Thursday, March 26, 2015

The 'obstetric dilemma' hypothesis unraveled

Right now I'm giving this talk at the annual meetings of the American Association of Physical Anthropologists. Here's the link to Session 15. Costly and Cute: How Helpless Newborns Made Us Human, where you'll see the participants, their talk titles and abstracts. This symposium evolved out of a seminar at the School for Advanced Research back in the summer of 2014 when I was very pregnant and on the cusp of facing my own obstetric dilemma.

Let’s see how far we can unravel this epic hypothesis in 15 minutes or less. To start, here are the two most important seeds I hope to plant today in hopes that you’ll read up on these things further:  One. The obstetric dilemma is a hypothesis, not a fact. And… Two. The obstetric dilemma is an elegant idea, and would be an extraordinary example of human exceptionalism if true.  But it bears the burden of proof. So what is the obstetric dilemma hypothesis? In a nutshell…

We use it to explain at least these two observations, and often there are more: First, childbirth is difficult and risky. And, second, humans are born with only about 30% of our adult brain size, but for chimps it’s 40%.  Why? The “obstetric dilemma” hypothesis. Constrained by bipedalism, the human pelvis constrains gestation length and fetal brain and body growth, while making childbirth difficult and dangerous.  Which, put differently, is the hypothesis that the unique human pelvis uniquely impacts human parturition and life history. Here’s a good example of how the obstetric dilemma is often presented as a common fact, even in a book that aims to have us rethink old assumptions:

“You can’t give birth to large-brained infants and also walk on two legs trouble-free, no matter how hard you try.” Alright so, locomotion’s running the show, but does it deserve to? That is, does selection for bipedalism limit the birth canal from widening and relieving a laboring mother’s difficulty?

As of now, even taking a new dynamic approach over the old static one, like Anna Warrener and her colleagues do, there is no strong history of support for the idea that wide or women’s hips are costlier or less efficient than narrow or male’s hips. This is, at present, the best approach to the question of whether women are presently at an upper limit in pelvic dimensions that selection for bipedalism will allow. And although it does not test the obstetric dilemma directly, it demonstrates that our perspective on sexual dimorphism has been biased toward seeing female bodies as compromised compared to a male ideal. 

Relatedly, recent additions to the hominin fossil record have lifted us from the temptation to see a progressive march through time to arrive at our bipedal perfection. There are numerous reasons the hominin pelvis evolved over the last several million years, making it unlikely that our present anatomy is what is “required” for bipedalism as is so often phrased. We can only assume that the few pelvic specimens in the hominin fossil record are testament to the variable morphology that has worked both for bipedalism as well as childbirth over the last several million years. Still, if bipedalism isn’t limiting the birth canal and causing childbirth difficulty, what is? Or, asked another way, there is an explanation for why chimpanzees have it so much easier than we do. So what is it?

The answer could still be that selection for bipedalism is constraining the pelvis and it has not yet been demonstrated.  The biomechanics of pregnancy may adaptively constrain pelvic dimensions, along the lines of what Katherine Whitcome and colleagues described for the spine. Further, limitations to tissue strength and other such properties of soft tissues of the pelvic floor may constrain pelvic dimensions, especially during pregnancy or as a result of it. But it must be noted that pelvic prolapse occurs in cows and sheep, not just bipeds.   Other significant contributors to childbirth difficulty include, but are not limited to: position of the laboring woman, position of the fetus and the umbilical cord, function of the placenta, uterus and cervix, muscular and bodily weakness, slow labor progression, multiple fetuses, preeclampsia, gestational diabetes, and age at first birth, young or old. At present, our impressions of hellish human childbirth are biased to a degree by Hollywood as well as the medical industry. So we may have to place some heavy blame on culture for our having it so much harder than chimpanzees. However, the rise in c-section rates from 5% to around 30% over the last several decades is not entirely due to unnecaesareans, as there has been a concurrent secular increase in neonatal size in hospitals as well. And that recent trend may be best explained by work being done by Jonathan Wells and his colleagues…

Although the tight fit between fetus and mother’s pelvis may have occurred millions of years prior, agriculture has had a remarkable and perhaps the most dramatic effect. Agriculture’s influence on diet and overall health has affected both the development of the female pelvis and the growth of her fetus during gestation. Undernourished mothers can birth relatively large babies due to adaptive responses to protect fetal growth during pregnancy, and regardless of maternal condition, larger babies are associated with longer labors and higher incidences of medical interventions. It is highly likely that there was never more childbirth difficulty than there is now and in recent history.  Plasticity has been both a blessing and a curse. Okay, so if childbirth difficulty may be new and is due to a complex vortex of proximate and ultimate explanations, not due solely or largely to locomotor selection on birth canal size, then what about our being born with only 30% of our adult brain size? 

Could the obstetric dilemma explain our relative immaturity at birth?  Which is to say, does the human pelvis uniquely influence gestation length and fetal growth? 

