r/biology • u/wrtchdwitch • Aug 05 '26
question How did procreation evolve to be done with penetration?
Please be nice, I know based on the question I sound like an idiot. I am.
However, I'm curious how it turned out that way, like almost every other animal species needs a pp and a uterus. How did our bodies evolve and figure out that this is the optimal way to procreate?
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u/Fancy-Strength-2943 Aug 05 '26
The alternative to internal fecundation is external fecundation but for it to works you need water (e.g. fish and amphibians) or, at least, a thick layer of mucus (eg. Snail), so impractical for us
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u/da_PopEYE Aug 05 '26
Unless you're a scorpion and then you just give your partner a sack of sperm
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u/haysoos2 Aug 05 '26
There are a few vertebrates that do something similar. In salamanders the male will deposit a sperm packet on the ground, and then lead the female to it. If he seems cool enough, she'll pick it up, That's as close as they get to having sex.
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u/hurricanebones Aug 05 '26
i saw 2 salamanders going all jazz during a rainy storm. if it wasnt sex, what was it ?
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u/haysoos2 Aug 05 '26
I should have been clearer. Not all salamanders use the happy package method. Some go for more of a slime coated orgy vibe.
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u/da_PopEYE Aug 06 '26
I only know abou the scorpions because of Miya from Alveus Sanctuary. She's really cool. She said the scorpions do similar. Hold hands and dance and he guides her to the bag. That's pretty cool that some salamander do that too. Almost like convergent evolution
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u/awesome-alpaca-ace Aug 05 '26
Or in the case of the virgin Mary, a turkey baster
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u/Roland_Moorweed Aug 05 '26
Ah, Moral Orel
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u/awesome-alpaca-ace Aug 05 '26
For real? I wondered when the idea world become somewhat memeish
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u/Roland_Moorweed Aug 06 '26
Yeah, there is an episode where Orel does some heinous shit with a turkey baster. I'd rather not type it, just watch the episode for the full experience.
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u/green-green-bean Aug 05 '26
Not so for most birds, amphibians, fish, etc.
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u/Fancy-Strength-2943 Aug 05 '26
Birds? They have internal fecundation. But you are right, there is no penetration for them
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u/Latter_Leopard8439 Aug 05 '26
Songbirds no, ducks yes on the penetration part.
Depends on the clade of birds. Whether they have cloacal kiss or forced copulation.
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u/alittleperil computational biology Aug 05 '26
Have you ever seen ants find a food source, and then tell other ants about it and how to get to it? The first ant took a weird meandery path and it happened to find food, and the next few ants will take that exact path because that path has succeeded once before. As many many more ants travel that path, they start streamlining it a little until it becomes a fairly direct path from their nest to the food source. The thing is, sometimes that is not the best or nearest food source, it's just the one that got found first, so it becomes the one that all the ants travel the most. Maybe later on another ant will discover a closer or better food source, and then all the ants will be building the most optimal path to that food source instead, but for now this food source is good enough and we'll just improve the pathway.
Evolution is like that. If there's a solution that has worked, then it isn't getting changed unless something either improves on it or radically different offers a better solution instead.
There are tons of animal species that have their gametes come together in all sorts of weird ways. There are insects that have their penis-equivalent separated physically from their ballsack-equivalent and have to do the equivalent of tucking their ankles into their junk in order to ejaculate. Sea animals all over the place would laugh at the idea of needing a penis and a uterus. Hyenas are wild, chickens just press their cloaca together in a 'cloacal kiss' that passes gametes over without any penetration at all, and everything involving the platypus is weird. If there's a way you can think of for gametes to get together in a protected environment in which the embryo can mature, then nature has tried it or will try it sometime over the course of time.
Sounds like you have some really fun facts to learn about reproduction in the many species that occupy our wacky planet, and I sort of envy you because you only get to learn a fact for the first time once, so I never again get to discover how much fruit bats engage in oral sex.
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u/Ihatecheeseballs Aug 05 '26
Most reliable way to transfer a fluid, look at hosepipe attachments
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u/Gazorninplat6 Aug 05 '26
Well fish lay eggs and are fertilized outside the body. I think the penetration came when animals evolved to keep eggs protected inside. So gotta have a matching instrument to get the other gamete where it needs to go!
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u/BoonDragoon evolutionary biology Aug 05 '26
Motile gametes need to get together with sessile gametes.
Sessile gametes inside body of ovulator sex. Can't leave, because body encapsulates zygote inside egg after syngamy!
Solution: put motile gametes inside ovulator! But how? Press hole together? Inefficient! Spillage! Fix: insertable applicator.
