at some point they backtracked on scrotality
October 3, 2018 9:59 AM   Subscribe

Natural selection has sculpted the mammalian forelimb into horses’ front legs, dolphins’ fins, bats’ wings, and my soccer ball-catching hands. Why, on the path from the primordial soup to us curious hairless apes, did evolution house the essential male reproductive organs in an exposed sac? It's like a bank deciding against a vault and keeping its money in a tent on the sidewalk.Why are testicles kept in a vulnerable dangling sac? It’s not why you think.
posted by sciatrix (70 comments total) 58 users marked this as a favorite
 
Fascinating. I've not known the history of testicles so this article made a vas defrens in my understanding.

Seriously, though, very interesting article.
posted by Joey Michaels at 10:15 AM on October 3 [86 favorites]


Great article, fascinating and well written. Thanks for posting.
posted by YAMWAK at 10:17 AM on October 3 [1 favorite]


Note, this is from 2013. Still interesting, just wondering if there's any update. I had no idea that mammals varied so much in scrotality.
posted by Nelson at 10:18 AM on October 3 [1 favorite]


but then I met Richard Dawkins...

...who proceeded to trot out the only evo-psych hypothesis in the article. Quelle suprise.

It's like a bank deciding against a vault and keeping its money in a tent on the sidewalk.

The way evolution usually works, the bank would have started out in the tent, but built the enormous brick edifice as a marketing ploy.

Cool article though.
posted by klanawa at 10:18 AM on October 3 [13 favorites]


And again with the titles that have me snorting at my desk!
posted by Making You Bored For Science at 10:21 AM on October 3 [2 favorites]


" The fact that such a ridiculous appendage evolved twice surely means we should be able to get a handle on it. "

I see what he did there.
posted by Kabanos at 10:25 AM on October 3 [13 favorites]


balls
posted by Reclusive Novelist Thomas Pynchon at 10:29 AM on October 3 [4 favorites]


Is this the new politics thread
posted by RobotVoodooPower at 10:35 AM on October 3 [39 favorites]


I thought it was because Evolution anticipated the need for “Balzac!” Jokes....
posted by GenjiandProust at 10:37 AM on October 3 [3 favorites]


proceeded to trot out the only evo-psych hypothesis

It's actually not an evo-psych hypothesis: handicap theory is certainly something that captivates evolutionary psychologists (who may or may not understand how to test for it or what the competing hypotheses are), but its home is firmly in the realm of evolutionary biology, population genetics, and sexual selection.

N.B., my research literally focuses around going "excuse me, this phenomenon that is largely used as an argument for demonstrating handicap signaling/good-genes hypotheses in nature? um, yes, here is a reason that phenomenon might evolve that has absolutely nothing to do with handicap signaling or explaining female preference?" so that particular body of work is something I am very familiar with. I am happy to criticize both it and evo psych, but I want to do so on fair grounds.

more on the cutting edge of testicular evolution research when I emerge from teaching in a few hours.
posted by sciatrix at 10:38 AM on October 3 [48 favorites]


more on the cutting edge of testicular evolution research when I emerge from teaching in a few hours.

So I guess we're all just dangling here, waiting for you to descend slowly (yet repeatedly) into the thread, much as one dips dried Camellia sinensis leaves into water.
posted by zombieflanders at 10:46 AM on October 3 [35 favorites]


Am I the only testicularly-equipped individual who had to squirm to relieve a psychosomatic physical uncomfortableness through various parts of the article?

Oh, yeah, just me? Good to know.
posted by hanov3r at 10:51 AM on October 3 [4 favorites]


Marsupials' testicles hang in front of their penises.

imagine if it had gone the other way, and we'd got front-facing ballsacks instead
posted by BungaDunga at 10:51 AM on October 3 [6 favorites]


What do you mean. BungaDinga? There's one in the White House right now.
posted by Quindar Beep at 10:53 AM on October 3 [33 favorites]


Clearly this means that the bathroom scene from There's Something About Mary was actually an instance of marsupial cross-devolution and not a physically impossible scenario.

