Internal fertilization

Internal fertilization is the union of sperm and egg inside a parent’s reproductive tract. It occurs across many animal groups and often reduces gamete exposure to environmental hazards.

Gamete: A reproductive cell, such as a sperm or egg, that carries one set of chromosomes. Sperm and eggs are the cells that unite during fertilization.
Copulation: Sexual intercourse involving the transfer of sperm into a partner’s reproductive tract. Copulation is a common route for sperm transfer in mammals and many other animals.
External fertilization: Fertilization in which sperm and egg unite outside the parents’ bodies, usually in an aquatic or moist environment. It is the main alternative, exposing gametes to the external environment.
Sexual selection: Natural selection arising from differences in mating success and access to reproductive partners. Internal sperm transfer can intensify competition over mates and fertilization.
Mating: Reproductive behavior or physical interaction that enables sperm transfer between organisms. In many species, mating places sperm inside a partner’s reproductive tract.
Spermatophore: A sperm-containing package produced by a male animal and transferred to a partner or deposited for uptake. Some animals achieve internal fertilization by transferring a spermatophore rather than copulating.
Parthenogenesis: Development of an embryo from an unfertilized egg. It produces offspring without the sperm–egg union that defines fertilization.
Sperm competition: Competition between sperm from different males to fertilize the same eggs. Sperm may compete inside the reproductive tract after multiple matings.
Sperm: A male reproductive cell specialized to reach and fertilize an egg. Sperm must travel through the reproductive tract to reach the egg.
Cloacal kiss: A brief contact between the cloacas of many birds that transfers sperm without intromission. Many birds transfer sperm internally through this specialized mating behavior.

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