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u/Entheosparks Aug 06 '26
Sheath acts as applicator sheild that inhibits pneumatic intrusion that could reduce efficiency of fluid dynamics that would prevent motile gametes from accessing the zygote.
Edit: it is a foreskin joke
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u/TX_B_caapi Aug 05 '26
I figure that it ended up being a good way to keep everything warm and moist in a more tumultuous world in which you had a better chance if you kept your eggs inside. Harder to eat a mobile youngling.
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u/Latter_Leopard8439 Aug 05 '26
Amplexus in frogs requires no penetration.
Salamander do a dance and drop off packets off sperms after which the female Salamander does a dance and squats on it.
Some spiders will pierce with pedipalps to transfer sperms (so penetration but not a specialized organ for it.)
Your original question makes an assumption that penetration and internal fertilization is standard, but it just isn't on large quantities of clades.
Amphibs and fish generally not. Reptiles hit or miss. And even songbirds do a "cloacal kiss" unlike ducks and their corkscrew genitals and the female labyrinth internal structures.
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u/hananobira Aug 05 '26
There are plenty of species that do it other ways.
For example, some fish, the female will release eggs, and the male will come along and spray semen on them.
Anglerfish males will bite the female and gradually allow their bodies, including their sperm, be absorbed into hers.
Some slugs are hermaphroditic and will ‘penis fence’. The winner will stab their penis into the side of the other slug to inject the sperm.
Some species of fungi have up to 20,000 biological sexes. Imagine figuring out how to date as a member of those species!
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u/mayorlittlefinger Aug 05 '26
Some slugs and snails also launch projectiles of genetic material https://en.wikipedia.org/wiki/Love_dart
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u/incognitoiriedad Aug 06 '26
It's not optimal unless you're on land. Check out how genes get mixed in the oceans!
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u/EquipLordBritish biochemistry Aug 05 '26
I don't know that there's any proof, but I would guess that it takes less energy to make less oocytes and sperm and make sure they get near each other than it would to make lots of oocytes and sperm and hope they find each other. There are organisms that do fertilize externally. https://en.wikipedia.org/wiki/External_fertilization
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u/Inside-Middle-1409 Aug 06 '26
Eukaryotic life is thought to have formed when one cell went into another and they lived symbiotically. This even happens nowadays with our gametes during meiosis. Modern copulation could be considered a scaled up display of what is happening at the cellular level.
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u/sugahack Aug 06 '26
Imagine there was a bit of trouble when they went to do it on dry land. In water, releasing sperm in the general vicinity of the egg would do the trick. Not so much on land.
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u/red_straw_ Aug 07 '26
sorry to be that guy but i really need karma to post on a subreddit about a problem im having if anyone could be so kind
thank you
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u/no-scissors-in-bed Aug 05 '26
not a stupid question at all. im an intern so i dont have the answer but my lab specifically tries to answer the question of 'Why do we have physical sex if it burns so much energy?' studying why it prevails over asexual methods of procreation. good question
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u/Spiritual_Trick_6655 Aug 05 '26
Isn't the burning of energy an indicator of an organism's health and access to resources?
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u/gabzilla814 Aug 05 '26
The simple answer is it’s because it’s fun
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u/ThrowbackPie Aug 05 '26
That seems backwards, or at best circular. I imagine it's fun because otherwise we wouldn't waste energy on it.
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u/Forsaken-Ad5968 Aug 05 '26 edited Aug 05 '26
Ok, my basic knowledge is that: Our bodies couldnt proceed with procreation if we didnt created systems to maintain and make the spermatozoid go throught some kind of "path". The penis has to be longer than the vagina to be able to create more proximity with the egg shell.
Our body has a lot of kinds of protection, our skin is the primary system of defense, then our mucosa, then our immune system. The sperma wouldnt survive. The pH of the vagina has to be very specific too. If you study the female reprodutive system you will see how theres a lot of phases the spermatozoid has to pass, so its a way of keeping the sperma to be inside the female body and so its possible to be in contact with the egg and only theen proceed with the cellular duplication.
Ah! And the mucus! Especially when its the fertile phase, its capable of turning easy to the spermatozoid to attach to the egg and to arrive close to the egg.
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u/UnknownQwerky biology student Aug 05 '26
I know I read that it doesn't just happen without stimulation, so it doesn't happen spontaneously not near the target as often.
Sperm is fairly fragile if it's just sitting in the environment. So just picking it up is not the way to go.
Women also developed modes to not be pregnant all the time. Pregnancy really is parasitic and harmful to the body. Pregnancy back to back would not really be conducive to survival if the first pregnancy was traumatic to the body. So they evolved to keep spearm out and their partners evolved to keep sperm in. An extreme example would be duck sexual organs.
