Mammals A mammal () is a vertebrate animal of the class Mammalia (). Mammals are characterised by the presence of milk-producing mammary glands for feeding their young, a broad neocortex region of the brain, fur or hair, and three middle e ...

are the only animals in which the

testes A testicle or testis ( testes) is the gonad in all male bilaterians, including humans, and is homologous to the ovary in females. Its primary functions are the production of sperm and the secretion of androgens, primarily testosterone. The ...

descend from their point of origin into a

scrotum In most terrestrial mammals, the scrotum (: scrotums or scrota; possibly from Latin ''scortum'', meaning "hide" or "skin") or scrotal sac is a part of the external male genitalia located at the base of the penis. It consists of a sac of skin ...

. Concurrently, mammals are the only class of vertebrates to evolve a prostate gland starting with prostate evolution in monotreme mammals.

Testicular descent The development of the gonads is part of the prenatal development of the reproductive system and ultimately forms the testicles in males and the ovaries in females. The immature ova originate from cells from the dorsal endoderm of the yolk sac. ...

occurs to a variable degree in various mammals, ranging from virtually no change of position from the abdominal cavity (

monotremes Monotremes () are mammals of the order Monotremata. They are the only group of living mammals that lay eggs, rather than bearing live young. The extant monotreme species are the platypus and the four species of echidnas. Monotremes are typified ...

, elephants, and

hyraxes Hyraxes (), also called dassies, are small, stout, thickset, herbivorous mammals in the family Procaviidae within the order Hyracoidea. Hyraxes are well-furred, rotund animals with short tails. Modern hyraxes are typically between in length a ...

); through migration to the caudal end of the abdominal cavity (

armadillo Armadillos () are New World placental mammals in the order (biology), order Cingulata. They form part of the superorder Xenarthra, along with the anteaters and sloths. 21 extant species of armadillo have been described, some of which are dis ...

whales Whales are a widely distributed and diverse group of fully Aquatic animal, aquatic placental mammal, placental marine mammals. As an informal and Colloquialism, colloquial grouping, they correspond to large members of the infraorder Cetacea ...

, and

dolphins A dolphin is an aquatic mammal in the cetacean clade Odontoceti (toothed whale). Dolphins belong to the families Delphinidae (the oceanic dolphins), Platanistidae (the Indian river dolphins), Iniidae (the New World river dolphins), Pontopori ...

); migration just through the

abdominal wall In anatomy, the abdominal wall represents the boundaries of the abdominal cavity. The abdominal wall is split into the anterolateral and posterior walls. There is a common set of layers covering and forming all the walls: the deepest being the vi ...

(

hedgehog A hedgehog is a spiny mammal of the subfamily Erinaceinae, in the eulipotyphlan family Erinaceidae. There are 17 species of hedgehog in five genera found throughout parts of Europe, Asia, and Africa, and in New Zealand by introduction. The ...

mole Mole (or Molé) may refer to: Animals * Mole (animal) or "true mole" * Golden mole, southern African mammals * Marsupial mole Marsupial moles, the Notoryctidae family, are two species of highly specialized marsupial mammals that are found i ...

seals Seals may refer to: * Pinniped, a diverse group of semi-aquatic marine mammals, many of which are commonly called seals, particularly: ** Earless seal, or "true seal" ** Fur seal * Seal (emblem), a device to impress an emblem, used as a means of a ...

); formation of a sub-anal swelling (

pigs The pig (''Sus domesticus''), also called swine (: swine) or hog, is an omnivorous, domesticated, even-toed, hoofed mammal. It is named the domestic pig when distinguishing it from other members of the genus '' Sus''. Some authorities cons ...

rodents Rodents (from Latin , 'to gnaw') are mammals of the order Rodentia ( ), which are characterized by a single pair of continuously growing incisors in each of the upper and lower jaws. About 40% of all mammal species are rodents. They are n ...

); to the development of pronounced scrota (

primates Primates is an order of mammals, which is further divided into the strepsirrhines, which include lemurs, galagos, and lorisids; and the haplorhines, which include tarsiers and simians ( monkeys and apes). Primates arose 74–63 ...

dog The dog (''Canis familiaris'' or ''Canis lupus familiaris'') is a domesticated descendant of the gray wolf. Also called the domestic dog, it was selectively bred from a population of wolves during the Late Pleistocene by hunter-gatherers. ...

ruminants Ruminants are herbivorous grazing or browsing artiodactyls belonging to the suborder Ruminantia that are able to acquire nutrients from plant-based food by Enteric fermentation, fermenting it in a specialized stomach prior to digestion, principa ...

