Adaptation for Single Births
The propensity for primates, including humans, to have only one fetus at a time.
Mammalian infants are born at either of two stages: altricial or precocial. Altricial infants are those that are born immature and helpless, e.g., rodents. Precocial infants are born able to move about soon after birth, e.g., deer. Animals that are born precocial, like primates, are K-selected, meaning that they give birth to a smaller number of offspring at a time (MacArthur and Wilson 1967; Martin 1983; Pianka 1970). Humans are part of a group of primates known as “higher primates” which typically give birth to single offspring at a time (Martin 2007). Having fewer offspring at a time allows the fetus to have a larger body size at birth and be more developed at birth in order to increase the chance of survival (Martin 2013). The adaptation for having a single fetus at a time may be a result of the ancestral morphology, or shape, of the uterus.
During pregnancy, a fetus develops and grows in the uterus. Placental mammals have a uterus which has two chambers, or spaces, allowing for multiple fetuses to gestate, or grow, at a time (Martin 2013). Women are one of the few mammalian exceptions that have a uterine morphology where the uterus only has a single chamber (Martin 2013). The single-chambered uterus develops early in-utero from the fusion of reproductive tracts (Martin 2013). Today, a low percentage of women (0.033%) are born with a two-chambered uterus (Martin 2013). Although these women can still have viable pregnancies, their babies are typically born premature and via medical interventions, such as cesarean section (Martin 2013).
The adaptation for humans to have single births may be a result of the overall space provided in the uterus to support developing fetuses. Primates are born at a more developed, or precocial, stage and thus are typically born at a relatively larger body size compared to altricial mammals. A fetus that is larger therefore affords less space in the uterus to support multiple fetuses to develop (Hamlett and Wislocki 1934). With fewer fetuses in the uterus at a time, a fetus can have a longer gestation, or pregnancy, period. A lengthened gestation period provides extra time for the fetus to grow in body size. Gestation length in precocial species is almost three times longer than that of altricial species (Martin 2007). This trend extends within precocial species, such as the primates. Relative to maternal body size, apes have longer gestation lengths than monkeys. Humans, as apes, have a gestation period of, on average, 40 weeks, whereas some monkeys have gestation periods of less than half that amount of time.
Another way to view how the gestation period influences the number of offspring is to consider the amount of energy it takes to gestate and grow a fetus. Increased gestation lengths in humans and the other apes increase the energetic burden placed upon the mother through the prolonged nutrient supply to the fetus. The timing of birth is now thought to be due to when the maternal metabolic limit is reached, and the mother can no longer support the fetus’s caloric needs (Dunsworth et al. 2012). In single birth, this metabolic limit is reached at 40 weeks and then birth occurs (Dunsworth et al. 2012). In multiple births, however, the mother’s metabolism must support more than one fetus, increasing her metabolic demands throughout the gestation. This decreases the length of gestation, as the Mother’s metabolic limit will be exceeded sooner rather than when the Mother is only providing for one fetus. Women who have twins have an average gestation length of 37 weeks, three weeks earlier than what is seen in an average singleton birth (Martin 2013). An infant born this early also affects the birth weight, coinciding with the standard threshold for premature infants (Martin 2013).
Decreased time in utero decreases the birth weight of the infant. The average birth weight of a single human infant is 7.5 pounds whereas in twins, the average birth weight decreases to, on average, 5.25 pounds (Martin 2013). Premature births and lower birth weights are associated with greater health risks such as coronary heart disease and hypertension (Trevathan 2010). Additionally, premature infants require more direct care from the Mother in order to survive than those infants born fully developed. In premedicinal times and throughout our evolutionary history, this would have caused excess strain and burden on the Mother and may have resulted in a higher infant mortality.
Evolutionarily, primates are adapted to giving birth to one infant at a time because it is beneficial for their reproductive success. The metabolic limit for fetal growth is determined by the mother (Dunsworth et al. 2012), and having a single offspring allows for a longer gestation period and a healthier infant. A single fetus can grow larger and more developed in utero without conceding space to multiple fetuses.
Humans typically give birth to one offspring at a time due to the morphology of the human uterus only supporting one embryo at a time. The single-chambered uterus evolved early in primate evolution and allows the fetus to be born more developed, as well as larger in body size. These adaptations ensured a better chance at survival in the evolutionary environment in which we, as primates, evolved, as well as in today’s modern world.
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