A mating plug, also known as a copulation plug, sperm plug, vaginal plug, or sphragis, is gelatinous secretion used in the mating of some species. It is deposited by a male into a female genital tract and later hardens into a plug or glues the tract together.[1] While females can expel the plugs, the male’s sperm still gets a time advantage in getting to the egg, which is often the deciding factor in fertilization.
The mating plug plays an important role in sperm competition and may serve as an alternative and more advantageous strategy to active mate guarding. In some species, such a passive mate-guarding strategy may reduce selection on large male size.[2] Such a strategy may be advantageous because it would allow a male to increase reproductive success by spending more time pursuing new female mates rather than active mate-guarding.[2]
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The mating plug of the Bombus terrestris was chemically analyzed and found to consist of palmitic acid, linoleic acid, oleic acid, stearic acid, and cycloprolylproline.[3] It was found that the acids (sans cycloprolylproline) were sufficient by themselves to create the plug. Researchers hypothesize that cycloprolylproline reduces female receptivity to further breeding.
Mating plugs are used by many species, including several primates,[2] bees, rats, reptiles, squirrels,[4] scorpions,[5] mice,[6] and spiders.[7]
Use of a mating plug as a strategy for reproductive success can also be seen in a few taxa of Lepidoptera and other insects and is often associated with pupal mating.[8]
Male-male matings have been observed in species of acanthocephalan worms. Upon completion of the act, the penetrating partner will seal off the recipient's genital region with a mating plug.[9] Once sealed, the anal cavity becomes impervious to further infiltration.