Fascinating!
posted by grumpybear69 at 10:59 AM on October 3 [2 favorites]




This was a fascinating article.

My only problem is that I'd like to post a humorous comment about it, but any joke I could make would be low-hanging fruit...
posted by Greg_Ace at 11:05 AM on October 3 [23 favorites]


oh i assumed this was going to be about RDR2
posted by poffin boffin at 11:07 AM on October 3 [1 favorite]


finding out that marsupials have front-facing scrota has in turn led me toward questions about the positioning of kangaroo vaginas and clitorises that let me tell you I never thought I’d be asking.
posted by Reclusive Novelist Thomas Pynchon at 11:07 AM on October 3 [7 favorites]


Thank you for subscribing to Sac Facts!
posted by davelog at 11:13 AM on October 3 [8 favorites]


It's actually not an evo-psych hypothesis

I'll consider myself schooled then.

But I think we can all agree that Dawkins is enough of a prick that he'd use it that way.
posted by klanawa at 11:21 AM on October 3 [6 favorites]


Like the standing vs sitting wipers, the facing the shower vs facing away shower takers, the askers vs the guessers... There is another divide we rarely talk about.

Adult testicle havers who can push their testicles back into their, to varying degrees still open, inguinal canals, leaving an empty and very stretchy sack available for puppetry and practical jokes, and those who can't.

When you are verifying which group you belong to later, also try to guess if the larger testicle, if available, is the one that hangs lower or higher. You may be surprised.
posted by Dr. Curare at 11:23 AM on October 3 [6 favorites]


questions about the positioning of kangaroo vaginas and clitorises

they have a tripartite vagina--it splits into two channels for fertilizing and one for dropping joeys along its length--and I don't know if anyone has looked at kangaroo clits, but tammar wallabies have as far as I can tell chosen to extend the option to extend the clit out such that you can pee down it. No apparent vulva at this reading, but I would expect the vaginal opening to be located anterior to the clitoris in any case.

fuck I'm gonna be late for my class but worth it

posted by sciatrix at 11:24 AM on October 3 [31 favorites]



questions about the positioning of kangaroo vaginas and clitorises


I believe the chicken picture precedent puts you in the clear.
posted by Dr. Curare at 11:30 AM on October 3 [1 favorite]


Marsupial genitalia: ❤️
posted by Kabanos at 11:36 AM on October 3 [1 favorite]


Dr. Curare - can you tie 'em in a knot ? Can you tie 'em in a bow ?
posted by k5.user at 11:39 AM on October 3 [4 favorites]


Only simple knots.

Been working on an icicle hitch*, but it is taking forever.

a knot that is excellent for connecting to a post when weight is applied to an end running parallel to the post in a specific direction. This type of hitch will hold its place even when holding a substantial load on a smooth surface. One can even suspend from a tapered post (such as a marlinspike) with this knot (hence the name "icicle hitch")
posted by Dr. Curare at 11:50 AM on October 3 [3 favorites]


Vervet monkey is a meme just dying to happen.
posted by Halloween Jack at 11:50 AM on October 3 [1 favorite]


The author ignores the neutral hypothesis: Perhaps they just fell down one day, and only a few animals bothered to put them back.
posted by clawsoon at 11:50 AM on October 3 [4 favorites]


The arc of evolution is long, but it bends toward dangly bits.
posted by Mr.Encyclopedia at 11:52 AM on October 3 [5 favorites]


Not seeing much PNAS corroboration ...
posted by tocts at 11:56 AM on October 3 [6 favorites]


Vervet monkey is a meme just dying to happen.

Would you say it's a meme that's teeming with productive potential, just waiting to fertilize online minds that will give birth to a multitude of ideas?
posted by Greg_Ace at 11:57 AM on October 3 [1 favorite]


I just want to know how many generations of selective breeding it will take for humans to get exploding penises like ducks have.

