No, not the obvious.
Specifically, what evolutionary advantage is served by having sperm created outside the rest of the body, under a lower temperature?
For whatever reason, mammalian sperm operate better at a lower temperature than is optimum for other bodily tissues, such as muscles. Therefore, hanging them away from the main body makes it easier to maintain this lower temperature.
I believe marine mammals like dolphins and whales do this by having blood from near their skin, which is cooled by their environment, pass by the testicles on its way back to the heart, which serves to keep them at a lower temperature.
From an evolutionary viewpoint, males that had better mechanisms of keeping sperm at lower than body temperature left more descendants, perpetuating the mechanism(s) used.
I remember an interview with an evolutionary biologist several years ago. The reporter asked her “If you could ask God one question, what would it be?”
Her response was your OP.
Spermatogenesis (the process of making new sperm) is less efficient at lower or higher temperatures; if a man can’t create as many sperm, it’s less likely he’ll impregnate a partner each time they have intercourse, and so it’s less likely he’ll be able to reproduce and prolong his genes.
Although it’s always struck me as strange that there’s an evolutionary advantage to keeping the testicles outside the body, where they can be accidentally damaged or deliberately targeted by attackers so easily - if they’re damaged to the point where you can’t reproduce, that’s an evolutionary game over, so it’s a pretty big risk keeping them in such a vulnerable place (think of all the activities that make men infertile simply by accident - bike riding, lots of hot baths, sitting laptops on the lap, etc). But it seems that’s where the advantage lies, so who am I to argue…
And here’s some discussion from wikipedia which suggests two alternative hypotheses for why testicles are outside the body (namely, that we are descended from an animal that needed testicles too large to fit inside its body, meaning we are stuck with spermatogenesis enzymes that function better outside the body; and that it protects the testes from abdominal cavity pressure changes).
[QUOTE=Testicle - Wikipedia]
The basal condition for mammals is to have internal testes. Only the Boreoeutherian land mammals, the large group of mammals that includes humans, have externalized testes. Indeed their testes function best at temperatures lower than their core body temperature. Their testes are located outside of the body, suspended by the spermatic cord within the scrotum. The testes of the non-boreotherian mammals such as the monotremes, armadillos, sloths, elephants remain within the abdomen.[7] There are also some Boreoeutherian mammals with internal testes, such as the rhinoceros.
Marine boreotherian mammals such as whales and dolphins, also have internal testes, but it has recently been shown (e.g., for dolphins) that they use elaborate vascular networks to provide the necessary temperature lowering for optimum function. As external testes would increase drag, many boreotherian aquatic mammals have internal testes which are kept cool by special circulatory systems that cool the arterial blood going to the testes by placing the arteries near veins bringing cooled venous blood from the skin.
There are several hypotheses why most boreotherian mammals have external testes which operate best at a temperature that is slightly less than the core body temperature, e.g. that it is stuck with enzymes evolved in a colder temperature due to external testes evolving for different reasons, that the lower temperature of the testes simply is more efficient for sperm production.
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More efficient. The classic hypothesis is that cooler temperature of the testes allows for more efficient fertile spermatogenesis. In other words, there are no possible enzymes operating at normal core body temperature that are as efficient as the ones evolved, at least none appearing in our evolution so far.
The early mammals had lower body temperatures and thus their testes worked efficiently within their body. However it is argued that boreotherian mammals have higher body temperatures than the other mammals and had to develop external testes to keep them cool. It is argued that those mammals with internal testes, such as the monotremes, armadillos, sloths, elephants, and rhinoceroses, have a lower core body temperatures than those mammals with external testes.
However, the question remains why birds despite having very high core body temperatures have internal testes and did not evolve external testes.[8] It was once theorized that birds used their air sacs to cool the testes internally, but later studies revealed that birds’ testes are able to function at core body temperature.[8].
Some mammals which have seasonal breeding cycles keep their testes internal until the breeding season at which point their testes descend and increase in size and become external[9]. -
Irreversible adaptation to sperm competition. It has been suggested that the ancestor of the boreoeutherian mammals was a small mammal that required very large testes (perhaps rather like those of a hamster) for sperm competition and thus had to place its testes outside the body.[10] This led to enzymes involved in spermatogenesis, spermatogenic DNA polymerase beta and recombinase activities evolving a unique temperature optimum, slightly less than core body temperature. When the boreoeutherian mammals then diversified into forms that were larger and/or did not require intense sperm competition they were stuck with enzymes that operated best at cooler temperatures and had to keep their testes outside the body. This position is made less parsimonious by the fact that the kangaroo, a non-boreoeutherian mammal, has external testicles. The ancestors of kangaroos might, separately from boreotherian mammals, have also been subject to heavy sperm competition and thus developed external testes, however, kangaroo external testes are suggestive of a possible adaptive function for external testes in large animals.
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Protection from abdominal cavity pressure changes. One argument for the evolution of external testes is that it protects the testes from abdominal cavity pressure changes caused by jumping and galloping.[11]
[/QUOTE]
Just a heads up, many, (most, almost all?) mammals can retract their testes, into the body cavity in times of stress. Only humans can’t, because our upright posture causes the retracting ligament to become entangled in our peritoneum. That said, under real stress, the human males’ body still tries, “Ow … hernia.”
