Primate obstetrics: The biology of birth

Abstract

A review of recent studies of cephalo-pelvic relationships in primates at birth reveal that body size is the primary determinant of cranial dimensions at term. Variation in intraordinal size relationships are used to predict the course of labor, and nonintrusive observational studies are required to test these predictions.Key words: primates, parturition, cephalo-pelvic relationships, encephalization, neonatal development INTRODUCTIONParturition requires a complex interaction of morphological, endocrine, and behavioral components for the transition to extrauterine life to be successful. Despite the opportunities offered for elucidating the operation of natural selection at this critical phase of reproduction, comparative studies of birth have infrequently been undertaken as a deliberate research strategy. Even so, it is widely believed that this event is much more strenuous, with greater risk to life, for humans than for any other mammal. A related view that has gained a measure of acceptance in recent years is that humans have unwittingly added to the trauma of birth by adoption of attitudes and procedures which are poorly informed from a biological standpoint. This theme emerges repeatedly in the literature on naturalistic childbirth, of which Haire's [1973] indictment of western obstetricians on a wide range of practices is a prominent example.Part of the reason for controversy surrounding human birth practices may be that the different approaches lack sufficient grounding in empirical data. The pervasive influence of culture on birth is amply demonstrated in the ethnographic literature [see, eg, Richardson & Guttmacher, 19671 as well as in western obstetric practice [Atwood, 19761. It is at this juncture that studies of phylogenetically close relatives, having a minimum of cultural expectations, may aid in defining the biological parameters of parturition, and thereby contribute to a more informed human obstetrical practice.Relevant to this question are data which, when used comparatively, reveal laws which govern the unfolding of major life cycle events. While the search for these laws has a rather long history in biology, it is primarily within the last decade that primates have come under close scrutiny from this perspective. The implications for parturition of laws governing ontogenic events are best evaluated in light of major evolutionary adaptations which characterize the primates. The cephalo-pelvic relationships which prevail at 194Lindburg birth and which therefore affect the course of labor and delivery are a product of locomotor adaptations in the pelvis and of phyletic changes in body size, encephalization trends and rates of brain maturation, all of which are believed to affect size of the neonatal cranium.A second, potentially useful kind of information derives from observational studies which document intraordinal variation in the birth event itself. Unfortunately, because births often occur at night [Jolly, 19721, information THE PELVISStudies of the primate pelvis have traditionally fo...

The evolution of modern human childbirth

“…9 in Martin, 1983). Bowden et al (19671, Leutenegger (1982a,b), and Lindburg (1982) also observed that variation among primate species in size of the infant's cranium relative to the mother's pelvic inlet is correlated with difficulty during parturition. In callitrichids, cebids (except the howler monkey), cercopithecids, hylobatids, and humans, the maternal pelvis is only slightly larger than the infant cranium if at.…”

Evolution of human serial pairbonding

“…"he evolution of secondary altriciality of the hominid infant could also have been a ductive burden, not by producing young with a longer infancy than that of pongids but by producing neonates that were more helpless during infancy than are those of apes (Fisher, 1982). There is a significant increase in altriciality in hominds relative to pongids (Trevathan, 1987;Lindburg, 1982;Gibson, 1981;Gould, 1977;Montagu, 1961), and a standard explanation for this secondary altriciality is that it occurred in hominid evolution when maternal pelvic diameters-in association with the corresponding diameters of the fetal cranium-no longer permitted successful parturition, favoring delivery of the fetus at an earlier stage of development (Washburn, 1950;Montagu, 1961;Gould, 1977). By 2 my B.P.…”

Size of the Anterior Fontanelle: Three‐Dimensional Measurement of a Key Trait in Human Evolution

“…One of these hypotheses contends that the persistent presence of the AF, combined with unfused sagittal, coronal, and metopic sutures of the cranial vault, functions to allow cranial deformation in the birth canal. The potential for deformation of the cranium during birth would help to solve the "obstetric dilemma" posed by the competing pressures of bipedal locomotion narrowing the pelvis, effectively making the birth canal smaller (Washburn, 1950(Washburn, , 1960Campbell, 1966;Pinkerton, 1973;McHenry, 1975;Zihlman, 1978; but see Dunsworth et al, 2012), and increasing encephalization in the newborn (Washburn, 1960;Lindburg, 1982;Tague and Lovejoy, 1986). Thus, these competing constraints imposed by bipedality and brain size evolution in the human lineage may have necessitated a modification in frontal neurocranial ossification (Falk et al, 2012), including retention of the AF.…”