Or are we born early to experience a rich period of stimulation and learning, crucial to the development of human cognitive and neuromuscular function as Portmann hypothesized? According to the extra-uterine spring hypothesis, the pelvis does not limit gestation and fetal growth, the benefits to life outside mother do. These two hypotheses for gestation length and fetal development have humans born early or underdeveloped. A third does not. 

When the energetic and metabolic costs of pregnancy and lactation are considered, gestation ends and the birth process is initiated when pregnancy reaches a critical point at which the mother can no longer support her growing fetus. Fetal energy demands (black circles) increase exponentially during gestation.  Maternal energy expenditure (grey squares) rises during the first two trimesters but reaches a metabolic ceiling in the third, as total energy requirements approach 2.0x basal metabolic rate.  Projected fetal energy requirements for growth beyond 9 months (dark, dashed line) quickly exceed the maximum sustainable metabolic rates for human mothers (horizontal, dashed line).  After parturition, infant energy demands increase more slowly, and maternal energy requirements while lactating do not exceed the maximum sustainable metabolic rates. The hormonal cascade that is involved in triggering human birth has been described by Peter Ellison’s metabolic crossover hypothesis. We suggested that although the data are far better for humans, this broad and more ultimate hypothesis that we called EGG likely applies to other species since, among placental mammals, maternal body size (a proxy for metabolism) is a good predictor of gestation length, fetal mass, and fetal brain mass as the work of Bob Martin and others work has demonstrated. Still, this doesn’t answer the question as to whether humans are born early or underdeveloped. But it doesn’t need to because folks have known for a while that ….

Human gestation is not short and seems to be even longer than expected for a primate of our body size. Relatedly, a human mother does not invest less in pregnancy than expected; she bears a large infant with a large infant brain for a primate of her body size. Humans are born with absolutely larger brains than other primates. How can we explain all this? Here’s one hypothesis:

[Text redacted to avoid spoilers. See abstract for Herman Pontzer's talk this morning here.]

One of the most common reactions to the EGG hypothesis and in defense of the obstetric dilemma is that …

… lactation is even costlier than gestation!

And this is coming, I think, from the point of view of lifetime life history theory where we assume that mammals maximize gestation length to put off lactation unless there is something adaptive about shortening gestation and sucking up those extra lactation costs. For many, this is assumed to be why we push gestation right up to pelvic limit. But then what’s adaptive about starting lactation early for all the other great apes which don’t gestate to the pelvic limit? Why not, instead, hypothesize that pregnant apes reach their EGG limit prior to their pelvic one and pregnant humans reach our EGG limit closer to our pelvic limit. In other words, our birth canal has been selected to be as adequately capacious but not as comfortable as theirs. Another common reaction to the EGG hypothesis has been…

… but the tight fit is too much of a coincidence to ignore. And I have a cheeky retort that involves my nostril and my finger, but kindly and seriously…Okay…

Well then let’s not ignore the other coincidences in other primates that clearly aren’t equipped with our bipedal pelvis. What explains their tight fits at birth? Could those explanations apply to humans? That would be the spirit of the EGG hypothesis which we have only modeled for humans so far but deserves to be tested by building the same model with other primates. Yet another common reaction I hear is…

… but why doesn’t the birth canal get bigger to make childbirth easier? And I’ve touched on this already in different ways. But another way to address this is…

It has. That we have sexual dimorphism in the bony dimensions of the birth canal and that everyone in here knows which one is male and which one is female speaks directly to the notion that the female pelvis (and not the male’s) has adapted to accommodate our relatively big babies. The adaptation works terrifically in a sort of no pain, no fitness gain kind of way. And this last common reaction I want to share is, again, all about that 30% of adult brain size at birth.

… but humans are clearly born premature!

 Well then, so are other primates and with and without a tight fit at birth, without habitual bipedalism, and without our level of encephalization. Comparing humans to altricial mammals (like carnivores, rodents, and lagomorphs) is arguably more poetic than scientific. Among anthropoids, Fragaszy and colleagues have described capuchins as relatively altricial too, given how they are only able to thermoregulate within a narrow temperature range, and have less postural control and locomotor ability just after birth compared to Old World monkeys and many platyrrhines with comparable available studies. It cannot be coincidence that neonatal capuchins have the smallest relative brain size (~50%) of all primates save for chimpanzees (~40%) and humans (~30%), meaning that they, like us, have more postnatal brain growth to accomplish compared to other primates. Now, for fun, let’s take humans out of the equation and marvel at chimpanzees instead…

Why are chimpanzees born with only 40% of their brain size when capuchins are born with 50%?! Among anthropoids, chimpanzee infants are even more helpless with their slow development and long period of dependency. For the first few months of their lives, chimpanzee infants are actively alert for only 10 percent of the day. As we’ve seen, it’s not their pelvis forcing them out early. It could be the extrauterine spring hypothesis; they’re born when they’re born to receive adaptive stimulation outside of the womb. But it could boil down to the fact that pregnant placental mammals can only grow a fetus up to a limit and then getting out there into the world is a wonderful thing for developing all there is to mammalian sociality as well. Given the trend toward greater altriciality among encephalized primates, capuchins, chimps, and humans might be better described as “less precocial” rather than singled out as somehow altricial. 

