Penis fencing

Penis fencing is a mating behavior engaged in by many species of flatworm, such as Pseudobiceros hancockanus.

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.

[6] Parturition, while necessary for successful offspring production, requires a considerable parental investment in time and energy, and according to Bateman's principle, almost always burdens the mother.

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.

[8] Commonly, many hermaphroditic species mutually inseminate, or trade sperm, rather than compete, Chelidonura sandrana as an example.

Two flatworms of the species Pseudobiceros bedfordi preparing for penis fencing. P. bedfordi is exceptional in that it applies sperm onto the partner's skin rather than injecting it.