) in mammals.Setchell B.P. (1978). The Mammalian Testis. Cornell University Press, Ithaca, New York. Since the descent of the testes into a scrotal pouch subjects the animal to enhanced risk of accidental damage and/or vulnerability from predators and rivals, presumably there must be some

evolution Evolution is the change in the heritable Phenotypic trait, characteristics of biological populations over successive generations. It occurs when evolutionary processes such as natural selection and genetic drift act on genetic variation, re ...

ary adaptive advantage to testicular descent. It has been proposed that the scrotum may act as a form of sexual decoration.Portman, A. (1952). Animal Forms and Patterns, Faber & Faber, London. A scrotal location also exposes the testes to a reduced temperature below that of the body,Moore C.R. (1923). On the relationship of the germinal epithelium to the position of the testis. Anatomical Record 34: 337-358. which has been suggested to reduce the spontaneous rate of germ cell mutations.Ehrenberg L., von Ehrenstein G., Hedgram A. (1957)
Gonad temperature and spontaneous mutation-rate in man
Nature 180: 1433-1434.

Mechanism for the sperm storage region of the epididymis promoting testicular descent

An alternative proposal is that testicular descent was preceded, and possibly influenced, by migration of the sperm storage region of the

epididymis The epididymis (; : epididymides or ) is an elongated tubular genital organ attached to the posterior side of each one of the two male reproductive glands, the testicles. It is a single, narrow, tightly coiled tube in adult humans, in length; ...

to a cooler extra-abdominal location.Heller, R.E. (1929)
New evidence for the function of the scrotum
Physiology Zoology 2: 9-17.Glover T.D. (1973)
Aspects of sperm production in some East African mammals
Journal of Reproduction and Fertility 35: 45-53.Bedford H.M. (1978)
Anatomical evidence for the epididymis as the prime mover in the evolution of the scrotum
American Journal of Anatomy 152: 483-508. The evolutionary adaptive advantage of testicular descent into an extra-abdominal position may be related more to the enhanced sperm storage capacity of the epididymis at lower extra-abdominal temperatures than to the testis itself. Greater sperm storage capacity in the epididymis has been associated with enhanced fertility. In this context, the proportion (26% of total) of mature sperm stored intra-abdominally in the monotreme epididymisDjakiew D. & Jones J.C. (1981)
Structural differentiation of the male genital ducts of the echidna (Tachyglossus aculeatus)
Journal of Anatomy 132: 187-202. is considerably less than the proportion of mature sperm stored in the epididymis of many eutherian mammals (50-75% of total) with descended testes.Dott H.M. & Skinner J.D. (1967). A reassessment of extragonadal spermatozoa reserves in Suffolk rams. Journal of Agricultural Sciences 69: 293-295.Orgebin-Crist M.C. (1968)
Gonadal and epididymal sperm reserves in the rabbit; Estimation of the daily sperm production
Journal of Reproduction and Fertility 15: 15-25.Amann R.P., Johnson L., Thompson D.L. & Pickett B.W. (1976)
Daily spermatozoal production, epididymal spermatozoal reserves and transit time of spermatozoa through the epididymis of the rhesus monkey
Biology of Reproduction 15: 586-592. Moreover, this increase in scrotal storage of sperm corresponds with

epididymis evolution from reptiles to mammals The epididymis, which is a tube that connects a testicle to a vas deferens in the male reproductive system, evolved by retention of the mesonephric duct during regression and replacement of the mesonephros with the metanephric kidney. Similarly, du ...

. The mechanism by which sperm storage in the epididymis is enhanced at lower extra-abdominal temperatures has been shown to be a consequence of the biophysics of oxygen availability and sperm oxidative respiration.Djakiew D. & Cardullo R. (1986). Lower temperature of the cauda epididymidis facilitates the storage of sperm by enhancing oxygen availability. Gamete Research 15: 237-245. Reprint available from the author. The cauda epididymis, where sperm are stored, can be up to 7 °C below abdominal temperatures. For a reduction in temperature of 7 °C the respiration rate of sperm declines by one half, and the solubility of oxygen in solution increases by approximately 10%. Hence for a reduction in temperature of 7 °C the availability of oxygen is doubled, and hence twice as many sperm can be stored per unit volume of epididymal duct. This increased sperm reserve at lower extra-abdominal temperatures has been related to enhanced

fertility Fertility in colloquial terms refers the ability to have offspring. In demographic contexts, fertility refers to the actual production of offspring, rather than the physical capability to reproduce, which is termed fecundity. The fertility rate ...

which provides an evolutionary advantage to the survival of the species. In conclusion, the evolution of descended testes was promoted by the lower extra-abdominal temperature of the cauda epididymis which increased oxygen availability to sustain and store more sperm.

References

{{reflist Testicle

Testes A testicle or testis ( testes) is the gonad in all male bilaterians, including humans, and is homologous to the ovary in females. Its primary functions are the production of sperm and the secretion of androgens, primarily testosterone. The ...