Edward M. Miller
Professor of Economics and Finance
University of New Orleans
New Orleans, LA 70148
October 27, 1995

Presented at the August 1993 Binghamton meeting of the Human Evolution and Behavior Society

Abstract

Female breasts enlarge during pregnancy and lactation. Since it is during these periods that provisioning the mother most benefits the child, males evolved to be more generous providers for women with enlarged breasts. Also, dominant males evolved to regard enlarged breasts as sexually unattractive. While this promoted their own reproductive success, it left such women free to be courted by the non-dominant males. Their courting involved provisioning the women, an act which also promoted their children’s well being. Thus, women with enlarged breasts were better provisioned. Since, for the two reasons given above, women with enlarged breasts were better provisioned, women with the genes for preferentially depositing fat in their breasts left more descendants, causing such genes to be selected. In non-human primates where male provisioning was unimportant, preferential fat deposits in the breasts did not emerge because such deposits could not lead to better provisioning.

[para 1]

Key words: human breasts, lactation, dominance, provisioning, sexual selection

I Introduction

A casual answer to the question, Why do women have breasts? is to nurse babies. However, most breast tissue is merely fat, not milk-producing tissue. At puberty the human breasts enlarge through the development of both glandular tissue and fat. The human is the only primate whose mammary glands (breasts) are visibly enlarged in the absence of pregnancy or lactation (Neville,1983, 105). This paper is directed to why non-lactating women have conspicuous breasts. While the fat in their breasts supports nursing, so does fat elsewhere. [para 2]

There has been earlier discussion of why women's fat is concentrated in their breasts (see Barash, 1982;Caro, 1987; Fisher, 1982; Harris, 1990; R. Smith, 1984; Montagna, 1985, for instance). Cant (1981) has proposed that large breasts provide evidence of abundant fat reserves. Gallup (1982) argues that breasts provide evidence of ovulatory potential. N. Smith (1986) suggests it is to provide softness for babies to hang on to. Low, Alexander and Noonan (1987) have proposed that fat deposits in the breasts make them appear able to produce more milk. Initially, males who chose to devote mating effort to women with prominent breasts got mates who were better able to nurse their children, causing the males to leave more surviving offspring. This led to a male preference for women with breasts. However, males could not distinguish large breasts due to fat from large breasts that really indicated much glandular tissue. Thus, women with breast fat deposits were more successful at attracting males. This selected for breast fat. [para 3]

This argument was disputed by Anderson (1988) and by and Sellen (1990) and defended by Low (1990). The author is inclined to doubt the Low, Alexander, and Noonan argument because women probably evolved to have the optimal amount of glandular tissue, with any additional such tissue reducing their fitness in some way. Males would have relatively little to gain from preferring women that had more than the optimal glandular tissue. [para 4]

In theory, males might gain by selecting women with more than optimal glandular tissue if the cost to the female of the extra tissue was increased mortality (perhaps reduced running ability, greater exposure to disease, depletion of her food reserves, etc) while the benefit of the extra milk went to the child. The male's genes, benefitting more from the child's health than the mother's health, might benefit if the mother could produce more milk. However, there are several reasons for doubting that male and female interests conflict much. [para 5]

The survival of a man's children normally required the mother's survival. Any adverse effect of excess glandular tissue on the mother's health would probably appear within the first few years of a child's life, adversely affecting the child. [para 6]

Theoretically, the genes for excessive glandular tissue could reduce her probability of reaching child bearing age, thus creating a conflict between male and female interests. However, breasts develop just before they are needed, i.e. at puberty, rather than in childhood. The genes affecting lactation are unlikely to have a significant effect on survival before they are turned on at puberty. Thus a serious conflict between male and female interests is unlikely. [para 7]

In the absence of a large scale conflict of interests, it is unlikely that the female fat distribution would have been selected to deceive males. However, this paperTs purpose is not to add to the discussion of the Low et al. hypothesis, but to propose a couple of new hypotheses. [para 8]

The Role of Sexual Selection

A slightly more sophisticated answer for why large breasts than Rto produce milkS is to attract men and to induce them to invest in her offspring. The argument starts with the hypothesis that men like breasts, an observation that has been reported not only in western cultures but in ones as different as Indonesia (Marlinata,1984). Flat chested women were unable to attract as much male investment, and thus their genes died out. Such sexual selection is capable of explaining how a male preference for females with breasts was maintained once men came to respond to them. Men who selected flat chested women as mates would leave flat chested daughters. These would be less attractive to men, and would be less well provisioned, leaving fewer descendants. Thus, once a male taste for female breasts emerged, sexual selection could maintain this taste, and maintain breasts. However, the question remains why such a male taste emerged. [para 9]

