Protection of obstetric dimensions in a small‐bodied human sample

Abstract

In human females, the bony pelvis must find a balance between being small (narrow) for efficient bipedal locomotion, and being large to accommodate a relatively large newborn. It has been shown that within a given population, taller/larger-bodied women have larger pelvic canals. This study investigates whether in a population where small body size is the norm, pelvic geometry (size and shape), on average, shows accommodation to protect the obstetric canal. Osteometric data were collected from the pelves, femora, and clavicles (body size indicators) of adult skeletons representing a range of adult body size. Samples include Holocene Later Stone Age (LSA) foragers from southern Africa (n = 28 females, 31 males), Portuguese from the Coimbra-identified skeletal collection (CISC) (n = 40 females, 40 males) and European-Americans from the Hamann-Todd osteological collection (H-T) (n = 40 females, 40 males). Patterns of sexual dimorphism are similar in the samples. Univariate and multivariate analyses of raw and Mosimann shape-variables indicate that compared to the CISC and H-T females, the LSA females have relatively large midplane and outlet canal planes (particularly posterior and A-P lengths). The LSA males also follow this pattern, although with absolutely smaller pelves in multivariate space. The CISC females, who have equally small stature, but larger body mass, do not show the same type of pelvic canal size and shape accommodation. The results suggest that adaptive allometric modeling in at least some small-bodied populations protects the obstetric canal. These findings support the use of population-specific attributes in the clinical evaluation of obstetric risk.

The Skeletal Phenotype of “Negritos” from the Andaman Islands and Philippines Relative to Global Variation among Hunter-Gatherers

“…The Andaman Islanders exhibit a very narrow pelvis (BIB) similar to the Khoisan, which is both absolutely narrow and narrow relative to limb lengths. Previous studies have investigated the narrow pelvic dimensions of the Andaman Islanders and Khoisan/ LSA Southern Africans (Kurki 2007(Kurki , 2011 but it is somewhat surprising that this characteristic is not shared with other small-bodied foragers such as the Aeta. Pelvic dimensions have recently been shown to carry phylogenetic information relevant to human population history (Betti et al 2012(Betti et al , 2013; however, we know very little about the mechanisms controlling other skeletal dimensions.…”

Sexual dimorphism in the size and shape of the os coxae and the effects of microevolutionary processes

“…While the selection of a large, obstetrically capacious pelvis might conflict with the thermoregulatory needs of a narrow pelvis in hot climates, such a dilemma might not present itself in cold climates, where a large pelvis would be climatically favored (Weaver and Hublin, 2009). Obstetrical constraints, if stronger in small-bodied populations as suggested by Kurki (2007), could increase pelvic dimorphism in these populations in respect to others characterized by larger body size. In a larger study, Kurki (2011) found only two pelvic measurements that appeared to be more sexually dimorphic in small-bodied populations than in larger-bodied ones: the anterior-posterior dimensions of the midplane and outlet, both obstetrically relevant dimensions.…”

“…Kurki (2007) compared body size and pelvic dimensions in three populations that ranged from small (KhoeSan), through medium (Portuguese), to large body size (European-Americans). Kurki's results showed that some dimensions of the birth canal, especially of the midplane and outlet, are not significantly smaller in KhoeSan women relative to the other two populations, despite the former being significantly smaller in body size and in most of the other pelvic measurements (Kurki, 2007).…”

“…Kurki (2007) compared body size and pelvic dimensions in three populations that ranged from small (KhoeSan), through medium (Portuguese), to large body size (European-Americans). Kurki's results showed that some dimensions of the birth canal, especially of the midplane and outlet, are not significantly smaller in KhoeSan women relative to the other two populations, despite the former being significantly smaller in body size and in most of the other pelvic measurements (Kurki, 2007). Kurki interpreted the results as an indication that some obstetrical dimensions are evolutionarily protected and preserved independently of body size, due to strong stabilizing selection acting on the size of the female pelvic canal to maintain viable dimensions in small-size populations.…”

Predictors of the narrowest pelvic cavity diameter in live females and males

“…Th e relationships of these dimensions in males are not presented in literature. In addition, the most part of the studies on the relationships among parameters was performed on skeletal collections without taking into account pelvic joints [5,6,10,17]. Th erefore, this study focuses on two aspects of the prediction of the bispinous diameter: 1) the relationship between the false pelvis and the true pelvis, which is not explored in males; 2) the sexual dimorphism in this prediction, which seems to be topical for measures in live humans.…”