Testicle Saga: Escape from the Abdomen
Genetic evidence casts new light on descended testes in mammals.
Posted Jul 25, 2018
My 2013 book, How We Do It, addressed an enduring mystery in mammal reproduction: “It is often claimed that the word ‘testify’ comes from an ancient Roman custom in which a man would clutch his testes in his right hand before giving evidence in court....it is undisputed that the Latin word testis originally meant ‘witness,’ and any human male can testify that the testicles are located in a risky place....Hazardous location of testes in pouches outside the main body cavity is a truly peculiar adaptation that demands explanation.”
Seeking an answer, a useful first step is to trace the evolution of descended testes. Testes remain high up in the abdomen in egg-laying mammals (monotremes), but descend in all marsupials, conspicuously so in kangaroos, and in most placentals, including humans and all other primates. Two alternative hypotheses exist: Either the common ancestor of marsupials and placentals lacked testis descent, which originated later, or descent already existed before those two mammal groups separated over 125 million years ago. Either way, independent changes — acquisitions or losses — in multiple lineages must have occurred.
In How We Do It, I felt obliged to leave the issue unresolved. Emphasizing prevailing uncertainty, an evolutionary reconstruction by Lars Werdelin and Åsa Nilsonne (1999) seemingly confirmed the second hypothesis, whereas a later analysis by Karel Kleisner and colleagues (2010) supported the first. Fortunately, as in other striking cases such as the evolution of mammal milk, new genetic evidence has opened up an entirely new perspective.
How testes descend
In all mammals, reproductive and urinary systems begin development in tight association. So males’ testes and females’ ovaries start out alongside the kidneys, near the backbone in the abdominal cavity. To end up near the base of the penis, the testes must hence shift away from the kidneys. In most mammals, they descend at least as far as the belly wall. To migrate completely outside the abdomen — as in primates, carnivores and hoofed mammals — testes pass through a pair of inguinal canals into a pouch-like scrotum. My preference for that word’s origin is the Latin scrautum, designating a quiver for arrows and evoking an image of Eros in action.
Complete descent of a testis occurs in two distinct stages. Both involve a ligament attached to the base of the testis, the gubernaculum (from the same Ancient Greek stem that gave us “governor”). In phase one, involving partial descent, the gubernaculum swells and anchors the testis low in the abdominal cavity. For full descent in phase two the gubernaculum bulges into the inguinal canal, migrates to the scrotum and drags the testis in.
Testis descent and temperature
Descent of testes is surely connected with the fact that all mammals are “warm-blooded”, typically having high core body temperatures around 37oC (98.6oF). Going further, many authors have baldly stated that sperm production is impossible at temperatures that high. But many counter-examples exist. For instance, testes never descend in birds, yet they are also warm-blooded and actually have an even higher average body temperature than mammals. Moreover, although full testis descent occurs in many mammals, there are numerous exceptions. In extreme cases, the testes stay close to their original location near the kidneys, but in a common intermediate condition the testes descend partially but still remain within the abdomen. This is, for instance, seen in two groups of mammals that have become secondarily adapted for fully aquatic life: dolphins + whales and sea-lions + seals.
Mammals that keep their testes in the abdomen do not consistently have lower core body temperatures than mammals with descended testes, so clearly an elevated body temperature does not automatically block sperm production.
The claim that sperm production cannot proceed at a raised body temperature is often backed with reference to occasional failure of testis descent in humans (cryptorchidism). Undescended testes are usually abnormally small and, if they are still in the abdomen after puberty, surgical relocation to the scrotum is needed for sperm production to occur. But human testes, as in all other primates, normally reside in an external scrotum at a lower temperature, a condition presumably already present in their common ancestor at least 70 million years ago. After such an extensive evolutionary history, aberrant retention of the testes in the abdomen can be expected to suppress sperm production regardless of the original reason for the evolution of descent.
But scientists may have been looking in the wrong direction. As British biologist Michael Bedford noted in 1978, several lines of evidence indicate that it is storing sperms rather than producing them that benefits from a lower temperature in a scrotum. Until their release, mature sperms are stored in the tail end of the epididymis, a coiled structure usually located alongside the testis. In some mammals, although the testis itself does not descend, the tail of the epididymis migrates to end up near the belly wall. Even in mammals in which testis and epididymis descend together, the epididymis leads and always migrates furthest. It is also notable that scrotal skin covering the epididymis, but not the testis, is often hairless and more easily cooled.
Genes for descent
The vital new breakthrough from genome research regarding testis descent followed on from multiple comparisons of DNA sequences that clearly identified four major groups of placental mammals. Three of those groups neatly correspond to findings from classical anatomical evidence, but the fourth — aptly named Afrotheria — is a newly recognized cluster of native African mammals: aardvarks, elephants, elephant-shrews, hyraxes, tenrecs, golden moles and manatees. Remarkably, evolutionary biologists had never previously suggested that all of these mammals shared a single common ancestor. Following recognition of Afrotheria, one mammal expert quipped that the only anatomical feature they have in common is a funny nose. Far more fundamental, however, is the universal feature of undescended testes. In most cases (uniquely among placental mammals), the testes remain in their original location next to the kidneys, although they partially descend in the aardvark and some tenrecs.