This is a common problem for a sexual selection theory. It is easy to explain how a trait found attractive by one sex is maintained once it emerges. The problem is to explain how a trait, and a preference for it, first emerges. [para 10]

Since most mammals lack large breasts except when suckling (Ford and Beach,1951, 48), we cannot merely say that a male preference for them had been inherited from the human ancestors. It is necessary to explain positively why males developed this preference. [para 11]

The key argument to be made here is that large breasts originally indicated a pregnant or lactating female. This in turn implies two facts: (1) She is in a stage of her life cycle when additional nutrition is most valuable. (2) She is less likely to be fertile. From each of these implications emerges a possible selective mechanism for large breasts. [para 12]

The first part of the paper will discuss the implications of the lactating female's need for additional nutrition. The second part of the paper will deal with the implications of the non-lactating female's greater fertility. [para 13]

II. Lactation as an Indicator of Nutritional Need

The Importance of Lactation

Lancaster and Lancaster (1987, ,194-195) state that, RA strong argument can be made that, among the higher primates, access to energy to support a long lactation is the principal concern of the adult female. Lactation is more energy demanding even than gestation (Anderson 1983, 27). It is likely that a child's survival has frequently been determined by the mother's ability to supply adequate milk after gestation. [para 14]

The problem of obtaining adequate nutrition during lactation is made more difficult because nursing itself consumes time that could be spent in food gathering, and because lactating women are accompanied by a vulnerable infant whose presence may interfere with food gathering. Indeed, among the Ache hunter-gatherers of Paraguay, lactating females gather fewer calories on foraging trips than non-lactating females (Hurtado et al.,1985). This was due to both lower efficiency and less effort. The lower efficiency was shown by gathering only 2601 calories per hour of palm starch pursuit versus 3524 for non-nursing females (their Table VII). [para 15]

Additional evidence that nutrition has frequently been difficult for lactating mothers is that women build up stores of body fat that are drawn on during lactation, and that ovulation is inhibited if the required fat deposits have not been built up (Frisch, 1989; Lancaster, 1986, 26-32). Fat reserves are built up during pregnancy even at the expense of the growing fetus. The accumulation of such fat reserves presumedly indicates that food intake during lactation was frequently inadequate. However, after a discussion of the literature in which some studies showed effects of nutrition on milk production and others did not, Casey and Hambidge (1983, 219) state Rit is difficult to come to any definite conclusion as to whether Tfeeding the nursing motherU does in fact Tfeed the childU.S However, common sense would suggest that at least severe maternal malnutrition would adversely affect the child. [para 16]

Admittedly, even if malnourished, the mother may be able to maintain lactation by drawing on fat reserves. To the extent this happens, the time needed to rebuilt her fat reserves would delay the next pregnancy. Indeed, evidence exists that supplemental feeding shortens the period of postpartum infertility. In a Gambian study it was reported that, RBefore supplementation, 19% of new conceptions occurred before 18 months after the birth of the previous child, whereas after supplementation, the proportion was 33%,S (Casey and Hambidge, 1983, 219). A male who expected to father a lactating female's next child (presumably because he had or was creating a pair-bond with her) would promote his reproductive success by provisioning her. [para 17]

Finally, there is anecdotal evidence from the siege of Leningrad and the hunger winter in Holland that the mother's capacity for breast feeding was maintained, but the duration of lactation was apparently considerably shortened (Casey and Hambidge, 1983, 226). Such a shortening would, of course, adversely affect a non-provisioner's child. [para 18]

Given that males restricted their provisioning to likely mothers of their children, male provisioning during lactation was selected for. Males provided extra provisioning during lactation because they had evolved to view swollen breasts as attractive. In turn, females were selected for large breasts to attract such provisioning. [para 19]

At the same time there was selection against any male tendency to find swollen breasts unattractive, since males with such feelings would have tended to stop provisioning their mates just when it was most needed. Prior to the emergence of male provisioning, it would have been in the male's reproductive interest not to court non-ovulating females whose swollen breasts evidence their non-ovulating status. This male avoidance of females with swollen breasts would have been selected against for males who provisioned the females they were attracted to. Thus it should have gradually disappeared. [para 20]

