Signalling in Human Breasts

Part B (Technical Report): The morphology of human breasts is unique amongst mammals. What, if anything are breasts signalling and why?

The structure of human breasts is unique amongst mammals, which Darwin (1871) suggested evolved to enhance mating success by attracting mates. Sexually selected morphological traits, such as breasts, emerge at sexual maturity as a result of sex hormones, timed precisely to impact the course of mate selection (Cronin, 1991; Gould & Gould, 1989). Breasts are uniquely human; more than five thousand mammals occupy our planet, yet homosapiens are the only being with permanent breasts (“The science of why human breasts are so big”, 2018). All other mammals develop temporary breasts during ovulation or when feeding an infant, with milk production being their sole purpose. However, the breasts of a female human develop during puberty, suggesting a difference in the evolutionary process. The reason for human breasts being permanently engorged is not clear, however there are several suggestions as to why this is. Human breasts signalling fertility, reproductive ability, and sex will be covered in the course of this technical report.

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Female breasts have been suggested to signal sexual fertility, showcasing an ability to conceive children to potential male mates. This signal is reported to be the most powerful at the beginning of the reproductive career, which is consistent with the belief that female sexual signals evolved to promote reproductive ability rather than fecundity (Symons, 1979). Linking to this, Frisch & McArthur (1974) and McArthur et al., (1976) found evidence for critical fat levels as a necessity for the maintenance of menstrual cycle and ovulation, thus suggesting the presence of breasts signals fertility through menstrual cycle maintenance. This positive relationship between the maintenance of fat deposits and female fertility has also been found by Wilmsen (1978). However, it has been suggested that it is not the presence of the breasts that signals fertility, but the symmetry of the breasts; Thornhill & Gangestad (1994) found breast symmetry is positively correlated with fertility and replicated this in two samples of males. Therefore, it can be suggested that despite the little research, and existing research being contradictory, female breasts do signal fertility to males. However, an abundance of research has suggested that female breasts are not a signal of fertility, but a signal of a female’s ability to produce an abundance of offspring.

Breast size has been linked to reproductive potential in females, signalling the ability of an individual to produce several healthy offspring. Research using hormone levels, including E2 and progesterone, which signal reproductive potential in females, has shown that women with large breasts have higher fecundity levels than individuals with smaller breasts (Jasienska, Ziomkiewicz, Ellison, Lipson, & Thune, 2004). However, it has been suggested that breast size is not directly linked to reproductive potential through hormones, but a sign of an ability to lactate, thus larger breasts have a higher possibility of successful lactation for infants, resulting in healthy offspring (Lowe, 1987). On the other hand, Minchin (1985) found no correlation between the size of non-pregnant breasts and subsequent success at or during lactation. Rather than as a use for lactation, it has been suggested that breast fat is an adaptation for making the breast large and soft enough to be convenient for an infant to reach and feed (Anderson, 1998), thus playing a role in breastfeeding, but not impacting the amount of milk that can be produced by an individual. Several hypotheses have been proposed regarding the role of human breasts; one such hypothesis is the Nubility Hypothesis (Marlowe, 1998). The Nubility Hypothesis (Marlowe, 1998) proposes that permanently engorged breasts evolved in hominid females act as an honest signal of reproductive value. Marlowe (1998) continues to suggest that breasts advertise age by their size and shape in order to signal reproductive health, with firm breasts signalling a young and mature female, while sagging breasts signal that the female is older (Marlowe, 1998). These honest signals of reproductive value include fat reserves which reflect a woman’s ability to survive difficult times when lacking resources, and invest in offspring (Cant, 1981; Gallup, 1982; Huss-Ashmore, 1980), thus larger female breasts signal an ability to survive and reproduce healthy offspring, even in times when food and other vital resources are limited, making the female more attractive to the male. The Nubility Hypothesis (Marlowe, 1998) has been linked to previous research which suggested that sexual selection via mate choice has enhanced the expression of permanently engorged breasts in women (Barber, 1995). This link proposes that permanently engorged breasts in humans have been maintained due to the reproductive signals they are releasing to males during the sexual selection process. The Nubility Hypothesis (Marlowe, 1998) has provided a large amount of knowledge into the signalling of human breasts and has similar results to other research studies into the same topic, thus its findings and propositions are strongly evidenced. However, these findings have been opposed by the suggestion that the role of breasts is to conceal reproductive status, rather than displaying reproductive value. Smith (1984) proposed that permanently enlarged breasts give females the ability to hide their reproductive status, gaining more attention from males, and increasing their likelihood to reproduce. However, this suggestion has very little empirical supporting evidence, and has been opposed because if a female is pregnant, other signs will be present, not just enlarged breasts. In opposition to this suggestion that female breasts signal reproductive ability, it has been suggested that female breasts signal sexual content to males, caused by continual exposure to media platforms (Gow, 1995; Grauerholz & King, 1997; Krassas, Blauwkamo, & Wesselink, 2003; Lin, 1997).

