Depending on the Species of animals, spermatozoa can fertilize ova externally or internally. In External fertilization, the spermatozoa fertilize the ova outside the female organisms which expelled the ova. Female fish, for example, spawn ova into their aquatic environment. Those ova meet with spermatozoa which have been carried away inside semen secreted by male fish.
By Internal fertilization, on the other hand, ova get fertilized inside a female organism. Internal fertilization takes places after Insemination of a female by a male through Copulation. In low vertebrates (amphibians, reptiles or birds and Monotreme mammals), copulation is cloacal. In Marsupial mammals and placental mammals, copulation is vaginal.
Mammalian semen is a whitish, milky fluid, very viscous, containing Water and small amounts of salt, Protein, and Fructose. It varies from being without scent and taste, to having a strong taste and smell of Ammonia. Many people claim that one's diet can alter the taste of semen for better or worse, but this has not been scientifically proven.
Seminal plasma of humans contains a complex range of Organic and inorganic constituents. They include metal and salt ions, sugars, lipids, Steroid hormones, enzymes, Prostaglandin hormones, amino acids and basic amines. The purpose of the seminal plasma is to provide a nutritive and protective medium for the spermatozoa during their journey through the female reproductive tract. The normal environment of the vagina is a hostile one for sperm cells, for it is very acidic (from the native microflora producing Lactic acid), viscous, and patrolled by immune cells. The components in the seminal plasma attempt to compensate for this hostile environment.
Basic amines such as Putrescine, Spermine, Spermidine and Cadaverine are responsible for the smell and flavor of semen. These alkaline bases counteract the acidic environment of the vaginal canal (which is harmful to sperm), and protect DNA inside the sperm from acidic denaturation. Salts and metal ions in the semen help to create a more hospitable environment for the sperm in the vaginal canal. A typical ejaculation can contain up to 5 mg of Zinc. Zinc serves to help to stabilize the DNA-containing Chromatin in the sperm cells. A zinc deficiency may result in lowered fertility because of increased sperm fragility. Zinc deficiency can also adversely affect Spermatogenesis.
The simple sugar Fructose is the main energy source of sperm cells, which rely entirely on sugars from the seminal plasma for energy. Other components of semen (mucus and texturizing proteins) serve to increase the mobility of sperm cells in the vagina and cervix by creating a less viscous channel for the sperm cells to swim though, and preventing their diffusion out of the semen. Prostaglandin hormones are involved in suppressing an immune response by the female against the foreign semen.
Other semen components shown to spur an immunosuppressive effect are seminal plasma and seminal lymphocytes. Note that any kind of sexual or other skin contact with the semen of a person infected with HIV should be avoided, even by persons already infected with the virus, as this may cause harmful re-infection.
In some pagan circles, semen is viewed as a magical fluid which harnesses the power within the male. It is not uncommon in some communities for a pagan Priestess to copulate with two or more males at the same time to harvest their semen. It is believed that in doing so an increase in power is transferred from the males into the body of the female (Priestess). Harvesting of the ejaculate and subsequently mixing it within the body of the female is believed to greatly increase the power within the female. (Garrot-1998)
Similarly, the white tantrism (or "sex magic"), espoused by "Christian" gnostic master Samael Aun Weor in over 60 books including The Perfect Matrimony, requires that a man never spill his semen in order to activate and harness his kundalini energy.