The scrotum is a pouch of skin and muscle that hangs below the penis and houses the testes, the epididymides, and the lower portions of the spermatic cords. Its location outside the body cavity is not accidental. The entire structure exists, in large part, to keep the testes cooler than the rest of the body, because sperm production fails at core body temperature. But the scrotum is more than a passive bag: it contains layers of muscle and connective tissue that actively regulate testicular temperature, and its development, nerve supply, and evolutionary history are all more intricate than most people realize.
What the Scrotum Is Made Of
If you could peel the scrotum apart layer by layer, you would find it is not one tissue but several nested ones. The outermost layer is regular skin, thinner and more pigmented than most skin on the body, with scattered hair follicles and abundant sweat glands. Just beneath the skin sits a thin sheet of smooth muscle called the dartos muscle, which is fused into the subcutaneous tissue. Unlike the muscles you consciously flex, the dartos operates automatically. When it contracts, the scrotal skin wrinkles and tightens; when it relaxes, the scrotum hangs loose and smooth. The dartos also forms the scrotal septum, a midline wall that divides the interior into left and right compartments, one for each testis.
Deeper than the dartos, each testis is wrapped in its own set of coverings that it acquired during its descent from the abdomen. These include layers of connective tissue and the cremaster muscle, a thin set of skeletal-muscle fibers that loop around the spermatic cord and testis. Unlike the dartos, the cremaster is a voluntary-type muscle, though most of its action is reflexive. If you stroke the inner thigh, the cremaster on that side contracts and pulls the testis upward, a response doctors test during neurological exams. Together, the dartos and the cremaster give the scrotum a surprising degree of active control over where the testes sit at any given moment.
The innermost covering is the tunica vaginalis, a thin membrane that forms a closed sac around most of each testis. A small amount of fluid between its two layers lets the testis glide freely. When that fluid accumulates abnormally, the result is a hydrocele, a painless swelling caused by lymphatic or fluid-drainage problems.1PubMed Central. Classifying Hydroceles of the Pelvis and Groin: An Overview of Etiology, Secondary Complications, Evaluation, and Management
Why the Testes Need to Be Cooler
Sperm production, or spermatogenesis, is exquisitely sensitive to temperature. In humans, optimal spermatogenesis occurs at roughly 34 to 35 degrees Celsius, which is about 2 to 3 degrees below normal core body temperature.2PubMed Central. Effect of Temperature on the Development of Stages of Spermatogenesis and the Functionality of Sertoli Cells In Vitro Some estimates place the required temperature drop as high as 6 degrees, depending on the stage of sperm development being measured.3PubMed. The effects and molecular mechanism of heat stress on spermatogenesis and the mitigation measures Either way, the scrotum’s external position is what makes this temperature gap possible. The thin scrotal skin radiates heat efficiently, and the sweat glands on the surface provide evaporative cooling.
When the testes are not exposed to this cooler environment, fertility suffers. Cryptorchidism, where one or both testes fail to descend into the scrotum, is a well-known cause of reduced sperm counts. Analysis of patients with undescended testes shows that even mild warming can damage both the stem cell population in the testes and the later stages of sperm maturation.4PubMed Central. Lifestyle impact and the biology of the human scrotum Varicoceles, enlarged veins within the scrotum that raise local temperature, can have a similar effect on sperm quality.
The Built-In Heat Exchanger
Hanging outside the body is only part of the cooling story. The scrotum also benefits from a sophisticated vascular trick inside the spermatic cord, the rope-like structure that connects each testis to the abdomen. Warm arterial blood flows down to the testis through the testicular artery, while cooler venous blood returns upward through a mesh of small veins called the pampiniform plexus. These vessels run in close contact with each other, and heat passes from the incoming arterial blood to the outgoing venous blood before it ever reaches the testis. This is a countercurrent heat exchanger, the same basic principle used by the flippers of whales and the legs of wading birds to conserve or shed heat.
The system is remarkably efficient. In rams, most of the arterial cooling happens in the coiled portion of the testicular artery where it is surrounded by the pampiniform plexus, and the venous blood warms by a corresponding amount.5Journal of Reproduction and Fertility. Relation of Vascular Heat Exchange to Temperature Regulation in the Testis of the Ram Experiments in bulls found that the mean efficiency of this heat transfer reached about 91 percent under controlled conditions, meaning very little of the arterial warmth actually makes it all the way to the testis without being shunted back.6PubMed. Efficiency of the countercurrent transfer of heat and 133Xenon between the pampiniform plexus and testicular artery of the bull under in-vitro conditions The transfer appears to work mainly by diffusion between the closely packed vessels rather than through any direct connections between arteries and veins.7PubMed Central. Presence of Arteriovenous Communication between Left Testicular Vessels and Its Clinical Significance
How the Scrotum Responds to Exercise and Cold
You have probably noticed that the scrotum looks very different depending on conditions. In cold air or cold water, it tightens dramatically, pulling the testes close to the body. In warm conditions, it hangs low and loose. This is not passive stretching. The dartos muscle in the scrotal wall and the cremaster muscle around each spermatic cord are actively adjusting testicular position to maintain the right temperature window.
