Given the phenomenon of male lactation, why are women’s breasts so much larger than men’s? Does the size contribute to greater volumes of production of milk? Is size related to milk production at all and acts only as sexual cues?
There are almost as many theories about this as there are breasts; male lactation is prettry irrelevant to them all though, AFAIK - I think it’s just a phenomenon arising out of the fact that we’re constructed using mostly the same set of biochemical tools.
Top theories I’ve heard are:
-It’s a sexual signal, stimulating the visual responses in the male that would normally be triggered by the sight of buttocks - and this because of upright posture.
-It’s to enable babies with flatter faces to suckle; and they have flatter faces because of larger cranial capacity.
-It’s a visual clue that the female is healthy and well-fed.
The question of why female breasts are the way they are comes up wityh some regularity here on the SDMB. Must be on a lot of people’s minds.
Breast size isn’t related to milk production. See Desmond Morris, or any number of sources. Breast size seems to be a sexual attraction feature, and I’ve argued the case for it being the case many times. Here’s a relatively recent thread:
http://boards.straightdope.com/sdmb/showthread.php?t=318808&highlight=Breasts
And in terms of removing negative pressures, humans can get away with physiological “flaws” that animals cannot, because we’re bright enough not to get killed because of them.
Two? 
It is my understanding that much of material in a breast is fatty tissue - presumeably not a factor in milk production. Besides having more fat, is there also more milk gland (in either size or number)?
Other apes produce plenty of milk without having female breasts any larger than male breasts – it’s not as if the boob is a big milk storage bladder. As I understand it, there’s only a little milk stored, sort of to get the baby started. The glands continue to manufacture milk as the baby nurses.
Larger breasts seem unrelated to this. (Morris claims that large human breasts actually get in the way of babies nursing, opposite to what Mangetout states. But you have to look critically at what Morris says, and I haven’t examined this myself.)
Morris does seem to be the first to have suggested that female breasts are “buttock mimics”, and a sexual signal to males. There’s actually a great deal more to this than that simple statement implies. It’s met with a surprising amount of resistance, but I find it very convincing. One reason I do is that in at least one other ape the female chest region resembles the buttocks/vulva as seen from behind, and in that case the chest most notably does not look like buttocks. It’s the gelada baboon, which has a characteristic “necklace” pattern of skin pigmentation and raised areas both on the chest and around the rear. An undoubted case of sexual signalling, and the placement is the same as in humans, but it doesn’t physically look the same.
I submit that the reason human females have rounded breasts is because human females have rounded buttocks. Humans are the only apes with such pronounced buttocks, and it’s a direct result of the engineering required to help us walk upright --those glutei maximi are needed to help pull and keep you upright.
There’s also a subsidiary feature that such sexual signals also indicate the state og health. Rooster’s coxcombs are thought to do this, because the (apparently) otherwise useless structure is well-defined and brightly colored when the rooster is healthy and well-fed, and “deflated” and dull otherwise.
So the reasion female breasts are bigger than men’s has nothing to do with lactation, male or female, and much to do with upright-walking females being biologically set up to attract upright-walking males with sexual signals mimicking the sex organ region and at the same time sending a signal that she is young and healthy. At least, that’s the theory.
Women’s breasts, like beer, are proof that God wants us to be happy. --apologies to Ben Franklin
I’m wondering about that breasts-mimicing-buttocks theory. I’m more attracted to cleavage than ass. Am I an assman in denial?
All I can say is, I don’t know the evolutionary theory behind big round boobs, but it damn sure works for me! I was attracted to my wife, in part (ok, a LOT), because of her bounteous bosom, with the end result being three kids. And I still wanna hit it every night. Sure, I’m a big, dumb, gullible, responds-live-Pavlov’s dog-to-big-titties kind of guy, but I’m a very happy man, lemme tell ya!
This is probably a derail, but I wonder too. The first I ever heard of it was in the BBC series “The Office” where David Brent tells a blind date “presumably, we used to do you from behind, and …”
well, apes do “do it from behind”, as do most animals. I’ve heard it said that the only other apes to mate face-to-face are bonobos (“pygmy chimpanzees”). I’ve also heard that bonobos also engage in oral sex. They’re our closest relatives – we share more genetic code with them than with anything else.
actually, there’s another interesting thing. Chimps may not have the pronounced buttocks of humans, but when they go into heat, female chimps’ hindquarters swell up and turn bright pink. It’s not particularly appealing – hot chjimp butt is irregular and lumpy, but male chimps are apparently into sexy , well-rounded butts at mating season. I don’t know if the same happens with bonobos. So there was possibly a predisposition for a well-rounded backside, and our ancestors’ choice of walking upright maybe just reinforced that. With human year-round fertility, there wasn’t a need for seasonal swelling. But a lady’s bottom is ceertainly more well-rounded than a man’s, and rounder than engineering requires. The breasts, I submit, just followed suit.