And maybe we drove ourselves to this state. Maybe we’re relatively helpless as infants because we can be, that is, because of the relaxed selection afforded by hominin caregivers. Apes are infant coddlers, and assuming our shared ancestors were too, this could have ramped up long ago, with meddling, manipulative parents and alloparents relaxing selection on infant independence, allowing them to evolve paddled feet, weak muscles, fat heavy bodies, and huge heavy heads. A scenario like this would push the origins of helicopter parenting back, potentially, into the Pliocene.

Many of our questions, even the most ultimate evolutionary ones, would be answered if we knew what triggered labor in humans and other primates. Ellison’s metabolic crossover seems to be the strongest hypothesis, but this is a research area that begs our attention. . Of course the clinical implications are significant but such knowledge would also shed light on why chimpanzees and many other primates and mammals give birth so far in advance of reaching a pelvic limit. If there is one labor trigger for all of us, how do we explain this variation in birth weight and gestation length in this large human sample? Is it metabolism and energetic throughput alone? Most probably not. It’s more likely that labor is a reaction to any number of stressors and included in that could be the size of the pelvis, but then this trigger would be a unique development in humans that would not explain the end of gestation in any species without a tight fit. Getting even more evolutionarily complex… if it all comes down to a fetal timer, then each and every species would have its specifically timed fetal timer to end gestation, and that, although not impossible, just doesn’t seem like a strong hypothesis when compared to a maternal metabolic and energetic constraint.

There is so much more to discuss, there are so many remaining questions, but I’ll end here and leave you with this: Without increasing ability to make nonhuman primate comparisons, there is little hope for knowing how bipedalism, adiposity, taillessness, encephalization, culture, etc. contribute to the evolution of pregnancy, childbirth, infancy, and how we parent and alloparent.  Live primate studies (and those of all placental mammals that pertain to these questions) are crucial if we are to explain how humans have such costly infants yet have much shorter inter-birth intervals than the rest of the hominoids. Without more detailed comparison we cannot know how costly our pregnancies and the resulting infants truly are. Primates that birth twins and practice cooperative breeding, like marmosets, hold great potential for answering many questions about reproductive physiology, metabolic limits and behavioral correlates. But for now, as far as the obstetric dilemma hypothesis goes, intense childbirth and intensive parenting are not so easily explained by bipedal pelvic anatomy. Gestation length, fetal growth, childbirth processes, and neonatal helplessness are all connected to the bipedal hominin pelvis one way or another, but whether they are fundamentally influenced by it is not an easy argument to make. The way I see it, as far as human evolutionary hypotheses go, the obstetric dilemma has gone from top dog to underdog. Support for the obstetric dilemma requires much deeper digging. And we anthropologists are great at that. Thank you.

References and relevant resources
Abitol MM. 1987. Obstetrics and posture in pelvic anatomy. J Hum Evol. 16(3): 243–255.

Abitol MM. 1993. Adjustment of the fetal head and adult pelvis in modern humans. Hum Evol. 8(3): 167-85.

Abitol MM. 1996. The shapes of the female pelvis. Contributing factors. J Reprod Med. 41(4): 242-50.

Aiello LC, Wells  JCK. 2002. Energetics and the evolution of the genus Homo.
Ann Rev Anthropol. 31: 323-38.

Arsuaga J-L, et al. 1999. A complete human pelvis from the Middle Pleistocene of Spain. Nature. 399: 255-258.

Basso O. 2014. Reproductive epidemiology in an evolutionary perspective: Why bigger may not be better. Curr Epidemiol Rep. DOI 10.1007/s40471-014-0008-2.

Berge C, Goularas D. 2010. A new reconstruction of Sts 14 pelvis (Australopithecus africanus) from computed tomography and three-dimensional modeling techniques. J Hum Evol. 58: 262–72.

Berge C, Orban-Segebarth R, P Schmid. 1984. Obstetrical interpretation of the Australopithecine pelvic cavity. J Hum Evol. 13: 573-87.

Betti L, von Cramon-Taubadel N, Manica A, Lycett SJ. 2013. Global geometric morphometric analyses of the human pelvis reveal substantial neutral population history effects, even across sexes. PLoS ONE. 8(2): e55909.

Billat ,V., Lepretre, P.M., Heugas, A.M., Laurence, H.M., Salim, D., & Koralsztein J.P. (2003). Training and Bioenergetic Characteristics in Elite Male and Female Kenyan Runners. Med Sci Sports Exerc, 35, 297–304.

Bogin, B. (2006). Modern human life history: the evolution of human childhood and fertility. In: K. Hawkes & R.R. Paine (Eds). The Evolution of Human Life History. (pp. 197-230). Oxford, England: K. James Currey Ltd.

Borries C, Gordon AD,  Koenig A. 2013. Beware of primate life history data: A plea for data standards and a repository. PLoS ONE. 8(6): e67200.

