It is evident that sexual dimorphism (differences in physical appearance) exists between human males and females. Such differences are attributable mainly to the effects of sex hormones, beginning at puberty (Wells, 2007). Respective morphological characteristics are distinct, although there is overlap between the sexes in individual cases. Such dimorphism is not confined to humans; it is exhibited by many other species, and to a varying degree. For example, it is barely detectable in snakes, and yet extraordinary in peacocks (Ridley, 2004). It is a long-held view that the effect is greater among polygamous species, in which paternal investment in parenting is relatively low (Darwin, 1871, as cited by Desmond & Moore, 2004, p. 254).
THE HOURGLASS FIGURE
One marked anthropological difference is body shape. Conspicuous in females only is the so-called ‘hourglass figure’. This is a combination of large breasts and broad hips contrasting with a relatively narrow waist (Dindia & Canary, 2006, p. 127). In silhouette, this shape resembles an hourglass, hence the metaphor. The existence of such a fundamental distinction invites a number of questions: What are the exact physical characteristics? Can they be explained satisfactorily with reference to Darwinian natural selection? Has sexual selection played a rôle? Are there biochemical or pathological associations? Does interracial variation exist? Is the ageing process relevant?
Overall body shape is determined by a combination of skeletal bone, fat distribution pattern and, to a lesser extent, musculature (Bloomfield & Fitch, 1995, p. 5). Each component is sexually dimorphic.
The relative broadness of the female hips is, largely, a manifestation of pelvic morphology. Several characteristics distinguish the gynoid version from the android: both the inlet and outlet are larger; the sacrum is shorter and broader; and the pubic arch exceeds 90° (Tortora & Anagnostakos, 1987, p. 172). Consequently, the complete frame has a more rounded appearance. Fat distribution augments this distinction. Females have a greater tendency to deposit adipose tissue around the hips and buttocks, as opposed to the torso and limbs, and such a mechanism is purported to be of genetic origin (Jamison, 2001, p. 329).
WAIST-HIP RATIO (WHR)
WHR is determined anthropometrically by circumferential measurement of the smallest part of the waist and the broadest part of the hips. However, clinical studies are frequently conducted using photographs of subjects with reference to pre-recorded WHR data (Tovée & Cornelissen, 2001; Schützwohl, 2005). The principal reason for such interest in this simple quotient is that, among females, it is considered to have adaptive significance (Singh, 1993).
Evidence suggests that it reflects general health and male desirability (Singh & Young, 1995). If so, then it follows that, in the eyes of males, one or more optimum values could exist. Sanderson (2001, p. 180) reported data from several studies which suggested a universal ideal WHR of 0.7, and this was confirmed by current evidence from Hong, Park, Lee and Suh (2008). Hence, a waist circumference of 24 inches would predicate an aesthetically-ideal hip measurement of approximately 34 inches. This WHR preference was seen to be independent of either breast size or total body weight.
In the light of these findings, the following question is raised: Does the most sexually-attractive WHR confer any other advantages on the female?
Firstly, given that the female pelvis is instrumental in childbirth, and the breasts in neonatal feeding, it would seem reasonable to suppose that WHR has particular significance with regard to the child-bearing years. Anthropological data show that gynoid fat distribution is normally present only between the approximate ages of 15 and 45 years (Ridley, 1994, pp. 154-156) which coincides with the fertile period. Moreover, it is absent in males and prepubertal females.
Garaulet et al. (2002) assessed fat distribution in both pre- and post-menopausal females and concluded that the gynoid pattern yielded to a more android type as menopause progressed. This confirmed conclusions drawn by Ley, Lees and Stevenson (1992) who studied post-menopausal women, some of whom were undergoing hormone-replacement therapy (HRT). These subjects exhibited a more persistent gynoid fat pattern than those of the non-medicated control group.
Although the basic hourglass shape is of comparable geographic distribution among young females, there exists some racial variation. Novotny et al. (2007) compared fat distribution among 11-to-12-year-old girls of Asian, Hispanic and Caucasian descent throughout the United States of America. Whites were found to have proportionally more gynoid fat than either Asians or Hispanics, despite Asians having the lowest mean waist circumference.
EFFECT ON FECUNDITY
Clinical investigations have been undertaken to assess whether or not fat distribution type affects pregnancy rate. Zaadstra et al. (1993) studied 500 women who were undergoing (their first programme of) artificial insemination. The number of required insemination cycles was recorded and conception probability calculated for each subject. After making adjustments for age, weight and smoking, it was found that a WHR increase of 0.1 (above 0.7) produced a 30% reduction in conception probability. That is, android fat distribution was negatively associated with successful treatment, and more so than either increasing age or obesity.
