For all living organisms, successful reproduction is measured by how many offspring survive to adulthood. Some animals such as flies and clams produce millions of eggs, but few live to reproduce (r-strategy). Birds and mammals however have a K-strategy where they parent a small number of offspring, in which they invest more and have a higher survival rate. Animal strategies are limited by the complexity of developmental pattern. Longer periods of development require more energy to support that period of maturation and growth. The eggs of anmiote vertebrates like reptiles, birds and mammals have the added burden of supplying the embryo with water, the egg is the perfect adaptation for enclosing and conserving water for the fetus. Homeothermic vertebrates have the added burden of keeping developing embryos within a certain temperature range; this is accomplished by burying or brooding the eggs. In some animals, the mother maintains a constant temperature for the eggs within her body until they are ready to hatch, also known as viviparity, or live-birth (Langdon, 2005: 247-8)
Mammals have three different reproductive strategies: Monotremes lay eggs, marsupials retain the egg to draw nourishment from the mother for one ovulatory cycle then emerge from the birth canal and live in a pouch to continue develop by drinking the mothers milk. Placental mammal (like ourselves), maintain the endometrium through an extended gestation period. The pituitary continues to secrete gonadotropins to sustain the corpus luteum, production of progesterone from the ovary maintains the uterine lining. There are multiple kinds of placentas, humans posses a hemochorial placenta like all other Haplorhine primates. This placenta is more invasive to the endometrium, causing the mothers tissue to break down so that her blood is in direct contact with the chorion. This arrangment offers more efficient transfer of oxygen and nutrients to the fetus. The placenta aggressively takes up nutrients from the mother’s blood, most importantly glucose. (Langdon, 2005: 248).
Delivery for most mammals appears quite effortless. Humans however are an exception to due difficulty caused by brain expansion. There is a clear compromise that we can see develop throughout the fossil record. Some evidence for this compromise is evident for the level of human altriciality. This is a condition in which much of the brain growth of the fetus occurs after birth, human are one of the most altricial species, delivering completely helpless young, and still our babies barely fit through the birth canal. Rosenberg and Trevathan explain the how human birth differs from modern non-human primate birth in 3 fundamental ways. 1) the neonatal head and body generally pass through a series of rotations during birth in response to the close correspondence between neonatal head and shoulder dimensions and maternal pelvic dimensions; 2) the neonate usually exits the birth canal in an occiput anterior position; and 3) human birth occurs in a social context with others in attendance (Rosenberg and Trevathan, 2002)
This third fundamental way in which human birth differs from other primates is very significant. Humans employ obligate midwifery as a social evolutionary adaptation to increase mother and infant mortality rates in hu
mans. Since the fit is so tight between fetal head and mother’s pelvis the neonate usually exits in an occiput, anterior facing position. This is different than that of primates and can result in damage of the spinal chord or death of the child if the mother tried to pull her baby out herself. It is necessary for someone to be ready to catch the baby when it emerges and assistance is often needed to cut the umbilical chord and clean the baby’s air passages out, because the mother is usually very exhausted and delirious from the excruciating pain of labor.
Medicalization of BirthEdit
Walrath warns us to keep in mind that the perception of human birth as pathology and the value-laden phrases used by scholars to describe birth are products of social and cultural ideas and could be skewed or biased. On the question of whether human birth is monotypic Walrath argues, “birth mechanisms vary now and probably did so in the past. The notion of a monotypic birth mechanism has been imported into paleoanthro- pological discourse from typological thinking in Euro-American biomedical practice and text” (Walrath, 2003).
It is appropriate to characterize human birth is monotypic in regard the historical situation. Referring to the process of rotation the fetus has to go through to be successfully delivered naturally. This selective pressure was reinforced throughout human history, because of the tight fit between encephalized infant fetus heads and small bipedal pelvises. However, over the last hundred years or so the medicalization of human birth has released this selective pressure. Now fetuses that are in strange positions during labor can be safely extracted via cesarean section, not to mention other procedures that can save premature and unhealthy babies. This process of the medicalization of birth will change our species dramatically. It is hard to say what human birth will be like a thousand years from now. Throughout our evolutionary history, a delicate balance has existed between the encephalization of the human cranium and the morphology of our pevis, currently however this balance has been disrupted in “modernized” societies and it will be interesting to see the effects this change has on our species as it races into the future.
Franciscus, R. G. (2009). When Did The Modern Human Pattern Of Childbirth Arise? New Insights From An Old Neandertal Pelvis. Proceedings of the National Academy of Sciences, 106(23), 9125-9126.
Langdon J. H. (2005) The Human Strategy: An evolutionary Perspective on Human Anatomy. Oxford University Press. Pp 244-280.
Rosenberg, K. and Trevathan, W. (2002) Birth, obstetrics and human evolution. GJOG: an International Journal of Obstetrics and Gynaeocology. (109) 1199-1206.
Walrath, D. (2003) Rethinking Pelvic Typologies and the Human Birth Mechanism. Current Anthropology. 44(1) 5-31.