Testicles that feel cool to the touch are completely normal and, in fact, a sign that your body is doing exactly what it should. The testes sit outside the body cavity for a reason: they need to stay a few degrees cooler than your core temperature of about 37 °C (98.6 °F) to produce sperm properly. That cooler-than-expected feeling you notice in the shower, after a swim, or on a chilly morning is your body’s thermoregulation system working as designed. The sensation becomes more interesting once you understand the surprisingly sophisticated machinery behind it.
Why Testicles Need to Be Cool in the First Place
Sperm cells are finicky about heat. The process of creating them, which takes roughly 64 to 72 days per cycle, depends on a temperature window a few degrees below what the rest of your organs operate at. When the testes get too warm, sperm production slows down and sperm quality drops. This isn’t a minor preference; above-normal temperatures are consistently detrimental to sperm across species.1PubMed Central. The Physiological and Evolutionary Ecology of Sperm Thermal Performance The scrotum evolved specifically to hold the testes away from the body’s warmth, and virtually all mammals share this arrangement.2PubMed Central. Lifestyle impact and the biology of the human scrotum
So when you feel that your testicles are cold, what you’re really feeling is the result of an ancient biological compromise. Your body runs hot enough to fight infection and sustain metabolism, but your sperm factories can’t handle all that heat. The solution: hang them outside the body in a temperature-controlled sac. That “cold” sensation is simply the difference between your core (around 37 °C) and the scrotal target (roughly 33–35 °C).
How Your Body Keeps Them Cool
The cooling system isn’t passive. Your scrotum has multiple active mechanisms working in concert, and understanding them explains why your testicles can feel noticeably cold at certain moments and warmer at others.
The most visible mechanism is the cremaster muscle, which raises and lowers the testes relative to the body. When you’re cold, the cremaster contracts and pulls the testicles closer to the abdomen for warmth. When you’re warm, it relaxes and lets them hang lower, increasing the distance from body heat and the surface area exposed to air. You’ve seen this in action every time you’ve gotten into cold water and noticed everything tighten up.
Just beneath the scrotal skin sits a thin layer of smooth muscle called the tunica dartos. This muscle responds directly to temperature. When the scrotum cools, dartos muscle fibers contract, wrinkling the skin and reducing the surface area to conserve heat. Research has shown that this muscle responds to cooling from about 33 °C down to 20 °C by increasing its tension, and a cold-sensitive receptor called TRPM8 plays a key role in maintaining that cold-induced muscle tone.3PubMed Central. Temperature-dependent contractility of rat tunica dartos muscle: Contribution of cold, menthol-sensitive TRPM8 TRPM8, if you’ve ever used menthol cream, is the same receptor that makes menthol feel cool on your skin. The scrotal version uses it to sense temperature changes and fine-tune muscle contraction accordingly.
The third and perhaps cleverest mechanism is a built-in heat exchanger in the spermatic cord. The testicular artery, which carries warm blood from the aorta down to the testes, is coiled and wrapped inside a mesh of veins called the pampiniform plexus. Cooler venous blood returning from the testes absorbs heat from the incoming arterial blood before it ever arrives. In rams, where this has been measured precisely, arterial blood cools by about 5 °C as it passes through this exchanger, and the venous blood warms by a similar amount on the way back.4Journal of Reproduction and Fertility. Relation of Vascular Heat Exchange to Temperature Regulation in the Testis of the Ram The testicular artery is spiraled and completely surrounded by this venous network, making the arrangement a remarkably efficient countercurrent cooling system.5PubMed. Morphology and innervation of a testicular ‘rete mirabile’ in the guinea-pig In humans, the same basic architecture exists: the pampiniform plexus absorbs heat from the testicular arteries, helping to pre-cool blood before it reaches the testes.6PubMed Central. Presence of Arteriovenous Communication between Left Testicular Vessels and Its Clinical Significance
When the Sensation Gets More Noticeable
Some situations make the cold sensation more pronounced. Cold weather is the obvious one. During cold exposure, the dartos muscles contract and the cremaster pulls the testes upward, but scrotal temperature still drops because there’s only so much insulation a thin sac of skin can provide.7PubMed Central. Regulation of ram scrotal temperature during heat exposure, cold exposure, fever and exercise You feel the cold more acutely because the gap between ambient air and your scrotal skin has widened.
