Because they need to be cooler than the rest of you. The testicles run a few degrees below core body temperature, and the temperature-sensitive steps that make testosterone and sperm only work properly inside that margin.
The Scrotum Is a Cooling System
The external placement is not an oversight in the design. The scrotum's whole job is to hold the testicles below core body temperature, and every part of it is built for that one task [1].
The skin is thin and sweats readily, which sheds heat by evaporation the same way sweat cools the rest of you.
Underneath it, a thin sheet of muscle pulls the testicles closer to the body when the air is cold and lets them drop away when it is warm, which changes how much surface area is exposed and how far they sit from your core [1].
It is a thermostat with a small range. It handles a warm afternoon, and it does not handle everything a modern day puts in front of it.
What the Cooler Temperature Is For
Two jobs run inside that margin. Leydig cells, which sit in the testicles and make the vast majority of a man's testosterone, rely on temperature-sensitive enzymes to turn cholesterol into the hormone.
And spermatogenesis, the process that makes sperm, runs through its own series of steps that work best below core temperature [2].
Push the temperature up and both slow down. Heat stress on those cells is well established, and on the sperm side it shows up as lower counts and worse movement [3]. Even a modest rise across an ordinary day tracks with poorer semen quality [4].
What Warms Them Up in Ordinary Life
Mostly the dull stuff. Sitting with a working laptop on your thighs measurably raises scrotal temperature, and closing your thighs together does a good part of that on its own, before the machine contributes anything [5].
A long drive does the same [6]. So does getting dressed: in every posture researchers tested, being clothed ran warmer than being naked [7].
None of these is dramatic. They are just constant, which is the problem. The system was built to hold against a warm day, not against eight hours in a chair.
Deliberate Heat Is the Sharper Version
A sauna is the same problem with the volume up. Whole-body heat is genuinely good for men, and the men in the Finnish research who used a sauna most often were about 40 percent less likely to die of any cause than the men who went once a week [8].
But the testicles are a part that does better staying cool. Sauna exposure measurably affects spermatogenesis, and the effect reverses once the heat stops [9].
That doesn't mean skip the sauna. The full cycle from stem cell to mature sperm takes about 74 days [9], so recovery runs on that clock rather than overnight, which is an argument for managing the temperature during the session instead of giving up the session.
What to Wear Against It
Oakman is built specifically for active men looking for scrotal cooling all day and want the option of a cold pack for cooling in the sauna or after heat exposure. The boxer briefs are built around that one problem. The removable cold pack sits in a medical-grade nylon shell in a front pocket during a sauna session or another seated stretch of heat, with a layer of soft fabric between it and the skin, cooling the testicles directly while you sit there.
The fabric is TENCEL MicroModal, naturally derived from wood pulp rather than plastic, blended with ROICA V550, a Japanese clean stretch fiber, rather than conventional spandex. Its fibers run about 20 percent finer than standard modal, and it doesn't shed microplastics like polyester underwear [10]. Dyes are bluesign approved, so what touches your skin is controlled all the way through the supply chain.
The fit is athletic with a gusset, an extra panel that gives the testicles room and airflow.
Join the Founding 100
Oakman is sauna cooling for men, launching early fall 2026. Founding members get first access and founding pricing locked in. Reserve your spot.
Frequently asked questions
How much cooler do the testicles need to be than the rest of the body?
A few degrees below core body temperature. The scrotum holds them there [1], and the temperature-sensitive steps of sperm and testosterone production run best inside that margin. Close it with heat, posture or tight clothing and those steps slow down [3].
Does what you wear really affect testicular temperature?
Yes. In every posture researchers tested, being clothed ran warmer than being naked [7]. Fabric and fit are among the few parts of this you actually control.
Can the body cool the testicles on its own?
Up to a point. The scrotal skin sweats, and a thin sheet of muscle raises the testicles toward the body when it is cold and lets them drop when it is warm [1]. That range is small, and it does not hold up against sustained heat from clothing, posture or a hot room [7].
How long do the testicles take to recover from heat exposure?
The effect of heat on sperm production is reversible. The full cycle from stem cell to mature sperm takes about 74 days, so recovery runs on that clock rather than overnight [9].
This is for general educational and informational purposes only and is not medical advice. It makes no claim about fertility, sperm, or testosterone outcomes from any product. Always talk with a qualified healthcare provider before making changes to your diet, exercise, or health routine. These statements have not been evaluated by the Food and Drug Administration. Oakman products are apparel and are not intended to diagnose, treat, cure, or prevent any disease.
References
- Ivell R. (2007). Lifestyle impact and the biology of the human scrotum. Reproductive Biology and Endocrinology (BioMed Central). [Link]
- Cai H, Qin D, Peng S. (2021). Responses and coping methods of different testicular cell types to heat stress: overview and perspectives. Bioscience Reports. [Link]
- Durairajanayagam D, Agarwal A, Ong C. (2015). Causes, effects and molecular mechanisms of testicular heat stress. Reproductive BioMedicine Online. [Link]
- Hjollund NHI, et al.. (2002). Impact of diurnal scrotal temperature on semen quality. Reproductive Toxicology. [Link]
- Sheynkin Y, Jung M, Yoo P, Schulsinger D, Komaroff E. (2005). Increase in scrotal temperature in laptop computer users. Human Reproduction (Oxford University Press). [Link]
- Bujan L, Daudin M, Charlet J-P, Thonneau P, Mieusset R. (2000). Increase in scrotal temperature in car drivers. Human Reproduction (Oxford University Press). [Link]
- Mieusset R, Bengoudifa B, Bujan L. (2007). Effect of Posture and Clothing on Scrotal Temperature in Fertile Men. Journal of Andrology (American Society of Andrology). [Link]
- Laukkanen T, Khan H, Zaccardi F, Laukkanen JA. (2015). Association Between Sauna Bathing and Fatal Cardiovascular and All-Cause Mortality Events. JAMA Internal Medicine. [Link]
- Garolla A, et al.. (2013). Seminal and molecular evidence that sauna exposure affects human spermatogenesis. Human Reproduction. [Link]
- Napper IE, Thompson RC. (2016). Release of synthetic microplastic plastic fibres from domestic washing machines: Effects of fabric type and washing conditions. Marine Pollution Bulletin (Elsevier). [Link]
