Mammals are the only animals in which the testes descend from their point of origin into a scrotum. Concurrently, mammals are the only class of vertebrates to evolve a prostate gland starting with

prostate evolution in monotreme mammals The monotremes (egg laying mammals) represent the order of extant mammals most distantly related to humans. The platypus (''Ornithorhynchus anatinus'') is indigenous to eastern Australia; the short-beaked echidna (''Tachyglossus aculeatus'') is i ...

Testicular descent The development of the gonads is part of the prenatal development of the reproductive system and ultimately forms the testes in males and the ovaries in females. The gonads initially develop from the mesothelial layer of the peritoneum. The ovar ...

occurs to a variable degree in various mammals, ranging from virtually no change of position from the abdominal cavity ( monotremes, elephants and hyraxes); through migration to the caudal end of the abdominal cavity (

armadillo Armadillos (meaning "little armored ones" in Spanish) are New World placental mammals in the order Cingulata. The Chlamyphoridae and Dasypodidae are the only surviving families in the order, which is part of the superorder Xenarthra, along wi ...

s, whales and dolphins); migration just through the abdominal wall ( hedgehogs, moles, seals); formation of a sub-anal swelling ( pigs,

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 nat ...

); to the development of pronounced scrota (

primates Primates are a diverse order of mammals. They are divided into the strepsirrhines, which include the lemurs, galagos, and lorisids, and the haplorhines, which include the tarsiers and the simians (monkeys and apes, the latter including huma ...

, dogs, ruminants) in mammals.Setchell B.P. (1978). The Mammalian Testis. Cornell University Press, Ithaca, New York. Since 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 evolutionary 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 (; plural: epididymides or ) is a tube that connects a testicle to a vas deferens in the male reproductive system. It is a single, narrow, tightly-coiled tube in adult humans, in length. It serves as an interconnection between the ...

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, ...

. 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 is 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 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