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A testicle, also called testis ( testes) is the male gonad in all gonochoric animals, including humans, and is homologous to the ovary, which is the female gonad. Its primary functions are the production of sperm and the secretion of Androgen, primarily testosterone.
The release of testosterone is regulated by luteinizing hormone (LH) from the anterior pituitary gland. Sperm production is controlled by follicle-stimulating hormone (FSH) from the anterior pituitary gland and by testosterone produced within the gonads.
However, the most accurate calculation of actual testicular volume is gained from the formula: (2025). 9783030523091, Springer Nature. ISBN 9783030523091
An average adult testicle measures up to . The Tanner scale, which is used to assess the maturity of the male genitalia, assigns a maturity stage to the calculated volume ranging from stage I, a volume of less than 1.5 cm3; to stage V, a volume greater than 20 cm3. Normal volume is 15 to 25 cm3; the average is 18 cm3 per testis (range 12–30 cm3).
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. As such, men with larger testicles produce on average more sperm cells in each ejaculate, as testicular volume is positively correlated with semen profiles.
Between tubules (interstitial cells) exist Leydig cellscells localized between seminiferous tubules that produce and secrete testosterone and other important for puberty (including secondary sexual characteristics like facial hair), sexual behavior, and libido. Sertoli cells support spermatogenesis. (2025). 9780702054785, Elsevier Health Sciences. ISBN 9780702054785 Testosterone controls testicular volume.
Immature Leydig cells and interstitial and epithelial cells are also present.
The function of the blood–testis barrier may be to prevent an auto-immune reaction. Mature sperm (and their ) emerge significantly after immune tolerance is set in infancy. Since sperm are antigenically different from self-tissue, a male animal can react immunologically to his own sperm. The male can make antibodies against them.
Injection of sperm antigens causes inflammation of the testis (auto-immune orchitis) and reduced fertility. The blood–testis barrier may reduce the likelihood that sperm proteins will induce an immune response. (2025). 9781461433354, Springer Science & Business Media. ISBN 9781461433354
The cremasteric muscle covers the testicles and the spermatic cord. 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. When cooling is required, the cremasteric muscle relaxes and the testicles lower away from the warm body and are able to cool. 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.
The cremasteric reflex will reflexively raise the testicles. The testicles can also be lifted voluntarily using the pubococcygeus muscle, which partially activates related muscles.
Testes follow the path of descent, from high in the posterior fetal abdomen to the inguinal ring and beyond to the inguinal canal and into the scrotum. In most cases (97% full-term, 70% preterm), both testes have descended by birth. In most other cases, only one testis fails to descend. This is called cryptorchidism. In most cases of cryptorchidism, the issue will mostly resolve itself within the first half year of life. However, if the testes do not descend far enough into the scrotum, surgical anchoring in the scrotum is required due to risks of infertility and testicular cancer.
The testes grow in response to the start of spermatogenesis. Size depends on lytic function, sperm production (amount of spermatogenesis present in testis), interstitial fluid, and Sertoli cell fluid production. The testicles are fully descended before the male reaches puberty.
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. There have also been some instances of their implantation in dogs.
Scientists are working on developing lab-grown testicles that might help infertile men in the future.
Stimulation of testicular functions via gonadotropin may enlarge their size. Testes may shrink or atrophy during Hormone therapy or through chemical castration.
In all cases, the loss in testes volume corresponds with a loss of spermatogenesis.
In the Middle Ages, men who wanted a boy sometimes had their left testicle removed. This was because people believed that the right testicle made "boy" sperm and the left made "girl" sperm. As early as 330 BC, Aristotle prescribed the ligation (tying off) of the left testicle in men wishing to have boys.Hoag, Hannah. I'll take a girl, please... Cherry-picking from the dish of life. Drexel University Publication.
Another theory says that testis is influenced by a loan translation, from Greek that is "two glands side by side".The American Heritage Dictionary of the English Language, Fourth Edition
There are multiple slang terms for the testes. They may be referred to as "balls". Frequently, "nuts" (sometimes intentionally misspelled as "nutz") are also a slang term for the testes due to the geometric resemblance. One variant of the term includes "Deez Nuts", which was used for a satirical political candidate in 2016.
In Spanish language, the slang term huevos is used, which is Spanish for which typically refers to unfertilized chicken eggs. Note that this is a non-sequitur or false analogy. The male gonad in mammals produces sperm (which transport male gametes to an egg). The male gonad does not produce the female gametes found in eggs. Perhaps the word {Lang|es|huevos}} is used as slang for male testes because the body of each epididymus within the scrotum has an overall shape similar to that of a common bird egg.
