Part one of this series defined transport response and how it has been studied in animals and how similar behaviors are seen in human infants. When carried by a walking caregiver infants relax physiologically and psychologically, making them easier to carry and reducing crying behavior. Here in part two, we’ll look at when transport response may have evolved in our ancestors, why it was conserved (remained a trait through other evolutionary changes), and how it applies to the use of infant carriers.
The Transport Response elicited by ambulatory carrying increases the chance for survival of both infants and caregivers by making infants easier to move as well as quiet, even when distress is present. Humans are one of very few species that combine riding (carrying our young with us) and altrical (helpless) infants. Normally, the combination is parking with altricial and riding with precocious, and that makes a lot of sense. Parkers have many babies at a time and keep them hidden in a den while mom hunts or forages. The babies can’t get up to much trouble since they are helpless and by being parkers they can go for longer stretches between feedings while their mother goes about her business. In the rare circumstance that a parker mother has to move her still-altricial offspring, the infants will elicit the transport response, helping the move go faster and undetected.
Riders generally have singleton pregnancies and must carry their infant with them for frequent feedings, but as the infant is precocious, they are able to cling on from birth, making the carrying easier. Should a precocious rider infant elicit the transport response they are likely to fall off their mother as she moves– which is fairly dangerous if mom does her grocery shopping in the treetops.
When a rider species has altricial infants (like with humans), it’s a problem, as riders must carry their babies with them yet their babies are unable to assist in the carrying by hanging on. Riders cannot simply revert to parking because the strategy is tied to their biology; once riding develops it is conserved (Ross). The problem is compounded when altricial infants are born at 5-6% of adult mass (chimps and gorillas have newborns 2-3% of adult mass) as bipedal infants have been since the time of Australopithecus Afarensis, four million years ago (DeSilva).
“Therefore, the infant calming responses may have the evolutionary function of increasing the survival probability of the infant in cases of emergency escape by the mother-infant dyad and ultimately work to support the mother-infant relationship. Conservation of this calming response in altricial mammalian species supports the adaptive value of this behavior in mother-infant relationships, and, as a consequence, infant survival.” (Esposito, 2013)
Transport Response may have emerged as the traits of riding, proportionally larger newborns, and bipedalism combined, during the time that Australopithecus Afarensis developed as a species. And perhaps, once carrying technology was created, those infants who were easier to carry in a carrier (by relaxing into it, instead of flailing like the numbed mouse pup in part one) were more likely to survive and so conserve the trait. This goes back to the potential for early screening for neurological disorders discussed in part one, from a natural selection perspective, infants who do not elicit the transport response or infants who, for whatever reason, work against their mother’s attempt to carry them are, sadly, less likely to survive to reproductive maturity until 1. infant carrying technology advanced beyond simple slings or thongs 2. semi-permanent settlements developed allowing infants to be culturally “parked”. Yet, even with these technological advancements, the trait for relaxing in response to being carried persists in human infants, even if the survival of an infant no longer depends on it (Bjorklund).
Parents and caregivers have always (since bipedalism freed them) had their hands full with things to get done. Yet, human infants, especially newborns, often cry when sat down or left alone, and relax when carried because that behavior will keep them alive– or at least, it did. In contemporary cultures which do not carry infants regularly, crying is a serious issue for parents and caregivers. In the west, there are medical diagnoses for excessive crying “paroxysmal fussing” or “colic”, with treatments including drugs and surgery. Crying has become maladaptive for many infants, costing them their lives due to unsafe pharmaceutical interventions to child abuse. When infants are left to cry-it-out, infants exhibit learned helplessness (so-called “self-soothing”) an alternative survival mechanism to the transport response: after a certain amount of time, crying will only attract predators and waste calories, therefore it is reasonable to shut down psychological and physiologically to conserve energy until, hopefully, someone or something will come along and provide care.
With a goal of reducing crying behavior, and the associated stress, abuse, and neglect that can come with it, many studies on have been done to determine a biologic “norm” of newborn crying patterns; on what actions will reduce crying from rocking frequency and direction to paying more attention to newborns, (discussed in more detail in part one). Again and again, the research shows that infant crying is culturally-based and that infants who are carried more cry less. There are two reasons for this, the first is the transport response, infants who are crying will typically stop crying and relax when carried upright by a walking caregiver– though studies typically emphasize maternal carrying and that crying will resume when the walking stops if the stimulus triggering the crying is still present, such as with pain. The second is that infants who are in close proximity to a caregiver can communicate their needs with non-crying cues. Neither the infant nor the caregiver(s) gets into the habit using infant crying as a baseline communication.
Understanding how the evolutionary pressures of our ancestors influence infant behavior today can help prevent both “excessive” crying behavior and the inappropriate caregiver responses to it. The use of an infant carrier hold an infant while the caregiver goes about their day, especially while walking, can reduce crying duration if not prevent some instances of it (Pederson, 1973; Hunziker, 1986; Esposito, 2013). Humans are biologically riders, adults are meant to move their bodies and infants were meant to be moved with them. Even if our technology has allowed many of us to be cultural parkers, babywearing can help caregivers find a compromise between our infant’s biology and our culture.
Bjorklund, David F. “The Role of Immaturity in Human Development.” <em>American Psychological Association Psychological Bulletin</em> 122.2 (1997):153-169.
Bonichinia, et al. “Infant Crying and Maternal Holding in the First 2 Months of Age: An Italian Diary Study.” Infant and Child Development 17 (2008): 581-592.
Esposito, Gianluca, et. al. “Infant Calming Responses during Maternal Carrying in Humans and Mice.” Current Biology 23.9 (2013): 739-45. Web. 15 July 2015.
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.
Hunziker, A. U., and R. G. Barr. “Increased Carrying Reduces Infant Crying: A Randomized Controlled Trial.” Pediatrics 77 (1986): 641-48.
Korner, Anneliese, and Evelyn B. Thoman. “Visual alertness in neonates as evoked by maternal care.” Journal of Experimental Child Psychology 10.1 (1970): 67-78.
Pederson, David R. “The Soothing Effects of Vestibular Stimulation as Determined by Frequency and Direction of Rocking.” Ontario Mental Health Foundation 84.1 (1973). University of Western Ontario, London. Dept of Psychology.
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.
Yoo, Kyung-Hee. “The Effects of Auditory and Vestibular Stimulation of Stress Hormones in Preterm Infants.” Journal of Korean Academic Fundamental Nursing 11.2 (2004): 203-212.