It seems to me that most mammals carry their testes on the outside. I know that sperm production is best (at least for humans) when the temperature is a few degrees cooler than body temperature, which is why the factories are hanging off the body, away from the heat. But I’ve got two questions:
What exactly is it about sperm production that requires a lower temperature in the first place?
Why is it set up this way? I can’t figure out exactly what advantage there is to having sperm produced best at a cooler temperature. Is it just that there’s no disadvantage, in the evolutionary sense, and so humans haven’t evolved internalized male gonads? If so, why are most mammals (I think) set up the same way?
Which wouldn’t make a blind bit of difference if no-one carried their nuts on the outside (except the “cooling” part).
I like the bit about “wars of old” - if I’m standing there with a club and someone else comes at me and brandishes his testicles at me, I’m sure I’ll think of something to do.
My WAG: I’ve always thought that Body Temprature was one of the factors for “activating” sperm. Ie: once introduced into the female, it’s one of the factors that helped the sperm with motility and driving them towards the egg itself.
So I’d wager a guess that if you’re making the suckers, it’s best to keep 'em at cooler Temperatures so you don’t have them going off? But then again, sperm aren’t stored in the Testis, more in the epididymis… Which is still located in the scrotum though.
That’s basically what’s going on when the Sperm is in vivo in the female.
So the gist of it being:
More fluid membrane = increased permeability to Ca2+ = increase in motility.
The key there is basically the increase in motility business, that’s a key factor for sperm finding the egg, and you don’t want that going on while the sperm is just sitting in your Scrotum. And one easy way to increase fluidity of an object is to increase the temperature, as that makes things more fluid (including cellular membranes).
So you keep your sperm stored in a cooler area so they don’t have fluid membranes and start trying to get motile before they enter the female reproductive tract. If they were inside YOU, then the sperm would basically be already motile, and you wouldn’t really be able to store sperm for as long, as they’d use up their energy reserves while still in you, and basically not have enough energy for traveling through the uterus.
So there ya go! A WAG + Wikipedia, and I think I may have actually stumbled upon the answer!
Science: Gotta love it!
You are begging the question. Had we evolved such, then there would be room. Women keep their ovaries on the inside, so it is possible, from a space organization standpoint, to have the gonads internal.
While the funny answers are entertaining, gentlemen, I’m really looking for a factual answer to this one.
Ok, a nice enough theory, but isn’t prostatic fluid full of simple sugars, to help give the sperm energy for their trip? They’re not really running on their own reserves, so “conservation of energy” doesn’t really make sense to me.
Good questions. I don’t know the exact answers, but I suspect they’re based on two factors:
[ol]
[li]There’s no disadvantage to having them on the outside that is sufficient to cause natural selection against it.[/li][li]Sexual reproduction started with some proto-animal that had the sperm organ on an external organ or part. That animal was certainly water-based, and released its sperm into the water next to the eggs ejected by the female. [/li][/ol]
In short, you’re looking for an “intelligent design” (note the lower case). Biology doesn’t exactly work that way. It’s more a matter of starting somewhere, no matter how strange, and then seeing if it works most of the time. If it does, and nothing else comes along, then that’s what you get.
Note that the “something else” has to give some initial survival or reproductive advantage. This something else must lead to many more young individuals with that feature, who go on to reproduce much better. That way, their genes “win” in the genome.
I can’t think of any fully land animal where the egg is fertilized outside the female (any examples?). The water method of dropping them off nearby and seeing what happens doesn’t work on land. So eggs are inside, sperm must get inside, at least that part of the apparatus has to be on the outside. So it makes some sense that the whole shebang started on the outside and never made its way back in.
Except that the ancestral location of male gonads is on the inside. Mammals are really the exception, since reptiles, amphibians, and fish all have internal gonads. Given the hazards that a testicle faces during development and descent, I’d say there would have to be some positive selective advantage for an external location in mammalian ancestry.
I think this is really it. The danger of losing reproductive ability to testicular injury is apparently outweighed by the advantage of increased reproductive ability granted by the increased motility of chilled testicles.
It was a joke. I have no idea why we evolved to have our gonads on the outside but for whatever reason it seems to work. I suppose that’s probably the best reason right there. It simply works and if it ain’t broke why fix it?
I agree. Have you ever seen a woman try to scratch her ovaries? I think I was watching some video, and I swear I saw some woman trying to do just that. It seemed she was having so much trouble reaching them with her fingers, she kept moaning and moaning. Eventually, she resorted to some elongated vibrating scratching device. When she finally got to the itch, she moaned loudly and then looked very relaxed.
IANAnAnimalReproductionSpecialist, but, from what I understand, no one is exactly sure why they need to be at cooler temperatures. As others have pointed out, every other kind of animal has internal testes. We just know that things go wrong when mammal’s get too warm. And it’s not just as simple as energy conservation: Sperm production stops at the cellular level when the scrotal temperature is too high for the species. Bulls kept in the high temperatures of California’s central valley will stop cellular activity in the testes during the summer months.