In , the testicle on the right side is usually larger. In many bird and mammal species, the left may be larger. Fish usually have two testes of a similar size. 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.
There are several hypotheses as to why most boreotherian mammals have external testes that operate best at a temperature that is slightly less than the core body temperature. One view is that it is stuck with enzymes evolved in a colder temperature due to external testes evolving for different reasons. Another view is that the lower temperature of the testes simply is more efficient for sperm production.
The classic hypothesis is that cooler temperature of the testes allows for more efficient fertile spermatogenesis. There are no possible enzymes operating at normal core body temperature that are as efficient as the ones evolved.
Early mammals had lower body temperatures and thus their testes worked efficiently within their body. However, boreotherian mammals may have higher body temperatures than the other mammals and had to develop external testes to keep them cool. One argument is that mammals with internal testes, such as the monotremes, armadillos, sloths, elephants, and rhinoceroses, have a lower core body temperatures than those mammals with external testes.
Researchers have wondered why birds, despite having very high core body temperatures, have internal testes and did not evolve external testes. BIOLOGY OF REPRODUCTION 56, 1570–1575 (1997)- Determination of Testis Temperature Rhythms and Effects of Constant Light on Testicular Function in the Domestic Fowl (Gallus domesticus) It was once theorized that birds used their air sacs to cool the testes internally, but later studies revealed that birds' testes are able to function at core body temperature.
Some mammals with seasonal breeding cycles keep their testes internal until the breeding season. After that, their testes descend and increase in size and become external.
The ancestor of the boreoeutherian mammals may have been a small mammal that required very large testes for sperm competition and thus had to place its testes outside the body. This might have led to enzymes involved in spermatogenesis, spermatogenic DNA polymerase beta and recombinase activities evolving a unique temperature optimum that is slightly less than core body temperature. 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. This position is made less parsimonious because the kangaroo, a non-boreoeutherian mammal, has external testicles. Separately from boreotherian mammals, the ancestors of kangaroos might have also been subject to heavy sperm competition and thus developed external testes; however, kangaroo external testes are suggestive of a possible adaptive function for external testes in large animals.
One argument for the evolution of external testes is that it protects the testes from abdominal cavity pressure changes caused by jumping and galloping.
Mild, transient scrotal heat stress causes DNA damage, infertility and abnormal embryonic development in mice. DNA strand breaks were found in spermatocytes recovered from testicles subjected to 40 °C or 42 °C for 30 minutes. 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.
Among the Hominidae, have little female promiscuity and sperm competition and the testes are small compared to body weight (0.03%). have high promiscuity and large testes compared to body weight (0.3%). Human testicular size falls between these extremes (0.08%).
Testis weight also varies in seasonal breeders like , , and .
Location
Internal
The basal condition for mammals is to have internal testes. The testes of monotremes, , and remain within the abdomen (testicondy). There are also some with external testes and mammals with internal testes, such as the rhinoceros.Schaffer, N. E., et al. " Ultrasonography of the reproductive anatomy in the Sumatran rhinoceros (Dicerorhinus sumatrensis) ." Journal of Zoo and Wildlife Medicine (1994): 337-348. Cetaceans such as whales and dolphins also have internal testes. (2025). 9780429063626 ISBN 9780429063626 As external testes would increase drag in the water, they have internal testes, which are kept cool by special circulatory systems that cool the arterial blood going to the testes by placing the arteries near veins bringing cooled venous blood from the skin. (1998). 9781489901613, Springer US. ISBN 9781489901613 In and , the location of the testes is para-abdominal, though have scrotal testes.
External
land mammals, the large group of mammals that includes humans, have externalized testes. Their testes function best at temperatures lower than their core body temperature. Their testes are located outside of the body and are suspended by the spermatic cord within the scrotum.
Size
The relative size of the testes is often influenced by . Testicular size as a proportion of body weight varies widely. 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. The testes of the right whale are likely to be the largest of any animal, each weighing around 500 kg (1,100 lb).
Internal structure
and most fish do not possess seminiferous tubules. Instead, the sperm are produced in spherical structures called sperm ampullae. These are seasonal structures, releasing their contents during the breeding season, and then being reabsorbed by the body. Before the next breeding season, new sperm ampullae begin to form and ripen. The ampullae are otherwise essentially identical to the seminiferous tubules in higher vertebrates, including the same range of cell types.
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