Most men have probably never given much thought to why their testicles are outside their body.
The reason is temperature.
Sperm production depends on an environment cooler than the body's core. The testes are generally a few degrees cooler, and the body has several ways of helping keep them there.
The scrotum places the testes away from the warmer abdomen. Blood flow helps regulate heat. Muscles can draw the testes closer to the body when it's cold and allow them to hang farther away when it's warm.
It's a surprisingly elaborate system for something most men barely notice.
What happens when things get too warm?
Researchers have studied what happens when the testes are exposed to additional heat.
In one experiment, men wore specially designed underwear that held the testes closer to the body for about 15 hours a day, raising their temperature by approximately 2°C. Over four months, researchers observed changes in sperm count, motility and other sperm measurements. After the heating stopped, those changes eventually reversed.
This wasn't ordinary underwear or an ordinary day. It was a deliberate experiment designed to increase testicular temperature.
But it demonstrated something important: sperm production is sensitive to heat.
Read the experimental heating study
So what about underwear?
That's harder to answer.
Some research has found small differences in scrotal temperature depending on what men wear. Other research hasn't found a clear relationship. Real life is difficult to isolate. Underwear is only one layer. Trousers, posture, activity, outside temperature and time spent sitting can all affect the environment around the testes.
There are some interesting findings.
In a 2018 study of 656 men attending a fertility center, men who reported primarily wearing boxers had 25 percent higher sperm concentration and 17 percent higher total sperm count than men who didn't primarily wear boxers.
Those numbers attracted attention. But the study was observational. It didn't establish that underwear caused the differences, and the researchers didn't measure scrotal temperature or control for the fabric the men were wearing.
And there's an important distinction: sperm concentration and sperm count are measurements. They are not the same thing as fertility, and neither proves a greater likelihood of conception.
So the science doesn't give us a simple rule that loose underwear is good and tight underwear is bad.
It gives us something more interesting to think about.
Why Aquve started asking
Aquve co-founder John Sartor became interested in these questions through his own fertility experience.
It led him to look more closely at testicular temperature, sperm research and something much more ordinary: the underwear men put on every morning.
The more he learned, the more he began questioning something most of us simply accept.
If the body is built to manage the position and temperature of the testes, why should underwear hold everything tightly and flat against it?
That question helped shape Aquve.
Our response was to give the male body considerably more room. Rather than pressing everything flat against the body, the Aquve pouch gives the anatomy space.
There's more to that design than room alone. We'll get to that.
Worth thinking about
The research on underwear isn't definitive. The biology of temperature is much clearer.
The body takes testicular temperature seriously.
What we put around it all day is worth thinking about.