Brown EA, Ruvolo M, Sabeti PC. 2013. Many ways to die, one way to arrive: how selection acts through pregnancy. Trends in Genetics. 29(10): 585-92.

Burton GJ, et al. 2015. Human evolution: brain, birthweight and the immune system. Phil Trans R Soc B 370: 20140061

Capellini, I., Venditti, C., & Barton, R.A. (2011). Placentation and maternal investment in mammals. Am Nat, 177(1), 86-98.

Chene G, Tardieu AS, Trombert B, Amouzougan A, Lamblin G, Mellier G, Coppens Y. 2014. A species; odyssey: evolution of obstetrical mechanics from Australopithecus Lucy to nowadays. Euro J Obstetrics & Gynecology and Reprod Bio. 181: 316-20.

Cho, S.H., Park, J.M., & Kwon, O.Y. (2004). Gender differences in three dimensional gait analysis data from 98 healthy Korean adults. Clinical Biomechanics, 19, 145–152.

Cunnane SC, Crawford MA. 2003. Survival of the fattest: fat babies were the key to evolution of the large human brain. Comp Biochem and Phys Part A. 136: 17-26.

Daniels, J., Krahenbuhl, G., Foster, C., Gilbert, J., & Daniels, S. (1977). Aerobic responses of female distance runners to submaximal and maximal exercise. Ann NY Acad Sci, 301, 726–733.

Davis-Floyd R, Cheyney M. 2009. Birth and the big bad wolf: An evolutionary perspective. Childbirth Across Cultures. The History of Non-Western Science. Selin H ed. DOI 10.1007/978-90-481-2599-9_1

DeSilva J, Lesnik J. 2006. Chimpanzee neonatal brain size: Implications for brain growth in Homo erectus. J Hum Evol. 51: 207-12.

DeSilva J. 2011. A shift toward birthing relatively large infants early in human evolution. Proc Natl Acad Sci USA. 108(3): 1022-27.

Dixson A. 2009. Sexual Selection and the Origins of Human Mating Systems. Oxford, England. Oxford University Press.

Dolea C, AbouZahr C. 2003. Global burden of obstructed labor in the year 2000: Evidence and information for policy (EIP), World Health Organization, Geneva, July 2003. Global Burden of Disease. pp. 1-17.

Douglas PH. 2014. Female sociality during eht daytime birth of a wild bonobo at Luikotale, Democratic Republic of the Congo. Primates. 55: 533-42.

Dufour, D.L., & Sauther, M.L. (2002). Comparative and evolutionary dimensions of the energetics of human pregnancy and lactation. Am J Hum Biol, 14(5), 584–602.

Dunsworth HM, Warrener AG, Deacon T, Ellison P, Pontzer H. 2012. Metabolic hypothesis for human altriciality. Proc Natl Acad Sci USA. 109(38): 15212-16.

Dunsworth, H.M. (2006). Proconsul heseloni feet from Rusinga Island, Kenya. Doctoral dissertation, The Pennsylvania State University.

Eide MG, et al. 2005. Size at Birth and Gestational Age as Predictors of Adult Height and Weight. Epidemiology, 16(2),  175-181 doi: 10.1097/

Ellison P. 2001. On Fertile Ground: A Natural History of Human Reproduction. Cambridge, Mass. Harvard University Press.

Epstein, H.T. (1973).  Possible metabolic constraints on human brain weight at birth.  Am J Phys Anthropol, 39, 135-136.

Falk D, Zollikofer CPE, Moromoto N, Ponce de Leon MS. 2012. Metopic suture of Taung (Australopithecus africanus) and its implications for hominin brain evolution. PNAS.

Ferber, R., McClay Davis, I., & Williams, D.S. (2003). Gender differences in lower extremity mechanics during running. Clinical Biomechanics, 18, 350–357.

Figlio D, Guryan J, Karbownik K, Roth J. 2014. The effects of poor neonatal health on

Fragaszy DM, Visalberghi E, Fedigan LM. 2004. The Complete Capuchin: The biology of the genus Cebus. Cambridge, England. Cambridge University Press.

Frank, L.G., & Glickland, S.E. (1994). Giving birth through a penile clitoris: parturition and dystocia in the spotted hyaena (Crocuta crocuta). J Zoology, 234(4), 659-665.

Garwicz M, Christensson M, Psouni E. 2009. A unifying model for timing of walking onset in humans and other mammals. PNAS USA. 106(51): 21889-93.

Gittleman JL, Thompson SD. 1988. Energy allocation in mammalian reproduction. Am Zool 28: 863-865.

Gould SJ. 1977. Ontogeny and phylogeny. Cambridge, Mass. Harvard University Press.

Grabowski MW, Polk JD, Roseman CC. 2011. Divergent patterns of integration and reduced constraint in the human hip and the origins of bipedalism. Evolution. 65: 1336–56.

Grabowski, M.W. (2012). Hominin obstetrics and the evolution of constraints. Evol Biol 10.1007/s11692-012-9174-7.