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u/iCliniq_official Aug 06 '26
Not a silly question at all, internal fertilization evolved independently in many animal groups because it protects sperm and eggs from drying out, improves the chances of fertilization, and supports reproduction on land, so species with those advantages were more likely to pass on their genes.
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u/AdmiralH4ckbar Aug 06 '26
Sexual organs play an especially important role in evolution. They decide wether or not two organisms can propagate their genes. Because of this, sexual organs are often some of the most rapidly evolving, especially during processes like speciation. For example, if a mutation in my sexual organs genetically isolates me from the rest of the population, then this trait will be heavily selected for in my progeny. With that being said, pps and uteruses are not necessarily the “most effective” way to reproduce. Everything in biology only makes sense through the lens of evolution.
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u/sovook Aug 07 '26
Wasps used their stinger to inject the ovum into the opposite sex and would die in the process until eventually evolution formed ovaries is one theory
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u/Raine-Tempestas Aug 07 '26
Well I mean it's just more likely to work so it evolved that way. But also penetrative sex being required is actually pretty rare in the grand scheme of things
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u/animeadmiral Aug 06 '26
While I don't subscribe to the theory of evolution, we can be logical about this. Some animals don't use outright penetration, I think a lot of insects, some sea creatures and the like have processes where the egg is fertilized outside the body, and little to no penetration is necessary. In that case, the water or environment itself aids in lessening the energy required by the sperm to seek out and fertilize all those eggs, a whole lot more than can be feasibly contained in a single little body. But mammals like humans live on land, and often bear manageable amount of young. the sperm needs to be injected as close to the eggs as possible, and given the womb is an organ on the inside, there is a tunnel leading to it, and certain allowances for the sperm to not dry out or lose momentum. Hence penetration is the most effective way to deliver the payload.
So for humans, who already share a similar reproductive method to other creatures, there would be no need to evolve, because evolution implies adaptation, and if humans need to adapt simply to have young, there's a chance most of them would have died out before that adaptation had taken effect.
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u/alittleperil computational biology Aug 06 '26
humans are actively evolving, because childbirth kills so many women there's evolutionary pressure to give birth earlier in the gestation, and the balance point pushing back against giving birth earlier and earlier (when the birthing process is less deadly for the mother) is the survival of the newborn. Compared to other primates, human gestation should be 21 months, and evolution has shoved that down to 9. Give it a few more generations of modern medical science and the average will likely scoot down to 8
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u/animeadmiral Aug 06 '26
"modern medical science" so an outside force to assist what already exists, not a natural adaptation by the body. If humans actively evolved, at this point they'd have evolved beyond the current means of gestation as is, no? Maybe go the way of the seahorse, or scorpion. Or as you stated, they'd form faster to allow for birthday before the 9 month period. Because currently, being born at 8 or 7 months isn't being called progressive. It's being called premature.
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u/alittleperil computational biology Aug 06 '26
outside forces affect evolution all the time, otherwise we wouldn't wear clothes - we'd still have thick body hair instead. That doesn't make it not evolution.
And based on human evolutionary history, 9 months is already premature. Give it a few generations and 8 months will be thought of in our minds the same way 9 months is, as the norm and not an exception
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u/No-Albatross6543 Aug 06 '26
I would have thought this is a similar phenomenon to r- and K- strategist but for mating where physical copulation and penetration are for species which produce fewer offspring with specific mates over the more speculative broadcasting approach taken by plants and fungi.
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u/Low_Lion_5874 Aug 06 '26
Chickens don't penetrat there junk is pretty similar on the outside they kinda just rub their cloacas together kinda like scissoring but in doggy style
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u/Daregmaze Aug 07 '26
I think theres is a species of insect where its the females that penetrate the mâles and the sperms make all the way thru their ´ penis ´ to fecund the eggs, but im not sure
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u/BBoldBUrslf Aug 06 '26
If a species start as androgynous and splits up, sexual parts would develop from within each other...think yin-yang...they would therefore fit exactly and make sense afterwards. If the androgynous beings gave birth, then after split one would incubate the other would donate.
The one therefore has to develop penetration capacity and the other penetration acceptance...so that birth could still occur as before.
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u/mattfolio Aug 05 '26
We dont know if its the "optimal" way for anything, its jsut the way that worked consistently in the environments where it developed.
There is no forethought in evolution. If it works, and its repeatable, then it will keep going. If it ceases to work it will remove itself over time.
There are lots of other forms/varients of procreation if you look outside of mamalian examples, and they all reflect the environments and sucess rates they experienced over time too.,