Testicles Are Cool
Exploring why men’s family jewels are exposed outside the body
Posted October 8, 2014
In all mammals, reproductive and urinary systems are closely associated during development, so the testes start to form alongside the kidneys. To descend, each testis must migrate through an inguinal canal into an external scrotal sac. This occurs in all primates. In fact, most mammals have descended testes, whereas other vertebrates completely lack this strange feature.
One early explanation attributed descended testes to gravity. This provokes absurd images of other, heavier body organs — such as kidneys — dangling in pouches underneath the body. In 1996, Michael Chance suggested that testes descend to avoid concussion from intense activity, potentially provoking inadvertent ejection of semen. This notion was triggered by a report that prostatic fluid was detected in oarsmen’s urine after a race. But surely the spectacular belly-flops of breaching humpback whales (whose testes are undescended) would generate far greater intra-abdominal pressure than merely pulling an oar? Swiss zoologist Adolf Portmann, mindful of the flashy scrotum of some monkeys, had suggested instead in 1952 that testes descended for display. However, some nocturnal primates such as mouse lemurs have particularly large descended testes but do not flaunt them while scurrying around at night.
Keeping the testes out of warm’s way
For some time, it has been widely accepted that testes descend to escape the raised body temperature that is universal to mammals. It is often baldly stated that sperm production is impossible at body heat, and some evidence at first seems to point this way. For instance, some 3% of newborn boys have undescended testes (cryptorchidism). If testes remain in the abdomen after puberty, sperm production is suppressed and surgical intervention is needed to restore fertility.
But human testes descend because the common ancestor of primates showed this condition 80 million years ago. So it is hardly surprising that abnormal retention in the abdomen suppresses sperm production, even if descent evolved for some other reason. In any case, sperm production often occurs despite a raised body temperature. For example, testes never descend in birds, yet their average body temperature is even higher than in mammals. Furthermore, in some adult mammals, testes remain in the belly cavity, often in their original location near the kidneys, not only in whales and dolphins but also in rhinoceroses, elephants, hyraxes, manatees and many insectivores (e.g. hedgehogs).
Mammals whose testes remain in the abdomen do not have lower body temperatures than mammals with descent, so body temperature alone obviously does not block sperm production. Instead, as British reproductive biologist Michael Bedford noted in 1978, several findings indicate that descent is actually connected with storing the mature sperms in the epididymis, a coiled structure neighbouring the testis. In some mammals, the testis stays put, but the tail of the epididymis descends to lie against the ventral belly wall. Even in mammals in which the testis descends, the epididymis leads and consistently travels farther. The lower temperature outside the belly cavity apparently increases oxygen availability for stored sperms.
The temperature in a man’s scrotum is only about 4°F below the core body level, but that is evidently enough to make the difference between fertility and infertility. This suggests that mildly heating testes might reduce fertility, perhaps providing a simple birth control method. Fertility experts John MacLeod and Robert Hotchkiss addressed this in a 1941 paper. Dry heat was applied using a fever therapy cabinet enclosing the entire body of test subjects. A fortnight later, sperm counts declined markedly, and the effect persisted for about 2 months. In 1965, fertility researchers John Rock and Derek Robinson explored this topic further. In one experiment, for at least 6 weeks normal men wore insulating underwear that limited the difference between scrotal and abdominal temperatures to 2°F instead of 4°F. In all cases, sperm counts began to decrease after about 3 weeks. Low sperm counts persisted for 3 to 8 weeks after the men stopped wearing the insulating underwear, returning to normal by 3 months after treatment ended. The overall finding from several experiments was that heating the testes decreased sperm counts, whereas cooling increased production. Since that time, only a few scattered studies have examined testicular heating as a possible means of birth control. For instance, in 1992 surgeon Ahmed Shafik reported that wearing a polyester sling applied to the scrotum around the clock for an entire year suppressed sperm production after about 5 months. The effect was reversible, but no practical application has followed.
Heating the scrotum by a few degrees evidently impairs sperm production, so it follows that certain activities or occupations may depress sperm counts. For instance, gynecologist Carolina Tiemessen and colleagues reported in 1995 that tight-fitting underwear can diminish semen quality. Driving a vehicle for long periods can also overheat the scrotum, reducing sperm counts. A 1996 paper by industrial medicine specialist Irene Figà-Talamanca and colleagues reported that taxi drivers in Rome had a lower proportion of normal sperms, an effect that increased with time spent in the job. Gynecologist Berndt-Johan Procopé, appropriately working in Finland, showed that regular use of saunas can adversely affect sperm production.
A relatively new arrival on the testis-toasting scene is a hot laptop computer, as reported by urologist Yefim Sheynkin and colleagues in 2011. For 29 healthy male volunteers, temperatures of the scrotum, a laptop computer, and a lap pad were recorded during 3 separate 1-hour sessions, each with different conditions. Scrotal temperature increased under all 3 conditions, regardless of leg position or the use of a lap pad. The increase was lowest, only 2.5°F, when subjects sat with a pad beneath the laptop and their legs 70° apart. The greatest increase, almost 4.5°F, occurred when the legs were held closely together without a lap pad. This study did not investigate whether scrotal temperature adversely affected semen quality, but the temperature rise was enough to impair fertility with prolonged use. So the take-home message for using an aptly-named laptop is to keep the legs spread and to use a protective pad.
Bedford, J.M. (1978) Anatomical evidence for the epididymis as the prime mover in the evolution of the scrotum. Am. J. Anat. 152 :483-508.
Chance, M.R.A. (1996) Reason for externalization of the testis of mammals. J. Zool. Lond. 239 :691-695.
Heyns,C.F. & Hutson,J.M. (1995) Historical review of theories on testicular descent. J. Urol. 153 :754-767.
Mieusset, R. & Bujan, L. (1994) The potential of mild testicular heating as a safe, effective and reversible contraceptive method for men. Int. J. Androl. 17 :186-191.
Parazzini, F., Marchini, M., Luchini, L., Tozzi, L., Mezzopane, R. & Fedele, L. (1995) Tight underpants and trousers and the risk of dyspermia. Int. J. Androl. 18 :137-140.
Portmann, A. (1952) Animal Forms and Patterns. London: Faber and Faber Ltd..
Robinson, D., Rock, J. & Menkin, M.F. (1968) Control of human spermatogenesis by induced changes in intrascrotal temperature. J. Am. Med. Ass. 204 :290-297.
Rock, J.C. & Robinson, D. (1965) Effect of induced intrascrotal hyperthermia on testicular function in man. Am. J. Obstet. Gynecol. 93 :793-801.
Saikhun, J., Kitiyanant, Y., Vanadurongwan, V. & Pavasuthipaisit, K. (1998) Effects of sauna on sperm movement characteristics of normal men measured by computer-assisted sperm analysis. Int. J. Androl. 21 :358-363.
Setchell, B.P. (1998) The Parkes lecture: Heat and the testes. J. Reprod. Fertil. 114 :179-194.
Shafik, A. (1992) Contraceptive efficacy of polyester-induced azoospermia in normal men. Contraception 45 :439-451.
Tiemessen, C.H.J., Evers, J.L. & Bots, R.S.G.M. (1995) Tight fitting underwear and sperm quality. Lancet 347 :1844-1845.