Why Is the Scrotum External on Most Male Mammals?

The testes hang outside the body in most male mammals because sperm production requires temperatures a few degrees cooler than the body’s core. In humans, the scrotal environment sits roughly 2–4 °C below internal body temperature, and even modest warming can damage developing sperm cells. That thermal sensitivity is the central reason evolution favored an external pouch for the gonads rather than keeping them safely tucked inside the abdomen. But the story is richer than a simple thermostat explanation: not every mammal has a scrotum, the cooling hardware involves more than just dangling in open air, and the evolutionary path that led here turns out to be surprisingly ancient.

Sperm Production and the Temperature Problem

The cells that eventually become sperm go through a long, intricate process of division and maturation. That process is exquisitely sensitive to heat. In humans, the testicular temperature is maintained about 2–3 °C below core body temperature, and research on men whose testes fail to descend (a condition called cryptorchidism) shows that even this small difference matters enormously.¹ When testes are trapped at abdominal temperature, sperm counts drop, DNA damage in sperm cells increases, and the germ cells that supply new sperm can undergo premature die-off.²

Heat doesn’t just slow sperm production; it damages the cells at a molecular level. Studies in both humans and rams have shown that sustained scrotal heat stress raises DNA fragmentation in sperm, disrupts the protective proteins that package chromosomes, and activates programmed cell-death pathways in the testis.³ In rams, researchers found higher chromatin fragmentation and increased markers of cell stress in ejaculated sperm after heat exposure.⁴ The picture that emerges is consistent across species: sperm cells are unusually fragile when it comes to temperature, and the scrotum exists largely to keep them cool enough to survive.

How the Scrotum Actually Keeps Things Cool

The scrotum is not just a passive bag of skin. It has several active and passive mechanisms working together to regulate testicular temperature, and the engineering is more sophisticated than it looks.

The most important mechanism is a network of veins called the pampiniform plexus. Blood returning from the testes passes through this dense tangle of veins that wraps around the testicular artery carrying warm blood down from the body. The arrangement works as a countercurrent heat exchanger: cool venous blood absorbs heat from the incoming arterial blood before it reaches the testes, pre-cooling the supply.⁵ Modeling work has shown that this exchanger effectively pre-cools arterial blood when external temperatures are moderate or low, though its ability to compensate diminishes as ambient temperatures climb.⁶

Then there is the dartos muscle, a thin sheet of smooth muscle embedded in the scrotal skin. When temperatures drop, the dartos contracts and draws the scrotum up closer to the body, reducing surface area and heat loss. When it warms up, the muscle relaxes, and the scrotum hangs lower, increasing the surface area exposed to air and letting heat dissipate.⁷ A separate muscle, the cremaster, runs along the spermatic cord and can lift or lower the testes within the scrotum. Together, these muscles give the body a real-time way to adjust how close the gonads sit to the warm trunk. The scrotal skin itself also contributes: it’s thin, has relatively little insulating fat, and contains sweat glands. In some species, seasonal changes in the skin’s structure help fine-tune heat exchange. In camels and buffalo, for instance, the density of sweat glands in the scrotal skin changes with the season and with ambient temperature and humidity.⁸

How the Testes Get There During Development

In mammalian embryos, the testes start life deep in the abdomen, near the kidneys. Over the course of fetal development, they migrate downward through the abdominal wall and into the scrotum. This journey happens in two distinct phases, each governed by different hormonal signals.

During the first phase, a ligament called the gubernaculum thickens and effectively anchors the testis in the inguinal (groin) region while the rest of the abdomen grows around it. The key signal driving this thickening is a hormone called INSL3, produced by specialized cells in the developing testis. INSL3 binds to receptors on the gubernaculum and causes it to proliferate and remodel, pulling the testis from its original position near the kidney down toward the groin.⁹ In the second phase, androgens take over: testosterone acts both directly on the gubernaculum and indirectly through a nerve called the genitofemoral nerve, which releases a signaling molecule that guides the testis the rest of the way into the scrotum.¹⁰

When either phase fails, the result is cryptorchidism, one of the most common congenital conditions in baby boys. Undescended testes are stuck at core body temperature, and if the condition isn’t corrected early, the damage to germ cells begins within the first year of life. Cryptorchidism is a significant risk factor for both infertility and testicular cancer later on.² More than 95 percent of testicular cancers are germ-cell tumors, and the link to prolonged heat exposure of undescended testes is well established.

