Okay, the truth is that, because of evolution, we were completely half-assed together. No intelligent design was needed, and with it we’d’ve turned out a lot more efficient. Certain chemicals require that our balls stay a couple degrees cooler than what our other chemicals require the rest of our processes operate at? No problem, if you are driven by what has worked so far and not by some imaginary requirement that everything work at the same temperature.
We are not an assemblage driven by optimization. Our development, and that of all other living things, was driven by “It lived long enough to reproduce.” Whatever organism provides that is likely to be suboptimal. In other words, a piece of shit; like a beater car that gets you to your next date, but shouldn’t be trusted to take you to your alma mater for homecoming.
Oh, lighten up. We’ve pretty much established that the scientific community hasn’t reached consensus on the OP’s question, so nobody really knows. Everyone here who’s posting a theory, with or without cites, is guessing. So chill.
Maybe this *is *the advantage - keeping your nads outside your body, and in harm’s way, is your method of demonstrating your worthiness to breed. That is, only those individuals who are strong enough, aggressive enough, and good enough at protecting themselves (and, by extension, their clan) are able to retain their manhood long enough to impregnate the females.
If the family jewels were safely tucked inside the pelvis, any mope could keep them intact long enough to spawn a young’un. This way, only the strong keep their baby batter in tip-top condition.
They evolved so that females of the species, who tend to be smaller and weaker physically, would have a good place to kick in case they are attacked by a male.
First off the trait itself has no survival value. Other ostentatious displays of fitness all have a starting point that is a valuable indicator of fitness in and of itself. The display may run away from there, but there needs to be that initial starting point to work from. For example the ancestral peahen valued males with bright tails because bright plumage in itself indicates that the male and his offspring have a good resistance to parasites. As a result females that valued bright tails had more surviving young and the whole process could snowball from there.
In your example what’s the starting point? Why would the offspring of males with external testicles have survival advantage? They only seem to have an obvious disadvantage. As a result any female that favours males with external testes will have fewer offspring and the preference itself will become extinct in short order. Sexual selection traits with a negative evolutionary impact can’t just evolve out of whole cloth for that reason. They need to evolve from a progenitor that gave a distinct reproductive advantage to the females that favoured them.
The second problem is that it would be far simpler to cheat than to actually develop this trait. IOW if females did like external testicles then the optimal solution would not be to have your nads swinging in the breeze, with all the plumbing complications and risks associated with that. The simplest solution would be to evolve deposits of fat or connective that mimick external testicles, while keeping the testicle safely tucked away inside. While there’s no guarantee that any trait will actually survive, if we incorporate the objection that there is no inherent survival advantage in having external testicles then cheats would certainly have evolved in parallel and without all the limitations.
Things like the peacocks tail can evolve despite a considerable evolutionary cost because there is no simple way to mimick the effect without the cause. But when the effect is simply a bulge under the skin that can be accomplished so many ways that It is improbable that numerous cheats wouldn’t have evolved.
Humans also have this, a networks of capillaries that surround the spermatic cords and testes. The increased surface area for blood acts as a coolant system. It’s called the pampiniform plexus. (Cool name, huh?) An enlargement of one of these capillaries is called a variocele. Picture
Not really. While it’s true that the ovaries don’t connect via ducts to the fallopian tubes, they’re not entirely open to the abdomen as such. There is a membrane which sandwiches the whole set of structures - uterus, fallopian tubes and ovaries, like a double layer of plastic wrap. It’s called the mesovarium, *mesometrium *and *mesosalpnix at various places, (nice drawing of a cut-away side view here). And the fimbriae of the fallopian tubes are very very close to the ovary, so close that they appear to be touching when you dissect the thing. We take our teacher’s word for it that in a living woman, there’s some tiny space there. But the mesovarium et al keep ova from wandering off too far and keeps the whole set of structures anchored in place, while the wavelike motions of the fimbriae set up a current to “suck” the ovum into the fallopian tube. That’s why extra-uterine pregnancies are almost always stuck to the fallopian tube. As long as there’s no tears in the mesovarium et al, a fertilized egg won’t generally wander off to, say, a kidney, and implant there. (Thank goodness!) Anatomy pictures for laypeople always seem to leave off the thick layers and layers of mesenteries we’ve got in there keeping things separated and in place.
As for why they don’t connect, it’s because of the prenatal development of the Müllerian ducts into fallopian tubes. Could it be different? Sure. But so far, either no one’s mutated to merge their Müllerian ducts with their gonadal ridges, or it hadn’t helped them get pregnant more than those of us without that mutation.
*Yes, I know they’re not technically ova yet. Work with me.
Your science teacher was making things up. Overpopulation of the species has never, until modern times and even now, only in very specific areas, been a problem. Certainly not long enough to have evolved that sort of system to be a reproductive advantage.
Sperm live about 5 days inside an obliging, healthy fertile woman’s body. They don’t last longer because they run out of carbohydrates for fuel. That little head is mostly DNA, with a small packet of fuel attached. Run out of carbs, and the sperm dies.
Sperm don’t die at body temperature. That’s a common misconception. Sperm can’t properly *develop *at body temperature. It’s much more accurate to say that spermatids, or spermatocytes, can’t thrive at body temperature. Sperm do just fine at it. Spermatids require proteins for their development which denature (change their shape and become useless) at or before 98.6.
