Why aren't testicles inside the body?

October 3, 2004 7:44 PM

BallsFilter: Why is male semen best created and stored at temperatures below normal body temperature? Why keep those delicate ovoids outside the body where they're exposed to a good swift kick? Shouldn't they be tucked deep inside for safety?
posted by five fresh fish to health (30 comments total)

(Good point...begin the genetic engineering!)
posted by inksyndicate at 7:46 PM on October 3, 2004

It's a bad design, and clearly the test version. The female design, while it has issues of its own, at least addresses the problem of external genitalia fairly well by keeping the structures internal.
posted by biscotti at 7:56 PM on October 3, 2004

Well, as for creation and storage temp, they're kept cold until they need to do their job, and then die shortly afterwards with normal female body temperature. If they were stored at body temperature, they'd never die once they got into the female. As a female myself, that idea is just ooky. The less living sperm I have swimming around inside me, the better.
posted by evilbeck at 8:02 PM on October 3, 2004

I'd have surgery to get an inny.
posted by holloway at 8:10 PM on October 3, 2004

Not necessarily, evilbeck; they could just have short lifespans. Those things are a dime a dozen anyway.
posted by kenko at 8:16 PM on October 3, 2004

They hang outside so they can dissipate heat and retain the lower temperature sperm require. In other words, its a feature not a bug. More here

TEMPERATURE AND SPERM

A low ambient temperature is essential for normal spermatogenesis. Testicular temperature needs to be from 4­7°C cooler than core body temperature. This is why the testes are designed to drop out of the abdomen into the scrotal sac. Three mechanisms keep the scrotum cooler than the rest of the body:

1. scrotal skin is thin, so the testes easily lose heat into the surrounding environment

2. air circulating around the scrotum can cool the skin

3. the arteries bringing blood into the scrotum run alongside the veins taking blood away to form a sophisticated heat-exchange mechanism. Rather like a hot and cold water pipe running together, the hot arterial blood (coming from the abdomen) loses heat to the cooler venous blood (coming away from the testes), so blood is already partly cooled before entering the scrotum.

Even if the testes heat up by as little as 2°C, sperm formation is adversely affected. Sperm count will drop, the number of normal sperm will fall and the number of abnormal sperm will increase.

Semen quality is naturally lower in summer compared with winter. Although semen volume does not change significantly, the total sperm counts per ejaculation in 131 volunteers fell from 320 million in winter to 250 million during July and August. This is probably a temperature effect.

posted by skallas at 8:19 PM on October 3, 2004

This suggests that if you're really hot you should:

Take off your shoes and socks
Remove anything covering your ears and hands
Expose your scrotum
posted by kenko at 8:22 PM on October 3, 2004

Don't ask evolution "why" questions; the answers you'll get tend to be things like "because it works well enough." Not only is this answer distressingly mundane, it is also disconcertingly close to the reason Microsoft Windows has been so successful.
posted by kindall at 8:28 PM on October 3, 2004

Sex evolved in the sea. My guess would be, like blood reflects the salinity of the ancient seas, the sperm reflects the cool temperature of the sea water where it began to do its thing.
posted by JohnR at 9:14 PM on October 3, 2004

A low ambient temperature is essential for normal spermatogenesis.

True, but the question that was originally asked is, why should that be the case? Every other kind of cell (including the ovum) seems to be able to be produced just fine at body temperature.
posted by jjg at 9:15 PM on October 3, 2004

Ever-living sperm would be ooky, indeed. Over a dozen gallons of ooky, according to a some webpage or other I stumbled across.

But there's no reason they couldn't be quite short-lived, and your body would just absorb them.

The cool seawater idea is great, except that sea mammals don't, afaik, have hangin' sacs. Whales, seals and the like seem to have 'em tucked into the body.

Fish and birds and the like have their sacks in their bodies. Seems to me that there must have been some sort evolutionary pressure to make 'em drop.

Still can't imagine what it was.
posted by five fresh fish at 9:28 PM on October 3, 2004

fff, probably because humans are much more monogamous then their primate relatives. Gorillas pretty much have a harem and have small testicles which don't hang so low. So, they can shoot blanks all day and not really be in too much trouble. I think the human set-up evolved because when a human male shoots and there isn't ammo then the whole tribe is in trouble. Evolution probably chose genes which could produce testicles and sperm which gave the best sperm count after humans became monogamous.

I could be way off. This website talks about chimp, human, and gorilla mating habits with emphasis on sperm, testicle size, etc.
posted by skallas at 9:50 PM on October 3, 2004

The spontaneous rate of DNA mutation increases with temperature (this is basic chemistry) and the the role of the testicles is basically to store DNA for the next generation. Sperm do have built-in DNA repair mechanisms, but why risk overloading them when you can just descend the testicles and cool them down a bit?
posted by shoos at 10:14 PM on October 3, 2004

The proteins in the sperm coat denature as they get warmer (i.e. closer to 37C). If they are kept cooler (34-36C) they remain viable for a longer time. Theory is, once the sperm is deposited (intercourse), they sperm coat denatures, other proteins are exposed which allow the head of the sperm to make contact and interact with proteins in an egg's wall and allow a sperm to enter -- conception.

