Activation Hypothesis: Cool Balls and Warm Vaginas

Testicles are delicate little darlings, so it’s puzzling as to why they have evolved to be so beautifully, yet vulnerably displayed, hanging right out in front wearing nothing but a thin scrotal sac. 

It turns out that this arrangement may be due to the fact that sperm are extra sensitive to temperature changes and prefer to hang out in a slightly cooler environment than regular body temperature.  

Human body temperature is around 37 degrees Celsius, but because of the anatomical positioning and thermoregulatory mechanisms of testicles, scrotal temperature is on average 2.5-3 degrees Celsius lower than body temp.

Spermatogenesis (the production of new sperm cells) occurs most efficiently at 34 degrees Celsius, meaning that balls are the ideal place for sperms to really thrive and prepare for their upcoming journey of a lifetime/suicide mission.

Interestingly, even though they prefer to chill in this cool environment, when the ambient temperature is raised just a couple of degrees Celsius, sperm get super excited and become extra lively for up to 4 hours.   

As vagina owners or enthusiasts may have already noticed, the vagina is like a personal portable heater.  So we know that sperm leave their breezy scrotal home to venture in to this new toasty environment.

Gordon Gallup and his colleagues presented ‘Activation Hypothesis’ to elaborate on this process.  The hypothesis is that vaginal heat ACTIVATES sperm to become extra energetic on contact, and therefore more likely to reach their destination (egg).

Now, you may have observed the ‘wonky balls phenomenon’ (one hanging lower than the other) or ‘pruny balls state’ (when scrotal skin becomes super wrinkly and tight), and other peculiar traits of the nut sack and wondered what it’s all about.

Activation Hypothesis may help to explain, so wonder no more.

Rather than acting as boring, passive sperm fridges, scrota are constantly working to maintain an optimal temperature.  The cremasteric muscle is the real hero here.  It’s the muscle that retracts the balls in the cold, and relaxes them in the warmth.  Further more, anatomist Stany Lobo and colleagues argue that each testicle is doing it’s own little dance inside the scrotum to keep maximising scrotal surface area and therefore heat dissipation. 

Sperm are so sensitive, if they’re too cold they’re useless and if they’re too hot they’re useless, so the thermoregulatory properties of the testicles are absolutely crucial to the survival of our species.

Activation Hypothesis can explain so much about the quirkiness of balls, and really makes you appreciate the work that goes in to keeping sperm happy.


Gordon G. Gallup Jr., Mary M. Finn, and Becky Sammis, “On the Origin of Descended Scrotal Testicles: The Activation Hypothesis,” Evolutionary Psychology 7, no. 4 (2009).

Stany W. Lobo et al., “Asymmetric Testicular Levels in the Crotch: A Thermodynamic Perspective,” Medical Hypotheses 72, no. 6 (2009)