Why Are Testicles Wrinkly? The Biological Reason

The wrinkled appearance of the scrotum comes from a thin sheet of smooth muscle called the tunica dartos, which sits just beneath the scrotal skin and contracts or relaxes in response to temperature changes. When the dartos muscle contracts, it pulls the skin into folds and ridges, drawing the testicles closer to the body for warmth. When it relaxes, the scrotum hangs lower and smoother, letting heat escape. This constant adjustment exists because sperm production is remarkably sensitive to temperature, and the wrinkling is one piece of an elaborate cooling system that most people never think twice about.

The Muscle Behind the Wrinkles

The tunica dartos is not like the muscles you flex at the gym. It is a smooth muscle, meaning it operates involuntarily, without conscious control. It forms a continuous layer just under the scrotal skin, and its contraction is what gives the scrotum its characteristic puckered texture.¹ The dartos is innervated by the sympathetic nervous system, making it part of the same fight-or-flight circuitry that governs heart rate and blood vessel constriction.²

There is a second muscle involved in testicular positioning, the cremaster, but it works differently. The cremaster is a skeletal muscle that wraps around the spermatic cord and physically lifts or lowers each testicle within the scrotum. It is the muscle responsible for the “jump” you might notice when cold water hits or when the inner thigh is stroked. The dartos, by contrast, does not move the testicles themselves so much as it reshapes the skin around them. Together, the two muscles give the scrotum its dynamic behavior: the cremaster adjusts testicular height, while the dartos adjusts how tightly the skin is gathered.²

Physical exercise also triggers dartos and cremaster contraction, even without a temperature change. During running, for instance, increased sympathetic nervous system activity raises adrenaline and noradrenaline levels in the blood. The dartos muscle contracts in response to noradrenaline, which is why the scrotum often tightens noticeably during a workout.³ The result is a temporary increase in scrotal skin temperature as the testes are pulled closer to the warm body core, though this typically resolves once exercise stops.

Why Temperature Control Matters for Sperm

Sperm production, or spermatogenesis, is one of the most temperature-sensitive biological processes in the human body. The testes need to stay roughly 2 to 6 degrees Celsius below core body temperature for sperm to develop properly.⁴ In humans, this usually means keeping the testes around 33 to 35°C rather than the body’s internal 37°C.⁵

This is not a small margin. When scrotal temperatures climb even modestly above this range for extended periods, the consequences for sperm are measurable. Studies on sustained scrotal heat stress in men have found significant drops in sperm concentration, motility, and the percentage of normally shaped sperm. DNA fragmentation, a type of genetic damage inside sperm cells, also increases substantially during and after heat exposure.⁶ Animal studies have confirmed that the germ cells developing inside the testes at the moment of heat exposure are the most vulnerable, with spermatids showing higher DNA damage than cells at earlier stages of development.⁷

The wrinkling of the scrotum is essentially the body’s first line of defense against this kind of damage. By contracting the dartos, the body reduces the surface area of the scrotum exposed to the outside environment and pulls the testes up against the warmth of the lower abdomen. By relaxing it, the body maximizes the surface area available for heat to radiate away. The wrinkles are not just an incidental quirk of anatomy; they are the visible evidence of active thermal management.

The Vascular Cooling System You Cannot See

The dartos muscle gets all the visible credit, but there is a less obvious cooling mechanism at work inside the spermatic cord that matters just as much. The testicular artery, which carries warm blood from the body down to the testes, is tightly coiled and surrounded by a dense network of small veins called the pampiniform plexus. This arrangement functions as a countercurrent heat exchanger: warm arterial blood flowing toward the testes loses heat to the cooler venous blood flowing back toward the body.⁸

The effect is that blood arriving at the testes has already been pre-cooled before it even gets there. The returning venous blood absorbs heat conveyed by the arterial supply and carries it away.⁹ This vascular heat exchanger works in concert with the dartos muscle: the muscle controls how much surface area the scrotum presents to the outside air, while the pampiniform plexus handles internal temperature tuning within the blood supply itself.

Sweating From Below

The scrotum also has an unusually high density of sweat glands compared to most other skin surfaces. Research on boars found that the scrotal skin had a higher concentration of sweat glands than the leg or torso, and that evaporative moisture loss from the scrotal surface serves as a meaningful heat-loss mechanism.¹⁰ The wrinkled texture of the dartos-contracted skin may actually help here: the folds increase the effective surface area for evaporation in much the same way that intestinal villi increase the surface area for nutrient absorption.

Seasonal variation in scrotal sweat gland activity has been observed in some species. Camels, for example, show sweat gland density in the scrotum that tracks with environmental heat load, with activity increasing during hotter months.¹¹ While human scrotal physiology has not been studied as extensively in this regard, the general principle holds: the scrotum is built to lose heat, and its gland-rich, wrinkle-prone skin reflects that function.

How Clothing and Posture Shift the Balance

If the wrinkled scrotum is the body’s thermostat for sperm, modern life throws a lot of interference at it. Body position and clothing both have measurable effects on scrotal temperature. Standing naked produces the coolest scrotal temperatures, while sitting with legs crossed pushes temperatures up by an amount comparable to putting clothes on in the first place.¹² Clothing in any position raises scrotal temperature compared to the naked state, regardless of posture.

The type of underwear matters more than most people assume. In one researcher’s detailed self-study, deep scrotal temperatures were about half a degree Celsius lower in boxer shorts than in briefs, and a further 1.2°C lower in specially designed open-front underwear than in boxers. Running actually lowered scrotal temperature by about half a degree compared to sitting with thighs apart, likely because of increased airflow.¹³ These differences sound small, but they are meaningful in the context of the narrow 2 to 6°C window the testes require.