Grand TI. 1992. Altricial and precocial mammals: A model of neural and muscular development. Zoo Biol. 11: 3-15.

Green D, Alemseged Z. 2012. Australopithecus afarensis scapular ontogeny, function, and the role of climbing in human evolution. Science. 338(6106): 514-17.

Gupta, J.K, & Nikodem, C. (2000). Case report: Maternal posture in labour. Euro J Ob & Gyn and Reprod Biol, 92, 273-277.

Hagen, D.R., Shuey, C.P., & Watkins, J.L. (1984). Restriction of uterine space reduces litter size in feral ossabaw swine. Biol Reprod, 30, 423-426.

Hamilton, B.E., Martin, J.A., Ventura, S.J. (2011). Births: preliminary data for 2010. Natl Vital Stat Rep 60(2),1-25.

Hammond KA,  Diamond J. 1997. Maximal sustained energy budgets in humans and animals. Nature. 386: 457–62.

Hamner, S.R., Seth, A., & Delp, S.L. (2010).  Muscle contributions to propulsion and support during running. J Biomech, 43, 2709–2716

Hewett, T.E. (2005). Biomechanical measures of neuromuscular control and valgus loading of the knee predict anterior cruciate ligament injury risk in female athletes: A prospective study. Am J Sports Med, 33, 492–501.

Hewett, T.E. (2006). Anterior cruciate ligament injuries in female athletes: Part 1, mechanisms and risk factors. Am J Sports Med, 34, 299–311.

Hirata S, Fuwa K, Sugama K, Kusoniki K, Takeshita H. 2011. Mechanism of birth in chimpanzees: humans are not unique among primates. Biol Lett. 7(5): 686-88.

Holloway RL, Boadfied DC, KJ Carlson. 2014. New high-resolution computed tomography data of the Taung partial cranium and endocast and their bearing on metopism and hominin brain evolution. PNAS. 111(36): 13022-27.

Hrdy S. 2009. Mothers and Others. Cambridge, Mass. Harvard University Press.

Jandó, G., Mikó-Baráth, E., Markó, K., Hollódy, K., Török, B., & Kovacs, I. (2012). Early-onset binocularity in preterm infants reveals experience-dependent visual development in humans. PNAS USA, 109(27),11049–11052.

Kasarik LB, Adolph KE, TAmis-Lemonda CS, Bornstein MH. 2010.WEIRD Walking: Cross-cultural research on motor development. Behav Brain Sci. 33(2-3): 95–96.

Kibii JM, Churchill SE, Schmid P, Carlson KJ, Reed ND, de Ruiter DJ, Berger L. 2011. A partial pelvis of Australopithecus sediba. Science. 333(6048): 1407-1411.

Konje JC, Ladipo OA. 2000. Nutrition and obstructed labor. Am J Clin Nutr. 72: 291-97.

Konner, M. (2010). How childhood has evolved. Chronicle of Higher Education, May 9.

Krogman W. 1951. The scars of human evolution. Sci Am, 185, 54–57.

Kurki HK. 2013. Skeletal variability in the pelvis and limb skeleton of humans: Does stabilizing selection limit female pelvic variation? Am J Hum Biol. 25(6): 795-802.

Kuzawa CW. 1998. Adipose tissue in human infancy and childhood: An evolutionary perspective. Yrbk Phys Anthropol. 41: 117-209.

LaVelle M. 1995. Natural selection and developmental sexual variation in the human pelvis. Am J Phys Anthropol. 98: 59-72.


Leutenegger W. 1982. Encephalization and obstetrics in primates with particular reference to human evolution. Armstrong E, Falk D eds. Primate Brain Evolution: Methods and concepts. Plenum: New York. pp. 85-95.

Lewton KL. 2011. Evolvability of the primate pelvic girdle. Evol Biol. 39: 126–39.

Liston WA. 2003. Rising caesarean section rates: can evolution and ecology explain some of the difficulties of modern childbirth? J R Soc Med. 96: 559-61.

Long, J.A. (2012). Dawn of the Deed. Chicago, University of Chicago Press.

Lovejoy CO, Suwa G, Spurlock L, Asfaw B,  White TD. 2009. The Pelvis and Femur of Ardipithecus ramidus: The Emergence of Upright Walking. Science. 326: 71.

Lovejoy CO. 2005. The natural history of human gait and posture: Part 1. Spine and pelvis. Gait and Posture. 21: 95-112.

Lovejoy, C.O., Heiple, K.G., & Burstein, A.H. (1973). The gait of Australopithecus. Am J Phys Anthropol, 38, 757–779.

Lynch, M. (2007). The frailty of adaptive hypotheses for the origins of organismal complexity. PNAS, 104, 8597-8604.

Martin RD. 1983. Human brain evolution in an ecological context. Fifty-Second James Arthur Lecture on the Evolution of the Human Brain. New York. American Museum of Natural History.