Mammals That Keep Their Testes Inside

If cooler temperatures are so important for sperm production, how do elephants, manatees, and platypuses manage? These animals are “testicond,” meaning their testes remain deep inside the abdomen and never descend. The group of testicond species isn’t random. Elephants, manatees, hyraxes, tenrecs, elephant shrews, and golden moles all belong to or are closely associated with the Afrotheria, an ancient clade of mammals that split off from other placental mammals early in evolutionary history.¹¹

Genomic studies have revealed something striking about these animals. Four afrotherians that completely lack testicular descent — the tenrec, cape elephant shrew, cape golden mole, and manatee — have lost or inactivated both of the key genes involved in testicular descent: INSL3 (the hormone that drives the first phase of descent) and RXFP2 (the receptor it binds to).¹² These gene losses are independent — they happened separately in different lineages — which tells us that testicondy evolved multiple times, not just once. The genes didn’t break because the testes stopped descending; rather, once the testes stopped descending for whatever reason, the genes were freed from selective pressure and gradually accumulated mutations until they stopped working.

This raises a genuine puzzle. If these animals’ testes sit at full body temperature, how do they produce viable sperm? For monotremes like the platypus, the answer may be partly that their core body temperature is lower than that of most placental mammals (around 32 °C). But elephants have core temperatures in the normal mammalian range, around 36–37 °C. Researchers suspect that testicond species have evolved alternative cooling strategies or that their sperm-producing machinery has adapted to tolerate higher temperatures, but the mechanisms are not yet fully understood.

The Dolphin Solution

Dolphins and whales present an especially elegant case study. As fully aquatic mammals, they have no scrotum and their testes are internal, tucked near the abdominal wall. External testes would create drag and be vulnerable to injury in an aquatic environment. Yet cetaceans are clearly fertile. The trick is an internal countercurrent heat exchanger that uses an entirely different plumbing route than what land mammals have.

In bottlenose dolphins, veins returning cool blood from the dorsal fin and tail flukes — surfaces constantly shedding heat to the surrounding water — run alongside the arteries supplying the testes. The cool venous blood chills the arterial supply before it reaches the gonads, achieving the same net result as external testes: a testicular temperature below core body temperature.¹³ Colonic temperature measurements near the testes have confirmed that this system creates a measurable temperature gradient, supporting the hypothesis that the cool blood specifically regulates testicular temperature rather than serving some other purpose.¹⁴

The dolphin example is useful because it separates two ideas that often get conflated. The scrotum is not the only way to cool testes. It’s just the most common solution among land-dwelling mammals. What the body consistently needs, across species, is a way to keep testicular tissue below core temperature. The scrotum is one answer; internal vascular plumbing cooled by fins is another; lower baseline body temperature may be yet another. The underlying requirement is thermal, not anatomical.

Was the Scrotum the Original Condition?

For decades, biologists debated whether the ancestral mammal had a scrotum that some lineages later lost, or whether the scrotum evolved independently in several groups. Phylogenetic analyses now strongly support the first scenario. When researchers mapped testicular position across the mammalian family tree, the scrotal state appeared to be ancestral for placental mammals, and evolution generally proceeded from a scrotal condition toward progressively more ascrotal or testicond arrangements.¹⁵ A more recent comparative genomic study covering 380 genes associated with cryptorchidism reached the same conclusion: the ascrotal or internal-testes condition is derived, not ancestral.¹⁶

This is somewhat counterintuitive. You might expect that the “safer” internal arrangement came first and that the scrotum was a later innovation. Instead, the scrotum appears to have been present very early, and some lineages subsequently evolved ways to internalize the testes. The gene-loss pattern in Afrotheria reinforces this: these animals didn’t fail to evolve descent; they evolved away from it. The finding also means that the vulnerability inherent in external testes — to trauma, to temperature extremes, to herniation — was present from the beginning and has been tolerated because the fertility benefit of cooler sperm production outweighed the physical risks.

