Human Pelvis - Sexual Dimorphism

Sexual Dimorphism

Modern humans are to a large extent characterized by bipedal locomotion and large brains. Because the pelvis is vital to both locomotion and childbirth, natural selection has been confronted by two conflicting demands: a wide birth canal and locomotion efficiency, a conflict referred to as the "obstetrical dilemma". The female pelvis has evolved to its maximum width for childbirth — a wider pelvis would make women unable to walk. In contrast, human male pelves are not constrained by the need to give birth and therefore are optimized for bipedal locomotion.

The principal differences between male and female true and false pelvis include:

• The female pelvis is larger and broader than the male pelvis which is taller, narrower, and more compact.
• The female inlet is larger and oval in shape, while the male sacral promontory projects further (i.e. the male inlet is more heart-shaped).
• The sides of the male pelvis converge from the inlet to the outlet, whereas the sides of the female pelvis are wider apart.
• The angle between the inferior pubic rami is acute (70 degrees) in men, but obtuse (90-100 degrees) in women. Accordingly, the angle is called subpubic angle in men and pubic arch in women. Additionally, the bones forming the angle/arch are more concave in females but straight in males.
• The distance between the ischia bones is small in males, making the outlet narrow, but large in females, who have a relatively large outlet. The ischial spines and tuberosities are heavier and project farther into the pelvic cavity in males. The greater sciatic notch is wider in females.
• The iliac crests are higher and more pronounced in males, making the male false pelvis deeper and more narrow than in females.
• The male sacrum is long, narrow, more straight, and has a pronounced sacral promontory. The female sacrum is shorter, wider, more curved posteriorly, and has a less pronounced promontory.
• The acetabula are wider apart in females than in males. In males, the acetabulum faces more laterally, while it faces more anteriorly in females. Consequently, when men walk the leg can move forwards and backwards in a single plane. In women, the leg must swing forward and inward, from where the pivoting head of the femur moves the leg back in another plane. This change in the angle of the femoral head gives the female gait its characteristic (i.e. swinging of hips).