Category: Obstetrics

Was Birth Assistance Among Early Hominids a Gendered Activity?

4/25/2015

Depiction of male and female Australopithecines at the American Museum of Natural History.

I've always like Karen Rosenberg and Wenda Trevathan's (2002) paper "Birth, Obstetrics, and Human Evolution" (available here). It does a nice job of exploring the social implications of anatomical changes related to the emergence of bipedalism, focusing on how the shape of the pelvis in australopithecines would have complicated birth and transformed delivery from a solitary experience into one that often required assistance. I used a condensed version of the article (Scientific American 13:80-85, 2003) in my 200-level Human Origins class: it really helps students see how you can make connections between fossils and behaviors and how one aspect of human evolution can have implications for others.

While I was reading one of the student papers from that class this semester, I started to question the model I had in my head of australopithecine females assisting each other in birth. The student wrote something that seemed to imply (intentionally or not - I'm not sure) that males were the ones giving assistance at birth. It struck me as odd, which made me ask myself why it should strike me as odd: why did I assume females and not males?

I don't think Rosenberg and Trevathan ever specify that females would have been the ones providing assistance to other females, but they use the term "midwifery" several times in their papers. Though not technically defined today as excluding males, the term "midwife" carries a lot of history that associates it very closely with females. This is the etymology as provided by Wikipedia:

"The term midwife is derived from Middle English: midwyf literally "with-woman", i.e. "the woman with (the mother at birth), the woman assisting" (in Middle English and Old English, mid = "with", wīf = "woman")."

Based on a quick perusal of some of the web resources that pop up first (e.g.,this, this, and this) the idea of the ancient origins of the association between females and birth assistance is widespread. I have no reason to doubt that this is correct: I would presume a strong female bias in birth assistance could be amply demonstrated both historically and ethnographically.

So should we assume that females were also providing birth assistance among early hominids?

Maybe not.

It seems clear that our modern conceptions of who should provide assistance at birth are culture-bound. By "culture-bound" I mean connected to other shared, leaned aspects of human societies, behaviors, and symbolic systems. I don't think I'll get an argument if I state that there is zero evidence for anything like human culture among australopithecines. So what happens to our gendered conceptions of birth assistance if you remove all or most of their cultural underpinnings? Would birth assistance still be performed primarily by females?

For the sake of argument, consider how selection might act to reinforce the behavior of males assisting females during birth. If labor and delivery are dangerous for both the female and the neonate, any assistance a male mate can provide increases the chances his genes will be passed on. This would be true in a situation of stable male-female pair bonding, as pair-bonding decreases the uncertainty of paternity. The inclination of males to provide effective assistance during birth would be selected for if pair-bonding was present and birth was dangerous.

If we assume that pair-bonding among australopithecines would not have been the same thing as "marriage" (a human cultural behavior), why do we assume that australopithecine birth assistance would be the same thing as "midwifery"? Our historical and ethnographic record of what humans do now really only gets us so far in addressing questions like this: the universality of a cultural practice among modern humans does not necessarily mean it always existed in the same form deep into the past.

Rosenberg, K., & Trevathan, W. (2002). Birth, obstetrics and human evolution BJOG: An International Journal of Obstetrics and Gynaecology, 109 (11), 1199-1206 DOI: 10.1046/j.1471-0528.2002.00010.x

Update (5/3/2015): The response of one of my friends on Twitter to this post was that "Sexual selection says they should've been too busy getting busy to care." I wrote this quick post taking an informal look at ideas about sexual dimorphism and sexual selection among australopithecines. It looks to me like the jury is still out, as there's a lot of conflicting information about the degree of sexual dimorphism in the two criteria we look at most often in primates: canine size and body size.

Fetal Head Molding and Obstetrics in Late Pleistocene Humans

3/15/2015

Preface: This post presents some work I did as a graduate student at the University of Michigan in 2007. It was a poster for a class called "Evolution of the Genus Homo," taught by Milford Wolpoff. I chose the topic because of my interest in the culture, biology, and social organization of Middle/Late Paleolithic humans (see discussion of my 2014 SAA paper here and here, and my 2015 AJPA paper here). I had hoped to develop this into a paper either alone or with a collaborator, but I have never found the time to follow through. As the information I collected ages and I begin to focus on moving to a new job that will require a lot of attention up front to the archaeology of the southeastern United States, it seems less and less likely that I'll ever get around to turning this into a paper. So I'm going to put my analysis from the poster out there "as is" and hope it useful to someone. If you read this and think it's an interesting idea or one that you'd like to pursue, let me know!

I apologize for the state of the bibliography: there are some formatting errors that I will correct when I have the time.

