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...
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