All primates are riders, meaning that primate infants ride on the body, usually of their mother, as she goes about her day. However, riding is a strategy that evolved long before primates and is seen in other mammals. Parking, on the other hand, is considered the ancestral state. Parkers, like foxes, for example, leave their young in a nest or burrow while they forage or hunt.
Reproductive success depends on having as many offspring reach sexual maturity as possible. Parkers tend to have liters or larger groups of offspring which are kept in dens or burrows while their mother hunts. While parkers are left undefended in a den risk predation, or death from neglect if something happens to their mother, it is still more efficient to have multiple offspring at a time in the hopes at least one will survive to maturity. As a further measure, many species of parker give birth to altricial (helpless) offspring, so that when they are at their most defenseless they are unlikely to stray far from the safety of their den.
Riders, on the other hand, tend to have singleton pregnancies, with precocious newborns who can assist by clinging to their mother from birth. Rider infants are exposed to the environment and any danger their mother may face while searching for her own food– and the added weight of an infant slows the mother down when escaping predation. Carrying is also more energetically costly for the adult which increases the amount of food they must consume. For these reasons, riding would seem to be somewhat maladaptive, yet, as Ross describes, once riding develops it is conserved, meaning that it remains for their descendants:
“If non-riding is assumed to be the ancestral state, riding was conserved in nearly all lineages once it has evolved. This suggests that, once carrying has evolved it is difficult to lose, possibly because of behavioral and physiological co-adaptations that occur once infants are carried,” (Ross, 765).
Among the physiological co-adaptations of riding is breast milk composition. Riders’ breast milk is higher in carbohydrates and lower in fat when compared with parkers. Therefore riders’ require frequent, regular feedings, around four per hour for newborn chimps, similar to human neonates (Hedberg-Nyquist). It would be impossible for a chimp mother to find enough food to survive if she had to return to a nest or burrow to feed her infant every fifteen minutes. She would starve and/or her helpless newborn would be a victim of predation.
Fortunately for chimp mothers, their babies are proportionately small, around 3% of their mother’s body mass (DeSilva). Chimp infants are also developmentally precocious and can grasp strongly to their mother’s body hair with both hands and feet, allowing her to move freely in pursuit of food, self-defense, and socializing. Unfortunately for our bipedal ancestors, like Australopithecus Afarensis, their newborns were proportionately large, helpless, and with no grasping feet (DeSilva). Their babies could not cling effectively, yet as riders, they still had to be toted along with their mother for frequent feedings. How did they (and therefore, we) survive?
Without an infant carrier, a bipedal mother faced two terrible fates: leave her baby to go hungry (or become a meal for a predator) while the group moves on to hunt and forage; or slowly starve to death because she cannot possibly source enough calories to keep up with lactation and carrying her baby in-arms. Cruel as it may be, they would become victims of natural selection– one or both would die without the infant carrier. But there is another form of evolutionary selection, what Timothy Taylor calls technological selection: survival of the cleverest. Our ancestors would not have survived without technology and our species only exists because of it, and it all began with the infant carrier:
“This book insists that there was an actual moment when we became human. It was a moment long before we became intelligent in any modern sense. It was a moment seized by a female as, for the first time, she turned to technology to protect her child. In that moment, everything that we were going to become was made not just possible but inevitable” (Taylor).
Humans remain physiologically riders, yet technology, once supporting (literally) our physiology, now allows many humans to act as cultural parkers, leaving their young in dens (homes) while they go out into the world to forage (work)– but that is a discussion for a later post in the culture section. Yet for all of our technology, we still haven’t replaced the environment our species evolved to spend their infancy in: on the body of their caregiver. As yet, no mechanical device is able to mimic the sensations of being carried during a walk which elicits transport response in infants, regulating their breath, heart rate, reducing stress hormones, and crying.
DeSilva, Jeremy M. “A Shift toward Birthing Relatively Large Infants Early in Human Evolution.” Ed. C. Owen Lovejoy. Proceedings of the National Academy of Sciences of the United States of America 108.3 (2011): 1022-027. PNAS. Web. 19 Jan. 2015.
Hedberg-Nyquist, Kerstin. “Developmental Care: A breast-feeding perspective.” Research on Early Developmental Care for Preterm Neonates. Ed. Jacques Sizun and Joy V. Browne. John Libbey Eurotext. 2006.
Ross, Caroline. “Park or Ride? Evolution of Infant Carrying in Primates.” International Journal of Primatology 22.5 (2001): 749-71.Springer. Web. 26 Jan. 2015.
Taylor, Timothy. The artificial ape: how technology changed the course of human evolution. Basingstoke: Palgrave Macmillan, 2010.