Of course, the exact genetic mechanism by which male standards of beauty are coded is unknown. One possibility is that the system is a polygenetic one, similar to that controlling other behavior. Some alleles contribute to an attraction, and other to a repulsion. The net effect is a summation of the effects of these different alleles. In this case, selection against repulsion by enlarged breasts would also promote male attraction to women with enlarged breasts. Thus, it is possible that a male preference for women with enlarged breasts could emerge as a result of the disappearance of a male repulsion to such women. [para 21]

Female Rewarding of Male Provisioning During Lactation

Even in existing apes food provision can be rewarded with sexual access. Females appear to reward with copulations males who feed them, even having been seen to take meat from the mouth of the male copulating with her (Goodall, 1986, 483-484). Estrous females succeed more often at begging meat from males than non-estrous ones (Goodall, 1986, 483-484). Kano (1979) describes how bonobos (pygmy chimpanzees) presented and copulated with males with fruits they wanted. Once hominids had adopted hunting, males frequently found themselves with meat which could be distributed to females. Males naturally distributed this meat to females who in turn gave them sexual access. [para 22]

What type of provisioning should females reward most generously with sexual access? If the critical period for females is lactation, females paired with males who provisioned them during lactation would leave more descendants. Thus, females would prefer such individuals as mates, and would react negatively to any who RabandonedS them during this period. This would disadvantage any males who found enlarged breasts unattractive. Conversely, any who actually found enlarged breasts attractive would have a positive advantage. Such a male would endeavor to attach himself (by provisioning) to any female with enlarged breasts, including those who had been abandoned by partners who did not find enlarged breasts attractive. Since a male who provisioned a female during one lactation could be expected to provision her during subsequent lactations, women would benefit from forming permanent pair-bonds with such males. By attaching such males to them, they provide for their nutritional needs during future lactational periods. [para 23]

Thus, males who were attracted to females with enlarged breasts would be more successful in forming pair-bonds with such females. Even if the females were not fertile when the bond was formed, such males would have a long-run success in competing against males who abandoned their mates when lactation enlarged their breasts. [para 24]

It is possible that women's intelligence and memory played a role. Women might very well observe which men abandoned their lactating and pregnant mates, and avoid forming pair-bonds with them even if courted. They would reason (correctly) that they would in turn be abandoned. Certainly, this would be a successful strategy for women, and the mental requirements would be within the probable abilities of early hominids. Even today, women seem to be close observers of the reliability of other women's mates, and denounce inadequate providers. Indeed, the ability of women to identify such unreliable providers, remember who they were, realize that they would probably do the same to a new mate, and resist courting by such males, may have led to the evolution of female intelligence, (and hence of human intelligence). Finally, the child of a man who was attracted by enlarged breasts would be better provisioned than children of one who was repealed by them, and hence abandoned his mate when lactation enlarged her breasts. [para 25]

Evidence as to the plausibility of males competing by aiding females with child-care is provided by behavior in other primates. Smuts (1986, 393-394) observed that, RIn a number of species living in multimale groups, males appear to contribute to offspring survival through baby-sitting, protection, occasional carrying, and other affiliating behaviors. Male parental investment does not fully explain these behaviors because the infants are often unlikely to be the male's own offspring.S She then offers the hypothesis that male infant care sometimes represents mating effort rather than parental investment (see also Whitten, 1986, 357). [para 26]

Once some males preferred lactating women with their swollen breasts, and women had evolved to form attachments to men who provisioned them during their greatest need (when lactating), the proportion of males and females with these genes gradually grew. [para 27]

In this account, disappearance of a dislike for enlarged breasts and attraction to enlarged breasts is partially a device for extending male provisioning beyond a short run purchase of sexual access. It may even have been a necessary step, since without it males would continually replace their pregnant or lactating mates, with ones whose flat chests indicate they were capable of being impregnated. Long-term pair-bonding, or RmarriageS required that a male's attraction to a female continue past her pregnancy and lactational period. [para 28]

Emergence of Breasts

So far the argument has been devoted to showing that males should have evolved to preferentially provision females with enlarged breasts. [para 29]

Such a male preference will lead to the emergence of large breasted females. The mechanism is simple. Extra provisioning is in the female's reproductive interests at all times, not merely when she is lactating. [para 30]

A female with breast fat deposits appears attractive to men and receives extra provisioning. When she is not lactating, the extra food is still helpful and increases the number of her offspring (possibly by adding to body fat). When she is lactating, the extra fat will give her even larger breasts, and cause her to be preferentially provisioned over females whose breasts contain only tissues devoted to milk production. [para 31]