Martin RD. 1996. Scaling of the mammalian brain: the maternal energy hypothesis. News Physiol Sci. 11: 149-56.

Martin RD. 1998. Comparative aspects of human brain evolution: scaling, energy costs and confounding variables. The origin and diversification of language. Wattis Symposium Series in Anthropology: Memoirs of the California Academy of Sciences.  Jablonski NG, Aiello LC,  eds. No. 24. pp. 35-68

Martin, R.D. (2013). How We Do It. New York, NY: Basic Books.

Montagu, A. (1961). Neonatal and infant immaturity in man. JAMA, 178, 56.

Navarete, A., van Schaik, C.P., & Isler, K. (2011). Energetics and the evolution of human brain size. Nature, 480, 91-93

Naya, D.E., Spangenberg, L., Naya, H., & Bozinovic, F. (2013). How does evolutionary variation in basal metabolic rates arise? A statistical assessment and a mechanistic model. Evolution, 67(5), 1463-1476.

Neilson, J.P., Verkuyl,D.A., & Bannerman, C. (2005). Tape measurement of symphysis-fundal height in twin pregnancies. BJOG, 95(10), 1054-1059.

Nelson, R.C., Brooks, C.M., & Pike, N.L. (1977). Biomechanical comparison of male and female distance runners. Ann NY Acad Sci, 301, 793–807.

Neubauer, S., & Hublin, J-J. (2012). The evolution of human brain development. Evol Biol 10.1007/s11692-011-9156-1

Newman T, Lengyel C, Pavlicev M,  Muglia LJ. 2014. Human evolution, genomics and birth timing: New approaches for investigating preterm birth.
NeoReviews. 15:17.

Noakes, D.E., Parkinson, T.J., & England, G.C.W. (2001). Arthur’s Veterinary Reproduction and Obstetrics, eighth edition. Edinburgh, Scotland: Saunders.

Perez-Gomez, J., Rodriguez, G.V., Ara, I., Olmedillas, H., Chavarren, J., González-Henriquez, J.J., et al. (2007). Role of muscle mass on sprint performance: gender differences? Eur J Appl Physiol, 102, 685–694.

Peterson CC, Nagy KA,  Diamond J. 1990. Sustained metabolic scope. Proc Natl Acad Sci USA. 87: 2324–28.

Pfeiffer S, Doyle LE, Kurki HK, Harrington L, Ginter JK, CE Merritt. 2014. Discernment of mortality risk associated with childbirth in archaeologically derived forager skeletons. International J Paleopathology. 7: 15-24.

Pike I. 2005. Maternal stress and fetal responses: Evolutionary perspectives on preterm delivery. Am J Hum Biol. 17: 55-65.

Pinto-Correia, C. (1997). The ovary of Eve: Egg and sperm and preformation. Chicago, Ill.: University of Chicago Press.

Plavcan M. 2000. Inferring social behavior from sexual dimorphism in the fossil record. J Hum Evol. 39(3): 327-44.

Plunkett, J., Doniger, S., Orabona, G., Morgan, T., & Haataja, R., et al. (2011). An evolutionary genomic approach to identify genes involved in human birth timing. PLoS Genet, 7(4), e1001365.

Pontzer H, et al. This morning. Humans, the high-energy ape: hominoid energetics and life history evolution. Program of the 84th Annual Meeting of the American Association of Physical Anthropologists. March 25-28, 2015.

Pontzer H, Raichlen DA, Gordon AD, Schroepfer KK, Hare B, O’Neill MC,
Muldoon KM, Dunsworth HM, Wood BM, Isler K, Burkart J, Irwin M, Shumaker RW, Lonsdorf  EV, and SR Ross. 2014.  Primate energy expenditure and life history. Proceedings of the National Academy of Sciences USA. 111(4): 1433–1437.

Pontzer H, Raichlen DA, Shumaker RW, Ocobock C,  Wiche SA. 2010. Metabolic adaptation for low energy throughput in orangutans. Proc Natl Acad Sci USA. 107(32): 14048–52.

Portmann A. 1969. A Zoologist Looks at Humankind. Schwabe: Basel. Translated in 1990 German text by Schaefer J. New York, NY: Columbia University Press.

Powers, C.M. (2010). The influence of abnormal hip mechanics on knee injury: A biomechanical perspective. J Orthop Sports Phys Ther 10.2519/jospt.2010.3337.

Prentice AM, Goldberg GR. 2000. Energy adaptations inhuman pregnancy: limits and long-term consequences. Am J Clin Nutr. 71: 1226-32.

Ramsbottom, R., Nute, M.G., & Williams, C. (1987). Determinants of five kilometre running performance in active men and women. Brit J Sports Med, 21, 9–13.

Ridley, M. (1995). Brief Communication: Pelvic sexual dimorphism and relative neonatal brain size really are related. Am J Phys Anthropol, 97, 197-200.

Roberts AM, Thorpe SKS. 2014. Challenges to human uniqueness: bipedalism, birth and brains. J Zool. 292: 281-89.

