Doctor Science Knows

Tuesday, November 13, 2007

Breasts and Evolution

Lightly edited from comments I left in a post by Amanda at Pandagon,
If only someone could make a nifty little program…. about whether men are really more "visual" than women, and some of the fallacies of "Evolutionary Psychology".

Puttin' on my Actual Evolutionary Biologist™ hat here.

Breasts are an example of how human anatomy & physiology have some unique and biologically bizarre features, features that contradict each other *and* contradict what evolutionary biology leads one to explect. Examples:

1) human males average larger than females -- as is expected if there is significant sexual selection on males (=females are picky, not all males reproduce)

2) human females have enlarged breasts, of a type found in no other mammal. This suggests that biologically unusual sexual selection on females is occurring. I don't think it can be to make nursing easier, Ailurophile, because my experience is that it *doesn't* -- it makes "latching on" a good deal harder than it is for droopy-chested chimps.

3) both human sexes (in most populations) have extremely exaggerated head hair, the longest hair of any mammal. This implies heavy, unbalanced sexual selection on at least one sex.

Look at it this way. You know how many other species of bird or mammals have males larger than females, but females more brightly-colored in *any* respect than males?[1]

*None*. Zip, zero. It does not compute. Far from being explicable by trivially easy "Evolutionary Psychology" reasoning (as practiced by sociologists, ferchrissakes), human reproductive biology is really problematic.

This is why I'm starting to think that self-domestication may be the missing link in human evolution. It's probable that for a long time (at least 100,000 years) humans have been attempting to control each other's reproduction, via arranged marriages and the like. No-one's done the math yet, so this is really just an educated guess on my part.

growth of the breast isn’t to encourage group survival, but is a result of sexual competition - females with bigger breasts had an advantage over their flatter sisters, suggesting (again) that males favoured them for whatever reason. Evo psych rears its ugly head again.
And again, this is why I talk about “evo-psycho”, because they don’t actually understand biology.

For female mammals to have elaborate, permanent display structures like breasts is *bizarre*, even unprecedented (the role & significance of the sexual swellings in female baboons is hotly debated). It’s particularly weird when you recall that a woman’s breasts swell while she’s nursing — *and thus infertile*.

Frankly, I don’t know why human females have “display breasts”. One thing I would definitely ask is, who are they displaying *to*? Potential mates, or potential in-laws? Or other females?

[1] In most warm-blooded animals males are larger and less camouflaged than females. There are some groups in which females are larger: baleen whales, birds of prey, and rabbits, for instance. In most of these, the males are still less camouflaged (e.g. Kestrels), so sexual selection is probably still acting on males, but ecological forces make them smaller.

There are a few birds in which females are larger and more brightly-colored than males (e.g. Phalaropes), and in these species females are polyandrous -- that is, sexual selection is for once acting more strongly on them than on the males.

There are *no* species in which males are larger and females are less camouflagued.

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  • Seems to me that postulating that both genders are 'display' genders, and that sexual selection is acting on both, is something to investigate. It may be that if not all human females can be guaranteed a mate or offspring, the possibility of sexual selection of women can be hypothesized. Having both genders be display genders would be very unusual, but it might account for the physical differences of both genders.

    By Anonymous Anonymous, at 3:46 PM  

  • Although women get to choose their mate, during the birth and for a long time after, they are completely dependant on support of others. Having these display structures might be the cheese, for her to then work out who a good mate is.

    By Blogger Unknown, at 10:53 AM  

  • Breasts and buttocks are unique to humans and are not found in other primates. Although the buttocks of the male contain a significant amount of adipose tissue, the larger buttocks of the female are the result of added fatty tissue. Hominids evolved in the arid climate east of the African Rift Valley. In a hot climate, where heat exchange via the skin is essential, fat storage is confined to special organs such as the hump of the camel and zebu cattle, and the fat tails of the sheep. I suggested that buttocks developed first as energy stores in an arid, hot habitat with an unpredictable food supply. A larger energy store was required by the gestating and nursing human female waiting for provision of food by the male. This encouraged the evolution of extra fat stores in the breasts, buttocks and thigh of the hominid female. The increased risk of energy shortage in the desert in the Khoisanoid race (Bushmen and Hottentots) with their antiquity, enhanced the evolution of enlarged buttocks (steatopygia) in the female as a fat reservoir. The appropriate energy stores, which represented the female's nutritional state, were therefore closely linked with her fertility. Women’s specific fat stores (breasts, buttocks and thighs) are a prerequisite for fertility, and because they characterize the female and signal her fertility, they are used as sexual attractants. For reference see Arieli (2004) Breasts, buttocks and the camel hump. Israel J Zool 50:87-91.

    By Anonymous Ran Arieli, at 6:58 AM  

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