A 2025 study that measured scrotal skin temperature during running in unclothed men found that temperature actually rose at the onset of exercise. The researchers attributed this to contraction of both the cremaster and dartos muscles, which pulled the testes closer to the warm body. The sympathetic nervous system activates during exercise, raising levels of adrenaline and noradrenaline in the blood, and the dartos muscle contracts in response to noradrenaline. On top of that, the mechanical stimulation of the inner thigh during running may trigger the cremasteric reflex, compounding the effect.8PubMed Central. Scrotal temperature change during running in naked humans In other words, during vigorous activity the body temporarily prioritizes protecting the testes from bouncing around over keeping them cool, even though the temperature briefly rises as a result.
Once exercise continues and the body begins sweating more broadly, the scrotal skin’s sweat glands and the increased airflow help dissipate that heat. The interplay between muscle contraction for physical protection and relaxation for cooling is constant and mostly invisible to the person who owns the scrotum.
How the Testes Get There in the First Place
The testes do not start out in the scrotum. During fetal development, they form near the kidneys, high in the abdomen. Over the course of gestation, they migrate downward in a two-phase journey. In the first phase, a ligament called the gubernaculum swells and anchors the developing testis near the groin. In the second phase, the gubernaculum actively migrates out through the abdominal wall, essentially pulling the testis down into the scrotum.9PubMed Central. The role of the gubernaculum in the descent and undescent of the testis
As the testis descends, it pushes through a sleeve of tissue called the processus vaginalis, which is an outpouching of the abdominal lining. The smooth muscle content of this sleeve appears to play a role in propelling the testis through it. If that muscle content is reduced or disrupted before descent is complete, the testis may fail to reach the scrotum, resulting in cryptorchidism.10PubMed Central. Obliteration of the Processus Vaginalis After Testicular Descent After descent, the processus vaginalis normally closes off, leaving only the tunica vaginalis around the testis. When it fails to close, fluid from the abdomen can trickle down and collect around the testis, causing a communicating hydrocele, one of the more common reasons for scrotal swelling in infants.
The Scrotal Raphe and What It Tells You
The visible line that runs down the center of the scrotum, continuing forward along the underside of the penis and backward toward the anus, is called the perineal raphe. Many people assume it is a scar, but it is a normal seam formed during embryonic development. Early in gestation, the genital structures of males and females are identical. In males, rising androgen levels cause the labioscrotal folds to fuse in the midline, forming the scrotum. The raphe is the external trace of that fusion.
Histological studies of human fetuses show that this midline fusion connects to a deeper intermuscular septum, and by about 12 to 15 weeks of gestation the raphe becomes thickened and more defined as underlying structures like the bulbospongiosus muscle develop.11PubMed Central. Perineal raphe with special reference to its extension to the anus: a histological study using human fetuses The raphe varies in prominence from person to person. In some men it is barely visible; in others it is a raised, darkly pigmented ridge. Neither extreme is abnormal. It is simply a reflection of how strongly the tissue fused and how much pigment was deposited along the seam.
Why the Scrotum Evolved at All
Given that external testes are vulnerable to injury, cold, and all sorts of inconveniences, why did mammals evolve a scrotum in the first place? The question has driven decades of debate, and the answer turns out to be less straightforward than “sperm need to be cool.” While that is true, it raises another question: why did sperm become heat-sensitive to begin with?
One phylogenetic analysis argues that the scrotum likely evolved before modern mammals fully diversified, in concert with the evolution of endothermy (warm-bloodedness). As body temperature rose in early mammalian ancestors, the existing temperature sensitivity of sperm production became a problem that external testes solved. Under this view, the real puzzle is not why so many mammals have a scrotum but why some have lost it. Elephants, some marine mammals, and certain insectivores keep their testes inside the body. These lineages appear to have evolved alternative solutions, such as lower core temperatures near the testes or heat-resistant sperm, that made the costly process of testicular descent unnecessary.12PubMed. The evolution of the scrotum and testicular descent in mammals: a phylogenetic view
A separate analysis points to multiple selection pressures at work: the need for proper sperm development and storage, yes, but also the need to protect the gonads from physiological disturbance such as abdominal pressure changes during locomotion.13PubMed. The evolutionary history of testicular externalization and the origin of the scrotum Yet another proposal focuses on hormones rather than sperm. It suggests that testicular exteriorization was partly an adaptation to temper testosterone production. At core body temperature, catecholamines like adrenaline stimulate more testosterone from the Leydig cells of the testes. Lowering testicular temperature dampens this effect, which could prevent excessive masculinization of male offspring.14PubMed Central. Reappraising the exteriorization of the mammalian testes through evolutionary physiology The honest assessment is that the scrotum’s evolutionary origin is likely a product of several overlapping pressures, not a single neat explanation.
