Sex organ

A sex organ, or reproductive organ, is any body part involved in sexual reproduction. These organs form an organism's primary sex characteristics. Their jobs include making and moving gametes, enabling fertilization, and helping offspring develop and be born. Such organs appear in many animals and plants, though their forms differ by species.

Sex organs are usually grouped as male or female. In mammals, including humans, male organs include the testicles, epididymides, and penis; female organs include the clitoris, ovaries, oviducts, and vagina. The testicles and ovaries are the primary sex organs, while all other reproductive organs are secondary. External parts are called genitals or external genitalia, visible at birth; internal parts are hidden. In plants, male structures like stamens produce pollen, and female structures like pistils produce ovules and receive pollen. Mosses and ferns have gametangia as reproductive organs within the gametophyte. In flowering plants, sex organs sit inside gametophytes within pollen and ovules; the flowers themselves are not sex organs. Conifers also house their reproductive structures inside gametophytes within cones and pollen. Together, all sex organs make up the reproductive system.

The primary sex organs are the gonads—internal organs that become testicles in males or ovaries in females. Gonads generate gametes with inheritable DNA and produce most hormones that guide sexual development and behavior. Secondary sex organs are the rest of the reproductive system, internal or external. The term genitalia (or genitals) refers to externally visible sex organs. In zoology, male genitalia are defined as all structures inserted into the female or that hold her near her gonopore during sperm transfer; female genitalia are those parts that directly contact male genitalia or male products during or right after copulation.

Finding a common origin for gonads is difficult; they likely evolved independently several times. Testes and ovaries first arose through natural selection. Sexual selection is now seen as the main driver of genital evolution, with male genitalia showing divergent traits from this pressure.

In vertebrates, the visible genitals of male placental mammals are the penis and scrotum; for females, the vulva. Males urinate and ejaculate through one urethral opening; females have separate vaginal and urethral openings. Both male and female genitals have many nerve endings, making them sensitive and pleasurable to touch. In many human societies, especially conservative ones, exposing genitals is considered indecent. Human sex organs include the full set of male and female structures.

During typical prenatal development, sex organs start from a common primordium and differentiate into male or female. The SRY gene on the Y chromosome, encoding testis-determining factor, directs male differentiation; its absence allows ovaries to develop. Internal and external reproductive organs develop based on hormones from fetal gonads and the cells' response. Initially, fetal genitalia look female-like: urogenital folds with a small central protuberance and the urethra behind it. If the fetus has testes producing testosterone and the genital cells respond, the outer folds swell and fuse to form the scrotum; the protuberance grows into the penis; inner folds wrap around the penis and fuse to form the penile raphe. Each organ in one sex has a homologous counterpart in the other. Sexual differentiation also includes secondary sexual characteristics, like pubic and facial hair and breasts, which appear at puberty. Because sexual selection strongly shapes genital structure and function, this organ system evolves rapidly, leading to great variety among animals.

Many other vertebrates—all amphibians, birds, reptiles, some fish, and a few mammals (monotremes, tenrecs, golden moles, marsupial moles)—have a single posterior opening called the cloaca. It serves for reproduction, digestion, and excretion. Some invertebrates have analogous excretory systems also called cloacae. Penile and clitoral structures are present in various forms across these groups.