If the testes were inside the body, the sperm would be denatured extremely quickly (matter of hours), and more sperm would have to constantly be made (because technically, one should always be ready). Keeping the sperm storage outside the body decreases the rate at which sperm needs to be regenerated (it still breaks down eventually, the rate of which I'm not sure of), thus saving the body some energy.

Finally, on the issue of protection. Muscles that control scrotal contraction are not only responsive to changes in temperature, but also activity. You'll notice, the scrotum rarely gets in the way while jogging, preventing injury when sexual intercourse is unlikely. And as any guy will tell you, there is a very protective instinct about that area of the body which also serves to prevent any harm from coming to the testes.
posted by ruwan at 11:17 PM on October 3, 2004

I don't think it has much to do with temperature, shoos--but I could be wrong.

Yes, Ruwan is right: it's called the cremaster reflex. Lifto-change-o, scrotum sack lifts upon touching the inner thigh.
posted by gramcracker at 11:31 PM on October 3, 2004

wrong about what? let's hear it.
posted by shoos at 11:39 PM on October 3, 2004

See here for some insight.
posted by shoos at 11:52 PM on October 3, 2004

Because that other part of our body, the brain, is being relied upon to think nice thoughts about each other, thereby preventing ball kicking activity. The real question then, becomes one about the brain, and being civil, or not being civil, and so forth...
posted by juiceCake at 12:33 AM on October 4, 2004

what people who are explaining aren't making clear, and what people who don't seem to be understanding probably don't realise, is that sperm aren't made fresh daily. they're stored by the gazillion in your balls. seriously. they're already all there and waiting. so they're in a kind of "deep freeze" until required.

(i'm no biologist, but this is what someone taught me once - i hope it's right!).
posted by andrew cooke at 4:36 AM on October 4, 2004

Testicular temperature needs to be from 4­7°C cooler than core body temperature

Aaaaagh!

Are you missing a decimal place?
posted by Civil_Disobedient at 4:59 AM on October 4, 2004

Sperm are in fact made fresh daily, Andrew, on the order of 100 million per day. You're probably thinking of ova, which don't multiply after fetal development.
posted by shoos at 5:03 AM on October 4, 2004

gramcracker - cremaster!
posted by shoepal at 7:35 AM on October 4, 2004

Official Cremaster site (mnsfw)
posted by shoepal at 7:39 AM on October 4, 2004

And we can't forget their value as a manipulative object during sexual intercourse, which ultimately promotes the continuation of the species.
posted by juiceCake at 8:21 AM on October 4, 2004

ah. thanks, shoos. i wish i'd been more critical when i was at school... sorry for the misinformation!
posted by andrew cooke at 8:40 AM on October 4, 2004

Why would you think that "normal" human body temperature was somehow the "norm?" Sperm evolved billions of years before humans did; In a different body, in a different environment, in a different climate.

Whales pulled their testicles inside their body when they migrated to the ocean, so they had to evolve such that some blood vessels would take the long trip down to their tails, where the blubber is thinner, and cool, before flowing by the testicles to cool them.

pwb.
posted by pwb503 at 10:16 AM on October 4, 2004

Sperm evolved billions of years before humans did; In a different body, in a different environment, in a different climate.

Couldn't you say the same thing about every other functional element of the human body? Nevertheless, these elements do fine at body temperature. I like ruwan's answer, but shoos seems to be on to something as well. It sounded far-fetched to me at first, but then I found this paper, which suggests that the mutation rate effect would be significant even with the apparently small difference in temperature afforded by putting the testes outside of the abdominal cavity (look at the concluding paragraph).
posted by mr_roboto at 11:40 AM on October 4, 2004

fff, probably because humans are much more monogamous then their primate relatives. Gorillas pretty much have a harem and have small testicles which don't hang so low. So, they can shoot blanks all day and not really be in too much trouble.


Well, except chimps have low-hanging balls as well.

You can't really as "why" with evolution, since everything evolves at the same time. No simple answer will suffice on a lot of questions (Although some might have an overriding reason), and there's a lot of interaction.
posted by delmoi at 8:06 PM on October 4, 2004

What kindall and delmoi said. For something to be selected against it needs to either:

1) get the organisms who have it killed before they can reproduce

2) cause them not to reproduce (because, for instance, members of the opposite sex aren't interested)

We can call a given evolutionary invention bad design all we like, but if it has survived, it was good enough.
posted by Zed_Lopez at 8:55 PM on October 4, 2004

Very few trisomies or monosomies (chromosomal aberrations) are paternally derived in comparison to those that are maternally derived, and the older the mother, the more likely these are to occur. In contrast, paternal age is almost negligible as a factor in chromosomal aberration. It makes just as much sense, then, to criticize the female design as poor; but, in fact, evolution is simply "what worked" and does not easily yield up facile answers to questions like "why?"
posted by ikkyu2 at 11:21 PM on October 4, 2004