Complications and selection against wide pelves

[snorkack]

The consequences of narrow pelves are dire and obvious. Baby stuck in pelvis is fatal for both.

Alternatives also are bad. Small, preterm or low birth weight children are vulnerable.

Narrow pelvis is so bad that pretty bad alternatives are evolved against it.

Think of the importance of pelvis. Pelvis, like skull, consists of several bones, but these are fused into one rigid ring. Consider that pelvis carries the weight of man: the weight of backbone is cantilevered sidewards through sacroiliac joints to hip joints and thence to legs.

Yet the pelvis of a woman literally disintegrates during pregnancy: sacroiliac joints (and pubic joint) undergo subluxation and move apart a long distance.

Then they cannot support the weight! Women suffer intense and chronic back paint during pregnancy and often cannot walk.

Now, why are pelves narrow?
Narrow pelves are supposed to have evolved as man rose to walk, as adaptations to walk.

Precisely how is a narrow pelvis better adapted to walk? Women with narrower than average hips and pelves suffer the complications described above. But what is the trouble with wide hipped women - women who have naturally wide girth not due to fat but wider than average pelvis bones before they get pregnant? Does wide pelvis´ unsuitability for walking get expressed in some forms of illnesses that are specifically complications of being big boned? How does natural selection eliminate wider hipped mothers so that the narrow hipped women continue to get born despite often dying in childbirth?

 

[256bits]

What you are describing is the obstetrical dilemma.
https://www.physicsforums.com/showthread.php?t=751197

This also gives an introduction into the human pelvis.
http://en.wikipedia.org/wiki/Human_pelvis

Precisely how is a narrow pelvis better adapted to walk?

Upright,?

Energy expenditure might have something to do with it. Wider spacing should(?) generally result in more muscle and ligament mass in addition to supporting pelvic bone stucture and mass, though the evolutionary pressures in this regard is beyond my scope. Locomotion involves walking, running and jumping. If by having narrower hips does result in an increased agility and speed to evade predators or catch prey then surely that would at first appear to be favourable, but again that is a debate probably raging in the community.

Human babies are borm immature. They are not the only mammals that do so. Pups from dogs and of course kittens would also fall in this category, as well as other births from animals in the wild in various degree. On the other end of the spectrum are nearly fully mature-functioning births, such as the foal from a horse or a calf from a cow.

Do other great apes give birth to babies as premature as human babies are? I guess you would answer no. The normal reason given is that the development of upright walking necessitated a change in pelvic structure which in turn necessitated a premature birth and shortened gestation period from that which would have resulted in a fully developed infant, with again various reasons thrown in. Add in the increasing cranial size of humans ( shoulder exit is also a problem at birth ) during the evolutionary process and we do have a 'obstetrical dilemma'. It is interestng to note that this casual relationship could be turned on its head, and instead premature birthing was that which led to an evolutionary tract opening up for an ability of upright locomotion, through changes in pelvic structure that could be pursued. What I am hinting at is that evolutionary pressures might be other than, in addition to, the regular hunter/prey point of view. Evolutionary scientists themselves have difficulty discerning the facts and processes of phylogeny, and if it was not so, they surely they would quickly be out of a job. Most people have difficulty distinguishing phylogeny( the species ) with ontogeny ( the individual ), although the two do relate to one another in some ways.


From the wiki article not all female human pelves are the same. A main list is as follows:

- The gynaecoid pelvis is the so-called normal female pelvis. Its inlet is either slightly oval, with a greater transverse diameter, or round. The interior walls are straight, the subpubic arch wide, the sacrum shows an average to backward inclination, and the greater sciatic notch is well rounded. Because this type is spacious and well proportioned there is little or no difficulty in the birth process. Caldwell and his co-workers found gynaecoid pelves in about 50 per cent of specimens.

- The platypelloid pelvis has a transversally wide, flattened shape, is wide anteriorly, greater sciatic notches of male type, and has a short sacrum that curves inwards reducing the diameters of the lower pelvis. This is similar to the rachitic pelvis where the softened bones widen laterally because of the weight from the upper body resulting in a reduced anteroposterior diameter. Giving birth with this type of pelvis is associated with problems, such as transverse arrest. Less than 3 per cent of women have this pelvis type.

- The android pelvis is a female pelvis with masculine features, including a wedge or heart shaped inlet caused by a prominent sacrum and a triangular anterior segment. The reduced pelvis outlet often causes problems during child birth. In 1939 Caldwell found this type in one third of white women and in one sixth of non-white women.

- The anthropoid pelvis is characterized by an oval shape with a greater anteroposterior diameter. It has straight walls, a small subpubic arch, and large sacrosciatic notches. The sciatic spines are placed widely apart and the sacrum is usually straight resulting in deep non-obstructed pelvis. Caldwell found this type in one quarter of white women and almost half of non-white women.

I suppose you are referring to the platypelloid and the android pelvis and the difficulties associated with a female with these pelves giving birth. Perhaps genetics is the answer. For a trait to survive, not all offspring with the trait need to survive to reproduce, only some of them.

Probably all this does not answer your questions, but hopefully it is something for you to bite down on of some sort.