Fetal Head Molding and Obstetrics in Late Pleistocene Humans

Figure 1. Illustration of changes in head shape that occur during birth (from A.D.A.M).

Introduction

This study compares available data from fetal and neonatal crania from the Late Pleistocene to the mechanics of fetal head molding during birth in recent humans.   The small number of fetal and neonatal remains dating to the Late Pleistocene offer an opportunity to simultaneously explore issues of obstetrics, selection, and early brain growth. Most treatments of birth and obstetrics in Pleistocene humans have focused on pelvic anatomy (e.g., Rak and Arensburg 1987; Rosenberg 1998; Rosenberg and Trevathan 2002; Trinkhaus 1984). Studies of childhood growth and development after birth are limited mainly by the dearth of sub-adult skeletons, particularly those that pre-date Neandertals (see Anton 2002; Dean et al. 1986; Minugh-Purvis 1988, 2002; Nelson and Thompson 2002; Stringer et al. 1990; Tellier 1998; Trinkhaus and Tompkins 1990).   Fetal and neonatal remains, from Neandertals and other Late Pleistocene humans, have been described but have not been the subject of detailed, hypothesis-based research.

Deformation (molding) of the fetal cranium is an important part of successful birth in recent humans. This study examines the hypothesis that thickness of fetal cranial bone would have been an impediment to successful birth in Neandertals and other Late Pleistocene humans. Investigating the possible role of fetal head molding in Pleistocene obstetrics may help shed light on both anatomical trends in human cranium (i.e., the emergence of "modern" cranial morphology) and demographic variables and population genetics that may underlay the spread of anatomical "modernity."   Mortality and trauma during childbirth, acting on the mother and/or the fetus, would be an important selective force.

Two aspects of fetal head molding are emphasized: cranial vault thickness and head dimensions.    Vault thickness affects the response of the cranium to pressure during birth. Head size and shape affect both the degree of molding that is required for the fetal head to pass through the birth canal and the distribution of forces on the fetal cranium.

Hypothesis: Thick fetal cranial bone in Late Pleistocene archaic humans would have caused difficulties during childbirth (relative to recent humans) by inhibiting head molding during delivery.

Assuming uniformity in the size of the birth canal between archaic and recent humans, this hypothesis has two test implications:

1) the increased thickness of Late Pleistocene fetal cranial vaults would have a significant effect on elasticity of the cranium

2) the dimensions of the fetal cranium are such that significant molding is required for delivery

In other words, fetal head molding must be shown to be both necessary (by the dimensions of the cranium) and significantly impeded (by the in-elasticity of the vault) in order to fail to reject the hypothesis. If either one of these test implications is rejected, then the hypothesis can be rejected.

Pleistocene Obstetrics: Previous Research

Much research focused on questions of obstetrics in Pleistocene humans have emphasized the selective constraints between locomation and birth mechanics in the pelvis (Rak and Arensburg 1987; Rosenberg 1998; Rosenberg and Trevathan 2002; Ruff 1995; Trinkhaus 1984). Based on pelvis remains, most researchers conclude that birth in Pleistocene humans was much like birth in recent humans (Rosenberg 1998; Rosenberg and Trevathan 2002). Subsequent to the description of the Kebara 2 pelvis (Rak and Arensburg 1987), most ideas about an unusually long gestation periods (Trinkhaus 1984) and rapid in utero brain growth (Dean et al. 1986) in Neandertals have been rejected (see Stringer et al. 1990:148).

While pelvic inlet size during the Pleistocene appears to be, overall, similar to modern humans, cranial capacity increased. during the Middle and Late Pleistocene. Stasis in pelvic inlet size and increase in head size produces an "obstetric dilemma" where the fetal head is larger than the birth canal. Rosenberg and Trevathan (2002:1205) state that

"Two changes could have allowed an increase in adult brain size to occur: human infants could have been born with a smaller percentage of adult brain size (resulting in greater infant helplessness) and/or there could have been an alteration of the shape of the pelvis concomitant with a change in the mechanism of birth."

There is a third possibility: fetal head molding. The possible importance of fetal head molding in Neandertals is raised by similarities in both pelvic inlet size and adult cranial capacity to recent humans. Minugh-Purvis (1988:260) speculated that the thicker vault bone observed in Neandertal fetal remains would have posed a problem if delivery required a "considerable degree of head molding." The possibility was also discussed by Friedlander and Jorndan (1994).

Figure 2.

