Closet Skeketons: Re-Examining Ideas About Pelvic Inlet Shape
Proceedings of the 1989 Conference of the American Association of Physical Anthropologists
Posted: 23 May 2018
Date Written: April 5, 1989
As an object of scholarly interest, the pelvis has received top priority in obstetrics. It also ranks number one in sexing archaeological material and ranks second behind the cranium in studies of human evolution. But even within one of these disciplines, successive views on the pelvis do not use the same jargon, nor do they divide the pelvis into the same categories. Most importantly, they embody fundamentally different ideas about the nature of causality, especially the causes of the apparent sexual dimorphism in the pelvis. The implications of the differences in these perspectives are very salutary.
It had long been thought that the differences in pelvic inlet shape between males and females were innate differences due to a primary genotypic distinction. The expectation of a close structure/function relationship in organic form led to the idea that the pelvis might be adapted to parturition. A close relationship between structure and function is not found in the pelvis, as there was a significant increase in functions with the transition to a completely bipedal form of locomotion. This multiple utility inhibits any specialisation, belying any close structure function/relationship in pelvic form.
One implication of this approach is that we can expect to observe ontogenetic changes in pelvic form throughout the lifetime of an individual. In populations with adequate sources of fats and carbohydrates, the levels of nutrition would not only reduce the age of menarche, but also might delay the onset of menopause. It is not known whether higher level of circulating oestrogens would mean the individual retains the broad ‘female’ type of pelvic inlet shape after reproductive years. And some archaeological sites have shown a tendency for older males towards the female type of pelvis, as at the Gallen Priory site, supposedly an all-male domain.
A second implication is that what we think of as a typically female type of pelvis as being permanently quite dimorphic outside of pregnancy and lactation may be a relatively recent phenomenon.
A third implication in adopting a more quantitative perspective on sexual differences is it may be appropriate to create an intermediate gender in the analysis of archaeological populations. It is difficult for archaeologists to re-consider whether there is a good biological bedrock for gender constructs because of the omnipresence of gender division in line with socially organised kinship systems in every culture. The sophisticated statistical discrimination function analysis seems to give some faith in the segregation of male and female skeletal data, but as Black pointed out, our results are more correctly considered as consistent, rather than accurate, when making sexual diagnosis.
The last implication of accepting that pelvic Inlet shape as such is not particularly dimorphic is for identifying the sex of early hominids. If pelvic inlet shape is more of a population characteristic, and the sexual dimorphism of it is of a dynamic nature, then it is not surprising that nearly all the australopithecines show one type, seemingly the ‘female’ or adolescent type of pelvis. A mature male australopithecine pelvis has yet to be identified. The rare Homo erectus pelvis proves show more male types, but as they are considered mostly adolescent this should be viewed with some reserve.
This consideration shows the way in which the perspectives from obstetrics and physical anthropology result in robustly different views about the significance of pelvic inlet shape. This study is largely abstracted from my PhD thesis, which should be referred to in the first instance.
Keywords: pelvis, dimorphism, sex differences, bipedalism, parturition, adaptation
JEL Classification: Z00
Suggested Citation: Suggested Citation