Physical activity also plays a role. Walking produces lower scrotal temperatures than sitting, likely because the swinging motion increases air circulation around the scrotum.8Human Reproduction. Influence of the type of undertrousers and physical activity on scrotal temperature If you’ve ever noticed that your testicles feel cooler after a walk on a brisk day, that combination of movement and cold air is the reason. Conversely, long periods of sitting, especially with your legs together, trap heat and push scrotal temperatures upward. The effects of daily activities on scrotal temperature are real but not always well understood.9PubMed. The relation between daily activities and scrotal temperature
What you wear matters, too. Tight-fitting underwear holds the scrotum closer to the body and restricts airflow, producing higher scrotal temperatures. Loose-fitting boxers or going commando both allow more cooling. A large study at a fertility center found that men who most frequently wore boxers had higher sperm concentration and total sperm count compared to men who didn’t, consistent with the idea that looser underwear keeps things cooler.10PubMed Central. Type of underwear worn and markers of testicular function among men attending a fertility center That same type-of-undertrousers study confirmed that tight-fitting underwear was associated with higher scrotal temperatures than loose-fitting or no underwear, across different activities.8Human Reproduction. Influence of the type of undertrousers and physical activity on scrotal temperature
Is There Such a Thing as Too Cold?
Here’s where people tend to worry unnecessarily. Under everyday conditions, your testicles aren’t going to get dangerously cold. Walking around on a winter day or sleeping without covers in a cool room will make them feel chilly, but the cremaster and dartos muscles are continuously adjusting to prevent the temperature from dropping to a point where tissue damage could occur. Your body treats testicular temperature regulation as a high priority.
Extreme and prolonged cold exposure is a different story, but it would have to be genuinely extreme, the kind of cold that puts your fingers and toes at risk of frostbite. The scrotum has a good blood supply, and the same countercurrent heat exchanger that cools incoming blood can also warm it when the environment is very cold. The system works in both directions.
That said, cold weather does have one clinically relevant connection to testicular health: torsion. Testicular torsion occurs when a testicle twists on the spermatic cord, cutting off its blood supply. It’s a medical emergency. A retrospective study in a tropical climate found a significant association between cold-weather months and higher rates of testicular torsion. Roughly 73% of cases occurred during fall and winter, when mean temperatures were lowest.11PubMed Central. Cold weather is a predisposing factor for testicular torsion in a tropical country. A retrospective study The suspected mechanism is that cold triggers vigorous cremaster contraction, and in boys or men whose testes are anatomically predisposed to torsion (often due to a loose attachment called a “bell-clapper deformity”), that contraction can initiate the twist. The cold itself doesn’t cause torsion in normal anatomy, but it can be a trigger in someone already susceptible.
The takeaway: if your testicles simply feel cool, that’s fine. If one testicle suddenly becomes extremely painful, swollen, or sits at an unusual angle, especially in cold weather, that’s not a temperature problem; it’s potentially torsion, and you need emergency care.
When Abnormal Temperature Might Signal Something Else
While cold testicles are almost always normal, warmer-than-expected scrotal temperatures can sometimes point to an underlying issue. A varicocele, which is an enlargement of the veins inside the scrotum (similar to varicose veins in the leg), disrupts the pampiniform plexus heat-exchange system. Research has shown that varicoceles cause a statistically significant bilateral elevation of both testicular blood flow and temperature compared to controls, and that surgical repair returns those values to normal.12PubMed. Varicocele: reversal of the testicular blood flow and temperature effects by varicocele repair If you notice that one side of your scrotum feels consistently warmer than the other, or you can feel a soft mass of enlarged veins (often described as a “bag of worms”), it’s worth mentioning to a doctor. Varicoceles affect roughly 15% of adult men and are the most common correctable cause of male infertility.
General cold sensitivity throughout your body, not just in the scrotum, could be a sign of hypothyroidism. An underactive thyroid reduces your body’s ability to generate heat in response to cold. When thyroid hormone levels are restored to normal, cold-induced heat production roughly doubles.13PubMed Central. Resolution of Hypothyroidism Restores Cold-Induced Thermogenesis in Humans If your cold testicles are accompanied by fatigue, weight gain, dry skin, and feeling cold everywhere, thyroid function is worth checking. But if the cold sensation is limited to the scrotum and happens in situations where cooling is expected, there’s no reason to suspect a systemic problem.
Cold Testicles and Fertility
Given how central temperature is to sperm production, a reasonable follow-up question is whether cold temperatures might actually improve fertility. The relationship turns out to be more nuanced than “colder is always better.”
Heat is clearly the bigger threat. Elevated scrotal temperatures from laptops, hot tubs, saunas, prolonged sitting, or fever have all been linked to reduced sperm quality. But extreme cold isn’t necessarily helpful either. A large multi-center study of over 33,000 men found an inverted U-shaped relationship between temperature anomalies and sperm quality. During hot seasons, anomalous heat was negatively associated with sperm concentration and motile sperm count. But anomalous cold during the cold season was also negatively associated with sperm motility.14PubMed Central. Effects of temperature anomaly on sperm quality: A multi-center study of 33,234 men In other words, there appears to be a sweet spot, and falling too far below it isn’t beneficial either.
