INTRODUCTION
Sexual dimorphism of the human pelvis is linked intimately with its adaptive functions.
The peculiarly shaped hominid pelvis represents the total response to the diverse
forces that have moulded its structure. These diverse forces are requirements for
efficient bipedalism and parturition. In some respects, the structural demands of
these unrelated functions have been in conflict. The morphological response to the
dominant requirement, bipedalism, is clearly discernible, while the changes serving
the needs of parturition are seen as compensatory modifications as reflected with
greater emphasis for pelvic sexual dimorphism in the female. In addition, sexual selection
has made sexual dimorphism even more pronounced. The female buttocks have undergone
sexual elaboration through mate choice by males. Thus, total pelvic architecture is
a mosaic constituted of the aggregate of differential responses to different functional
goals.
There are complications during parturition that have been repeatedly prevented or
interceded by medical technology, close monitoring, surgical practices such as caesarian
sections, and other strategies. With these complications, one could hypothesize that
these natural operators, which have exerted their influence since the beginning of
mankind, might be becoming increasingly destabilized, attenuated or stochastic.
The Male and Female Pelvic Blueprint and Anatomic Variations
In general, the structure of the male pelvis is significantly heavier and thicker
than that of the female. The male pelvic bones are also adapted to fit a more massive
and sturdy body architecture. For example, the male acetabulum has been designed to
fit a bigger femur. Though a large amount of the sexual dimorphism of the pelvis is
accounted for by size differences, sex-linked shape variation is also very conspicuous
and cannot be considered an allometric consequence of differences in body size between
the sexes (1). These variations in shape are demonstrated by the more rounded frame
of the female pelvis. The sciatic notches are broader, the greater pelvis is shallower,
the lesser pelvis is wider and the pelvic inlet and outlet are larger (longer pubic
bones and a greater degree of curvature of the pectineal line). The female hipbones
are also different in traits associated in the position of the sacroiliac joint in
the iliac bone (2). As a result of this ‘flattened’ appearance, the female obturator
foramen is more elliptical. The pubic arch is formed by the conjoined rami of the
pubis and ischium of the two sides. These rami meet at the pubic symphysis to form
the subpubic angle. The subpubic angle is nearly a right angle in females and is considerably
less in males; approximately 30° narrower (3).
Superficial to the skeleton and musculature of the pelvis, sexual dimorphism in pelvic
morphology is most apparent in body fat distribution as measured by waist hip ratio
(WHR). The WHR has been shown to be independent of overall body weight and an accurate
predictor of risk for various diseases, premature mortality, degree of estrogenicity
and fecundity of women (4). Undoubtedly, healthy women have a greater propensity to
possess rounder hips and a lower WHR compared to most men (4).
Nonetheless, the human pelvis is not always distinctly dimorphic. It has been well
established that nature has allowed individual anatomical variation and departures
from set norms within each sex. Hence, one could infer that it is possible to find
any of the previously defined archetypal features in the ‘wrong’ sex. In addition,
there is metric and morphologic variation in the expression of sexual dimorphism between
racial phenotypes and populations (5). In other words, the final shape of the female
pelvis is affected by multiple etiological factors - cultural, environmental and genetic.
The gynecoid pelvis (rounded shape) is said to be the normal female type while the
android pelvis (heart-shaped) is often designated a male variant. Women with the android
pelvis do not typically present with hyperandrogenism, and signs of hyperandrogenism
have been similarly encountered in patients with gynecoid and other types of pelvis.
This android pelvis was also found more frequently in women exposed to strenuous physical
activity during adolescence and observed more often when the acquisition of erect
posture was delayed beyond the usual age of 14 months, while a platypelloid pelvis
was more frequent when erect posture was acquired before 14 months (6).
In addition to being developmentally discriminating, the distribution of the anthropoid
pelvis between both sexes and frequency is ethnically discriminating as well. It was
observed that there were significant differences in the accuracy of sex determination
from pelvic morphology between both males and females, and whites and blacks. Pubic
bone shape was the easiest to assess and was the most consistently reliable morphological
indicator of sex in both sexes and population groups. However, in blacks, the greater
sciatic notch form allowed the highest separation. This demonstrates that racial differences
significantly affect the expression of sexual dimorphism (5).