Robson SL, van Schaik CP,  Hawkes K. 2006. The derived features of human life history.  The Evolution of Human Life History. Hawkes K, Paine RR, eds. Oxford, K. James Currey Ltd. pp. 17-44

Rosenberg K, Trevathan WR. 2002. Birth, obstetrics, and human evolution. BJOG. 109(11): 1199–1206.

Rosenberg K, Trevathan WR. 2007. An anthropological perspective on the evolutionary context of preeclampsia in humans. J Reproductive Immunology. 76: 91-97.

Rosenberg K, Trevathan WR. 2014. Evolutionary obstetrics. Evolution, Medicine, and Public Health, p. 148. DOI: 10.1093/emph/eou025

Rosenberg KR, Trevathan WR. 1995. Bipedalism and human birth: The obstetrical dilemma revisited. Evol Anthropol, 4(5), 161-168.

Rosenberg KR. 1992. The evolution of modern human childbirth. Yrbk Phys Anthropol. 35: 89-124.

Roy RP. 2003. A Darwinian view of obstructed labor. Obstetrics and Gynecology. 101(2): 397-401.

Rozzi, S.L., Lephart, S.M., Gear, W.S., & Fu, F.H. (1999). Knee joint laxity and neuromuscular characteristics of male and female soccer and basketball players. Am J Sports Med, 27, 312–319.

Ruff CB. 2010. Body size and body shape in early hominins: implications of the Gona pelvis. J Hum Evol. 58: 166-78.

Ruff, C. (1998). Evolution of the hominid hip. In E. Strasser, J. Fleagle, A. Rosenberger & HM McHenry (Eds). Primate locomotion: recent advances. New York, NY: Plenum Press.

Ruff, C.B. (1995). Biomechanics of the hip and birth in early Homo. Am J Phys Anthropol, 98, 527–574.

Rutherford JN, Tardiff SD.2008. Placental efficiency and intrauterine resource allocation strategies in the common marmoset pregnancy. Am J Phys Anthropol 137(1): 60-68

Sacher GA, Staffeldt EF. 1974. Relation of gestation time to brain weight for placental mammals: implications for the theory of vertebrate growth. Am Nat. 18(963): 593-615.

Sakai T, et al. 2012. Fetal brain development in chimpanzees versus humans. Curr Biol, 22(18), R791–R792

Sayer D, Dickinson SD. 2013. Reconsidering obstetric death and female fertility in Anglo-Saxon England. World Archaeology. DOI 10.1080/00438243.2013.799044.

Schultz A. 1949. Sex differences in the pelves of primates. Am J Phys Anthropol, 7(3), 401-424.

Scott, J.R. et al. (editors). (1994). Danforth’s Obstetrics and Gynecology, 7th edition. Philadelphia, PA.: J.B. Lippincott Co.

Shorten, A., Donsante, J., & Shorten, B. (2002). Birth position, accoucheur, and perineal outcomes: Informing women about choices for vaginal birth. Birth, 29(1), 18-27.

Simpson SW, et al.  2008. A female Homo erectus pelvis from Gona, Ethiopia. Science, 322, 1089–1092.

Smith, L.K., Lelas, J.L., & Kerrigan, D.C. (2002). Gender differences in pelvic motions and center of mass displacement during walking: stereotypes quantified. J Women Health Gend Based Med, 11, 453–458.

Tague RG, Lovejoy CO. 1986. The obstetric pelvis of A. L. 288-1 (lucyLucy). J Hum Evol. 15: 237-94.

Tague RG. 2011. Fusion of coccyx to sacrum in humans: Prevalence, correleatees, and the effect on pelvic size, with obstetrical and evolutionary implications. Am J Phys Anthropol.

Tague, R.G. (1989). Variation in pelvic size between males and females. Am J Phys Anthropol, 80, 59-71.

Tague, R.G. (1992). Sexual dimorphism in the human bony pelvis, with a consideration of the Neandertal pelvis from Kebara Cave, Israel. Am J Phys Anthropol, 88, 1–21.

Thibault, V., Guillaume, M., Berthelot, G., El Helou, N., Schaal, K., Quinquis, L., Nassif, H., Tafflet, M., Escolana, S., Hermine, O., & Toussaint, J.F. (2010). Women and men in sport performance: the gender gap has not evolved since 1983. J Sports Sci Med, 9, 214-223.

Thompson ME. 2013. Comparative reproductive energetics of human and nonhman primates. Annu Rev Anthropol. 42: 287-304.

Thorburn, G.D., Hollingworth, S.A., & Hooper, S.B. (1991). The trigger for parturition in sheep: fetal hypothalamus or placenta? J Dev Physiol, 15(2),71-9.

Trevathan W, Rosenberg K, eds. In review. Costly and Cute: How helpless babies make us human. SAR. Santa Fe.

Trevathan WR, Rosenberg K. 2000. The shoulders follow the head: postcranial constraints on human childbirth. J Hum Evol. 39: 583-86.