Competing and Complementary Hypotheses

Temperature regulation is the dominant explanation, but it’s not the only hypothesis that has been proposed, and some researchers argue the full picture is more layered.

One alternative is the “activation hypothesis,” which suggests that sperm need to be stored at cool temperatures so that they can be activated by the warmer environment of the female reproductive tract. Recent work on a calcium channel in sperm called CatSper lends some support to this idea. Researchers found that CatSper opens at a specific thermal threshold, allowing calcium to flood in and trigger the vigorous swimming pattern sperm need to reach an egg. Below that temperature, the channel stays closed.¹⁷ If sperm were already at body temperature during storage, this thermal trigger might not work as reliably.

Another idea focuses on the dangers that internal body movements pose to delicate tissue. Abdominal pressure fluctuations from running, jumping, or coughing could compress the testes against other organs. By moving them outside the body cavity, the scrotum may also protect the gonads from mechanical pressure. Still others have suggested that scrotal externalization may serve a signaling function — in some primates, scrotal color is a sexually selected trait. In rhesus macaques, for instance, males that sired more offspring had darker, redder scrotums, and scrotal color intensity correlated with testosterone levels and social rank.¹⁸

None of these hypotheses are mutually exclusive. The most balanced reading of the evidence is that temperature regulation is the primary driver, but multiple selection pressures have shaped the diversity of testicular positions across mammals.¹⁷

Modern Life and Scrotal Temperature

If the whole point of the scrotum is to keep the testes cool, modern habits that warm the groin area are worth paying attention to, especially for men concerned about fertility. One of the more striking findings in this area is how quickly sitting in a chair raises scrotal temperature. Researchers have recorded increases of up to 3 °C within just 20 minutes of sitting on commonly used chairs, and they have suggested that prolonged sitting in sedentary societies may contribute to the declining semen quality observed over recent decades.¹⁹

The concern extends beyond office chairs. Laptop computers placed on the lap, tight-fitting underwear, hot tubs, and heated car seats can all raise scrotal temperature into a range that impairs sperm production. Studies have documented that even a few months of sustained scrotal heat stress significantly reduces sperm concentration, motility, and the proportion of normally shaped sperm, with effects on DNA integrity persisting for the full duration of heat exposure.³ The good news is that these effects are generally reversible once the heat source is removed. Sperm take roughly 74 days to fully mature in humans, so after eliminating the heat exposure, most men see recovery within two to three months.

For men who are actively trying to conceive, the practical advice is straightforward: avoid prolonged heat exposure to the groin. That doesn’t mean you can never sit down or take a warm bath. But if you’ve been struggling with fertility, it’s worth evaluating whether daily habits like long commutes, hours at a desk, or regular hot-tub use might be quietly warming your testes past their comfort zone. The cooling system the scrotum provides is robust, but it wasn’t designed for an era when humans sit for eight or more hours a day with their thighs pressed together.

Scrotal Color as a Sexual Signal

Beyond thermoregulation, the scrotum appears to have been co-opted for another purpose in some species: sexual signaling. In many primates, the scrotal skin is brightly or conspicuously colored, and this coloration can change with hormonal status and social context. Male vervet monkeys are famous for their vivid blue scrotums, and the intensity of the color is linked to testosterone levels and dominance rank.

The rhesus macaque data provide some of the clearest quantitative evidence that scrotal appearance matters for reproductive success. Males with redder and darker scrotums sired more offspring, and the best statistical model for predicting reproductive output included the interaction between scrotal redness, testosterone, rank, and age.¹⁸ Whether this means females are directly choosing mates based on scrotal appearance, or whether scrotal color simply reflects the same hormonal profile that drives competitive behavior and mating access, is still debated. Either way, the scrotum has become a billboard of hormonal status in these species — a secondary function riding on top of the primary thermoregulatory one.

In humans, scrotal color doesn’t appear to play the same conspicuous signaling role, likely because upright posture, clothing, and reduced reliance on visual cues in mate choice have made it irrelevant. But the primate examples are a reminder that once a structure exists for one purpose, evolution can repurpose it. The scrotum started as a cooling device and became, in some lineages, something closer to a badge of reproductive quality.