The scrotum sits outside the body because sperm production requires temperatures 1° to 8°C lower than core body temperature, depending on the species. At internal body temperature, the cellular machinery that repairs DNA during sperm development breaks down, and sperm production can grind to a halt. This temperature sensitivity is so extreme that artificially warming the testes to core body temperature can render a male functionally sterile, even though sex drive remains unaffected.
Why Sperm Can’t Handle Body Heat
Developing sperm cells go through a complex process called meiosis, where chromosomes pair up, swap genetic material, and divide. This process involves deliberately breaking and repairing DNA strands. At temperatures around 37°C to 38°C (normal body temperature for most mammals), the repair machinery fails to assemble properly on those broken DNA sites. The result is an accumulation of unrepaired double-strand breaks, which is one of the most dangerous types of DNA damage a cell can sustain.
Cells with this level of damage trigger a built-in self-destruct program. A 2022 study published in Communications Biology tracked exactly what happens when testicular tissue is exposed to increasing temperatures. At 38°C, developing sperm cells were wiped out while earlier-stage cells survived. At 40°C, no germ cells remained at all. Importantly, the supporting cells that nourish sperm were fine at every temperature tested. Heat sensitivity is specific to the sperm-producing cells themselves.
In the most severe cases of prolonged heat exposure, all sperm cells except the most primitive stem cells are lost within a couple of weeks. Even the transition from stem cell to actively developing sperm cell gets blocked. The testes don’t lose their ability to make sperm permanently, but recovery takes time, and the damage at each stage is real.
How the Scrotum Stays Cool
The scrotum isn’t just a passive sac hanging in open air. It has an active temperature control system built from muscles and blood vessels that continuously fine-tune testicular temperature.
The most elegant piece of this system is a network of veins called the pampiniform plexus. These veins wrap tightly around the artery carrying warm blood from the body’s core down to the testes. As cool venous blood flows back up, it absorbs heat from the incoming arterial blood before that blood ever reaches the sperm-producing tissue. This countercurrent heat exchange works the same way an efficient radiator does: warm fluid and cool fluid flow in opposite directions, transferring heat between them.
Two sets of muscles add a second layer of control. The cremaster muscle can pull the testes closer to the body when it’s cold, borrowing warmth from the abdomen. The dartos muscle wrinkles the scrotal skin to reduce its surface area, minimizing heat loss. In warm conditions, both muscles relax, letting the testes hang lower and the scrotal skin stretch smooth, maximizing the surface available for heat dissipation. Unusually for a striated muscle, the cremaster actually contracts more strongly in response to heat, reflecting its specialized role in temperature regulation rather than ordinary movement.
Competing Theories Beyond Cooling
The cooling hypothesis is the most widely supported explanation, but researchers have proposed several alternatives over the decades, each highlighting a different selective pressure.
The “galloping hypothesis” suggests that externalization originally protected the testes from pressure spikes inside the abdomen caused by running, jumping, and leaping. In a galloping animal, abdominal pressure fluctuates rapidly, and delicate reproductive tissue could be damaged by repeated compression. Moving the testes outside the body cavity would shield them from these mechanical forces.
The “training hypothesis” takes a different angle entirely. Because the scrotum receives relatively poor blood supply compared to internal organs, sperm develop in a low-oxygen, resource-scarce environment. The idea is that this hostile setting acts as a filter, screening out weaker sperm and producing fewer but higher-quality cells capable of surviving the demanding journey to fertilize an egg.
A “display hypothesis,” proposed by the zoologist Adolf Portmann in 1952, suggested that the scrotum serves as a visual signal of reproductive fitness, explaining why some primate species have brightly colored or prominently sized scrota. This evolved into the broader “handicap hypothesis,” which frames costly physical traits as honest signals of genetic quality. Experimental evidence for this idea, however, has been consistently disappointing.
A 2021 comparative genomics study found molecular support for both the cooling hypothesis and the training hypothesis, suggesting these pressures may not be mutually exclusive. The galloping hypothesis also has logical merit for certain lineages. It’s plausible that multiple forces pushed testes outward, with temperature sensitivity being the strongest and most universal.
Mammals That Keep Their Testes Inside
Not every mammal has a scrotum, and these exceptions are informative. Elephants, manatees, hyraxes, and several other members of a group called Afrotheria retain their testes inside the abdomen. So do whales, dolphins, and seals. Monotremes like the platypus also have internal testes. These animals are called “testicond” mammals.
Phylogenetic mapping suggests that internal testes are actually the ancestral condition for all mammals. The scrotum appears to have evolved independently at least twice: once in marsupials and once in the large group of placental mammals called Boreoeutheria, which includes primates, rodents, carnivores, and hoofed animals. Several lineages within that group then lost the scrotum secondarily, a trend especially common among Laurasiatheria (the branch containing bats, whales, and some insectivores).
How these testicond species manage to produce viable sperm at body temperature remains an active question. Some appear to have evolved heat-resistant versions of the proteins involved in sperm development. Others may use internal cooling strategies, such as specialized vascular networks near the kidneys or, in the case of marine mammals, blood flow patterns that route cooled blood from fins and flukes past the reproductive organs.
What Happens When Descent Fails
Cryptorchidism, where one or both testes fail to descend into the scrotum, is the most common birth defect affecting male reproductive anatomy in humans. It provides a natural case study for why external positioning matters.
When testes remain inside the body, the sustained higher temperature impairs sperm development. In boys born with both testes undescended, fertility outcomes depend heavily on whether certain precursor cells (called Ad spermatogonia) are present. Among those who had these cells in at least one testis, 55% went on to develop normal sperm counts after corrective surgery. But when these precursor cells were depleted in both testes, no patient achieved a normal sperm count, even after successful surgery brought the testes into the cooler scrotal environment. The damage from prolonged internal heat exposure, particularly during early development, can be irreversible if critical cell populations are lost.
This clinical reality underscores the biological logic of the scrotum. The narrow temperature window for healthy sperm production is not a minor preference but a hard requirement, one that shaped the anatomy of most male mammals over more than 100 million years of evolution.