Temporal Development of Sexual Dimorphism of the Pelvis
Sexual dimorphism begins as early as the fetal stage of development. Indeed, the appearance
of morphological differences, such as the greater inter-ischiatic distance in fetal
females after the 26–27th week of gestation, could indicate the presence of pathological
fetal development (7). Evidently, sexual dimorphism does become increasingly pronounced
and divergent as a child grows into an adult. While there are significant sex differences
in breadth of the ischium and acetabular regions by 8 years of age, most of the sexual
dimorphism in the pelvis develops during the adolescent growth spurt, during which
both male and female pelves undergo growth remodeling of the pelvic cavity. Over the
same time period, males show significantly greater growth in the acetabulum, and females
show greater growth in the pelvic cavity (8).
By age 18, the pelvis demonstrates a posterior-to-anterior gradient of increasing
dimorphism within the inlet of the pelvic birth canal. Canalization of growth of the
transverse diameters of the sacrum, inlet, anterior inferior iliac spines, and breadths
of the ilium and ischium during puberty can be attributed to the effects of stabilizing
selection operating on both males and females. On the other hand, over time, there
is evidence of increased variation and discordant change within each sex and differential
growth between the sexes for the interacetabular diameter, breadths of the anterior
superior and posterior inferior iliac spines, public length, and ilium height. These
are patterns indicating effects of disruptive selection on the pelvis (8). Growth
studies have indicated that the linea terminalis may be unique by continuing to grow
in early adulthood in females but not in males. This growth occurs at the medial border
of the pubis. The selective advantage of a later age at maturation of the pubis in
females than males is that the period of growth is prolonged, thereby contributing
to sexual dimorphism in pubic length, linea terminalis length and pelvic inlet circumference
(9).
Natural Adaptive Forces in Shaping Sexual Dimorphism of the Pelvis:
Bipedalism versus Parturition
An evolutionary pattern towards bipedalism, taking into account selective pressures
of reproduction, has been demonstrated by the increase of critical dimensions of the
pelvis as the maternal skeleton becomes larger. One such dimension, the distance between
the ischial spines, defines the pelvic midplane and is an important consideration
in hominid reproduction. It was found that a correlation exists between skeletal frame
size and the distance between the ischial spines in females. In females, but not in
males, weight and femoral head diameter are excellent predictors of the distance between
ischial spines. However, the femoral head diameter, in females, does not predict weight
as well as it does in males (10).
These adaptations to ease parturition as reflected in the sexual dimorphism in the
human pelvis and femur are disadvantageous to women in terms of mechanics of locomotion.
The mechanical variables that primarily contribute to dimorphism are the moment arm
of the gluteus medius and the torque produced by the abductors at the hip. These mechanical
aspects of hip function produce greater pressure on the femoral head in females (11).
The divergent selective forces shaping the pelvis remain destabilized primarily because
of the rare and capricious growth of the human brain. The pelvis has to resculpt itself
repeatedly to deliver babies with a disproportionately large head. Perpetual brain
expansion is a consequence of both natural and cultural evolution that favours intelligence,
learning, tool making and an elaborate culture. Naturally, it is easy to see that
the speed and uniqueness of this encephalization process must have been driven by
some sort of positive feedback mechanism. One of the postulated processes is the runaway
social competition model1 which implies that our brain developed as a result of social
competition between fellow men. Humans got better at deceiving and outmaneuvering
one another which, as a result, selected for men who were able to see through deceptive
plans. Such a ‘runaway’ process would result in a rapid positive feedback mechanism
that enhances human brain power. Another ‘runaway’ model that could explain this encephalization
is the runaway gene-culture co-evolution model2. This model assumes that the emergence
of modern culture that depended heavily on the manipulation of tools and materials
meant that only individuals who were creative and capable enough to develop tools
and harness or exploit materials survived. These individuals were naturally those
with higher cognitive abilities and were thus able to further improve and complicate
the material culture, which in turn selected for even brighter men (12).
As humans have evolved from a unique line of primate ancestry, which accentuates the
need for brain expansion, innovation and intelligence in place of physical strength,
agility, speed and endurance, the present-day fetal head is painfully massive and
cumbersome for the mother during childbirth. This is substantiated by the classic
comparative relation, which states that primate species, like humans, with a large
neonatal brain size relative to the birth canal, tend to have more sexually dimorphic
pelves (13).
Mate Selection in Sculpting the Human Body:
How do the Waist-Hip-Ratio and Body Mass Index Signify Healthiness?
