The obstetric dilemma: An ancient game of Russian roulette, or a variable dilemma sensitive to ecology?

Abstract

The difficult birth process of humans, often described as the ''obstetric dilemma,'' is commonly assumed to reflect antagonistic selective pressures favoring neonatal encephalization and maternal bipedal locomotion. However, cephalo-pelvic disproportion is not exclusive to humans, and is present in some primate species of smaller body size. The fossil record indicates mosaic evolution of the obstetric dilemma, involving a number of different evolutionary processes, and it appears to have shifted in magnitude between Australopithecus, Pleistocene Homo, and recent human populations. Most attention to date has focused on its generic nature, rather than on its variability between populations. We reevaluate the nature of the human obstetric dilemma using updated hominin and primate literature, and then consider the contribution of phenotypic plasticity to variability in its magnitude. Both maternal pelvic dimensions and fetal growth patterns are sensitive to ecological factors such as diet and the thermal environment. Neonatal head girth has low plasticity, whereas neonatal mass and maternal stature have higher plasticity. Secular trends in body size may therefore exacerbate or decrease the obstetric dilemma. The emergence of agriculture may have exacerbated the dilemma, by decreasing maternal stature and increasing neonatal growth and adiposity due to dietary shifts. Paleodemographic comparisons between foragers and agriculturalists suggest that foragers have considerably lower rates of perinatal mortality. In contemporary populations, maternal stature remains strongly associated with perinatal mortality in many populations. Long-term improvements in nutrition across future generations may relieve the dilemma, but in the meantime, variability in its magnitude is likely to persist. Yrbk Phys The notion that maternal pelvic dimensions are subject to powerful competing demands from reproduction and locomotion is widely accepted in the biomedical and anthropological literature (Krogman, 1951;Washburn, 1960;Schultz, 1969;Rosenberg, 1992;Rosenberg and Trevathan, 1996;Wittman and Wall, 2007;Walsh, 2008;Franciscus, 2009;Trevathan, 2011). The maternal pelvis is frequently considered to be subject to two counteracting evolutionary forces: decreased height and increased mediolateral breadth in order to optimize the biomechanics of locomotion, and increased anteroposterior dimensions in order to enable birth of the unusually encephalized human infant. The compromise imposed by these antagonistic demands manifests as a difficult passage of the fetal head through the birth canal (Wittman and Wall, 2007), resulting in the birth process being a more complex and lengthy procedure in humans than in closely related species of ape (Tevathan, 1988;Rosenberg, 1992;Rosenberg and Trevathan, 2002). The antagonistic interaction of bipedalism and encephalization has been assumed to have followed the emergence of the large Homo brain within the last 2 million years (Martin, 1983).In addition to shaping the unusual mechanism of birth, h...

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

“…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. Pelvic dimensions, for example, may vary with ecological parameters, diet, and other aspects of phenotype (Wells et al 2012). In addition, lower limb length, in particular, the dimensions of the tibia, appears to be relatively sensitive to environmental variation (Trinkaus 1981;Holliday 1997;Stock 2012).…”

The evolution of the human pelvis: changing adaptations to bipedalism, obstetrics and thermoregulation

“…africanus had a 'human-like movement of rotation and flexion of the fetal skull' during birth ( [56], p. 262; see also [36,75]). Neonatal head size relative to the size of the birth canal in australopithecines was probably closer to apes than humans ( [33,75 -77]; but see [1,36]), and the M -L wide and A-P narrow pelvis is seen by most authors as a barrier to birth of infants with relatively large brains (see [16] for a review of encephalization and birth in fossil hominins).…”

“…Adult body size in H. heidelbergensis and the Neanderthals, which was within or above the modern human range, changed little during the Middle Pleistocene, but adult brain size did increase [24,60,74,113,114,116]. Fossil evidence suggests that Neanderthal infants were born with a body and head size similar to modern newborns, implying a similar level of obstetric difficulty in Neanderthals and modern humans [1,16,74,77,124] and probably 'obligate midwifery' (cf. [35,130]).…”

Covariation between human pelvis shape, stature, and head size alleviates the obstetric dilemma

“…However, a recent study showed that neonatal size and human gestation length are limited not only by pelvic dimensions but also by maternal metabolic capacity (16). Phenotypic plasticity of pelvic dimensions and head size in response to changes in nutrition, poor food availability, and infectious disease burden, among others, might influence the severity of the obstetric dilemma (17)(18)(19).…”