No, we’ve established that we’ve got a public message board, where lots of partially educated lay-people are as welcome to post their understandings of the issue as people who know know more about it.
Look, there’s no question at all as to the evolutionary purpose of testicles: to keep things cool because sperm develop better at a temperature slightly lower than that in the abdomen. We know that because men with undescended testicles or men who wear tight underwear and have a higher than normal body temperature have deformed sperm, and far fewer sperm overall. This is not a medical mystery.
No, not in this case. There’s no evidence (that I know of) that shows external gonads linked chromosomally with another advantageous trait. External testes *are *better, for humans. If you don’t have them, you don’t have healthy sperm. With fewer healthy sperm, you have fewer healthy offspring. Enter Darwin, stage left.
Now…why did sperm evolve to develop best under the influence of proteins which denature before 98.7 degrees? Ah…now* that’s* a mystery**!
**Other than the always true “because a successful mutation made it so”, of course.
Ack, I just read this again and need to correct one more thing.
No, the woman’s immune system does not react to the ovum as a “wrong” or intruder cell. Immune response is based on proteins in the cell membrane, not the DNA. Since the ovum was made by the woman’s body, it has her marker proteins on its cell membrane, and so is not be attacked by her immune system.
Besides which, the cell membrane of the ovum is never exposed to the interstitial fluid of the woman. The ova is surrounded by hundreds of her cells which contain lots of calcium ions. These cells are collectively called the corona radiata. When the first sperm burrows its way through these calcium packets and through the surface membrane of the ovum, the ovum releases chemical ions which causes the corona radiata cells to burst, releasing their calcium ions. This makes the surface of the cell impenetrable to the rest of the sperm.
Arkcon, you seem like you have a good head for science which is hampered by not knowing enough facts. You’ve reached a lot of erroneous conclusions not because you’re not bright (obviously, you are!) but because you don’t have the foundation of knowledge you need. I really suggest you take an anatomy/phys class or two at your local community college. I think you’d find it just fascinating and even fun. I know I have!
I’m thinking, WhyNot, that they’re referring to a cooling system slightly different to the pampiniform plexus (which is what many mammals, not just humans) have. Remember, the pampiniform plexus works when the testicles are outside the body, the system they’re talking about is for keeping testicles that are inside the body cooler. And the pampiniform plexus also cools the blood before it supplies the testicles.
And regarding the fimbria not touching the ovary, in some species, I think they do. Or at least, their connective tissues are more developed so yes, the ova have no way to go but to the oviduct. In other species (like humans), the connective tissue is there, but not as well formed so that the possibility of the ova going somewhere else is there (small chance, but it is there).
And btw, I’m not as sure in the ovary, but in the case of the spermatids, they ARE kept separate from the blood (blood-testicle barrier, IIRC), and if there is a break of that barrier, the body will mount an immune reaction against the immature sperm.
Thanks for your well-informed and interesting post. I just wanted to mention that when I said there was “no scientific consensus,” I meant the very mystery you mention here. Not “Why are they external?” but “Why are we set up so they need to be?” which is how I understood the OP’s question.
Yes, that’s what I was taught, too. What I wasn’t taught was why; none of my teachers know why the immune system is triggered by what are genetically the male’s own cell membranes in a man without an autoimmune disorder. Do you know? (If it isn’t too much of a hijack, of course.)
Because they do not necessarily express the same cell markers than the other ones, and also, because the immune system didn’t “grow up” recognizing the cell surface proteins on those cells as “self” (because of the barrier). Which, I would think, would be the same case in the ovary (if there is a similar blood-ovary barrier analog to the blood-sperm barrier, I forgot).
Hmmm…no one mentioned one, but that’s certainly not definitive. So the barrier is there because…the barrier is there. Spermatocytes need protection because they’ve always had protection. Huh. Well, guess it makes as much sense as the anatomy of the knee!
There’s loads of other activities that can harm the testes though - for instance, something like 1 in 20 men suffer ‘testicular torsion’ at some point in their lives, which is where an abrupt temperature change or physical impact twists the testicle and restricts its blood supply. It’s a medical emergency that requires an operation within 24 hours to prevent the testicle dying and turning gangrenous. So there must be a pretty serious advantage to keeping them outside the body, if evolution is willing to take that risk!
I don’t think this is a fair characterization of evolution. You’re right that technically evolutionary forces aren’t driven by optimization, they’re driven by ‘what works’ - but when species are fiercely competing in an environment with scarce resources for millions of years, ‘what works’ starts to come pretty close to ‘what works best’. When it comes to hugely significant, macro-level traits like the testicles being outside the body, you can almost guarantee that there’s a specific evolutionary reason for it. Human bodies have been honed closer and closer to perfection over millions of years, in highly competitive circumstances. They’re not ‘beater cars’, they’re Ferraris.
Well, these are cells that are undergoing meiosis and dividing their chromosome number in half, and like I said, the body does not recognize those as self because, when the immune system was getting geared up, either the gonads had not developed completely or the barrier was already there. It is possible, that if the immune system had been prepped to those dividing meiotic cells from the beginning, it would have no problem recognizing them.
Heck, in animals the immune system will sometimes recognize viruses as “self” and won’t mount a response if the fetus is infected during a certain time in gestation.
Why it developed that way, I don’t know. Repro tracts and immune tracts failed in communicating basics?
