Male sex organ, which also provides the channel for urine to leave the body. Three long columns of tissue extend through its length, covered by elastic tissue and a thin layer of skin. One expands at the tip into a mushroom-shaped structure (glans penis) and contains the urethra (see urinary system), which ends in a slitlike opening. In sexual arousal, blood fills spaces in the tissue, and blood vessels constrict to hold it there, enlarging and hardening the penis in an erection. The foreskin, a circular fold of skin covering the glans, is often removed (see circumcision). Seealso reproductive system.
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In mammals, all intact developmentally typical males have a penis, but the clitoris in the females of the following species is sufficiently enlarged that it is usually termed a pseudo-penis: spotted hyena, squirrel monkey, lemur, and binturong. The labia of the spider monkey are elongated and may be similarly confused during display. Elongated labia are also observed in humans.
The mammalian pseudo-penis appears to be simply for display, although the hyena is an exception: erection of the pseudo-penis (which is voluntary in both sexes) is certainly a display of submission in both male and female spotted hyena, but the female hyena additionally uses her pseudo-penis for urination, copulation, and childbirth. In addition, this makes it difficult for males to mate without the full cooperation of females, which helps the female-dominated society of spotted hyenas to eliminate forced sex.
A notable example of a bird with a pseudo-penis is the Red-billed Buffalo Weaver, which can still mate even if its pseudo-penis is disabled. Only 3% of bird species have a phallus: the common urogenital arrangement for both males and females is the cloaca, through which all elimination and reproduction (via juxtaposed cloacas) occurs, but certain bird species, particularly ratites, screamers, waterfowl, and cracids (a family of aboreal galliformes) exhibit a phallus in the male, a structure derived from the internal cloacal wall which is generally associated with sperm competition. Popularised species include the Ruddy duck, and the particularly gifted Argentine lake duck.