Penis fencing is a mating behavior engaged in by certain species of flatworm, such as Pseudobiceros hancockanus. Species which engage in the practice are hermaphroditic, possessing both eggs and sperm-producing testes.
The species “fence” using two-headed dagger-like penises which are pointed, and white in color. The “winner” is the organism that inseminates the other. The sperm is absorbed through pores in the skin, causing fertilization in the “loser”.