Fetal Head Molding in Recent Human Birth

In recent humans, the fetal cranium is a flexible structure that deforms during birth because of pressures between the fetal head and the cervical walls (Lapeer and Prager 2001; McPherson and Kriewall 1980a, 1980b) (Figure 2). Pressures and deformation are greatest at the sub-occipito bregmatic plane (Lapeer and Prager 2001; Rosenberg and Trevthan 2002).

During a normal labor, the parietal bones undergo the most significant changes in shape, being compressed towards each other and elongating in the axial plane (Lapeer and Prager 2001; McPherson and Kriewall 1980b). The occipital bone is relatively rigid and undergoes little change during molding (McPherson and Kriewall 1980b:18; Rosenberg and Trevathan 2002:1201). The frontal, occipital, and parietal bones interlock at the sutures after a certain limit of deformation occurs, preventing excessive molding and protecting the brain within a more rigid structure (McPherson and Kriewall 1980a:15).

The risk of excessive molding is greater in pre-term deliveries, where cranial bone is not sufficiently thick to prevent excessive molding (McPherson and Kriewall 1980a). Clinical studies have shown that excessive molding during birth (i.e., where too much deformation occurs) may be linked to psycho-neurological disorders, mental retardation, cerebral palsy, and death (see McPherson and Kriewall 1980b).

Fetal cranial bone must be thin enough to allow sufficient deformation of the cranium, but thick enough to form a rigid structure to protect the brain. Optimal thickness values would vary for different portions of the fetal cranium depending on the pressures that are exerted and the required responses to those pressures.

Parietal Thickness, Span, and Deformation Under Load

Parietal bones grow outward from a center of ossification that later becomes the parietal eminence (Ohtsuki 1980). The bones are thickest at the eminence, thinning towards their margins (McPherson and Kriewall 1980b). Ohtsuki (1977) reported a mean thickness of 0.54 +/- 0.13 mm for term (9-10 month) fetal parietal bones at the center of ossification and a thickness of 0.40 +/- 0.10 for term fetal frontal bones at the center of ossification (n = 10). McPherson and Kriewall (1980a:10) reported term fetal parietal bones that varied in mean thickness from 0.71-0.86 mm.

In their analysis of the mechanical properties of fetal parietal bone, McPherson and Kriewall (1980a:11) found that differences in thickness and the orientation of the bone fibers affected the elastic modulus (the resistance to deformation when a load is applied). Thicker cranial bone requires more force to deform. Figure 3 shows the relationship between thickness and elastic modulus in the data supplied by McPherson and Kriewall (1980a:10,13), using only the parietal bones with fibers orientated parallel.

Using the formulae provided by McPherson and Kriewall (1980:11), we can use the estimates of elastic modulus to estimate the loads that would be required to bend segments of bone of varying length and thicknesses (Figures 4 and 5). Other things being equal, longer "beams" of bone require less force to bend, while thicker "beams" require more. To have the same resistance to bending force, a longer "beam" must be thicker.

Figure 3. Thickness of fetal cranial bone plotted against elastic modulus (data from McPherson and Kriewall 1980a). The regression (R2 = 0.78) is: 4.13 + 2.86(log of thickness in mm)

Figure 4. Plot of force required to cause a deflection of 1 mm in "beams" of bone of varying thickness (assuming a beam length of 75 mm - approximately that of a modern human term fetus). While the absolute values of these calculations may not be accurate (parietal bones vary in thickness in cross-section and do not behave simply as "beams") the calculations show that the resistance to force changes dramatically when thickness increases from 1 mm to 2 mm.

Figure 5. Plot of force required to cause a deflection of 1 mm in "beams" of bone of varying length (assuming a beam thickness of 1 mm).

Fetal Vault Thickness, Dimensions, and Molding in Late Pleistocene Homo

The Late Pleistocene fossil record contains numerous remains from sub-adult specimens. Of interest here are those remains that preserve portions of the cranial vault, particularly the frontal and parietal bones. Fetal and neonatal remains of Neandertals have been recovered from La Ferrassie (Heim 1982) and Hortus (Lumley-Woodyear 1973). Rremains of two Neandertals less than about a year old have been reported from Shanidar (Trinkhaus 1983) and Krapina (Minugh-Purvis 1988). Neonatal remains attributed to anatomically modern Homo sapiens have been reported from Cro-Magnon (Minugh-Purvis 1988), Qafzeh (Tillier 1999), and Abri Patuad (Minugh-Purvis 1988). Krapina is the earliest site, dating to the late Riss/early Wurm (Wolpoff 1999). La Ferrassie, Shanidar, Qafzeh, and Hortus date to Wurm I/Wurm II. Abri Pataud and Cro-Magnon date to Wurm III/IV (Wolpoff 1999).

