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.

Are Humans Evolved for Polygamy or Monogamy?

Are humans naturally monogamous or polygamous?Well, what's science have to say about it?

Since there's more than one kind of polygamy, I'll split it up into multiple male partners per female (polyandry) and multiple female partners per male (polygyny).

First, a little background:  Gorillas mate in harems.  One male per 3-6 females.  Chimps on the other hand have more of a free love mating scheme.

Polyandrous Scale


Are there signs that our ancestors had many male partners?  Yes, but not we're paltry compared to some.  Here's how we can tell: let's say you're a female that has many men to choose from but you only sleep with a few.  In that case male displays become quite pronounced (think peacock tail).  On the other hand, what if a species not only has many males for the female to choose from, but she ends up sleep with many of them.  The competition between males is no longer how can I impress the girl, but how can I get my sperm to the egg more effectively or faster than the other males.

I've heard some discussion that the shape of the male glans is actually a scoop to remove other male's semen while thrusting.  So, that's one way, but a more common one among many species is a simple numbers game--overwhelm the competition with an army of sperm.  How would that change the morphology of an organism?

It would give it huge testicles.

In short, if there are many males sleeping with one female the one with the largest testicles has the best chance of winning.

So, how do human testicles compare to other species?

Gorilla: 1 ounce testicles at .02% of body weight with 50 million sperm per ejaculate.
Chimpanzee: 4 ounces testicles at .3% of body weight at 600 million sperm per ejaculate.

Human: 1.5 ounces testicles at .04-.08% body weight with 250 million sperm per ejaculate.

Summary: we're semi-promiscuous.  Our females fidelity is some where in between gorillas and chimps (but more toward the gorillas side).

There's more to the equation, though!


(Another interesting solution to this problem is to made a vaginal plug out of your semen: http://en.wikipedia.org/wiki/Mating_plug)


Polygynous Scale


Ever wonder why men are taller than women?  Why men can bench more than women?  Why they compete separately during the Olympics?

There's an evolutionary reason for that!

Here's the idea:  when there's a group of males competing for a limited number of females the strongest will have an edge.  The bigger you are, the more intimidating you are, the stronger you are, the better you can fight the more females you can win.

This is exemplified by the gorilla.  We've already said they have small testicles, so that isn't how the males have reproductive success.  What is it?  They get so big and muscular that they can kick all the other male's asses.  Gorilla males have a very, very, very well guarded harem of females.

They don't need big nuts when they've got big guns.

So, the more sexually dimorphic (skewed male/female size ratio) the more polygynous the species.  How do humans stack up?

Gorillas: males are up to 400lbs. Females are around 200 lbs. Males are 100% heavier.
Chimpanzees: males are 170-185. Females are 135-145 lbs.
Humans: males are 190.  Females, 160.  Males are 26% heavier.

Summary: we're more polygynous than chimps, but less than gorillas.

Much more could be said here, but that's something to chew on.  (Aside--high IQ has been correlated with increase monogamy in humans)



It seems conceivable that since human females put so much effort into displaying to males (makeup, cosmetic surgery, clothing, etc.) that males are often the choosers in our species.  Why be choosy? Only because you have something to lose from picking a genetically inferior female - time, effort, resources.  If there was nothing to lose then you'd have no standards.  Being choosy just may indicate that we are a particularly monogamous and caring kind of male species...On the other hand, maybe human females aren't trying to win a male so much as they are trying to keep one.

(Later addition below)
If it's such a "man's world", as some say it is, then why is polygamy out lawed in most countries?  At first blush that doesn't make any sense!  If men rule the world then you'd totally expect polygamy to be in vogue!   Is it that women prevent it to avoid exploitation?   Well, unlikely.  Think about it.  Polygamy is in the best interest of women evolutionarily.  From an evolutionary perspective what do when want out of a man?  Resources to care for a child and superior genes.  In a stratified society (which can lead into polygamy) woman can get both more resources and better genes from being with a highly successful money-bags.  Also, in a polygynous society all the women get a mate (in theory) since even the lower quality ones will be taken since a low quality woman is better than no woman.  Men on the other hand (on average) end up worse off.  One reason being what I just mentioned--in a polygynous society not all of the men get a woman.  In fact, it's interesting to note that monogamy may very well be one of the first steps towards democracy.  The average man started to collectively say that they were no longer going to let a few successful men take all the women from them.  So, contrary to some popular opinions monogamy wasn't a defense of women's rights, but of instead of men's.  I find that fascinating.