Extra breast fat involves little costs. Females require large fat stores to carry themselves through the strains of pregnancy and lactation. If the fat is not in the breasts, it must be in other areas. Thus, a shift of fat to the breasts appears to impose no major costs. Because it motivates preferential male provisioning, it is selected for. [para 32]

III Lactation as a Fertility Indicator

Unattractiveness as a Female Strategy

This brings us to the second characteristic of lactating females (besides their need for nutrition). They are less likely to be ovulating. [para 33]

Smith (1984, 641) asks why human females have pendulous breasts, noting "Pendulous breasts should initially have been sexually repulsive, at least to high status males, because they would have signaled lactation, amenorrhea, anovulation, and therefore a female of little or no current reproductive value." He is correct, and why men came to prefer large breasts provides an interesting puzzle. [para 34]

However, this unattractiveness to dominant males may not have been a disadvantage once male provisioning had appeared. Instead, not being attractive to dominant males can benefit females (Strassman, 1981). Let us explore this paradox since it could explain human breasts. [para 35]

Prior to the evolution of hunting and male provisioning, hominids probably lived in multimale groups with social organizations similar to today's chimpanzees (see Goodall, 1986, for a description). With only limited scope for paternal investments, males maximize their reproductive success by devoting their efforts to maintaining dominance hierarchies and to preventing other males from mating with desirable females. While not completely successful at monopolizing desirable females, the dominant males generally are the most successful at reproduction. [para 36]

What strategies are open to a non-dominant male? Prior to the emergence of provisioning, the best a non-dominant male could do would have been to steal an occasional copulation, or to wait until he himself became dominant (Goodall, 1986; van Waal, 1982, 1989). However, once hunting is common and males have food to distribute, non-dominant males can compete by offering more and better provisioning. The male may offer to invest in the offspring of a single female (or possibly a small number of females). His per capita investment is greater than the dominant male can offer, and this can persuade females to cooperate in mating with him, possibly even going on consortships. [para 37]

A low dominance male's optimal strategy is to concentrate his provisioning on a single female. Only by concentrating his provisioning can he make mating with him attractive enough to obtain female loyalty (see Miller, 1993 for a more detailed account). [para 38]

Thus, with each non-dominant male concentrating his meat on a single female, females get more meat if paired with a non-dominant male than if in a dominant male's Rharem.S [para 39]

If female reproductive success requires male provisioning, females will be selected for those characteristics that lead to them being mated by the non-dominant males. These are the characteristics that make them relatively unattractive to dominant males. [para 40]

One of the characteristics dominant males should be selected to find relatively unattractive is enlarged breasts, since these indicate a temporarily infertile female. Dominant males would spend very little time copulating with such females, or trying to prevent other males from copulating with them. Thus non-dominant males would enjoy relatively unhindered access to such females. Goodall (1986) has described how dominant male chimpanzees take the females closest to ovulation, leaving the other females for the remaining males. The same behavior probably occurred in early hominids. Since pregnancies occasionally occurred, non-dominant males were selected to find such females attractive, and to court them. Once males have provisioning available as a mating tactic, and females are exploiting it by forming long term attachments, the benefits of courting the females neglected by the dominant male extend beyond an occasional successful impregnation. Bonds formed then can carry over to a later time when the female is again ovulating. [para 41]

Since large breasted females are better provisioned than other females, they leave more surviving offspring. Thus, the genes for fat deposits in the breasts are selected for. When lactating, these females will be among the largest breasted females, and will be better provisioned than other lactating females. When not lactating, they wonUt be as conspicuously Rflat-chestedS as other females, and may benefit from the preferential provisioning that results from being considered relatively unattractive by the dominant males. Thus, females came to be selected for large breasts, both when lactating and when not-lactating. [para 42]

Smith (1984, 641) argues that "the evolution of perennial pendulous breasts could have initially evolved to provide females facultative polyandry." It is certainly possible that if a female's mate regarded her as less attractive because of enlarged breasts, she would find it easier to mate with others. Unfortunately, her mate would provision her less well, and in general this effect would outweigh the advantage of being able to more easily mate outside the pair-bond. However, if large breasts are viewed not as a device to cheat on a RhusbandS but as a device for avoiding unwelcome attention from a dominant male (such as rape), one has the theory of this paper. [para 43]

After the first draft of this paper had been written, it was discovered that Knight (1991, 219) also argued that it was in early women's interest to avoid inclusion in a dominant male's harem, and that large breasts might help her do so. [para 44]

