Men have testicles because they serve two essential functions: producing sperm and manufacturing testosterone. These are the only organs in the body that do both jobs, and without them, reproduction and many aspects of male health would not be possible. What makes testicles unusual is their location outside the body, which turns out to be critical to how well they work.
Two Jobs, One Organ
Testicles handle sperm production and hormone production simultaneously, using different types of cells for each task. Specialized cells called Leydig cells, scattered in the tissue between the sperm-producing tubes, are responsible for making testosterone. The brain’s pituitary gland sends a hormone signal through the bloodstream that lands on receptors on these cells, triggering a chain reaction that converts cholesterol into testosterone.
The testosterone concentrations inside the testicles are staggering compared to the rest of the body: 25 to 125 times higher than what circulates in the blood. That local concentration isn’t an accident. Sperm cells cannot develop past their earliest stages without testosterone present at those levels. In the absence of testosterone or the receptors that detect it, sperm development stalls before the cells ever mature. So the testicles essentially create the hormone environment they need to do their other job.
Testosterone also drives changes far beyond the testicles themselves. It’s responsible for the development of muscle mass, bone density, body hair, a deeper voice during puberty, and the sex drive that persists throughout adulthood.
Why They Sit Outside the Body
The most common question about testicles isn’t what they do, it’s why they hang outside the body in such a vulnerable position. The answer comes down to temperature. Human sperm production requires an environment 2 to 3°C (about 4 to 5°F) cooler than normal core body temperature. The scrotum provides exactly that.
This isn’t a passive arrangement. The scrotum actively regulates its own temperature using a layer of smooth muscle beneath the skin. When the environment gets cold, this muscle contracts, wrinkling the scrotal skin and pulling the testicles closer to the body for warmth. When things heat up, the muscle relaxes, letting the scrotum hang lower and allowing more heat to dissipate. A second muscle, the cremaster, works alongside it to raise and lower the testicles within the scrotum. Together, these muscles act like a thermostat, keeping conditions in a narrow range suitable for sperm development.
This cooling requirement has been recognized since at least 1926, when the physiologist Carl Moore proposed that the scrotum evolved as an adaptation to the higher body temperatures that came with being warm-blooded. Higher internal temperatures would damage developing sperm and increase the rate of genetic mutations in those cells. Moving the production site outside the body solved both problems.
Not All Mammals Need a Scrotum
If temperature is the reason testicles are external, you might wonder why some mammals keep theirs inside. Elephants, manatees, rock hyraxes, and cape golden moles all retain their testicles deep within their abdomens. Some of these animals, like tenrecs and golden moles, have body temperatures low enough that internal sperm production isn’t a problem. But elephants run warm enough that they should, theoretically, be sterile. Researchers still don’t fully understand how elephants manage it.
This has led to competing theories about why external testicles evolved. One alternative idea, the “cool storage hypothesis,” suggests the real issue isn’t sperm production but sperm storage. The epididymis, a coiled tube sitting on top of each testicle where sperm mature and wait before ejaculation, may need cooler temperatures to keep stored sperm viable. Another proposal, the “training hypothesis,” suggests that the slightly lower oxygen levels in the scrotum stress sperm cells in a beneficial way, improving their metabolic fitness and making them better swimmers.
How Sperm Get From Production to Exit
Inside each testicle, sperm are manufactured in tightly coiled tubes. The cells produced there are immature and can’t yet swim or fertilize an egg. They move into the epididymis, the coiled structure on top of the testicle, where they spend several days maturing and gaining the ability to move.
During ejaculation, sperm are pushed from the tail of the epididymis into a long tube called the vas deferens, which travels up through the spermatic cord, into the pelvic cavity, and behind the bladder. There it joins with fluid from the seminal vesicles, passes through the prostate (which adds more fluid), and exits through the urethra. The fluid from the seminal vesicles and prostate makes up most of the volume of semen, providing nutrients and a medium for sperm to swim in.
What Happens When Testicles Don’t Descend
Testicles form inside the abdomen during fetal development and normally descend into the scrotum before or shortly after birth. When one or both fail to make that journey, the condition is called cryptorchidism, and it’s one of the most common issues identified in newborn boys. It illustrates exactly why location matters so much.
An undescended testicle sits at core body temperature, which is too warm for normal sperm development. The elevated heat inhibits the development of sperm-producing cells, leading to smaller sperm-producing tubes, depletion of developing sperm cells, and eventually scarring of the tissue. Cryptorchidism is a well-established risk factor for infertility.
It also raises the risk of testicular cancer significantly. Men with a history of an undescended testicle face a risk 5 to 10 times higher than the general male population. About 10% of all testicular germ cell tumors occur in men who had this condition. The risk is even greater when the testicle remains in the abdomen rather than getting stuck partway down in the groin area. This is why surgical correction is typically performed early in life.
Heat and Everyday Sperm Quality
Because the system is so temperature-sensitive, everyday habits can nudge things in the wrong direction. Sitting for six or more hours a day, wearing tight underwear to bed, using electric blankets, and working in hot environments have all been associated with reduced sperm motility. Sauna use has received particular attention: studies of men who used saunas twice a week for three months at 80 to 90°C showed measurable changes in sperm quality, though some smaller studies found no significant difference.
The effects of occasional heat exposure are generally reversible. Sperm take roughly 74 days to develop from start to finish, so temporary disruptions from a hot tub or a fever tend to resolve within a couple of months once the heat source is removed. Chronic or occupational heat exposure is a different story and can cause more persistent changes in sperm count and motility. For men actively trying to conceive, keeping the testicles at their preferred temperature is one of the simplest and most controllable factors in the equation.