Copulatory plugs (plugs of coagulated semen produced by male accessory gland secretions after copulation) are found in Pan and Pongo, but are absent among humans and gorillas. The two main gel-forming seminal proteins are seminogelin 1 and 2 that can cross-link to form a coagulum behind the first, sperm-rich fraction of the ejaculate. This pattern may be due to the relative lack of male-male competition for mates in the uni-male gorilla (and human?) groups, which is in contrast to chimpanzees and orangutans, where there is more male competition during mating. Chimpanzees have an expanded semenogelin gene compared to humans and gorillas, (caused by the expansion of tandem repeats resulting in a protein twice the size of that in humans). Gorillas show evidence for loss-of function mutations (stop codons) in both semenogelin genes. The function of the copulatory plug may to prevent back-flow of semen and/or to interfere with the ejaculates of other males.
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Carnahan S.J., Jensen-Seaman M.I., 2008. Hominoid seminal protein evolution and ancestral mating behavior. Am J Primatol. 70(10):939-48.