For anyone actively trying to conceive, the practical implication is straightforward: avoid prolonged sitting with legs pressed together, choose looser-fitting underwear, and be mindful of extended exposure to heat sources like laptops on the lap or hot tubs. The dartos muscle can only do so much to compensate when external conditions work against it.

When the Thermostat Breaks

One of the most common clinical disruptions to scrotal thermoregulation is a varicocele, an abnormal enlargement of veins within the scrotum. Varicoceles affect a significant percentage of men and are a well-recognized cause of subfertility. The problem is fundamentally about heat: the enlarged veins impair the countercurrent heat exchange system, causing scrotal temperature to rise above the normal range.¹⁴

The elevated temperature does not just passively inconvenience sperm production. It triggers oxidative stress, which in turn drives germ cell death through programmed cell destruction pathways. The heat and oxidative stress are closely linked: as scrotal temperature rises, so do markers of cellular damage and germ cell apoptosis.¹⁵ Mathematical modeling of varicocele patients shows that the disease alters heat exchange among the scrotal skin layers and the deeper testicular structures, and that even asymptomatic varicoceles can produce measurably higher baseline scrotal temperatures.¹⁶ In these cases, the dartos muscle may contract normally, but the underlying vascular problem means temperature control is compromised from within.

Why Testes Ended Up Outside the Body

The question of why mammalian testes are external in the first place has a surprisingly contentious history in evolutionary biology. The most common explanation is the “cooling hypothesis,” which says that spermatogenesis evolved to function best at cooler temperatures, and the scrotum evolved to provide that environment. But there is a chicken-and-egg problem: did sperm become temperature-sensitive first, forcing the testes outward? Or did the testes move outward for other reasons, and then sperm adapted to the cooler environment?

One line of evidence suggests the scrotum evolved independently at least twice in mammalian history, once in marsupials and once in the group that includes most placental mammals. Multiple lineages have subsequently lost the scrotum, suggesting it is not universally indispensable. In mammals that have lost external testes, like elephants and some marine mammals, alternative mechanisms for managing testicular temperature appear to have evolved.¹⁷ The pattern suggests multiple selection pressures at work: temperature is one, but protection of the gonads from physical or physiological disturbance is another.

An alternative theory proposes that testicular descent was driven not by temperature sensitivity of sperm, but by the need to temper testosterone production. In warm-blooded mammals, the stress-response pathway involving adrenaline can stimulate the testes to overproduce testosterone. Moving the testes to a cooler external location dials down this stimulation, avoiding excessively high testosterone that could lead to problems during fetal development. Birds, which keep their testes internal, do not exhibit this same adrenaline-driven stimulation of testicular testosterone production because their adrenal gland anatomy is structured differently.¹⁸

Whatever the original evolutionary pressure, the scrotum and its wrinkled architecture appear to have emerged early in mammalian evolution, likely alongside the development of warm-bloodedness. One analysis placed the origin of the scrotum before the divergence of modern mammals, with many lineages subsequently losing it when they evolved alternative solutions to the temperature problem.¹⁹ The wrinkled skin, then, is a feature with deep evolutionary roots, not a quirk of recent human anatomy.

How the Scrotum Forms Before Birth

The scrotum’s development in the womb offers another clue to its wrinkled design. Early in fetal development, male and female genitalia begin from the same precursor structures. In males, the bilateral genital folds fuse along the midline, forming the scrotal raphe, the visible seam that runs from the underside of the penis down the middle of the scrotum and continues toward the anus. At ten weeks, this raphe is thin and wavy. Over the following weeks, as the underlying muscles and spongy tissue develop, the raphe thickens and straightens.²⁰

The dartos muscle develops as an integral part of this fusion process, embedding itself beneath the scrotal skin from early in development. The scrotum does not become wrinkled because the skin stretches and sags over time, the way facial wrinkles form. Instead, the wrinkling is built into the anatomy from the start, a consequence of the dartos muscle being physically woven into the skin layer. Even in newborns, the scrotum shows its characteristic texture.

Aging, Appearance, and What Cosmetic Medicine Is Doing About It

While the wrinkled appearance of the scrotum is functional, not everyone is content with it, particularly as aging adds laxity that goes beyond what the dartos muscle was designed to manage. Like skin elsewhere on the body, scrotal skin loses collagen and elasticity with age, and the dartos muscle itself weakens. The result is a scrotum that hangs lower and wrinkles more prominently than it did in earlier decades.

This has spawned a niche within cosmetic urology sometimes referred to as scrotal rejuvenation. Interventions include surgical procedures to reduce laxity, and there has been speculation about injecting botulinum toxin into the dartos muscle to reduce wrinkling, essentially paralyzing the muscle that causes the folds.²¹ The irony is worth noting: the same muscle contraction that exists to protect fertility is the cosmetic “problem” these procedures aim to diminish. For men considering such treatments, the thermoregulatory trade-off is something to discuss with a urologist rather than a cosmetic surgeon alone.

Broader research on how men perceive their genital appearance suggests that dissatisfaction tends to center on size rather than texture, with studies of large populations finding that nearly half of cisgender men wished for a larger penis, a concern amplified by media and social comparison.²² Scrotal wrinkling, by contrast, rarely registers as a top concern in survey data, though the emergence of dedicated cosmetic procedures indicates that cultural attention to it is growing. For most people, the wrinkles are simply the visible sign that a quietly sophisticated thermoregulatory system is doing exactly what it was built to do.