A further suggestion is that female breasts do not signal anything to males, but it is how males perceive them. In more recent times, it has been suggested that breasts are nothing but a sexual symbol to males (Latham, 1997), being perceived as sexual objects rather than signalling fertility or reproductive ability. Mainstream media, pleasure centres in the brain, and western culture ideologies have been offered as the cause of males perceiving female breasts as sexual objects. The traditional image of the housewife has been replaced by representations of women as sexual objects in today’s media, with a focus on their appearance and sexual appeal. These images have emerged as a central presence in all media outlets, including prime time television shows (Grauerholz & King, 1997), adverts (Lin, 1997), music videos (Gow, 1995), and magazines (Krassas, Blauwkamo & Wesselink, 2003). Despite being fairly old research, the same ideals are still found in current media outlets, and it can be argued that this is more relevant in current times due to an increase in media usage. Exposure to such media types has been shown to impact how men treat and respond to women in subsequent real-world situations, increasing the sexualisation of their behaviour. Rudman & Borgida (1995) found men exposed to sexist and objectifying adverts later recalled more about the female’s appearance and less about her personal background than controls, suggesting that exposure to media is able to change how a male perceives a female’s body, including her breasts. Thus, implying that it is not the breasts that are signalling to the male, but the males perception of the breasts that leads to a sexual objectification of female breasts. In regards to pleasure centres in the brain, research has shown that males shown videos of women’s breasts opted for a ‘smaller-sooner’ reward, compared to males shown a pastoral scene (Smith et al., 2009). This indicates that the reward centres of the brain, primarily the pre-frontal cortex, were involved in watching the videos, thus suggesting that men associate breasts with a reward and female breasts are therefore perceived as rewarding by males, increasing their sexual objectification. It has also been suggested that males perceive breasts as a sex symbol due to westernised culture. Breasts are less eroticised in cultures where women go topless than in western cultures where they are exploited in advertising (Ford & Beach, 1951). The fascination with pornography in western cultures also plays a part in how males perceive breasts; Dettwyler (2017) found that the use of breasts for the sexual pleasure of men is learned through western culture, including through pornography. This limited view of the breasts is pervasive and leads to the sexual objectification of female breasts, rather than perceiving them as what they are evolved for, feeding young.

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Overall, it can be concluded that what female breasts are signalling is ambiguous. It has been suggested that female breasts signal an ability to produce offspring, being produced during puberty, however there is a vast amount of contradicting research regarding whether breasts are to signal to a mate, or for feeding offspring. From these conflicting research findings, it can be suggested that the role of breasts in successful reproduction and provision for offspring is a topic in need of further research to provide an additional insight into the relationship between breast size and reproductive potential. The suggestion that the perception of female breasts by males may be more influential than what the breasts alone are signalling opens a large area for research to be conducted, proposing that female breasts may not be signalling anything at all to mates, but it is the way that males perceive female breasts that leads to fascination. Despite this suggestion, it can be concluded that female breasts portray many different signals, but further research needs to be conducted to make a final judgement as to what female breasts really do signal.

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