A mating plug, also known as a copulation plug, sperm plug, vaginal plug, or sphragis, is gelatinous secretion used in the mating

Mating

In biology, mating is the pairing of opposite-sex or hermaphroditic organisms for copulation. In social animals, it also includes the raising of their offspring. Copulation is the union of the sex organs of two sexually reproducing animals for insemination and subsequent internal fertilization...

 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. 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

Sperm competition

Sperm competition is a term used to refer to the competitive process between spermatozoa of two different males to fertilize an egg of a lone female. Competition occurs whenever females engage in promiscuous mating to increase their chances in producing more viable offspring...

 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. 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.

Composition

The mating plug of the Bombus terrestris

Bombus terrestris

Bombus terrestris, the buff-tailed bumblebee or large earth bumblebee is one of the most numerous bumblebee species in Europe. The queen is 2–2.7 cm long, while the workers are 1½–2 cm...

was chemically analyzed and found to consist of palmitic acid

Palmitic acid

Palmitic acid, or hexadecanoic acid in IUPAC nomenclature, is one of the most common saturated fatty acids found in animals and plants. Its molecular formula is CH314CO2H. As its name indicates, it is a major component of the oil from palm trees . Palmitate is a term for the salts and esters of...

, linoleic acid

Linoleic acid

Linoleic acid is an unsaturated n-6 fatty acid. It is a colorless liquid at room temperature. In physiological literature, it has a lipid number of 18:2...

, oleic acid

Oleic acid

Oleic acid is a monounsaturated omega-9 fatty acid found in various animal and vegetable fats. It has the formula CH37CH=CH7COOH. It is an odorless, colourless oil, although commercial samples may be yellowish. The trans isomer of oleic acid is called elaidic acid...

, stearic acid

Stearic acid

Stearic acid is the saturated fatty acid with an 18 carbon chain and has the IUPAC name octadecanoic acid. It is a waxy solid, and its chemical formula is CH316CO2H. Its name comes from the Greek word στέαρ "stéatos", which means tallow. The salts and esters of stearic acid are called stearates...

, and cycloprolylproline. 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.

Occurrence in nature

Mating plugs are used by many species, including several primates

Sexual dimorphism in non-human primates

Sexual dimorphism in non-human primates has long been observed in the primate order, with numerous studies performed to document and explain the phenomenon. Recent studies have mainly used the technique of comparative analysis to examine both the variation in the expression of the dimorphism among...

, bee

Bee

Bees are flying insects closely related to wasps and ants, and are known for their role in pollination and for producing honey and beeswax. Bees are a monophyletic lineage within the superfamily Apoidea, presently classified by the unranked taxon name Anthophila...

s, rat

Rat

Rats are various medium-sized, long-tailed rodents of the superfamily Muroidea. "True rats" are members of the genus Rattus, the most important of which to humans are the black rat, Rattus rattus, and the brown rat, Rattus norvegicus...

s, reptile

Reptile

Reptiles are members of a class of air-breathing, ectothermic vertebrates which are characterized by laying shelled eggs , and having skin covered in scales and/or scutes. They are tetrapods, either having four limbs or being descended from four-limbed ancestors...

s, squirrel

Squirrel

Squirrels belong to a large family of small or medium-sized rodents called the Sciuridae. The family includes tree squirrels, ground squirrels, chipmunks, marmots , flying squirrels, and prairie dogs. Squirrels are indigenous to the Americas, Eurasia, and Africa and have been introduced to Australia...

s, scorpion

Scorpion

Scorpions are predatory arthropod animals of the order Scorpiones within the class Arachnida. They have eight legs and are easily recognized by the pair of grasping claws and the narrow, segmented tail, often carried in a characteristic forward curve over the back, ending with a venomous stinger...

s, mice

Mouse

A mouse is a small mammal belonging to the order of rodents. The best known mouse species is the common house mouse . It is also a popular pet. In some places, certain kinds of field mice are also common. This rodent is eaten by large birds such as hawks and eagles...

, and spider

Spider

Spiders are air-breathing arthropods that have eight legs, and chelicerae with fangs that inject venom. They are the largest order of arachnids and rank seventh in total species diversity among all other groups of organisms...

s.

Use of a mating plug as a strategy for reproductive success can also be seen in a few taxa of Lepidoptera

Lepidoptera

Lepidoptera is a large order of insects that includes moths and butterflies . It is one of the most widespread and widely recognizable insect orders in the world, encompassing moths and the three superfamilies of butterflies, skipper butterflies, and moth-butterflies...

 and other insects and is often associated with pupal mating.

In homosexual matings

Male-male matings have been observed in species of acanthocephala

Acanthocephala

Acanthocephala is a phylum of parasitic worms known as acanthocephales, thorny-headed worms, or spiny-headed worms, characterized by the presence of an evertable proboscis, armed with spines, which it uses to pierce and hold the gut wall of its host...

n worms. Upon completion of the act, the penetrating partner will seal off the recipient's genital region with a mating plug. Once sealed, the anal cavity becomes impervious to further infiltration.