Obstetrical dilemma

The obstetrical dilemma refers to two conflicting trends in the evolutionary development of the human pelvis where the transition to walking upright, bipedal locomotion, required a decreased size of the bony birth-canal but the trend toward increased intelligence required a larger cranium, which would need a wider obstetrical pelvic area.^[1] The obstetrical dilemma has been studied widely because it has increased the difficulty of childbirth for modern day women. Because of this, complications in birth are more frequent as well as the need of assistance in childbirth. This change seems to have evolved over time because this dilemma does not seem to pose a problem for our most recent relatives, non-human primates, who still manage to give birth with little difficulty.

Evolution

Human ancestors seemed to originally give birth in a similar way that non-human primates do. Most primates have neonatal heads that are close in size to the mother’s birth canal. Because of this females do not seem to need assistance in birthing and seek seclusion.^[2] There is also no need for neonatal rotation because the canals are wide enough to accommodate the infant.^[3] Modern humans birthing methods greatly differ from this because of both pelvic shape of the mother and neonatal shape of the infant.

The diagram compares the size and shape of the pelvis as the infant's skull must move through it for the Chimpanzee, Australopithecus afarensis and Homo sapien sapien. It shows how the Obstetrical dilemma affected the evolution of the human species.

The obstetrical dilemma began when human ancestors started to evolve into a bipedal creature. Because humans are the only obligately bipedal primates, meaning their body shape requires them to only use two legs, major alterations had to be made to the shape of the female pelvis.^[1] A number of structures in the body changed size, proportion, or location in order to accommodate bipedal locomotion and allow a person to stand upright and face forward. To help support the upper body, a number of structural changes were made to the pelvis. The ilial pelvic bone shifted forward and broadened, while the ischial pelvic bone shrank, narrowing the pelvic canal. These changes were occurring at the same time as humans were developing larger craniums. Therefore, in order to successfully undergo childbirth, the infant must be born earlier and earlier, thereby making the child increasingly developmentally premature.^[4] The concept of the infant being born underdeveloped is called altriciality. Other ways of evolving to cope with bipedalism and larger craniums were also important such as neonatal rotation of the shoulders to allow the infant to fit through the canal, shorter gestation length which allows the infant to be born smaller, assistance with birth, and a malleable neonatal head which is softer and leaves the birth canal more easily.

Proposed solution to obstetrical dilemma

Due to the obstetrical dilemma, modern humans give birth very differently from their primate counterparts because of bipedalism and the evolution of larger brains. Humans have a much more difficult time of giving birth and have had to evolve to solve the constraint posed by the human pelvis. Neonatal rotation is when the body gets rotated to align the shoulders transversely in order to exit the birth canal. It is one important solution because growth in the size of the cranium as well as the width of the shoulders makes it more difficult for the infant to fit through the pelvic canal.^[1] Gestation length in humans is shorter than most other primates because it leads to altriciality which is a solution to the obstetrical dilemma. If the cranium of an infant is not fully developed at the time of birth, it will fit through the pelvic canal more easily. Therefore humans have evolved to give birth to a neonatal form that is underdeveloped and has not matured, whereas other non-human primates give birth to infants that are more fully matured.

Humans are born with an underdeveloped brain; only 25% of their brains fully developed at birth as opposed to non-human primates where the infant is born with 45–50% brain development.^[5] Human infants are also almost always born with assistance from other humans because of the way that the pelvis is shaped. Since the pelvis and opening of birth canal face backwards, humans have difficulty giving birth themselves because they cannot guide the baby out of the canal. Non-human primates seek seclusion when giving birth because they do not need any help due to the pelvis and opening being more forward.^[3] Lastly, humans are born with a very malleable fetal head which is not fully developed when the infant exits the womb.^[1] This allows for the head to develop more after birth and for the cranium to continue growing without affecting the birthing process. All of these lead to human infants depending on their parents much more and for much longer than other primates.^[4][6] Another result of the obstetrical dilemma is sexual differences in humans. Human males evolved narrower hips optimized for locomotion, whereas female hips evolved to be a wider optimization because of childbirthing needs.^[6][7][8]

Alternative hypotheses

Alternative hypotheses have been proposed for the obstetrical dilemma. Some studies have shown that higher brain growth rates happen earlier on in ontogeny than previously thought,^[9] which challenges the idea that the explanation of the obstetrical dilemma is that humans are born with underdeveloped brains. This is because if brain growth rates were largest in early development, that is when the brain size would increase the most. Premature birth would not allow for a much larger head size if most of the growth had already happened. Also, it has been suggested that maternal pelvic dimensions are sensitive to some ecological factors. There has been a lot of evidence linking body mass to brain mass and determining maternal metabolism as a key factor in the growth of the fetus. Maternal constraints could be largely due to thermal stress or energy available. A larger brain mass in the neonate corresponds to more energy needed to sustain it. It takes much more energy for the mother if the brain fully develops in the womb. If maternal energy is the limiting factor then an infant can only grow as much as the mother can sustain. Also, because fetal size is positively correlated to maternal energy use, thermal stress is an issue because the larger the fetus the more the mother can suffer heat stress.^[7]

Some studies suggest that other factors may exacerbate the dilemma. One of these is dietary shifts, possibly due to the emergence of agriculture. This can be both due to change in diet as well as the increase in population density since agriculture was developed; more people leads to more disease.^[7] Studies have also been done in twins to show that pelvic size may be due more to the environment in which they live than their genetics.^[10] Another study disproves the thought that narrower hips are optimized for locomotion because it was found that a Late Stone Age population Southern Africa that survived largely on terrestrial mobility had women who had uncharacteristically small body size with large pelvic canals.^[7]

The Energetics of Gestation and Growth (EGG) hypothesis offers another alternative to the obstetrical hypothesis, equating the constraints on gestation and parturition to the energy restrictions of the mother. It has been shown that using both professional athletes and pregnant women, there is an upper limitation to the amount of energy a woman can produce before it cause deleterious effects: approximately 2.1x their basal metabolic rate. During pregnancy the growing brain mass and length in the neonate correspond to more energy needed to sustain it. This results in a competing balance between the fetus’s demand for energy and the maternal ability to meet that demand. At approximately 9 months gestation, the fetus’s energy needs surpasses the mother’s energy limitation, correlating with the average time of birth.^[4] The newly born infant can then be sustained on breast milk, which is a more efficient, less energy demanding mechanism of nutrient transfer between mother and child.^[11] Additionally, this hypothesis demonstrates that, contrary to the obstetrical dilemma, an increased pelvic size would not be deleterious to bipedalism. Studying the running mechanics of males and females, it was shown that an increased pelvic size related to neither an increased metabolic nor structural demand on a woman.

Costs of the obstetrical dilemma

The obstetrical dilemma has many costs. Because humans are born with a malleable skull and an underdeveloped brain,^[1] human infants need to be taken care of for a lot longer than the infants of other animals, especially other primates. Humans spend a lot of their time caring for their children as they develop whereas other species stand on their own from when they are born. The faster an infant develops, the higher the reproductive output of a female can be.^[12] So in humans, the cost of slow development of their infants is that humans reproduce relatively slowly.

References