...A friend wants to know that. Yeah. That's it.
posted by delfin at 11:59 AM on October 3 [2 favorites]


Just noting that this article does the thing where gender and sex terminology are frequently conflated.
posted by tobascodagama at 12:09 PM on October 3 [5 favorites]


Not seeing much PNAS corroboration ...

The article does call it a hypothesis, and makes it sound like no actual testing has been done yet.
posted by Greg_Ace at 12:17 PM on October 3


scrotality (to use the scientific term for possessing a scrotum)

Mortal Kombat: Education Edition
posted by oulipian at 12:20 PM on October 3 [12 favorites]


For mammals that returned to the water, tucking everything back up inside seems only sensible; a dangling scrotum isn’t hydrodynamic and would be an easy snack for fish attacking from below.

The Aquatic Ape folks will surely be disappointed to hear this.
posted by tobascodagama at 12:34 PM on October 3 [5 favorites]


please let’s keep talking about delightful science facts or at least let’s not keep talking about the things that dr curare does to pass time
posted by Reclusive Novelist Thomas Pynchon at 12:35 PM on October 3 [1 favorite]


The Aquatic Ape folks will surely be disappointed to hear this.

Didn't we have a thread recently about men crab-walking into the bathtub in order to put the scrotum in last?
posted by clawsoon at 12:35 PM on October 3 [8 favorites]


What a dick move.
posted by Melismata at 12:37 PM on October 3 [1 favorite]


... i hardly know ‘em!
posted by Reclusive Novelist Thomas Pynchon at 12:45 PM on October 3 [2 favorites]




Me [to wife]: “Aha! A relevant post on which I have an excuse to talk about rats’ massive testicles - ”

Wife: “You don’t NEED any excuse. You talk about that with ANYBODY.”

Me: “I do NOT - ”

Wife: “You are actually known as ‘Rat Nuts Girl’.”

Well, then.

As my glamorous assistant Inspector Javert will now demonstrate for your edification, male rats are indeed the proud possessors of disproportionately large bollocks.
posted by Morfil Ffyrnig at 1:12 PM on October 3 [29 favorites]


Whenever the subject of evolution comes up I get a little irked about the phrasing of "why." There isn't necessarily going to be a reason. Evolution is not "here's the problem, how do we deal?" it's more like "let's throw a bunch of random crap at the wall and see what sticks." Can two pointless things evolve separately at two different times in the history of animals? Yes.
posted by picklenickle at 2:31 PM on October 3 [18 favorites]


I share the skepticism about the cooling hypothesis. One thing I've learned from my study of biochemistry and molecular biology is that evolution is remarkably good at fine-tuning enzymes and other important molecules to work best at their usual temperature. If the issue were merely that the core body temperature was a few degrees too hot for sperm, the most likely evolutionary response would be to tweak sperm biochemistry until they did work well at core body temperature, not to eject the testicles from the core. It seems much more likely to me that the scrotum evolved first, and then the testicles and sperm adapted to work best at their slightly cooler temperature.
posted by DevilsAdvocate at 2:39 PM on October 3 [7 favorites]


They hang low when you’re hot and cuddle tight when you’re cold - hard to account for that other than by some version of the cooling theory, surely?
posted by Segundus at 2:43 PM on October 3 [1 favorite]


"Why are testicles kept in a vulnerable dangling sac?" Obviously, the easier to suffrajitsu .
posted by King Sky Prawn at 2:43 PM on October 3 [4 favorites]


One kind of cool thing about scrota is that terrestrial forest monkeys, and monkeys living in the understory of forests, can sometimes end up with a sort of irridescent scrotum and other genitalia? That's how you get something like a mandrill (terrestrial), a lesula (understory), and some other monkeys whose scrota I could not find pictures of.
posted by ChuraChura at 2:44 PM on October 3 [4 favorites]


MetaFilter: some other monkeys whose scrota I could not find pictures of
posted by Kattullus at 2:45 PM on October 3 [15 favorites]


I'm no expert on the topic, but Dawkins' theory sounds like a load of bollocks.
posted by oheso at 2:54 PM on October 3 [1 favorite]


Vervets, too! Their scrotum is really what I think of when I imagine eau-de-nil. Fun fact: this is the only context I'm aware of in which mammals have bothered to evolve the ability to produce true blue coloration.

