In some species, females mate with multiple males, which provides an additional battleground for male-male competition: the battle among the sperm. Natural selection has produced a variety of male adaptations designed to fight this battle, including:
i More sperm: Males of non-monogamous species produce more sperm than males of monogamous species.
i Seminal plugs: Most common in insects, but also seen in some vertebrates, is the production of seminal fluid that solidifies into a plug to prevent subsequent males from mating — a strange but predictable adaptation to the problem of competition from other males' sperm.
i Toxic seminal fluid: Fruit flies are one species that employs this strategy. Their sperm contains toxic compounds that inhibit the sperm of subsequent males. Unfortunately, poison semen isn't too good for the female; head to the later section "The Battle of the Sexes: Male-Female Conflict" to find out more about what happens when what the male wants is at odds with what the female wants.
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