Sunday, July 17, 2011

External Testicles and the Cremaster/Dartos Reflex

What would Youtube and America's Funniest Home Videos be without clips guys getting hit in the crown jewels? (Wouldn't it be interesting to know what percent of youtube views are related to this genre?)

As you well know, male humans (and other boreotherian mammals, et al) have external testicles because their optimal temperature for sperm production is below that of our core body temperature--about 94 degrees Fahrenheit.  Why, though?

  1. Optimal Enzyme Temperature
    1. Enzymes are molecule machines that help reactions take place.  Enzymes have optimal temperatures because if they are outside of that golden medium then they change shape and don't work as well (or at all).  There are enzymes that are involved in spermatogenesis like spermatogenic DNA polymerase beta and recombinase activities that are optimized at 94 degrees F.
    2. Other temperature enzymes might not have evolved yet.   The obstacles for their evolution might be quite unlikely--you'd need a new enzyme and mutations that would cause internal testicles at the same time.   Seems easier to just keep doing what we're doing.
    3. Maybe our enzymes are a throw back to cold blooded reptile, lower temp mammal or before.  So, we got hot and our testicles got to the  business of  moving.
  2. Maybe it's a throw back to ridiculous sperm competition.
    1. Perhaps there was sperm competition so intense that one of our ancestors had to move its testicles outside of its body to house them and now it's just stuck around.
  3. Maybe it's protection from internal body cavity pressures changes like from galloping, running, etc.
Since we're kind of a menagerie of evolutionary mementos having external testicles can pose some negative consequences for humans, just one of which is inguinal hernias. Male testes descend from inside the body after birth. Why aren't they on the outside in utero? Because males and females are made from the same evolutionary template. Male testes come down from roughly where the female ovaries are (their equivalent) through the inguinal canal. This usually works fine, but because evolution isn't perfect it sometimes causes some nasty hernias to come through that same hole. I'll share the most PG of the pictures I found (freaky thing to Google).
Because our external temperature is not static, nor can our testicles be.  There are two  main muscle groups that control the movement of the testicles closer and further from our warm bodies.

  • Dartos Muscle
    • This is a layer of muscle right below the skin of the scrotum.

Dartos muscle can be seen as the red, muscular layer right below the skin.
  • Cremaster Muscle
    • Provides much of the up, down lift to thermoregulate and protect.

  • Cremaster and Dartos Reflexes
    • So, evolution has a problem.  It wants our testicles to be cool, but the further out the more vulnerable they become.  The musculature above isn't just to regulate temperature.  This can easily be demonstrated by the cremaster and dartos reflexes.  Many men (and women by extension) are unaware that if the inside of the thigh is touched it causes the testicle of that side it rise closer to the body for protection.  Many men will also relate to feeling a sympathetic pain if they hear/see something extremely painful or disgusting.  I don't know if this is cremaster or something else, but I've heard from others that they experience it.  (Comment anonymously below if you know what I'm talking about, can recount a time and physiological response.)

Dolphins--are warm blooded, but with internal testicles.  Apparently they actually have veins that run near the testicles after having circulated below the skin to cool down--a.c. for their testes.

Pictures from here, here, here, here, here.


  1. Very interesting and informative!

  2. Glad you think so! The body is a wonder-work!


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