Can two pointless things evolve separately at two different times in the history of animals? Yes.

Sure, but if they're costly things... why would they be maintained by drift alone? And why would they evolve twice, with different mechanisms of descent, and then be maintained for long periods of time? I mean, this is certainly a possibility--but it's also worth thinking about potential explanations critically, and drift/random chance is just one explanation among many.

(I tend to default to neutralist explanations, but this one has me at a bit of a loss.)

Okay, I promised an update on the literature. I will produce a summary as soon as I finish (and this is true) B.G. Lovegrove's opus Cool sperm.
posted by sciatrix at 2:54 PM on October 3 [6 favorites]


Segundus, that speaks to temperature regulation rather than cooling as such. I'm guessing that body cavity temperature would be much more uniform without the need for some sort of gonadic mobility.
posted by oheso at 2:57 PM on October 3 [3 favorites]


gonadic mobility

Sockpuppet name up for, er, grabs...
posted by Greg_Ace at 3:14 PM on October 3 [7 favorites]


they have a tripartite vagina
I don't know if anyone has looked at kangaroo clits
extend the clit out such that you can pee down it
No apparent vulva at this reading

Metafilter: I seriously can't decide.
posted by mrgoat at 3:20 PM on October 3 [6 favorites]


(I tend to default to neutralist explanations, but this one has me at a bit of a loss.)

I don't want to make a wall of text getting into the fine details, but basically I'm surmising that although there are many ways animals have been maintaining the scrotal descents now that it's there, and although there are various pros/cons to having descended scrotums, whatever initially caused the mutations didn't necessarily need a reason. Or, for that matter, be limited to only one reason. The cooling/galloping/handicap/etc hypotheses could all be contributing factors playing off of each other in varying degrees.

So yeah, mostly just weird about evolution being discussed in "why" terms as if it had some kind of mindfulness.
posted by picklenickle at 3:32 PM on October 3 [4 favorites]


It’s so you can scratch them.
posted by lucidium at 3:42 PM on October 3 [5 favorites]


where's the mammalian peacock, some species toting a pair of soccer balls?
There once was a man of Assizes
With bollocks of differing sizes
One ball was so small
It was no ball at all
But the other was huge, and won prizes.
posted by Joe in Australia at 3:53 PM on October 3 [18 favorites]


...as soon as I finish (and this is true) B.G. Lovegrove's opus Cool sperm.

These sperm, are they eldritch? Convex and concave at the same time?

I enjoyed this article a lot, but wish it had included a phylogenetic tree so I could follow along.
posted by acrasis at 4:01 PM on October 3


There once was a man from Madras,
Whose balls were made out of brass.
When he clanged them together,
They played "Stormy Weather",
And lightning shot out of his ass.
posted by homunculus at 5:22 PM on October 3 [13 favorites]


hi i'm on metafilter and i could overthink a plate of testicles
posted by medusa at 8:04 PM on October 3 [9 favorites]


...some other monkeys whose scrota I could not find pictures of.

I'm sure that in 1000 years 1000 monkeys would be more likely to type the complete works of Shakespeare rather than type that phrase.
posted by BlueHorse at 9:02 PM on October 3 [4 favorites]


One thing I didn't get from the article: why us it more plausible that some mammals ejected their testicles and then reeled them back in, rather than supposing that they only ever partially completed the process in the first place?
posted by Joe in Australia at 9:14 PM on October 3 [1 favorite]


hi i'm on metafilter and i could overthink a plate of testicles

Once they are on the plate, we call them prairie oysters.
posted by Dip Flash at 10:36 PM on October 3 [8 favorites]


Adult testicle havers who can push their testicles back into their, to varying degrees still open, inguinal canals, leaving an empty and very stretchy sack available for puppetry and practical jokes, and those who can't.

wut ?
posted by Pendragon at 12:32 AM on October 4 [1 favorite]


Joe in Australia: One thing I didn't get from the article: why us it more plausible that some mammals ejected their testicles and then reeled them back in, rather than supposing that they only ever partially completed the process in the first place?

