A mating plug, also known as a copulation plug,[1] sperm plug, vaginal plug, or sphragis (Latin, from Greek σφραγίς sphragis, "a seal"), is a gelatinous secretion used in the mating of some species. It is deposited by a male into a female genital tract, such as the vagina, and later hardens into a plug or glues the tract together.[2][3] While females can expel the plugs afterwards, 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.[4] In some species, such a passive mate-guarding strategy may reduce selection on large male size.[5] 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.[5]