top of page
Search

Why Are Testicles Outside the Body and What Makes Sperm Swim?

  • Writer: FibonacciMD
    FibonacciMD
  • Jun 19
  • 3 min read

Updated: Jun 23

The Fascinating Science Behind Sperm Motility and Testicular Location

Ever wondered why testicles are outside the body? Recent research sheds light on these intriguing biological questions and how the body works in concert to ensure successful fertilization.


testicles, sperm, spermatozoa, testicles are outside the body, CatSper

In a study published April 2025, researchers examined mouse sperm to try to determine the biochemical makeup and process that makes spermatozoa move or swim. (Sperm and spermatozoa are generally interchangeable in meaning, but sometimes the word sperm may refer to both seminal fluid and spermatozoa.)


Spermatozoa are activated by heat and in most mammals, the testes are outside of the body to keep the temperature 2 to 4 degrees C less than core body temperature.  Some animals have internal testes but have adapted to either keeping their body temperature or the area around the testes lower.  Some birds have internal testes and high core temperatures, but birds do not carry the CatSper gene.


Spermatozoa need to move to come in contact with the ovum in the oviduct to start fertilization.  CatSper is the name of a calcium ion channel that controls the level of intracellular calcium.  When CatSper is activated, calcium enters the spermatozoa flagella (tail on the spermatozoa that moves), and it starts to gyrate, allowing the spermatozoa to move or swim.  The researchers in this study found that as the temperature surrounding the spermatozoa increased, the CatSper ion channel was activated resulting in more flagellar movement.  However, above 38 degrees C (100.4 degrees F) flagellar activity diminished.   

They also found that the acidity of the spermatozoa’s environment affects CatSper activation. 


Spermatozoa are stored in the testes at a pH of 6.9 (acidic).  They reported that an acid environment suppresses heat activation of the CatSper channel.


Thus, when spermatozoa enter the vagina with its 37 degree C temperature (98.6 degrees F) and neutral pH of 7.4, much warmer and less acidic environment than the testes, there are ideal conditions for CatSper to activate and help start the spermatozoa’s movement to the egg.  However, there is one more system in place to prevent the spermatozoa from being activated too quickly.  In seminal fluid, which surrounds spermatozoa, there is a substance called spermine.  Spermine appears to blunt the temperature increase response of CatSper, which allows the spermatozoa to get closer to the egg target, before it activates (called capacitation).   Spermatozoa, once activated by CatSper, have a limited lifespan, so it is best if activation occurs in the oviducts.  The spermine gradually dissipates and is also removed by chemical reactions in the oviducts. This allows the spermatozoa to fully activate when the spermatozoa are closer to the ovum, increasing the odds of successful fertilization.


Comments:

Fertilization is an amazingly complex process with adaptations such as temperature, acidity of the environment, and biochemical reactions appearing to be part of the way it is controlled.  Other research has shown CatSper channel activation also occurs after exposure to female progesterone and prostaglandins (hormone-like substances), and certain odorants (chemical compounds that have a smell or odor).  It seems that CatSper activation is probably due to multiple factors.


Moving the flagella, or tail rapidly allowing travel of the sperm to the ovum is only a part of the process of fertilization, and this particular study determined that body temperature and pH are critical parts of CatSper activation.  In both mice and humans, non-functioning CatSper channels, due to mutations, can cause male infertility.


One of the reasons human testicles are outside the body appears to be an adaptation designed to decrease the temperature stored spermatozoa are exposed to and prevent premature activation.


updated 6/23/2025

References

bottom of page