Animal research paints a consistent picture. In mice, brief mild cold exposure (one minute) significantly increased total sperm motility compared to controls. But severe cold exposure for longer durations reduced progressive motility, the kind of forward-swimming movement that matters most for reaching an egg.15Fertility and Sterility. Acute cold exposure differentially affects sperm parameters in mice Moderate cooling helps; drastic cooling does not.
This is why researchers have explored scrotal cooling devices for men with fertility problems. In men with low sperm counts due to undescended testes (corrected in childhood), nighttime scrotal cooling using a gentle air stream lowered scrotal temperature by about 0.8 °C and produced significant increases in sperm concentration and total count after 8 to 12 weeks.16PubMed. Improvement of semen quality by nocturnal scrotal cooling in oligozoospermic men with a history of testicular maldescent Another study of scrotal cooling devices reported significant improvements in sperm motility and vitality, though it also noted that nearly a third of users found the devices uncomfortable, and about a fifth said the devices interfered with daily activities.17PubMed Central. Evaluation of patient compliance with the use of scrotal cooling devices The evidence here is still limited, but the principle is sound: for men whose scrotal temperatures run higher than ideal, modest cooling can improve things.
Why Mammals Evolved External Testes at All
It seems like a design flaw to have such a critical organ dangling outside the body, vulnerable to injury and temperature swings. Evolutionary biologists have debated this for over a century, and the answer involves more than just sperm. One prominent hypothesis frames external testes as a secondary adaptation to warm-bloodedness. When mammals evolved endothermy and maintained a steady internal temperature around 37 °C, sperm production struggled to keep up in the hotter environment. Natural selection favored individuals whose testes sat outside the body, where a few degrees of cooling restored healthy sperm output.18PubMed. The evolutionary history of testicular externalization and the origin of the scrotum
But the sperm-cooling hypothesis isn’t the whole story. A more recent analysis proposes that external testes also help regulate testosterone production. The stress-response pathway that kicks in during fight-or-flight situations can stimulate testosterone production via adrenaline-like signaling. At core body temperature, this stimulation is stronger. By housing the testes at a lower temperature, the body may temper testosterone production to prevent excessive masculinization of male offspring.19PubMed Central. Reappraising the exteriorization of the mammalian testes through evolutionary physiology Birds, which also run warm but keep their testes internal, avoid this problem through a different anatomical setup of their adrenal tissue. So the external scrotum may serve a dual role: protecting sperm from heat and preventing hormonal overproduction.
How Scrotal Temperature Is Actually Measured
If you’re curious how researchers study this topic, you might picture awkward thermometers, but the modern standard is infrared thermography. A thermal camera can read scrotal surface temperature without touching the skin, and studies have shown that surface readings correlate tightly with temperatures deeper in the tissue. In rams, scrotal surface temperature correlated with subcutaneous temperature at r = 0.95 and with deep testicular temperature at r = 0.91, both highly significant.20PubMed. Relationship of scrotal surface temperature measured by infrared thermography to subcutaneous and deep testicular temperature in the ram This non-contact approach has made it much easier to study how temperature fluctuates with posture, clothing, activity, and environment.
Some fertility clinics now use thermographic imaging as part of evaluations, especially when investigating varicoceles or assessing men who work in heat-intensive occupations. For everyday purposes, though, there’s no reason to measure your scrotal temperature. If the system is working, you feel cool. If something is wrong, the symptoms (pain, swelling, a palpable mass of veins, or systemic cold intolerance with other thyroid symptoms) are usually more informative than a number on a thermometer.
Asymmetry in Scrotal Temperature
One detail that catches some men off guard: the two sides of the scrotum aren’t always the same temperature. The left testicle typically hangs a bit lower than the right, and its venous drainage follows a slightly different path (the left testicular vein empties into the renal vein at a steep angle, while the right drains more directly into the large vena cava). This anatomical difference is part of why varicoceles are far more common on the left side, and it can also produce subtle temperature differences between the two testes. Studies measuring scrotal temperature by side have documented slightly different readings on the right versus left, with the right side tending to be marginally cooler during walking.8Human Reproduction. Influence of the type of undertrousers and physical activity on scrotal temperature
If you notice one testicle feels slightly cooler or hangs differently, that’s within the range of normal human asymmetry. A sudden, dramatic temperature difference between the two sides, especially with swelling or pain, is a different situation and warrants medical attention. But the mild asymmetry that most men notice is just anatomy doing its thing, unremarkable and harmless.