INTRODUCTION
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?
ESSENTIAL
ANATOMY
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.
ASSOCIATED
PATHOLOGY
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).
NATURAL
SELECTION
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.
SEXUAL
SELECTION
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.
CONCLUSION
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 expired
Copyright
© 2014 Paul Spradbery
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