Two marine flatworms engaged in a penis fencing match Two marine flatworms rearing up, preparing to fence (4)

The adaptive value of a behavior looks at the selective advantage of a behavior and how that contributed to evolution.

Penis fencing is a dangerous endeavor to embark on. One, or both, individuals will often leave the interaction with tissue damage. Hypodermic insemination can result in puncture wounds which can leave scarring and, in some cases, it can be even more damaging. If the flatworm is hit in the mid-front of the body, the mating can result in a complete loss of the posterior portion of the body. Although many can regenerate this lost tissue it comes at a high cost of energy (Arnqvist and Rowe 2005). That is why many species have evolved a different in copula sperm transfer. This form of mating where the male inserts the penis into the vagina is under strong female control as it does not result in the damage characteristic of intradermal hypodermic insemination (Michiels & Newman 1998).

Intradermal hypodermic insemination is possible in flatworms because they are hermaphroditic and therefore can play the role of both male and female. This suggests that the sexual conflicts that lead to mating behavior is not a result of between individual conflict but within the individual. Although the cost of being stabbed would select against penis fencing, the benefit that the "male" in the interaction receives would select for this behavior. In fact observations of this behavior in the Prudhoe marine flatworm support the idea that individuals will attempt to donate more sperm than they receive (Michiels & Newman 1998).

Further, in systems without heavy investment of parental care, male fitness is more closely linked to mating success than that of female fitness. This means females have a lower optimal mating rate because they have less of an incentive to mate multiple times. Males on the the other hand have a higher optimal mating rate. This is both because their fitness is more closely linked to mating success and thus evolutionarily males that mate more pass on there genes more to produce more males that will also mate more and because it cuts down on other male sperm competition. (Anthes et al. 2005). This would also lead to a behavior that encourages hermaphroditic flatworms to try to play the role of male as there is little parental care involved in mating.

Thus penis fencing evolved in some species of marine flatworms as the adaptive value of donating sperm is greater than that of receiving. Penis fencing is essentially the fight to be that male. The format of this fight is also under evolutionary control. The worms "rear up" which is an effective stance for both "striking and parrying." However, if both worms do not rear up, and one glides
away insemination is rare. Therefore, rearing up has adaptive value for sexually responsive individuals, the ones that get to pass their genes on (Michiels & Newman 1998).

It is likely that all of there hypothesis have played some role in the evolution of this behavior. Lack of parental care in marine flatworms likely made the advantage of being the male more attractive so it played a bigger role in evolution. As penis fencing became the normal way for some species of marine flatworms to reproduce the selective value of different fighting stances then became important.