The etymology of the word is somewhat colorfully based on Roman law. The aforementioned 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).
Both functions of the testicle, sperm-forming and endocrine, are under control of gonadotropic hormones produced by the anterior pituitary:
In healthy European adult human males, average testicular volume is 18cm³ per testis, with normal size ranging from 12cm³ to 30cm³. Measurement in the living adult is done in two basic ways:
The volume is then calculated using the formula for the volume of an ellipsoid: 4/3 π × (length/2) × (width/2) × (depth/2).
To some extent, it is possible to change testicular size. Short of direct injury or subjecting them to adverse conditions, e.g., higher temperature than they are normally accustomed to, they can be shrunk by competing against their intrinsic hormonal function through the use of externally administered steroidal hormones. Steroids taken for muscle enhancement often have the undesired side effect of testicular shrinkage. Similarly, stimulation of testicular functions via gonadotropic-like hormones may enlarge their size. Testicles may shrink or atrophy during hormone replacement therapy.
The function of the blood-testis barrier (red highlight in diagram above) may be to prevent an auto-immune reaction. Mature sperm (and their antigens) arise long after immune tolerance is established in infancy. Therefore, since sperm are antigenically different from self tissue, a male animal can react immunologically to his own sperm. In fact, he is capable of making antibodies against them.
Injection of sperm antigens causes inflammation of the testis (autoimmune orchitis) and reduced fertility. Thus, the blood-testis barrier may reduce the likelihood that sperm proteins will induce an immune response, reducing fertility and so progeny.
The testicles can also be lifted voluntarily using the pubococcygeus muscle, which partially activates related muscles. This can sometimes be triggered by tightening or sucking in the stomach or abdomen.
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 testicles to keep them cool. It is argued that those mammals with internal testicles, such as the monotremes, armadillos, sloths, elephants, and rhinoceroses, have a lower core body temperatures than those mammals with external testicles.
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 sacs 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 testicles 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 testicles 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.
Some prominent conditions include:
BACILLUS THURINGIENSIS INSECTICIDAL CRY1AB TOXIN DOES NOT AFFECT THE MEMBRANE INTEGRITY OF THE MAMMALIAN INTESTINAL EPITHELIAL CELLS: AN IN VITRO STUDY
Jan 01, 2006; SUMMARY The mammalian intestinal epithelium has been found, based on in vivo experiments, to be resistant to insecticidal Cry...