Flatworms Mate When a Male Stabs Another Male with Its Penis

    When flatworms mate, two "males" use their bifurcated penises to fence one another. The winner, or the flatworm who stabs the other with its penis, remains a male while the loser becomes female.

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    Flatworms are soft-bodied invertebrates that are grouped between parasitic that infect humans and animals, and non-parasitic types that mostly dwell in water.

    Some species of flatworm, like the Persian carpet flatworm, have an unusual way of mating; they engage in 'penis fencing' where two males duke it out with their penises in battles that can last up to an hour.

    The flatworm's penis has two dagger-like heads and the flatworm to be stabbed by the other's penis is injected with sperm, subsequently becoming a fertilized female while the other remains a male.

    According to Bateman's principle, childbearing is a burden for the mother that requires considerable time and energy. So, from a biological standpoint for a flatworm, it's preferable to be the father rather than the mother.

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