Data from subsequent in vitro fertilization assessments were published by Waas, Waldenström, Rössner and Hellberg (1997). Results showed that women whose WHR was between 0.70 and 0.79 had twice the pregnancy rate of those with a WHR of more than 0.80. No correlation was implicated between pregnancy rate and body mass index (BMI), which is defined as the ratio of body weight to the square of the height (kgm-2) (Eknoyan, 2008).
Endocrinological reasons underpinning increased reproductive potential were sought by Jasieńska, Ziomkiewicz, Ellison, Lipson and Thune (2004). Salivary concentrations of progesterone and the oestrogenic hormone 17β-oestradiol (E2) were quantified daily, throughout a single menstrual cycle, in more than 100 women. Those with narrow waists, relative to breast size, had significantly higher mean and mid-cycle E2 concentrations than any other groups. Hence, a causal link between hormone concentrations and WHR was postulated. Girls with low WHRs are known, also, to show earlier pubertal hormonal activity (Sanderson, 2001, p. 180).
Conversely, increased androgen concentration is believed to have the opposite effect: fat is deposited in the abdominal region (Guerrero & Floyd, 2006, p. 66), a characteristic associated more closely with males.
WHR has been linked with several clinical disorders within the last twenty years. Arguably the most significant is heart disease, the incidence of which is universally lower in females than in males (Holtzman, 2008, p. 80). Angiographic evidence has suggested that coronary artery disease increases with android fat distribution, more so in women over 60 years of age (Hartz et al., 1990). However, Rexrode et al. (1998) were unable to show that regional fat distribution was a greater risk factor than general obesity.
Relationships between WHR and plasma lipoprotein concentrations have been established. High-density lipoprotein 2 (HDL2) was found to be positively associated with android fat distribution (Ostlund, Staten, Kohrt, Schultz & Malley, 1990). More recent cardiovascular research carried out by Smith, Al-Amri, Sniderman and Cianflone (2006) involved analysis of plasma adiponectin concentration. A strong positive relationship with WHR was demonstrated. The general conclusion is that women with gynoid fat patterns are less prone to cardiac disease.
High WHR has also been implicated in the development of non-insulin-dependent diabetes mellitus (NIDDM) (Schmidt, Duncan, Canani, Karohl & Chambless, 1992). Similar findings were published by Carey et al. (1997), who studied more than 40,000 American women over an eight-year period. It was claimed that BMI and waist circumference were additional independent predictors of NIDDM risk. Interracial variation of NIDDM incidence with respect to WHR was assessed by Lovejoy, de la Bretonne, Klemperer and Tulley (1996). Results demonstrated a different degree of risk between Caucasians and African-Americans.
Given that serum oestrogen concentration is, firstly, inversely proportional to WHR (Jasieńska et al., 2004) and, secondly, directly proportional to breast cancer risk (Hulka & Moorman, 2001), it follows that gynoid fat distribution predisposes women to this type of malignancy. This hypothesis is supported by data from both pre-menopausal (Kumar, Riccardi, Cantor, Dalton & Allen, 2005) and post-menopausal women (Friedenreich, Courneya & Bryant, 2002). Therefore, other factors being equal, increased risk of cardiac disease correlates to reduced risk of breast cancer and vice versa.
High WHR has been associated, also, with polycystic ovary syndrome (PCOS). Kirchengast and Huber (2001) compared PCOS sufferers with healthy women of equivalent BMI and found that a significantly low proportion of the test group presented with a gynoid fat pattern. This is, perhaps, unsurprising: PCOS is a hyperandrogenic condition (Marshall & Bangert, 2008, p. 199). Other high-WHR-associated disorders include hypertension, stroke, menstrual irregularity and ovarian malignancy (Sanderson, 2001, p. 180).
The racial and geographic ubiquity of the hourglass figure (Diamond, 1998, p. 182) implies that this anthropological feature has an evolutionary basis. Several theories offer potentially valid reasons as to why this body shape might provide a selective advantage for the genes responsible.
Most recently, Lasseka and Gaulin (2008) contended that WHR affected cognitive ability in a woman’s offspring. It was thought that gluteofemoral fat deposits might provide valuable nutritional reserves in the form of long-chain polyunsaturated fatty acids. These are believed to contribute positively to neurodevelopment in utero (Agostini, Trojan, Bellù, Riva & Giovannini, 1995).
