Sperm production is controlled by follicle-stimulating hormone (FSH) from the anterior pituitary gland and by testosterone produced within the gonads.

Another method is to use calipers, a ruler, or an ultrasound image to obtain the three measurements of the x, y, and z axes (length, depth and width).

[1] The number of spermatozoa an adult human male produces is directly proportional to testicular volume, as larger testicles contain more seminiferous tubules and Sertoli cells as a result.

[3] As such, men with larger testicles produce on average more sperm cells in each ejaculate, as testicular volume is positively correlated with semen profiles.

[6] The sperm move into the vas deferens, and are eventually expelled through the urethra and out of the urethral orifice through muscular contractions.

[6] Within the seminiferous tubules, the germ cells develop into spermatogonia, spermatocytes, spermatids and spermatozoa through the process of spermatogenesis.

[12] Blood supply and lymphatic drainage of the testes and scrotum are distinct: Many anatomical features of the adult testis reflect its developmental origin in the abdomen.

[1][18] Large molecules cannot pass from the blood into the lumen of a seminiferous tubule due to the presence of tight junctions between adjacent Sertoli cells.

[13] Mature sperm (and their antigens) emerge significantly after immune tolerance is set in infancy.

[13] Injection of sperm antigens causes inflammation of the testis (auto-immune orchitis) and reduced fertility.

[22] When this muscle contracts, the cord shortens and the testicles move closer up toward the body, which provides slightly more warmth to maintain optimal testicular temperature.

[22] Contraction also occurs in response to physical stress, such as blunt trauma; the testicles withdraw and the scrotum shrinks very close to the body in an effort to protect them.

A majority of these genes encode for proteins that are expressed in the seminiferous tubules and have functions related to spermatogenesis.

[27] In humans, starting at about week 4, the gonadal rudiments are present within the intermediate mesoderm adjacent to the developing kidneys.

Testicular prostheses are available to mimic the appearance and feel of one or both testicles, when absent as from injury or as treatment in association to gender dysphoria.

The testicles of calves, lambs, roosters, turkeys, and other animals are eaten in many parts of the world, often under euphemistic culinary names.

Testicles are a by-product of the castration of young animals raised for meat, so they might have been a late-spring seasonal specialty.

[45] As early as 330 BC, Aristotle prescribed the ligation (tying off) of the left testicle in men wishing to have boys.

This led to the common practice of producing two witnesses, bribed to testify the same way in cases of lawsuits with ulterior motives.

Frequently, "nuts" (sometimes intentionally misspelled as "nutz") are also a slang term for the testes due to the geometric resemblance.

The primitive jawless fish have only a single testis, located in the midline of the body, although this forms from the fusion of paired structures in the embryo.

When the boreoeutherian mammals diversified into forms that were larger or did not require intense sperm competition, they still produced enzymes that operated best at cooler temperatures and had to keep their testes outside the body.

[68] Mild, transient scrotal heat stress causes DNA damage, reduced fertility and abnormal embryonic development in mice.

[69] DNA strand breaks were found in spermatocytes recovered from testicles subjected to 40 °C or 42 °C for 30 minutes.

[69] These findings suggest that the external location of the testicles provides the adaptive benefit of protecting spermatogenic cells from heat-induced DNA damage that could otherwise lead to infertility and germline mutation.

In the mammalian kingdom, there is a tendency for testicular size to correspond with multiple mates (e.g., harems, polygamy).

Production of testicular output sperm and spermatic fluid is also larger in polygamous animals, possibly a spermatogenic competition for survival.

[71] Among the Hominidae, gorillas have little female promiscuity and sperm competition and the testes are small compared to body weight (0.03%).

[72] Testis weight also varies in seasonal breeders like red foxes,[73] golden jackals,[74] and coyotes.

The ampullae are otherwise essentially identical to the seminiferous tubules in higher vertebrates, including the same range of cell types.