Vault Thickness

Data on vault thickness at the parietal and frontal eminences are available for six fetal/neonate skeletons and three young (<1 year) infants from the Late Pleistocene.

Figure 6. Top: Drawing of Neandertal neonatal parietal from Hortus 1b (adapted from Lumley-Woodyear 1973). Bottom: Neandertal fetal and newborn frontal bone fragments from La Ferrassie compared to frontals from recent humans (adapted from Heim 1982).

Figure 7.

The thickness of the frontal and parietal bones in this sample contrasts with the data from the modern sample provided by Ohtsuki (1977) (Figure 7), outside the 2 sigma range of his means for term fetuses. If the Late Pleistocene fetal and neonate remains are aged accurately, fetal parietal bone was generally 1.5 to 2 times thicker than that of modern humans.

Considerably more force would be required to deform these bones, assuming the geometry of the bones was otherwise equivalent (see below).

Test prediction 1 is supported: differences in fetal cranial vault thickness are sufficient to affect molding of the cranium.

Fetal Head Dimensions

The limited data available on very young Late Pleistocene individuals suggests that some aspects of fetal head geometry may have differed from that of more recent humans (Minugh-Purvis 2002; Stringer et al. 1990). Bregma-lambda distance appears to have been shorter in Neandertals than in recent humans throughout life (see Minugh-Purvis 2002:488-489; Gunz and Havarti 2007; Harvarti 2003; Trinkhaus 1983:371). In mature Neandertals, the shorter distance is associated with a lower position of bregma (see Harvarti 2003)

Figure 8.

A shorter bregma-lambda distance would reduce the cross-section of the fetal cranium in a dimension that is key to the necessity for fetal head molding. It appears that this distance may have been about 10 mm less in Neandertals at the time of birth relative to recent humans: perhaps 80 mm rather than 90 mm (see Minugh-Purvis 2002). A difference of ca. 10-12% in the bregma-lambda chord would be sufficient to account for a 3-7% reduction in the sub-occipito-bregmatic diameter (SOBD). Assuming equivalence in other dimensions of the head and pelvic inlet, this difference alone would significantly lessen the degree of fetal head molding that would be required for successful delivery.

Test prediction 2 is not supported: the dimensions of the fetal cranium are such that significant molding was probably not required for delivery.

Conclusions

Significant fetal head molding was probably not critical to successful Neanderthal birth. While thicker cranial bone would have reduced elasticity, a smaller SOBD would have negated or lessened the need for molding during birth.

Reduction in fetal cranial thickness may not have been a reproductive advantage for "modern" humans. Rather, cranial thinness associated with an increase in the SOBD may have increased the risks to the fetus during birth (i.e., though excessive molding) while reducing or maintaining the risk to both mother and fetus (i.e., through arrested labor). In the absence of selection for thinner bone associated with a flexibility requirement, thick fetal cranial bone would have offered protection to the fetal brain during delivery. Apparent stasis in pelvic anatomy suggests that smaller, thicker fetal crania may be the ancestral condition. An increase in SOBD, perhaps reflecting some difference in fetal brain growth, would have preceded selection for thinner cranial bone in this scenario.

The fetal cranium is a complex mechanical structure. Constructing a simulation model (similar to that of Lapeer and Prager 2001) of delivery in Neanderthals is possible with the available data. This model could be used to test hypotheses about obstetrics in a more sophisticated way than is possible by calculating simple ratios of head and pelvic size.

References Cited

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Dean, M.C., C. B. Stringer, and T. Bromage. 1986.    Age at death of the Neanderthal child from Devil’s Tower, Gibraltar and the implications for studies of general growth and development in Neanderthals. American Journal of Physical Anthropology 70:301-309.

Friedlander, N. J., and D. K. Jordan. 1994. Obstetric implications of Neanderthal robusticity and bone density. Human Evolution 9:331-342.

Gunz, Philipp, and Katerina Havarti. 2007.   The Neanderthal “chignon”: Variation, integration, and homology. Journal of Human Evolution 52:262-274.

Havarti, Katerina. 2003. The Neanderthal taxonomic position: Models of intra- and inter-specific craniofacial variation. Journal of Human Evolution 44:107-132.

Heim, Jean-Louis. 1982. Les enfants nJandertaliens de La Ferrassie. Paris, Masson.

Lapeer, R.J., and R.W. Prager. 2001. Fetal head moulding: Finite element analysis of a fetal skull subjected to uterine pressures during the first stage of labour. Journal of Biomechanics 34:1125-1133.

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Was Birth Assistance Among Early Hominids a Gendered Activity?

Fetal Head Molding and Obstetrics in Late Pleistocene Humans

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