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Bipedalism and Human Birth: the Obstetrical Dilemma Revisited

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Bipedalism and Human Birth: the Obstetrical Dilemma Revisited Evolutionary Anthropology 161 Bipedalism and Human Birth: The Obstetrical Dilemma Revisited KAREN ROSENBERG AND WENDA TREVATHAN . adaptation to bipedal locomotion decreased the size of the bony birth-canal at mals giving birth are rare, we know the same time that the exigencies of tool use selected for larger brains. This ob- that neonatal death resulting from stetrical dilemma was solved by delivery of the fetus at a much earlier stage of cephalopelvic disproportion, when a development. (Washburn’) fetal head is too large to pass through . there can be no doubt that many of the obstetrical problems of Mrs. H. Sapiens the mothers birth canal, is not uncom- are due to the combination of a narrower pelvis and a bigger head in the species. 2 mon in species such as marmosets, (Krogman ) squirrel monkeys,ll baboon8 and macaques. 12 Cephalopelvic dispropor- tion is also, of course, a relatively fre- Although Washburn described it as history of continual compromises, quent cause of death for human as a “dilemma” and Krogman called it adapting first to one constraint and infants and can be the cause of death a “scar of human evolution,” both then to another. for their mothers.13 Because the authors recognized that the unique cephalopelvic constraints in monkeys way that humans give birth is the re- BIRTH IN NONHUMAN PRIMATES are greater than those in great apes, the comparisons made herein be- sult of a set of constraints imposed by Contrary to popular perceptions, tween nonhuman primates and hu- several exclusively human attributes: humans are not the only animals that bipedalism, a large brain, and “secon- have difficulty during childbirth. One mans will use monkeys, rather than dary altriciality,” or the delivery of the important characteristic of primates the phylogetically closer great apes, to infant in a helpless state. Although hu- as a group is a large head and brain represent nonhumans. man birth has long been seen as a sin- relative to body size (i.e., a high It is not only the size of the pelvic gle compromise between locomotion encephalization quotient). For most aperture that is important to the birth process, but also its shape. The birth and brain size, we now know that the primates this means that their infants, canal can be seen as a passageway particularly complex series of twists at birth, have heads that are close to with an entrance (the inlet) and an exit and turns that human babies make in the size of the maternal birth canal through which they must pass. This is (the outlet) and a space between (the the process of emerging from their midplane) through which the infant mothers’ birth canals is the result of a especially true of monkeys, lesser apes, and humans. Schultz3,4 depicted must pass. For most primate species, this relationship in a classic figure, re- the entrance (see Table l), the exit, and drawn here as Figure 1, showing the the passageway between them are Karen Rosenberg is Associate Professor size of the maternal pelvic inlet and longer in the sagittal dimension than of Anthropology at the University of in the transverse dimension. The neo- Delaware. She is a paleoanthropologist the infant cranium in Ateles, Nasalis, who has worked on fossils in Europe, the Macaca, Hylobates, the great apes, and natal cranium is largest in the sagittal Near East and Eastern Asia. Her research humans. The close correspondence dimension in all primate species, in- interests are in the origin of modern humans and the evolution of modern between the size of the maternal pelvis cluding humans. Furthermore, the human childbirth. Wenda Trevathan is and the size of neonatal cranium is ob- back of the infant cranium is broadest, Associate Professor of Anthropology at so that it fits best against the broader New Mexico State University. Her major vious for monkeys, gibbons, and hu- research interests are the evolution of mans, whereas chimpanzees, gorillas, back of the monkey pelvis. 14 human female reproductive behavior, and orangutans appear to have spa- Until recent work by Stoller, it had including sexuality, pregnancy, childbirth, and menopause. Her major publications cious birth canals, probably as a result been assumed that in monkeys the include Human Birth; An Evolutionary of their large bodies. As one would ex- sagittal dimension of the infant cra- Perspective and articles on menstrual pect from examining these drawings, nium lines up with the sagittal dimen- synchrony, sexuality, and evolutionary medicine. birth is reported to be difficult for the sion of the pelvis, the infant facing the smaller-bodied primates (monkeys ventral side of the mothers body (Fig. Email: [email protected] and gibbons), as well as for humans,5-9 2) and