The testicle (from Latin testiculus, diminutive of testis, meaning "witness" [of virility], plural testes) is the male generative gland in animals.
The etymology of the word is based on Roman law. The Latin word "testis", witness, was used in the firmly established legal principle "Testis unus, testis nullus" (one witness [equals] no witness), meaning that testimony by any one person in court was to be disregarded unless corroborated by the testimony of at least another. This led to the common practice of producing two witnesses, bribed to testify the same way in cases of lawsuits with ulterior motives. Since such "witnesses" always came in pairs, the meaning was accordingly extended, often in the diminutive (testiculus, testiculi). After a while, it was reduced to balls.
Like the ovaries to which they are homologous, testes are components of both the reproductive system (being gonads) and the endocrine system (being endocrine glands). The respective functions of the testes are:
Both functions of the testicle, sperm-forming and endocrine, are under control of gonadotropic hormones produced by the anterior pituitary:
- producing sperm (spermatozoa)
- producing male sex hormones of which testosterone is the best-known
The basal condition for mammals is to have internal testes. Only the Boroeutherian 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, suspended by the spermatic cord within the scrotum. The testes of the non-boreotherian mammals such as the monotremes, armadillos, sloths, elephants remain within the abdomen. There are also some Boreoeutherian mammals with internal testes, such as the rhinoceros.
- luteinizing hormone (LH)
- follicule-stimulating hormone(FSH)
Marine boreotherian mammals such as whales and dolphins, also have internal testes, but it has recently been shown (e.g., for dolphins) that they use elaborate vascular networks to provide the necessary temperature lowering for optimum function. As external testes would increase drag, many boreotherian aquatic mammals 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.
There are several hypotheses why most boreotherian mammals have external testes which operate best at a temperature that is slightly less than the core body temperature, e.g. that it is stuck with enzymes evolved in a colder temperature due to external testes evolving for different reasons, that the lower temperature of the testes simply is more efficient for sperm production.
1) More efficient. The classic hypothesis is that cooler temperature of the testes allows for more efficient fertile spermatogenesis. In other words, there are no possible enzymes operating at normal core body temperature that are as efficient as the ones evolved, at least none appearing in our evolution so far.
The early mammals had lower body temperatures and thus their testes worked efficiently within their body. However it is argued that boreotherian mammals have higher body temperatures than the other mammals and had to develop external testes to keep them cool. It is argued that those 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.
However, the question remains why birds despite having very high core body temperatures have internal testes and did not evolve external testes. It was once theorized that birds used their air sacks to cool the testes internally, but later studies revealed that birds' testes are able to function at core body temperature.
Some mammals which have seasonal breeding cycles keep their testes internal until the breeding season at which point their testes descend and increase in size and become external.
2) Irreversible adaptation to sperm competition. It has been suggested that the ancestor of the boreoeutherian mammals was a small mammal that required very large testes (perhaps rather like those of a hamster) for sperm competition and thus had to place its testes outside the body. This led to enzymes involved in spermatogenesis, spermatogenic DNA polymerase beta and recombinase activities evolving a unique temperature optimum, slightly less than core body temperature. When the boreoeutherian mammals then diversified into forms that were larger and/or did not require intense sperm competition they were stuck with enzymes that operated best at cooler temperatures and had to keep their testes outside the body. This position is made less parsimonious by the fact that the kangaroo, a non-boreoeutherian mammal, has external testicles. The ancestors of kangaroos might, separately from boreotherian mammals, 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.
3) Protection from abdominal cavity pressure changes. One argument for the evolution of external testes is that it protects the testes from abdominal cavity pressure changes caused by jumping and galloping.
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