necessary for successful offspring production, requires a considerableparental investment in time and energy, and according toBateman's principle, almost always burdens the mother. Thus, from anoptimality model it is usually preferable for an organism to inseminate than to be inseminated. However, in many species that engage in this form of copulatory competition, each father will continue to fence with other partners until it is inseminated. InAlderia modesta, individuals willstore sperm from several "fencing matches" before laying their eggs, and smaller individuals will more often inseminate a larger partner, with larger individuals spending more energy on laying eggs when paired with a smaller partner on the occasion that they transfer sperm unilaterally.[7]
In the absence of potential mates, some species such asNeobenedenia melleni are capable of reproducing throughself-insemination.[8]
Commonly, many hermaphroditic species mutually inseminate, or trade sperm, rather than compete,Chelidonura sandrana as an example.[4][9] The tiger flatworm,Maritigrella crozieri, also transfers sperm bilaterally.[10] In many species that engage in bilateral insemination, sperm trading is conditional. If one partner "cheats", and does not transfer sperm, the other partner will either prematurely abandon the partner, or will engage in typical mating behavior without transferring sperm.[9][11] Other species will alternate which partner transfers sperm, engaging in multiple bouts of fencing with the same partner over time. InA. modesta, bilateral sperm transfer is the most common, especially in similarly sized mate pairs.[7]
The term is also applied, usually informally, to homosexual activity between two males amongbonobos; same-sex genital-genital rubbing is used in bonobo society to cement bonds, reduce conflict, and express communal excitement over food.[12] Severalwhale species also engage in penis fencing.[13]