In an inspired approach, Virag Sharma and colleagues examined two genes that trigger development of that key player in testis descent, the gubernaculum: RXFP2 and INSL3. Comparison of 71 placental mammal species revealed that 67 have intact versions of both genes, the sole exceptions being four members of Afrotheria completely lacking testis descent: elephant-shrew, tenrec, golden mole and manatee. Crucially, degenerating relics of those two genes tell us that functioning versions must have been present in the common ancestor of all placental mammals. So testicular descent was most probably the ancestral condition. This inference is bolstered by the finding that inactivation of the RXFP2 gene arose independently at least four different times during the evolution of Afrotheria. The authors conclude that their “results provide a molecular mechanism that explains the loss of testicular descent in afrotherians”. But this conclusion is problematic because elephant and hyrax, both of which have completely undescended testes, possess intact RXFP2 and INSL3 genes. It is hence possible that some other mechanism had already inhibited descent in the common ancestor of Afrotheria and that those two genes were later inactivated in some lineages as a sequel of disuse.
Why do testes descend?
Of course, the scrotumless elephant in the room is that we still have no generally accepted explanation for testis descent. One early author blamed abrupt body movement, arousing absurd images of other, heavier abdominal organs such as liver and kidneys dangling from the body in pouches. A wide-ranging 2008 commentary in Slate by Liam Drew effectively covers the technicolor array of available “explanations” for testicular descent. Published options include escape from abdominal pressure arising from intense activity (Michael Chance and Roland Frey), flashy genital displays (Adolf Portmann), and adaptation for sperm activation by a warmer vaginal environment (Gordon Gallup and colleagues). These one-off proposals generally make little mention of the influence of body temperature — the one aspect for which strong evidence exists — and fail to explain why some mammals have fully descended testes while others have only partial descent or none at all.
Thanks to Virag Sharma and colleagues, at least it is now evident that testes were descended in the common ancestor of marsupials and placentals. So we can narrow our search to finding reasons why some mammals secondarily lost this feature.
Bedford, J.M. (1978) Anatomical evidence for the epididymis as the prime mover in the evolution of the scrotum. American Journal of Anatomy 152:483-508.
Bedford, J.M. (2004) Enigmas of mammalian gamete form and function. Biological Reviews 79:429-460.
Chance, M.R.A. (1996) Reason for externalization of the testis of mammals. Journal of Zoology, London 239:691-695.
Drew, L. (2013) The scrotum Is nuts: Why are testicles kept in a vulnerable dangling sac? It’s not why you think. Slate
Freeman, S. (1990) The evolution of the scrotum: a new hypothesis. Journal of Theoretical Biology 145:429-445.
Frey, R. (1991) Zur Ursache des Hodenabstiegs (Descensus testiculorum) bei Säugetieren. Journal of Zoological Systematics & Evolutionary Research 29:40-65.
Gallup, G.G., Finn, M.M. & Sammis, B. (2009) On the origin of descended scrotal testicles: The activation hypothesis. Evolutionary Psychology 7:517-526.
Heyns, C.F. & Hutson, J.M. (1995) Historical review of theories on testicular descent. Journal of Urology 153:754-767.
Kleisner, K., Ivell, R. & Flegr, J. (2010) The evolutionary history of testicular externalization and the origin of the scrotum. Journal of Biosciences 35:27-37.
Lovegrove, B.G. (2014) Cool sperm: why some placental mammals have a scrotum. Journal of Evolutionary Biology 27:801-814.
Martin, R.D. (2013) How We Do It: The Evolution and Future of Human Reproduction. New York: Basic Books.
Portmann, A. (1952) Animal Forms and Patterns: A Study of the Appearance of Animals. London: Faber and Faber Ltd.
Setchell, B.P. (1998) The Parkes Lecture: Heat and the testes. Journal of Reproduction & Fertility 114:179-194.
Sharma, V., Lehmann, T., Stuckas, H., Funke, L. & Hiller, M. (2018). Loss of RXFP2 and INSL3 genes in Afrotheria shows that testicular descent is the ancestral condition in placental mammals. PLoS Biology 16(6),e2005293:1-22.
Short, R.V. (1997) The testis: the witness of the mating system, the site of mutation and the engine of desire. Acta Paediatrica Supplement 422:3-7.
Werdelin, L. & Nilsonne, Å. (1999) The evolution of the scrotum and testicular descent in mammals: A phylogenetic view. Journal of Theoretical Biology 196:61-72.