The role of temperature in sperm

When we think about fertility, we often picture hormones, eggs, sperm counts, and maybe even lifestyle factors like diet or stress. But rarely do we think about temperature. Yet, a new study published in Nature Communications reveals that sperm are exquisitely tuned to heat, and that their ability to sense temperature may be one of the most finely balanced systems in human biology.

The research, conducted by Polina V. Lishko, a cell biologist at Washington University School of Medicine in St. Louis, along with colleagues Dilip K. Swain, Citlalli Vergara, and Júlia Castro-Arnau, uncovers something remarkable: sperm carry a molecular thermometer. This thermometer is built into a protein channel called CatSper, and it plays a decisive role in whether or not fertilization can happen.

The unsung hero of fertility

Every sperm cell has a long tail, or flagellum, that it uses to swim. But swimming straight isn’t enough to reach an egg. At a certain point in the female reproductive tract, sperm undergo a dramatic behavioral shift called hyperactivation. Instead of gentle, symmetrical strokes, the tail starts lashing in wild, powerful arcs. This hyperactive motility allows sperm to push through thick fluids and barriers on the way to the egg.

Hyperactivation is triggered by calcium rushing into the tail through a specialized gateway: the CatSper channel. Without CatSper, sperm are effectively directionless. Mice bred without CatSper are infertile, and in humans, mutations in the CatSper gene are linked to male infertility. It’s not an exaggeration to say that CatSper is the gatekeeper of life.

Until now, scientists knew CatSper responded to changes in voltage, acidity, and in humans, even to hormones like progesterone. What wasn’t known was whether CatSper could also be switched on by heat.

The Washington University team used an advanced method called patch-clamp electrophysiology to measure CatSper activity directly in mouse sperm. What they found was that CatSper is temperature-gated. In other words, the channel flips open when the surrounding temperature reaches about 33.5 °C.

That threshold is not random. It is roughly the temperature of the scrotum in mice (and slightly below in humans). The fact that CatSper is sensitive right around this point suggests evolution has tuned it perfectly. If sperm get too warm while still inside the male body, CatSper could be triggered prematurely, leading to damage or dysfunction. By keeping testes slightly cooler than the rest of the body, nature has built a safeguard against accidental activation.

The researchers also calculated the Q10 value, a measure of how much a biological process speeds up with a 10 °C rise in temperature. CatSper’s Q10 was unusually high (about 5), meaning it is extremely responsive to even small changes in heat. This makes it one of the most temperature-sensitive channels known in sperm.

Why sperm don’t self-destruct

If heat can flip CatSper on, how do sperm survive the warm environment of the female reproductive tract? After all, once ejaculated, sperm are suddenly exposed to 37 °C, core body temperature. The researchers found that sperm are protected by two clever safety systems. First, acidic storage conditions in the male body. Before ejaculation, sperm are stored in the epididymis, where the environment is slightly acidic. Acid prevents CatSper from activating even if temperatures rise. Second, a chemical shield in semen. Seminal plasma (the fluid surrounding sperm after ejaculation) contains a molecule called spermine. At physiological levels, spermine inhibits CatSper’s heat sensitivity. In effect, it acts like a temporary lock, preventing the channel from opening too soon. Only later, when sperm undergo a final maturation process called capacitation inside the female reproductive tract, do they lose this protection. At that point, CatSper becomes poised to respond, but now in the right place, close to the egg.

The discovery that CatSper doubles as a molecular thermometer sheds light on a long-standing mystery: why most mammals keep their testes outside the body. For humans and many other mammals, testes hang in a scrotum that is 2–4 °C cooler than body temperature. Raise the temperature just a few degrees, as happens in conditions like varicocele, where veins in the scrotum enlarge and trap heat, and fertility often plummets. The new study provides a molecular explanation: overheating may trigger CatSper too soon, causing sperm to “burn out” before they ever reach an egg.

Interestingly, not all animals use this strategy. Birds, which have internal testes at much warmer temperatures, lack CatSper entirely. Marine mammals like dolphins and seals, whose testes are also internal, have evolved elaborate cooling systems to maintain the right conditions. And some marsupials, like the Madagascar tenrec, simply solved the problem by lowering their entire body temperature. Evolution has taken multiple routes, but the common theme is clear: sperm and heat must be carefully balanced.

This discovery has many potential ramifications. For example, understanding how heat disrupts sperm function could lead to new therapies for men with temperature-related infertility. Also, if scientists can design drugs that mimic spermine or otherwise lock CatSper shut, it may be possible to develop non-hormonal male contraceptives. In addition, for men facing cancer treatments or high-risk environments, strategies that control CatSper activity could improve long-term sperm storage.

At its core, this research is about timing. Fertilization is a race, not just against other sperm, but against the ticking clock of a sperm cell’s own lifespan. CatSper is both the starter pistol and the finish line. Activate too soon, and the sperm is doomed. Activate too late, and the egg may be gone. By showing that CatSper is wired to temperature, the team at Washington University has revealed just how finely tuned this system really is. It’s a delicate balance of heat, chemistry, and timing, one that quietly shapes the continuation of every mammalian species, including our own.

If you want to learn more, read the original article titled "Temperature-controlled switch activates sperm, is key to fertility" on Nature Communications at http://dx.doi.org/10.1038/s41467-025-58824-0.