 Their mating ritual involves penis fencing in which both try to stab to inseminate the other and at the same time avoid being stabbed.Being inseminated represents a cost because striking and hypodermic insemination can cause considerable injury; as a result, the conflict lies in adapting to be more adept at striking and parrying and avoiding being stabbed.Also the earthworm "Lumbricus terrestris" show behavior where both parts try to make sure as much sperm as possible is absorbed by their partner.