Why didn't humans evolve with the testicles safe inside the body?

Having a newborn son raises all sorts of interesting questions. During several examinations the doctor checked to make sure his testicles decended correctly - this makes me wonder why they need to be outside in the first place.

I know that right now the testicles have to be outside the body because sperm require slightly cooler temperatures in order to develop properly - but it seems it would make more sense for the testicles to be safe inside and able to produce sperm at normal body temperature.

Is there any evidence why it ended up this way? Is it that there was just no evolutionary pressure to go one way or the other? Or things just never mutated in a way where it would be an option?

I suppose it would make more sense to have them inside the body and just make them able to cope with the extra heat, but evolution isn’t able to plan ahead in this way.

That all mammals (or the vast majority of them) have external testicles would seem to indicate that the evolutionary branch that led to it happened very early on and that we’re pretty much stuck with it now.

Evolution works like that - the arrangement of stuff in/on the retina is far from optimal, but we’re stuck with it because we are now way past the point where it can be changed by a small amount of mutation.

Think of it this way; you’re driving and you come to a fork in the road - the left-hand fork will take you to the mountains, the right-hand fork will take you to the coast. You turn left.
Now as long as you’re quick about it, you can probably change your mind and detour in such a way as to eventually end up at the coast, but the further you go along the mountain road, the more committed you become; you might still be faced with choices, but after a certain point, none of them will lead you back to the coast road.
Again, that isn’t to say that evolution has some kind of higher purpose or goal in mind, only that the available options at any given point are necessarily restricted by the legacy of whatever currently exists with which to work.


Because sperm cells can be killed by elevated body temperatures.

So, they’re external to keep them cool.

BTW–high fevers have been known to cause sterility in men.

There must be more to this. Why are sperm cells killed by elevated temperatures that all other cells can survive perfectly well? Is it some special characteristic of the cell that’s useful for their function (like their size or special chemical composition), or is it just a chance thing? Does being killed in hot temperatures help sperm in some way (like competition between sperm seems to weed out less healthy sperm before they can fertilise the egg)?

Perhaps God had a sense of humor that included thinking it was funny when guys got kicked in the nuts.

They’re not killed per se, they just cannot be formed. Obviously, even with retained testicles, the tissue is still alive (and some cells still produce testosterone)… otherwise you’d have gangrene.

The cells that produce sperm happen to work better at lower temperatures. That said, the way testicles are, they already receive low oxygen levels (compared to other places). Increased heat increases metabolic demands, but does not increase oxygen blood flow, and the oxygen that arrives is not enough to meet the demands caused by higher temperatures. Thus the spermatogenesis shuts down.

Now, some animals do have testicles (marine mammals, elephants) inside the body, and at least some have developed an internal cooling system around the testes.

They can live in the female reproductive region (which is at or near 98.6 degrees) for up to 48 hours or slightly longer.
The cooler temperatures are needed for the production, not the support.

Jonathon Woodall, Ha, even so much as a little flick with a finger can induce the “Creeping Pain of Death”.

Me-thinks the reason for external testes is this;

Long time ago, our male mammal decendents had less real-estate (smaller bodies). Larger testes would produce more sperm resulting in more chance for offspring. Since real-estate was at a premium, the testes expanded outside of the body - resulting in the ever popular position that they now hold.

I think it involves the paradoxes of costly sexual display, i.e. a peacock’s feathers and a baboon’s red ass. Sort of, “A man must be incredibly capable to be able to afford to have one of the most important symbols of his masculinity dangle in the wind.” There is a reason that phrases like “balls of steel” become associated with successful risk taking in popular culture.

people always give the "they need to be cool’ answer. it seems so circular to me.

why did testicals evolve to be outside and vulnerable?
because they need to be outside to work.
why did they evolve the need to be outside?

Yeah, She’s a real joker… :smiley:

If I may I’d like to change “symbols” to “organs” because it’s the physical risk that creates the symbolic statement.

Because there’s no selective disadvantage to external testicles. Men born with testicles slightly closer to their body are not more likely to survive and reproduce than those with testicles slightly farther from their body.

We have them outside so your wife can fondle them.

Some people just want to take away all the fun. Next thing you know they’ll propose women have internal breasts too.

Or, as the case may be, your husband.
Perhaps things would be different if enough of our very distant ancestors had their testicals ripped off before reproducing. Fortunately, that wasn’t the case.

This reminds me of an experiment that was done where they had men soak their genitals in very warm water for nearly an hour each day as a possible means of birth controll and after a week of doing it the sperm count dropped to almost nothing. It went back up in a few days as soon a they discontinued though but it might be a practical means of birth controll in undeveloped countries without access to other forms of birth controll.

You don’t think testicles dangling outside the body are more susceptible to a potentially sterilizing injury vs. those tucked safely in the body cavity?

Well, while that is the obvious assumption, 6 billion people and countless billions of other mammals prove that this fear is thoroughly unfounded. Obviously, the risk of sterilizing injury is quite insignificant.

Men are just careless and foolhardy with their reproductive organs.

Also, they need to be in an obvious, easy-to-locate spot because men have a hard time finding things.

When you take the remote control and put it neatly on the shelf next to the TV, a man can no more find it than he could find something dumped to the very bottom of a mile-deep ocean canyon. Just to, their naughty bits must be right there in the open, handy for grabbing.

So on the one hand, we have vulnerability, but on the other hand we have the poor man’s lava lamp - perhaps the evolutionary weighting is understandable.

You quoted me quoting someone else so I can’t really decide if your arguing for his point or against mine. Could you elaborate on what it is that you do/don’t agree with because depending on what exactly your saying it sounds like you’re either denying an antecedent or affirming a consequent.