Scrotal Conditions Worth Knowing About
Because the scrotum is exposed and relatively thin-walled, it is susceptible to a handful of conditions that are uncommon elsewhere on the body.
Testicular torsion occurs when a testis twists on its spermatic cord, cutting off its own blood supply. This is a surgical emergency that can result in loss of the testis within hours. The underlying vulnerability is often a structural variant called the bell-clapper deformity, in which the tunica vaginalis wraps around the testis, the epididymis, and a length of the spermatic cord more completely than normal, leaving the testis free to swing and rotate like a clapper inside a bell.15PubMed. The bell-clapper deformity of the testis: The definitive pathological anatomy Torsion is most common in adolescent boys and young men, and sudden onset of severe, one-sided scrotal pain should always be treated as a potential case.
Scrotal lymphedema is a rarer condition in which lymphatic drainage in the genital region becomes blocked, causing the scrotum to swell, sometimes enormously. Causes include filarial infection (a parasitic worm disease common in tropical regions), prior inguinal surgery, radiation, and certain cancers. Left untreated, lymphedema leads to progressive fibrosis, recurrent infections, and significant functional impairment.16International Journal of Surgery Case Reports. Surgical management of the giant scrotal lymphedema: A case report at a tertiary facility in northern Tanzania In severe cases, surgical reduction or excision of the affected tissue is the only effective treatment.17PubMed Central. Giant lymphedema of the penis and scrotum: a case report
Chronic scrotal pain, defined as persistent or intermittent scrotal discomfort lasting three months or more, can arise from identifiable causes like epididymitis, varicocele, or nerve entrapment, but in many cases no clear structural problem is found. The scrotum has a dense and somewhat overlapping nerve supply drawn from several spinal segments, which helps explain why scrotal pain can be diffuse, hard to localize, and sometimes felt in the groin or lower abdomen instead of the scrotum itself.18PubMed Central. Anatomy and physiology of chronic scrotal pain
Seasonal Changes in Animals
In many seasonal breeders, the scrotum is not the same size year-round. Its dimensions track with reproductive readiness, expanding during breeding season and shrinking during the off-season. This is most studied in livestock. In Racka rams, for example, scrotal circumference was smallest in January and peaked in September, a difference of nearly 45 percent.19Small Ruminant Research. Seasonal changes of scrotal circumference, blood plasma testosterone concentration and semen characteristics in Racka rams Similar seasonal swings have been documented in mithun bulls, with the largest scrotal circumference observed in winter and spring and the smallest in summer, a pattern that correlates with testosterone levels and semen quality.20PubMed. Effect of season and age on scrotal circumference, testicular parameters and endocrinological profiles in mithun bulls
These seasonal changes reflect genuine growth and regression of the testicular tissue inside, not just scrotal skin stretching. Breeders and veterinarians use scrotal circumference as a quick, non-invasive proxy for fertility in male livestock. In humans, scrotal size does not vary with season to nearly the same degree, though temporary changes from temperature, hydration, and arousal are completely normal.
Mechanical Properties of Scrotal Tissue
Scrotal skin feels different from skin on the rest of the body, and this is not just a subjective impression. The tissue is thinner, more elastic, and more extensible than most skin, which lets it accommodate the wide range of positions the testes need to occupy. Until recently, though, the actual mechanical properties of scrotal tissue had not been rigorously measured.
A study using equine tissue found that scrotal tissue had a tangent modulus of about 4.8 megapascals, making it stiffer than penile sheath tissue (about 2.3 megapascals) but softer than vaginal canal tissue (about 10.2 megapascals).21PubMed. Comparative tensile properties of the equine vagina, penile sheath, and scrotum While these numbers come from horses rather than humans, they quantify something people intuitively know: scrotal tissue has a balance of stretchiness and resilience that allows it to expand and contract repeatedly without tearing or losing its shape. This mechanical profile matters for surgical planning, particularly for procedures like prosthetic implants that rely on the tissue to hold devices in place over years of use.