Cellularly, and biochemically, spermatozoa are pretty different from somatic cells. When you look at them under the microscope, they’re really only slightly bigger than bacteria … they look like periods at the end of a sentence, at max magnification, while regular cells look to be fingernail sized at that magnification. Spermatozoa have pumped out much of their water, and moved cellular structures around … in short, they’re very strange cells. I wouldn’t have anticipated those changes would require slightly lower temperature, but with hindsight, it seems a plausible explanation.
But ovaries don’t fare much better. What brilliant idea was that … “OK, these organs ripen and release ova periodically. Let’s put them near the fallopian tubes where the ova belong … but not touching” But you get the idea, they have to be kept separate, their products have only half the genetic complement, their cell surfaces read “wrong” to the immune system.
[EDIT]
Ah, wikipedia has a more through definition. Good stuff. But not really less speculative than mine. Heh.
Cite for any of this?
o_O
ut oh…
Not to mention, the dangly bits of quadrupeds are somewhat well-protected by their hind legs, and predators tend to attack the head & throat instead of the loins. And the good ol’ “swift kick in the nuts” trick really only became an issue once bipedal creatures evolved. 
As for the OP, the explanation I’ve heard is that it’s to prevent sperm from lasting more than a few days inside the womb – if sperm persisted longer, the female would get pregnant every time she ovulated, leading to severe overpopulation of the species. (No cite, it’s just what my science teacher told me.)
[quote=“Blake, post:7, topic:498290”]
[quote=“Arkcon, post:6, topic:498290”]
Just a heads up, many, (most, almost all?) mammals can retract their testes, into the body cavity in times of stress. Only humans can’t, because our upright posture causes the retracting ligament to become entangled in our peritoneum. That said, under real stress, the human males’ body still tries, “Ow … hernia.”
Sorry. If I wanted to give citations, I’d work for Wikipedia, or better yet for Cecil, or Snopes. This isn’t the first few posts, so a WAG is not disallowed.
When I had to kill and remove mouse testes, for a developmental biology class, I was told to avoid stressing the mice, or I’d not find testes. There. I went to school, and I know some stuff. Just ignore the text, if that is inadequate.
Sorry to flip out on you, but I’m sick and tired of the lame-ass “Cite” “Cite” “Cite” comments. I don’t have a citation. If it were a citation of value, I’d be in a text book, in a university library – not some lame ass web page, so you and I can’t reach it, right now. Those are the only type of citations you’ll find on wikipedia – the pages that sell holistic cleansing crystals are called “sources” and my recollection of a university textbook is “original research”, fine. Just please ignore me and my previous WAGs, seriously, my feelings won’t be hurt for being ignored. What’s the source of the wikipedia article quoted above? Just another forum.
I don’t think all mammals, or the majority of them, can retract their testicles, but some species, particularly rodents and lagomorphs (bunnies) have open inguinal rings and can and will retract their testicles (makes sexing them harder).
In regards to larger animals, bulls (and maybe horses) have stronger cremaster and tunica dartos, the muscles responsible for bringing the scrotum (and testicles) closer to the body. Useful during cold winters. It wouldn’t surprise, although nobody has told me, that other ruminants have similar mechanisms.
After all, they want the testicles cooler than the body temp, not forming little ice-testicles.
As to my cite? Books, classnotes, and some experience, consider I work with animals. 
:rolleyes:
IOW you have absolutely no evidence to back up your extraordinary claims. Not only that but they simply do not gel with what do know to be fact. It’s just some guff that you made up.
I’m with KarlGrenze on this one. The majority of mammals can’t retract their testicles into the body cavity. They are just like human sin that they can lift the testicles so they lie closer to the body, but they can’t be retracted into the body cavity.
I agree. Better citations are in the last two posts. I was mistaken, that most mammals retract the testicles, into the body cavity. There may be sources for other things I said, but I don’t have them handy. They are not made up. But I will not defend them.
First of all, textbooks and the like are perfectly valid cites, even if they’re not quite as convenient. Second, the cites of most value would be in scientific journals, all of which are online now anyway. Third, it’s acceptable to have statements without cites, but you should be aware that on this site, at least, a lack of cites in a post will be regarded as weakening that post. No, that’s not in the rules, but it is a part of the culture of this board.
Yeah, but what about Tiger Tanaka?
But God knows, there are times we try real hard. :eek:
I’ve wasted my life. Really, I have. I’m not stupid, but is my brain better used curing cancer or catching James Bond references? Yeah, obviously, the former, but my brain disagrees. 
Evolution doesn’t select for individual traits. All you ever get is a package deal.
A hundred million years ago, there was a freakish proto mammal with inside out nads. For some some reason (maybe he also had bigger teeth or could run faster) he was successful. His ancestors inherited the sharp teeth and super speed. They also got the weird balls.
The package deal was good enough.
Millions of years passed and air chilled testes became the norm. Not because they are intrinsically better than any other arrangement, but because nothing better turned up.
Those activities (particularly the last) have existed for a short time in evolutionary terms, though.
Cite?
Not questioning your veracity, I just gotta get me a gander at that prehistoric weird-balled wolverine. 