Why do human females have permanently prominent breasts?

Question

Humans seem to be the only animal whose females have breasts that permanently have a "full" appearance, ie a prominent amount of tissue even when not lactating, whereas other species' breasts seem to almost totally "deflate" when not longer lactating.

Why is that?

This feature has costs to the organism; significant biomass is dedicated and perhaps mobility is impacted. Given there are costs, for the feature to have come into existence, evolution/adaptation principles suggest there must have been some advantage.

Is there some biological advantage?
Are they vestiges of an adaptation that gave a significant advantage to some prior species or in some specific (presumably cold) prior environmental conditions? Is there any credible research to support certain reasons?

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My suspicion is that although genders can be distinguished easily enough, even from a distance, using other secondary characteristics, the answer may have an anthropological/social answer, rather than a biological one.

Answer 1

You may be interested in Breast asymmetry, sexual selection, and human reproductive success.

Abstract:

Breasts of human females are large compared to those of closely related primate species, and they can thus be hypothesized recently or currently to have been subject to directional sexual selection. Here we show that (1) large breasts have higher levels of fluctuating asymmetry than small breasts, (2) breast fluctuating asymmetry is higher in women without children than in women with at least one child, (3) breast fluctuating symmetry is a reliable predictor of age-independent fecundity, and (4) breast fluctuating symmetry appears to be associated with sexual selection. These conclusions were similar in studies from two cultures differing in fecundity and brenstfeeding traditions (Spain; New Mexico, U.S.A.). Choosy males that prefer females with symmetrical breasts may experience a direct fitness benefit in terms of increased fecundity and an indirect benefit in terms of attractive or fecund daughters.

Some quotes from the text:

The size of breasts in human females is large compared to that of other primate species, and there has thus been a recent evolutionary change in breast size. Human breasts are characterized by their large size and rapid development prior to and during puberty, when growth of other body parts also reaches a high level. They can thus be considered costly structures in terms of energy use. … Heavy investment in breast development during puberty may seem puzzling because the functional significance of breasts is far from obvious. There is no or little relationship between breast size and production and composition of milk in current industrial societies (reviews in Anderson 1983; Cant 1981; Low et al. 1987), and breasts have therefore been hypothesized to constitute a deceptive signal used by women to attract preferred mates (Low et al. 1987). In prehistoric human populations, conditions may have been different, when mother's milk was the sole source of nutrition for young children and when resources often may have been more limited than in present industrial societies. In this study we show that large breasts tend to be more asymmetric than small breasts. In prehistoric societies, where resources were likely to be less available or more unevenly distributed among women, breast asymmetries are likely to have been much larger than in present industrial societies with relatively easy access to resources. …

If the relationship between breast size and fluctuating asymmetry is robust, we hypothesize that a directional preference for large, symmetrical breasts, as apparently found in some cultures (Ellis 1954), can be maintained in the population because selection for large breasts will be balanced by selection against asymmetrical breasts

We also show that breast symmetry is a reliable predictor of age-independent fecundity, and that choosy males that use breast symmetry as a cue in their mate choice will experience an advantage in terms of reproductive success. Breast symmetry can thus be considered a reliable signal used in intersexual communication.

Answer 2

There are two theories that I find attractive regarding the prominence of the human breast. AS many have pointed out, human breast are really strange in mammals. Chimps are flat chested and have no problem nursing their young. And most of the female breast is fat tissue rather than milk glands (which grow during pregnancy and are mostly degraded after lactation stops)

theory 1 Human breast are under sexual selection. Men like big breast. So it gets selected for. There is much justification reported such as breast being a status of health (plenty of fat reserves) and estrogen levels, and thus a proxy for fertility.

However I have not seen any real data to back such an idea, such as A-B cup women being less attractive than C-D cup women. Nor data that women with small breast are less fertile than women with bigger breast.

theory 2 Anti smothering device. Human babies have no snout. So without a prominent breast to create an air space between the body of the mother and the nose of the baby, the baby would suffocate itself when nursing. And a chimp baby does have more of a snout than a human baby.

It is an intriguing idea. However it is also known that human breast does grow during pregnancy, becoming more prominent (particularly apparent for women with small breast). And such gains can be lost once lactation stops. So with that said... is there a reason why the female breast needs to be elevated all the time and not just after pregnancy. I have also not encountered any evidence that women with small breast are smothering their babies when breast feeding.

So at this moment I am leaning towards a hybrid idea...a hypothesis of my own.... that the human breast initially became more prominent for a biological reason.. perhaps a truly flat breast which chimps have really does smother babies without a snout. However once these small breast emerged, they came under the attention of men, and were sexual selected to a far larger size that we see today.

Answer 3

Even though human females' breasts appear full, they're only filled with milk after giving birth or parturition. Other times they're mostly made up of fat. And they're full mostime probably only to signal the males that they're capable of raising a baby.

Answer 4

I think I remember reading once from Desmond Morris, and possibly from the Naked Ape, that (at least) one of the reasons that humans have large breasts is because they resemble to the buttocks.

It's definitely because of sexual selection, and apparently, it's also connected to the fact that lot of our body language messaging comes from our front side. (Contrary to some other apes that can have larger or swollen buttocks for sexual messaging).