basically like. why do women just have breasts all the time when to my knowledge the only purpose of them is to feed children. why don’t they go away like other mammals’ when you haven’t had a child.
In: Biology
They kind of do. Breasts are made up of connective tissue, fatty tissue, and the “important part” that is the glands and ducts actually used to produce, store, and expel milk. Those mammary glands don’t fully develop until pregnancy. [Source on that](https://pubmed.ncbi.nlm.nih.gov/32816256/#:~:text=Most%20pregnancies%20cause%20the%20areola,glands%20to%20become%20more%20prominent.) Women often see their breast size increase noticeably due to the increased volume of mammary glands, plus some extra fat. But like primates, women can still breastfeed fine with small breasts as long as that mammary tissue can develop.
The breast that you see on a woman when she’s not far along in a pregnancy is mostly just fat. Like most of the weird things about humans, it was likely a random mutation that caused some women’s bodies to put extra fat on their breasts, and those women went on to have enough living children for the genetic mutation to propagate. It became a secondary sex characteristic AFTER it proved to be a not-harmful/ maybe helpful mutation, otherwise it would’ve died out from kinky males mating with women whose children wouldn’t survive well enough. I’d wager the extra fat helps women survive pregnancies and breastfeeding
I’m sure the fertility angle people have mentioned is a part of it but also the ability to feed. Same reason women are evolutionarily drawn to strong men. “He’s strong and he can catch lots of animals to feed me and my babies.” Men see large breasts or wide hips and the monkey brain thinks “She can bear many children and feed them plenty.” Most things in the world can get boiled down to reproduction if you really look at them. I think we’ve even seen these traits evolve over time where women have been attracted to men with money, he can take care of her and children, or even more recently the “dad bod” , looks like a dad so he can probably take care of children.
There’s a lot of theories, not a lot of certainties. Lindybeige has a [pretty solid video](https://youtu.be/xcrxNBlqrbM?si=-9S0A1WyuC1deF5M) speculating on why it could’ve been possible for breasts to evolve in this way.
If I recall correctly, he argues that a couple relatively unique human traits (walking upright, persistence predation) allowed for the mutation of permanent breasts to happen at all, and a different mutation happened where attraction for breasts was flipped from finding them unattractive to attractive, which allowed for the permanent breast trait to be sexually selected for.
Many of the other arguments among these comments are referring to developments and reinforcements after the fact, but could just as easily be true; there’s many influences and it could be any combination of them
Growing a baby inside you is already pretty energy extensive. Growing breasts at the same time seems pretty calorie intensive as well.
I suspect having large breasts can very much help you if you want to find a partner and get pregnant…
Evolution operates by natural selection, not by purpose. Sexual selection is one selection mechanism. In the case of humans it is hypothesized that full lips and breasts echo labia and buttocks. Our species is one of one of the few that mates face to face,
Cows have udders and my dog has nipples when not pregnant. It may be true that my dog’s mammaries are much smaller when not pregnant. They are still there.
So the premise of the question is false.
The mammary glands are all ready to go, having grown with secretion ducts and what not all in the correct organization. With reproduction active, they certainly can swell and “grow”, but this is not the same as growing a new body part.
Body growth is required to happen during normal development, because complex multicellular organisms do not spontaneously grow new body parts. That’s why mammaries grow before pregnancy. You would be correct that it is not necessary before pregnancy from a logical point of view. Puberty is the final step in development into a full grown organism. That’s simply how it works.
I’m anticipating someone will come along and say “the breasts are formed in the male as well, this proves they are already existing and don’t need to grow during puberty.” Again, the critical issue is that our bodies develop according to a plan. Gestation, childhood, puberty. Then you’re full grown. Puberty prepares you for reproduction.
Humans have had a unique evolution. My favourite hypothesis (because we don’t actually know) about Permanent breasts is that they evolved early in Homo Ergaster as a by product of other evolutions.
An increase in subcutaneous fat tissue as a response to a need to store more energy for the big human brain, and thermoregulation. More SFT increased hormone levels, including estrogen.
We had more and more evolutions to support the energy needs of our brain which increased fat deposits which increased hormone levels which caused more fat deposits.
These permanent fat deposits then became a secondary sex characteristic.
Probably related to hidden estrus.
https://en.wikipedia.org/wiki/Concealed_ovulation
A likely reason for both being to keep men around so they can protect the woman from bears and other wild animals. Look up the wiki for other hypotheses.
Of course, to get a sure answer we’d have to ask the alien astronauts that Intelligently Designed us (and if nobody did, we’ll just have to keep on guessing).
it’s because other mammals go through hormonal cycles (like having a mating season) whereas humans have evolved to have a ready structure for child rearing at any time (due to survival). It also helps that human society helped with this (like with wet nurses) as humans have evolved to continue producing milk for as long as their breasts are being suckled upon. To ELI5, breasts form during puberty and do not go away because humanity’s survival depends on having enough breasts to provide milk to suckle upon on, to provide adequate nutritional benefits for the tribe.
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