The obstetrical dilemma hypothesis: there's life in the old dog yet

Abstract

The term 'obstetrical dilemma' was coined by Washburn in 1960 to describe the trade-off between selection for a larger birth canal, permitting successful passage of a big-brained human neonate, and the smaller pelvic dimensions required for bipedal locomotion. His suggested solution to these antagonistic pressures was to give birth prematurely, explaining the unusual degree of neurological and physical immaturity, or secondary altriciality, observed in human infants. This proposed trade-off has traditionally been offered as the predominant evolutionary explanation for why human childbirth is so challenging, and inherently risky, compared to that of other primates. This perceived difficulty is likely due to the tight fit of fetal to maternal pelvic dimensions along with the convoluted shape of the birth canal and a comparatively low degree of ligamentous flexibility. Although the ideas combined under the obstetrical dilemma hypothesis originated almost a century ago, they have received renewed attention and empirical scrutiny in the last decade, with some researchers advocating complete rejection of the hypothesis and its assumptions. However, the hypothesis is complex because it presently captures several, mutually non-exclusive ideas: (i) there is an evolutionary trade-off resulting from opposing selection pressures on the pelvis; (ii) selection favouring a narrow pelvis specifically derives from bipedalism; (iii) human neonates are secondarily altricial because they are born relatively immature to ensure that they fit through the maternal bony pelvis; (iv) as a corollary to the asymmetric selection pressure for a spacious birth canal in females, humans evolved pronounced sexual dimorphism of pelvic shape. Recently, the hypothesis has been challenged on both empirical and theoretical grounds. Here, we appraise the original ideas captured under the 'obstetrical dilemma' and their subsequent evolution. We also evaluate complementary and alternative explanations for a tight fetopelvic fit and obstructed labour, including ecological factors related to nutrition and thermoregulation, constraints imposed by the stability of the pelvic floor or by maternal and fetal metabolism, the energetics of bipedalism, and variability in pelvic shape. This reveals that human childbirth is affected by a complex combination of evolutionary, ecological, and biocultural factors, which variably constrain maternal pelvic form and fetal growth. Our review demonstrates that it is unwarranted to reject the obstetrical dilemma hypothesis entirely because several of its fundamental assumptions have not been successfully discounted despite claims to the contrary. As such, the obstetrical dilemma remains a tenable hypothesis that can be used productively to guide evolutionary research.

Dynamic finite-element simulations reveal early origin of complex human birth pattern

“…It is therefore difficult to imagine that metabolic reasons alone would have prevented these early hominins from giving birth to infants with brain sizes of 166–184 g as predicted from newborn-to-adult brain size proportions of a general primate model. Hence, our findings of the presence of pelvic constraints on fetal head size even in the relatively small-brained australopithecines support the original obstetrical dilemma hypothesis 8 that secondary altriciality in hominins is primarily related to the anteroposterior shortening of the birth canal as an adaptation to bipedalism rather than to metabolic limitations of the placenta, although some combination of these two hypotheses is also conceivable 6 .…”

“…The rotational behaviour observed in our bony finite-element birth simulation of an average modern human also challenges previous assumptions that early hominins maintained a transverse head orientation at each pelvic plane during birth 18 , 22 . Australopithecines therefore seem to have likely evolved a more human-like birth pattern with secondary altriciality prior to the appearance of substantial encephalization characterizing the genus Homo , which is in line with the predictions of the obstetrical dilemma hypothesis 6 , 8 .…”

“…This is based on the assumption that fatty acids required for fetal brain growth do not cross the placenta efficiently (but see ref. 6 ). However, brain size in australopithecines was only marginally larger relative to body mass than that inferred for our common ancestor with chimpanzees 15 and there is consequently no reason to assume that the placenta in australopithecines was less efficient than that of chimpanzees.…”

Sex classification using the human sacrum: Geometric morphometrics versus conventional approaches

“…Biomechanical constraints related to bipedal locomotion might therefore dominate in the sacrum over reproductive selection pressure. Similarly, mediolateral width and anteroposterior depth of the pelvis might be more constrained by bipedal locomotion or factors like thermoregulation rather than obstetrics [see Discussion in 74 ]. The sacroiliac joints might also buffer the biomechanical pressure of weight transmission during bipedal locomotion, a hypothesis supported by the lack of sufficient evidence for sex-based or geographically related differences in the shape of the auricular surface noted in this study.…”

Ontogeny of learning and visual discrimination in zebrafish

“…The study of brain maturation and early cognitive development in humans has played a central role for the understanding of how the nervous system works. Humans are characterised by an extreme form of altriciality, which has recently evolved in response to mechanical and energetic constraints imposed by cranial capacity expansion and bipedal locomotion [1]. Newborn infants are helpless and highly dependent on their parents.…”