The Malthusian Argument for Lactation Being Critical

It has been shown that circumstances exist in which males benefit by preferentially provisioning females with enlarged breasts. This raises the question of whether the circumstances in which humans evolved are such that males would have benefited by preferentially provisioning females with breasts. If they were, it can be argued that human males may have evolved to preferentially provision females with enlarged breasts. However, if circumstances were such that male provisioning was unessential, an argument that females were selected for success at inducing such provisioning is less convincing. For instance, evidence that currently most females receive enough food for adequate lactation (with the aid of stored fat) might be used to argue that prehistoric female reproductive success was not facilitated by attracting male provisioning. [para 45]

There is a Malthusian argument that population density will be such that female reproduction is promoted by provisioning during lactation. Food will be abundant enough so that a female can provision herself when she is not lactating, but will have difficulty doing so when lactating. Consider the alternatives. [para 46]

Suppose food was so abundant that lactating females were adequately fed without male provisioning. The population would grow in such a food abundant environment. As the population grew, they would compete with one another for food. Eventually food scarcity would stop population growth. The mechanism might be through unassisted females being unable to gather enough food to support lactation (remembering they would be carrying infants while foraging). Alternatively, a prolonged period between pregnancies to rebuild fat reserves might be required. We have now arrived at the circumstances in which male provisioning could make a difference. [para 47]

Now consider circumstances where the population was so large that a non-lactating female could not quite support herself. She would lose weight and soon fall below the weight at which she could become pregnant. Any females who did become pregnant would have difficulty supporting lactation. The population would decline under these circumstances. Lower population pressure would gradually cause food to become more abundant. When population stopped declining, circumstances would again be such that male provisioning could make a difference. [para 48]

Putting these two sets of circumstances together, the population is likely to be neither growing nor shrinking only when it has reached a level such that the food supply is adequate to support non-lactating females (and perhaps to permit them to slowly gain weight), but not to support lactating females without their having to draw on their fat reserves. In these circumstances, male provisioning is likely to make a real difference to a lactating female's milk production, or to the speed with which she regains weight and resumes ovulating after pregnancy, and hence to her fitness. This benefit to the genes of the male who provisions her (and whose child likely benefits), is likely to exceed the benefit from provisioning a non-lactating female. These are the circumstances in which males are likely to be selected for attraction to enlarged breasts, and in which females are likely to be selected for visible breasts. [para 49]

Finally, once male provisioning is normal, population will increase to where non-provisioned females cannot reproduce themselves. Then, ability to induce male provisioning can determine reproductive success. [para 50]

III. Miscellaneous Considerations

Notice this model can explain why humans are the only female primates with prominent breasts. Human males are the only male primates who regularly provision females with large quantities of food, and are hence the only species whose males are likely to have evolved to provide provisioning when it is most needed. They would thus be the only species whose females increase male provisioning by simulating lactation. [para 51]

Supporting evidence that enlarged breasts serve to attract males rather than to serve some other purpose is that they appear as part of puberty rather than earlier. Attracting males prior to puberty would not contribute to reproductive success. [para 52]

VI. Conclusions

Female breasts enlarge during pregnancy and lactation. Since it is during these periods that provisioning most benefits the child, males evolved to be more generous providers for females with breasts. Also, dominant males evolved to regard enlarged breasts as not sexually attractive. While this promoted their own reproductive success, it left such females free to be courted by non-dominant males. These males courted by provisioning, an act which also promoted their childrenTs well being. Thus, females with enlarged breasts were better provisioned. [para 53]

For the two reasons given above, females with breasts were better provisioned. Thus, females with genes for preferentially depositing fat in their breasts left more descendants, causing such genes to be selected for. Thus, breast fat, by mimicking pregnancy and lactation, deceived males, (or non-dominant males at least) into providing better provisioning. [para 54]

Males were selected to find breasts attractive because this led to greater female provisioning when it was most needed, and greater male success in forming durable pair-bonds. These survived until the female was again ovulating. Of course, once the male preference for women with breasts emerged, the usual sexual selection feedback mechanism took hold. Having breasts showed possession of genes for breasts. Such female's daughters would inherit valuable male attracting genes. In non-human primates where male provisioning is unimportant, breast fat deposits did not emerge because they would not motivate better provisioning. [para 55]

[para 56]

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, T.M.and D.W.Sellen (1990). The Reproductive Advantages of Fat in Women. Ethology and Sociobiology 11, 51-66.

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