Sperm may be small, but they have strong preferences, especially regarding temperature. In mammals, including humans, these microscopic swimmers thrive in conditions slightly cooler than the body’s average temperature. Yet their ultimate destination—the female reproductive tract—is noticeably warmer.
So, how do these delicate, heat-sensitive cells survive the journey and successfully fertilize an egg in such a challenging environment?
Scientists have uncovered a surprising secret about sperm—heat plays a crucial role in their journey to fertilization. Research from Washington University School of Medicine in St. Louis reveals that warmer temperatures, like those inside the female reproductive tract, activate a special signal in sperm. This signal transforms their movements from gentle navigation to intense thrashing and twisting, helping them penetrate the egg.
The discovery could lead to breakthroughs in male contraceptives and infertility treatments. In experiments with mice, researchers identified a key protein found in all mammals that triggers this hyperactive state when sperm encounter the right temperature. This hyperactive state is key for successful fertilization.
Single molecule puts sperm on track
This work has identified a temperature-controlled switch in sperm that triggers these increased movements precisely when they are needed during fertilization.
All mammals have a special protein in sperm called CatSper, which helps control the movement needed for fertilization. Scientists originally thought CatSper was activated by pH levels in the female reproductive tract and, in primates, by the hormone progesterone. However, most mammalian sperm don’t react to progesterone, suggesting another factor was at play.
Temperature emerged as a likely trigger. Mammals have evolved ways to keep male reproductive organs cooler than body temperature—dolphins use their dorsal fins to cool blood before it reaches their testes, elephants use their ears, and humans store sperm in external testicles. Birds, which don’t have these cooling systems, also lack CatSper, reinforcing the idea that temperature is key to sperm activation.
The team used micron-scaled tools and techniques originally developed to study brain cells and the pattern of electric charges distinctive to CatSper’s activation in individual sperm cells. They also measured the spikes generated when the temperature surrounding the cell surpassed 38 degrees Celsius (100.4 degrees Fahrenheit).
Females supercharge sperm evolution, especially in animals
Once CatSper was activated, the sperm’s behavior switched from the relatively smooth motions they use for navigation to the hyperactive movements needed to enter the egg for fertilization.
Journal Reference:
- Swain, D.K., Vergara, C., Castro-Arnau, J. et al. The essential calcium channel of sperm CatSper is temperature-gated. Nat Commun 16, 3657 (2025). DOI: 10.1038/s41467-025-58824-0