While the natural forces discussed above are quintessential operators in hominid evolution,
the influence of mate selection on sexual dimorphism in the modern-day human are apparent
and cannot be discounted. Body traits classically considered to be attractive, such
as the lower Waist-Hip-Ratio (WHR) and Body Mass Index (BMI) of women, could be explained
by mate selective operators.
While mate choice takes much time and energy, sometimes to the point of impairing
survival or genetic perpetuation, it is very evolutionarily sound. A number of us
have probably contemplated delaying procreation, and running the risk of never being
able to propagate, simply because the ideal mate has not crossed our paths. Why are
we willing to suffer such risks? One possible response to this existential question
is that if we were to find an ideal mate with good genes, our offspring would likely
have higher survivability, and propagate its genes well. According to the “selfish
gene” hypothesis, animals with genes to select mates with good genes would hence produce
more viable offspring carrying those selective genes, and thus the genes for selectivity
would spread throughout the gene pool.
Evolutionary biology theories, such as the good genes model3, have suggested that
the most fundamental form of mate choice is selection for indicators of viability
and fertility, which may manifest in any easily perceivable bodily or behavioural
trait to reveal age, health, nutritional status, strength, dominance, social status
and disease resistance. These honest indicators would demonstrate the chances that
a potential mate has desirable genetic traits that would be passed onto offspring
and enhance their survival, or is capable of helping to provide for and protect offspring.
Some of these indicators that serve as major targets for selective mate choice by
males include facial neoteny, averageness and symmetry. Another functional model of
mate selection, the good provider model4, pertains to the more fluid and complex social
patterns of hominid civilization. Thus, indicators of social success and cognition
would also come into play as survival depends not only on individual strengths but
also on the ability to cooperate with and outmaneuver others (14).
These mate selection theories could explain the reason behind the morphological amplification
of the female breasts and buttocks. In order for females to solicit male attention
and investment, accentuated bodily features signaling youthfulness, healthiness and
fertility would have to be judged as ‘attractive’ to the opposite sex. Indeed, this
is a fundamental assumption of adaptive explanations of female attractiveness. In
particular, sexual dimorphism in body fat distribution has also been assumed to be
vital in mate selection. WHR is an accurate predictor of nutritional status, reproductive
age and degree of estrogenicity and parity of women, independent of overall body weight.
Finally, cross-cultural and historical data have suggested that the relationship between
WHR and female attractiveness is not culture-specific and not inculcated by what modern
Western fashion dictates or media (15).
The other putative cue to female physical attractiveness is BMI. It has been shown
that both males and females assigned higher ranking for attractiveness, youthfulness,
healthiness, reproductive capability and intelligence to normal weight figures with
low WHRs. Overweight figures were assigned low rankings for all these qualities except
reproductive capability. Underweight figures, regardless of WHR, were assigned low
ranking for reproductive capability and those underweight figures that had high WHRs
were assigned low ranking for healthiness (16,17). Female and male subjects, judged
heavier female target figures with low WHRs as more attractive and healthier than
thinner figures with higher WHRs. Female subjects perceived heavier female target
figures with low WHR to represent ideal female figures. It is proposed that female
attractiveness and ideal female shape may be more influenced by WHR than overall body
size (4).
More recently, it was revealed in another study that WHR was less important than BMI
as a predictor of attractiveness ratings for bodies. Viewers’ judgments were influenced
more by BMI than WHR (18). BMI was the primary predictor of attractiveness in both
front and profile, and the putative visual cues to BMI showed a higher degree of view-invariance
than shape cues (19). Interestingly, a study in 2004 developed the volume height index
(VHI), which is the body volume divided by the square of the height. This was heralded
as the most important and direct visual determinant of female physical attractiveness.
VHI is also a key indicator of health and fertility owing to its strong linear relation
to BMI (20).
COMPLICATIONS DURING CHILDBIRTH:
Are Female Hips too Small?
While, the influence of natural forces and mate selection on human anatomy have been
well documented for years, few have explored the possibility that rapid advancement
of medical practices might currently play an important role in increasing the frequency
of more atypical anatomical forms and consequently, re-shape many of our human characteristics,
such as the female pelvic anatomy.