I think - though I might be wrong - that it's based on phylogeny, i.e. who descended from whom. If everybody on one branch of an evolutionary tree has a dropped scrotum except one little twig down near the end of the branch, it's likely that the twig is a reversion.

For a more extreme example, you could think of the fins of fish and of whales. Based on evolutionary relationships, we know that the fins of whales are descended from the forelimbs of land mammals, rather than a partial modification of a fish's fin. They are ultimately descended from a fish's fin, since arms are descended from fins, but they're a reversion to fin shape after a detour through being an arm.
posted by clawsoon at 3:31 AM on October 4 [8 favorites]


So the paper I linked above is a 2014 phylogenetic assay taking a really close look at the cursoriality, temperature-regulating. Findings: no significant differences among inguinal, scrotal, and testicond species in body temperature as a function of size; scrotal and inguinal species (defined: no scrotum hanging in a skin sack, but testicles have definitely ventured away from the kidneys and opted to hang out around the inguinal area, often subcutaneously) are more or less identical in body temperature. Testicond species, who keep their gonads up by their kidneys, are a little cooler but not significantly so.

Lovegrove speculates that the temperature at which sperm is kept, on average, seems to be preserved surprisingly close to the ancestral eutherian mammals's testigondic core body temperature (based on the core temperature phylogenetic reconstructions). I have no idea whether he's right, but it's an interesting idea. I do note that both Lovegrove's paper and the more recent paper linked at the bottom are suggesting lots of evolutionary transition in testicle location, not necessarily a single evolution of scrotums and a few obvious losses. That fascinates me and suggests to me that selection is probably playing some kind of role, although I don't know why.

Another recent paper (2014) hypothesizes that the temperature differential is required in order to allow for mechanisms involved in sex-specific genomic imprinting. It's actually a pretty interesting thought, although I think we'd need to know more about imprinting and testicondy in more clades of mammals to be able to evaluate fully.

It is perhaps worth noting that birds, also endotherms with warm internal body temperatures, universally (to my knowledge) retain internal testes. To that effect, findings in 2016 that at least one heat shock protein involved in male fertility and spermatogenesis has been under strong positive selection in birds while remaining under strong purifying selection are pretty interesting. Note that birds do not display genomic imprinting in the same way as mammals.

(Apparently some male birds do develop cloacal protuberances during the breeding season, but this isn't due to gonadal descent--they just, er, store all the sperm they make until an opportunity for ejaculation presents itself. fascinating.)

Confusing everyone, a very recent study (only out three months ago!) found that testicond afrotherians--that is, elephants, tenrecs, manatees, aardvarks, and elephant shrews, among others--appear to possess inactivated copies of two genes that are known from mouse work to be involved in testicle descent. This throws a lot of confusion onto the question of the ancestral scrotal state of all placental mammals, suggesting that maybe the ancestral placental mammal did let its testicles hang low. Who knows!
posted by sciatrix at 7:46 AM on October 4 [18 favorites]


I don't know if anyone has looked at kangaroo clits

People being what they are, I am 100% confident SOMEONE has, the question is whether they were interested in documentary evidence of their exploration being available to others
posted by solotoro at 7:55 AM on October 4 [1 favorite]


Not seeing much PNAS corroboration

Do I have to be the person to point out that PNAS can be forced to rhyme with penis? Ctrl + f

....Apparently I do.
posted by bilabial at 12:59 PM on October 4 [2 favorites]


We were all joking about it the other day on Chura's dung beetle post. :D
posted by sciatrix at 10:45 AM on October 5


After RTFA, came back to say what Kabonos beat me to. :)
posted by pmburns222 at 4:40 PM on October 6


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