These same areas of fat storage are thought, also, to facilitate easier locomotion. By lowering an expectant mother’s centre of gravity, it is believed that balance, and hence stability, is enhanced (Pawlowski, 2001). This would render bipedal movement less dangerous. However, another change during pregnancy is breast enlargement. On average, breast mass increases nine-fold (Orshan, 2006, p. 448). This would counteract any gravitational benefit induced by increased fat storage around the buttocks and thighs. The importance of differential weight gain is, therefore, questionable.
A further point of interest is that many women habitually carry nursing infants on their hips (Leakey, 1981, p. 105). This places the infant in a convenient position to suckle, while the mother retains one free hand to carry out daily activities. Fitzgerald (1922) documented observations of Indian women simultaneously carrying babies on their hips and water pots on their heads. In Peru, Hern (2003) described the indigenous Shipibo women as being naked above the waist, apart from a looped shawl employed as a hip-carry.
A smaller WHR, and thereby greater waist concavity, would make infant transport less cumbersome. An extreme example is found among the Khoisan peoples of southern Africa. Khoi women exhibit massively increased fat deposition in and around the buttocks, a condition known as steatopygia (Lyons, 2004, p. 31). This protrusion provides a natural seat for their babies (Human Behavior and Evolution Society, 2008, p. 327). However, excessive adiposity might hinder movement or the ability to gather food, or even precipitate diabetes. Therefore, a trade-off could exist.
The evolutionary significance of this ‘pseudo-marsupial’ infant-carrying arrangement could be an associated reduction in environmental risk. That is, in a typical hunter-gatherer society, if a mother’s body shape were insufficiently curvaceous to carry her infant effectively and comfortably, the only other option might be to venture out alone, leaving the infant vulnerable to either nutritional neglect or predation. Ergo, the mortality rate of infants born to high-WHR females might, historically, have been relatively high. This is, however, conjecture.
Congenital breast cancer or heart disease, related to female body shape, could create selection pressure if incident before or during reproductive age. However, in most cases, both pathologies manifest themselves post-menopause (Andolina, Lillé & Willison, 2001, pp. 16-17; Nathan & Judd, 2006, p. 965). Consequently, as with Huntington’s disease, for instance (Winter, Hickey & Fletcher, 2002, p. 300), the associated genes are passed to the next generation before the onset of symptoms (and, eventually, death). This would imply little effect on gene frequencies.
The hourglass figure may be considered a sexual signal of reproductive potential. From a male viewpoint, large breasts suggest a plentiful supply of milk for his offspring. Small breasts imply a risk of inadequate lactation which might prove fatal. Broad hips symbolize a capacity for safe and uncomplicated childbirth (Low, Alexander & Noonan, 1987). However, milk is stored in glandular, as opposed to adipose, tissue; and birth canal size is not proportional to hip broadness (Diamond, 1998, p. 183). Therefore, the signal could be construed as deceptive (Dawkins & Guilford, 1991), although this would be immaterial to the female.
The accompanying waist slimness could be equally desirable. Ridley (1994, pp. 154-156) suggested that substantial abdominal fat might hinder foetal growth. However, women with copious android fat are quite able to conceive and reach full term. A different hypothesis is that a slim waist implies that a female is not already pregnant and, therefore, a prospective suitor would not be risking investment in the propagation of another male’s genes. This is, arguably, more logical, as a heavily expectant female would be incapable of deceptively signalling that she is chaste.
Therefore, males attracted to hourglass-figured females could increase the differential survival of their own genes.
There can be little doubt that evolutionary mechanisms account for the distinctive body shape of the human female. As cited by King (2000, p. 160), the late Marilyn Monroe (Figure 73.1) — vital statistics 36-24-34, WHR 0.7 (Guerrero & Floyd, 2006, p. 66) — once said:
‘Big breasts, big ass, big deal.’
It would appear so.
Figure 73.1: This iconic photograph of Marilyn Monroe, taken in 1955, shows clearly her narrow waist (w) contrasting with broad hips (h) and large breasts. The waist-hip ratio, r = w/h. An r value of approximately 0.7 is universally considered to be the optimum value with respect to both male preference and female health. Coincidence? I think not: males are genetically programmed to prefer healthy females.
Copyright © 2014 Paul Spradbery
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