passing straight through the but somewhat easier for the larger- birth canal without rotating. The in- 10 Key words: Birth, obstetrics, australopithecines bodied (great) apes. fant is born in a position obstetricians Homo, bipedalism Although observations of wild ani- describe as occiput posterior. Based 162 Evolutionary Anthropology ARTICLES Ateles and outlet lie perpendicular to each other in humans, whereas in other pri- mates they are parallel to each other. The human infants head, like the monkey’s, is greatest in the sagittal di- mension. The extremely close corre- spondence between the fetal head and the maternal pelvic dimensions re- quires that these dimensions line up so that the head enters in the trans- Inlet verse plane (facing to the side) (Fig. 2), Transverse Diameter but then rotates to emerge in the sagit- tal plane, usually facing to the back in a position obstetricians refer to as oc- ciput anterior (Fig. 3). The head must rotate again after it passes through the outlet so that the shoulders, which, as in the great apes, are fairly broad and rigid, can follow the head through the maximum diameters of the birth ca- nal. In addition to the differences in cross-sectional shape, human and Figure 1. A redrawing of Schultz’ classic diagram relating the size of the maternal pelvic inlet and nonhuman primate pelvic morpholo- the size of the neonatal head in different primate species. Maternal pelvic and infant cranial outlines are diagrammatic, but scaled so that the transverse diameter of all maternal pelvic inlets are constant and all dimensions are correctly scaled relative to one another. For each species, the outlined oval represents the average maternal pelvic inlet, the black oval represents the average infant’s cranium. Note that in the monkeys and gibbon, the dimensions of the infant cranium are only slightly smaller than the dimensions of the mother’s pelvis. In great apes, the pelvic inlet is relatively spacious. In humans, the infant cranium is actually longer than the anterior-posterior It is not only the size of dimension of the pelvic inlet, requiring the head to enter the inlet facing sideways. the pelvic aperture that is important to the birth on observations that included radio- Monkeys and apes generally give process, but also its graphs of laboratory animals during birth wherever and whenever other parturition, Stollerl4 has shown that group members and predators are shape. squirrel monkey and baboon infants least likely to be present. Laboring may rotate during birth, albeit in a dif- mothers typically seek seclusion, often ferent way than human infants do. In among trees, which provide protec- tion from terrestrial predators. Diur- the four baboons (Papio anubis) and gies differ in another way The broad- (Saimiri sci- nal species generally give birth at seven squirrel monkeys est part of the monkey pelvis is at the uretrs) Stoller studied, the fetuses en- night; nocturnal species tend to give 5 back, whereas the human pelvis is also tered the birth canal in various birth during the day.1 For nonhuman spacious in the front. As with mon- positions, but then rotated to exit face primates, birth generally is a solitary keys, the human infant’s occiput is the first (mentum anterior), facing the event. largest, most rigid part of its head. maternal pubic bones and with their heads in an extended position. Thus, rather than lining up with the back of its head against the maternal Monkey mothers usually deliver in Although, as shown in Figure 1, the a squatting position. As the infant is sacrum as monkeys do, the human in- close correspondence between the in- fant more commonly lies so that its born, the mother typically reaches fant cranium and maternal pelvis in down to guide it out of the birth canal occiput is against the pubic bones, and monkeys is also characteristic of hu- toward the front of the pelvis as it exits and toward her nipples, and may wipe mans, the orientation of the pelvic di- mucus from the baby’s mouth and the birth canal, its convex frontal bone 10 ameters differs. In monkey pelves, nose to assist its breathing. Other both the inlet and the outlet are great- passing along the concave anterior animals may observe the birth process est in the sagittal dimension. In con- surface of the sacrum. The smallest di- from a distance but do not assist the trast, the human birth canal is ameter of the human fetal head is the mother or infant. The newborns of “twisted” in the middle so that the sub-occipito-bregmatic diameter most monkey species have sufficiently. greatest breadth at the inlet is in the (from the back of the head to the top developed motor skills that an infant transverse dimension, while the outlet of the head). In order to make this can assist in its own delivery once its is largest in the sagittal dimension. In minimum cross-sectional diameter hands are free.
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