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

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 antagon...

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Autores principales: Haeusler, Martin, Grunstra, Nicole D.S., Martin, Robert D., Krenn, Viktoria A., Fornai, Cinzia, Webb, Nicole M.
Formato: Online Artículo Texto
Lenguaje:English
Publicado: Blackwell Publishing Ltd 2021
Materias:
Acceso en línea:https://www.ncbi.nlm.nih.gov/pmc/articles/PMC8518115/
https://www.ncbi.nlm.nih.gov/pubmed/34013651
http://dx.doi.org/10.1111/brv.12744
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author Haeusler, Martin
Grunstra, Nicole D.S.
Martin, Robert D.
Krenn, Viktoria A.
Fornai, Cinzia
Webb, Nicole M.
author_facet Haeusler, Martin
Grunstra, Nicole D.S.
Martin, Robert D.
Krenn, Viktoria A.
Fornai, Cinzia
Webb, Nicole M.
author_sort Haeusler, Martin
collection PubMed
description 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.
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spelling pubmed-85181152021-10-21 The obstetrical dilemma hypothesis: there's life in the old dog yet Haeusler, Martin Grunstra, Nicole D.S. Martin, Robert D. Krenn, Viktoria A. Fornai, Cinzia Webb, Nicole M. Biol Rev Camb Philos Soc Original Articles 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. Blackwell Publishing Ltd 2021-05-19 2021-10 /pmc/articles/PMC8518115/ /pubmed/34013651 http://dx.doi.org/10.1111/brv.12744 Text en © 2021 The Authors. Biological Reviews published by John Wiley & Sons Ltd on behalf of Cambridge Philosophical Society. https://creativecommons.org/licenses/by-nc/4.0/This is an open access article under the terms of the http://creativecommons.org/licenses/by-nc/4.0/ (https://creativecommons.org/licenses/by-nc/4.0/) License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited and is not used for commercial purposes.
spellingShingle Original Articles
Haeusler, Martin
Grunstra, Nicole D.S.
Martin, Robert D.
Krenn, Viktoria A.
Fornai, Cinzia
Webb, Nicole M.
The obstetrical dilemma hypothesis: there's life in the old dog yet
title The obstetrical dilemma hypothesis: there's life in the old dog yet
title_full The obstetrical dilemma hypothesis: there's life in the old dog yet
title_fullStr The obstetrical dilemma hypothesis: there's life in the old dog yet
title_full_unstemmed The obstetrical dilemma hypothesis: there's life in the old dog yet
title_short The obstetrical dilemma hypothesis: there's life in the old dog yet
title_sort obstetrical dilemma hypothesis: there's life in the old dog yet
topic Original Articles
url https://www.ncbi.nlm.nih.gov/pmc/articles/PMC8518115/
https://www.ncbi.nlm.nih.gov/pubmed/34013651
http://dx.doi.org/10.1111/brv.12744
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