Trevathan, W.R. (2010).  Ancient Bodies, Modern Lives: How evolution has shaped women’s health. Oxford, England: Oxford University Press.

Trevathan, W.R., Smith, E.O., & McKenna, J.J. (2008). Evolutionary Medicine and Health.  Oxford, England: Oxford University Press.

Trinkaus E. 1984. Neanderthal pubic morphology and gestation length. Curr Anthropol. 25(4): 509-14.

Turner MJ, Rasmussen MJ, Boylan PC, MacDonald D, Stronge JM. 1990. The influence of birth weight on labor in nulliparas. Obstetrics and Gynecology. 76(2): 159-63.

Walker A. 2009. The strength of great apes and the speed of humans. Curr Anthropol. 50(2): 229-34.

Wall-Scheffler CM, Geiger K, Steudel-Number KL. 2007. Infant carrying: The role of increased locomotory costs in early tool development. Am J Phys Anthropol. 133: 841-846.

Wall-Scheffler CM. 2012.  Energetics, locomotion, and female reproduction: Implications for human evolution. Annu Rev Anthropol. 41: 71-85.

Walrath D, Glantz MM. 1996. Sexual dimorphism in the pelvic midplane and its relationship to Neandertal reproductive patterns. Am J Phys Anthropol. 100: 89-100.

Walrath D. 2003. Rethinking Pelvic Typologies and the Human Birth Mechanism. Curr Anthropol. 44: 5-31.

Warrener AG, et al.  2015. A Wider Pelvis Does Not Increase Locomotor Cost in Humans, with Implications for the Evolution of Childbirth. PLOS ONE

Washburn, S. 1960. Tools and human evolution. Sci Am. 203: 3–15.

Watts E. 1990. Evolutionary trends in primate growth and development. In DeRousseau C (ed): Primate Life History and Evolution. New York: Wiley-Liss, p. 89-104.

Weaver TD, Hublin J-J. 2009. Neandertal birth canal shape and the evolution of human childbirth. PNAS. 106(20): 8151-8156.

Weiner S, Monge J,  Mann A. 2008. Bipedalism and parturition: An evolutionary imperative for Cesarean delivery? Clin Perinatol. 35(3): 469-478.

Wells JCK, Desilva JM,  Stock JT. 2012. The obstetric dilemma: An ancient game of Russian roulette, or a variable dilemma sensitive to ecology? Yrbk Phys Anthropol.  149(55): 40-71.

Wells JCK. 2015. Between Scylla and Charybdis: renegotiating resolution of the ‘obstetric dilemma’ in response to ecological change. Phil Trans R Soc B 370: 20140067.

Weyand, P.G., Sternlight, D.B., Bellizzi, M.J., & Wright, S. (2000). Faster top running speeds are achieved with greater ground forces not more rapid leg movements. J Appl Physiol, 89, 1991–1999.

Whitcome KK, et al. 2007. Fetal load and the evolution of lumbar lordosis in bipedal hominins. Nature, 450, 1075-1078.

Wildman DE, Uddin M, Romero R, Gonzalez JM, Than NG, Murphy J, et al. 2011. Spontaneous abortion and preterm labor and delivery in nonhuman primates: Evidence from a captive colony of chimpanzees (Pan troglodytes). PLoS ONE. 6(9): e24509.

Wittman AB, Wall LL. 2007. The evolutionary origins of obstructed labor: bipedalism, encephalization, and the human obstetric dilemma. Obstet Gynecol Surv. 62: 739-48.

World Health Organization. 2005. World Health Report: Make Every Woman and Child Count. Geneva. World Health Organization

Zanolli, C., Bondioli, L., Manni, F., Rossi, P., & Macchiarelli, R. (2011). Gestation length, mode of delivery and neonatal line thickness variation. Human Biology, 83(6), 3.

Zuk M. 2013. Paleofantasy. New York: W.W. Norton.


Nick Kilzer said...

Thanks for post this. Very interesting!

Ken Weiss said...

Another coup, Holly! I wish I could have been there to see the presentation, but I wasn't able to make the AAPAs this year.

Stacy said...

This was awesome! Thoroughly enjoyed being there live.

Maureen said...

This is fascinating and fits well with my Milk Hypothesis, outlined in a book entitled Milk matters: infant feeding and immune disorder. I think that we need to upend the idea of lactation as costly and infants as immature and see instead see lactation as protective (of infant and mother) and infants as being as mature as they need to be to do those essentials they need to do in order to adapt to the environment they are being born into. Milk is the unique bridge from womb to world - a sort of continuing umbilical cord for the newborn, but also the medium of exposure to the specific environment, and supporter of adaptation to that environment. As well, its mutual feedback mechanisms create bonds that ensure caregiving is personalised and ongoing, rewarding both partners. We have strayed so much from responsive breastfeeding that this seems quaint and unreal to some, but to women who have experienced the power and pleasure of normal breastfeeding for normal lengths of time, years not months, (deemed prolonged by many) it makes sense. Read the book and see how this resonates and fits with what you're discussing here.