It has been previously established that the size and shape of the pelvic inlet are
key during labour as it determines the ease in which the fetal head enters the lesser
pelvis. Along these lines, the size of the lesser pelvis has always been important
in obstetrics because it determines the size of the bony pelvic canal through which
the fetus passes during a vaginal delivery (3). More frequently than one might imagine,
some kind of difficulty or injury, across a spectrum of severity and recovery rates,
is encountered during human childbirth. Today, a woman with a narrow or misshaped
pelvis can, in fact, successfully give birth to a massive baby. Such a phenomenon
would probably never have existed before the age of aggressive medical and scientific
intervention in assisting the birthing process. In fact, for our ancestors, a grossly
mismatched maternal-fetal presentation was often sorrowfully fatal to both mother
and child.
Even with modern surgical and medical practices, childbirth, a rite of passage for
most women, is dangerous and can potentially lead to lifelong morbidity. During parturition,
extensive damage of varying severity can occur to the pelvic floor, which supports
the fetal head while the cervix of the uterus is dilating. The fetal head may compress
the nerves of the mother’s sacral plexus, producing pain in the lower limbs. Tearing
or stretching of the perineal body during childbirth may result in a permanent weakness
of the pelvic diaphragm. The perineal body is especially important in women because
it is the final support of the pelvic viscera and serves as the attachment for the
perineal muscles. As a result, prolapse of the vagina through the vaginal orifice
may occur after the support of the inferior part of the posterior wall of the vagina
is removed (3).
These ill-effects associated with childbirth are also exacerbated when the mother
faces obstructed labor. The major causes of obstructed labor is cephalopelvic disproportion,
which may be due to a small pelvis, a large baby, fetal malpresentation, a tight perineum,
or abnormalities or tumors of the uterus, ovary, or vagina. When obstructed labor
is unrelieved, the presenting fetal part is impacted against the soft tissues of the
pelvis and a widespread ischemic vascular injury develops that result in tissue necrosis
and subsequent fistula formation. (21). The processes of labour and vaginal delivery,
especially the former, can cause pudendal nerve damage. A heavier baby and a longer
second stage of labour were both associated with significant prolongation of pudendal
nerve latency. Pudendal nerve damage was also found after forceps delivery and perineal
tears (22).
Vaginal delivery has also been regarded as one of the commonest causes of stress urinary
incontinence and anal incontinence. This is primarily caused by injury to the levator
ani, in particular the pubococcygeus, which encircles and supports the urethra, the
vagina and anal canal, and the pelvic fascia, which may be torn during childbirth.
Stress urinary incontinence is further provoked by damage to the levator ani and pelvic
fascia that hold the position of the neck of the bladder and the urethra in place.
Furthermore, delivery causes partial denervation of the pelvic floor, with consequent
renervation, in most women having their first baby. For some, it is likely to be the
first step along a path leading to prolapse or severe stress urinary incontinence
(3,22,23). Anal incontinence has additionally been attributed to traumatic childbirths
and multiple deliveries, because of repeated stretching of the pudendal nerves and
subsequent nerve damage (24). The risk for anal sphincter injury alone could be used
as an argument for elective Cesarean section (25).
In light of the plethora of injuries associated with childbirth, it is easy for one
to postulate that in the absence of healthcare, incompatible pelvic-fetal anatomy
could result in significant infant and maternal mortality and morbidity. Although
concrete scientific evidence does not exist, one has to wonder if medical practice
is indirectly and insidiously introducing variability to the human anatomy.
CONCLUSION
The morphology of the human body is susceptible to perpetual modifications. This dynamic
nature is characteristic of most biological systems and testimony to traditional theories
of evolution. However, classic Darwinism never fully accounted for or addressed the
potency of the overbearing forces put forth by mate selection and social survival
strategies. Intelligence, which evolved as a natural survival strategy, heralds a
new tangential path in evolution. Henceforth, modern concepts of biocultural evolution
are ironically replacing Darwinism to address and explain present-day issues. Old
strategies of symbiosis, competition for ecological niches, survival of the fittest,
phenotypic variability and mutations have lost some of their pertinence in the dawn
of human civilization which seems to cherish altruism, cooperation, supra-ordinate
goals and social cohesiveness. It is also prudent for us to acknowledge that even
as we strive to analyze attractiveness and attempt to find functional patterns or
algorithms of beauty, clichés like ‘beauty is in the eye of its beholder’ and ‘beauty
is not only skin deep’ are timeless and accurate.