It’s like magic. A healthy newborn, inconsolable despite every effort, suddenly stops crying the moment we step outside for a walk. Is it the sky? Fresh air? Or is it the way I’m moving? Could the sensation of being held by a walking human trigger relaxation in a baby?
For altricial mammals, those who are helpless at birth, being picked up and carried by their mother elicits sudden relaxation making them easier to move. For example, when a newborn puppy is picked up by its mother, observers can see that the puppy’s rear legs and tail curl up, while their body becomes passive. Internally, the puppy’s heart rate drops and respiration slows similar to that of a puppy at rest. This relaxation behavior while being carried is called the Transport Response and it helps ensure the survival of the mother and her offspring by making infants easier to move.
“[Kittens, puppies, mouse pups] assume a passive and compact posture with the hind legs drawn up while being carried. This postural regulation has been studied experimentally in laboratory rates as ‘transport response.’” (Esposito)
Transport response is most often seen in parkers and then to the brief window in which their young are helpless. Parkers generally give birth to litters of offspring, which are left in a den while their mother hunts or forages for food. Their breast milk composition allows for fewer, less frequent feedings than those of riders. Only in life or death situations would a parker find the need to carry her young to a new location and then only during a short window of her offspring’s infancy when they are unable to follow on foot, or paws. Esposito’s research shows that human infants response to maternal carrying is similar to mouse pups: they become passive and compact, quiet and calm (as measured by reduced heart rate and crying).
Vestibular-Proprioceptive stimulation, a fancy phrase referring to the combination of touch and rhythmic rocking, is calming to mouse pups and human infants. In animal studies, the Transport Response could not be faked by having a researcher hold and move mouse pups in a similar manner to their mother, i.e. pinching scruff of the neck and slowly moving mouse pup. “Indeed animal studies find that the tactile sensation from maternal grasp and proprioception are required to elicit the carrying-induced calming responses”(Esposito). The same has been seen in human infants: being held in-arms of a seated caregiver or being rocked in a bassinet (or mechanical swing) will not produce the same effect on the infant as being carried during a walk.
“A human infant carried by his/her walking mother exhibits a rapid heart rate decrease, and immediately stops voluntary movement and crying compared to when he/she is held in a sitting position.”(Esposito)
If Transport Response exists in human infants, then there should be a measurable reduction of crying in infants who are frequently carried compared with infants who are not carried. In the 1980’s, Hunziker found that there was a significant reduction, 43% less crying overall and 51% less during the evening hours. However, subsequent studies did not show a reduction in crying with supplemental carrying. Why the difference? Failing to distinguish between holding an infant while stationary and holding while walking– and the walking is the key to trigger the transport response (Esposito). In one study, mothers were inadvertently discouraged from walking with their infants due to the requirement to keep a stuffed bear containing a 24 hr audio recorder within 100m of their infant at all times; even though the bear came with a carrying strap it was an added burden to carry along with their baby (St. James-Roberts, et al.). One also has to inquire whether participants in the increased “carrying” groups were taught how to use the infant carriers or slings that were provided and whether those carriers could be safely and comfortably used hands-free– obviously more research needs to be done.
In 2008, a study of Italian mothers seeking to confirm infant crying patterns, previously identified in North America and Northern Europe, notably a peak of infant crying at 5 weeks. However, the researchers failed to find a peak of crying in the Italian infants, instead, they found a peak of mothers holding their babies, carrying them around as they went about their day– especially in the evenings when North American and other Europeans noted the highest level of crying behavior. The researchers encouraged noting cultural differences in infant carrying before determining biological norms for crying behavior in infants (Bonichinia, et al).
Since the 1960’s there have been studies on the best direction and frequency of rocking for soothing infants (Pederson; Yoo) though always with the baby in a bassinet or swing rather than on the body of a walking caregiver. All the researchers noted that rocking movement soothed babies, with faster and larger amplitude of rocking being the most effective at soothing– and establishing regularity of breathing (MacKinnon). Korner and Thoman’s research demonstrated that vestibular stimulation in the upright position was more effective at soothing a human infant than physical contact alone. Elements of this research were used for the design of mechanical infant swings and seats– missing or ignoring the fact that infants were more contented when held by a walking human. Fortunately, we already have the technology of the infant carrier to make walking with our infants more energetically efficient and practical for caregivers who need the free use of their arms.
“Walking for humans may be the most ethologically similar stimulation as it contributes both tactile sensory input and ambulatory motion, which may render walking more effective in calming infants that other kinds of rhythmic motion such as mechanical rocking.” (Esposito)
Parents, caregivers, and infants all benefit from understanding the role of Transport Response in the reduction of crying– however, it has applications beyond that. If an infant does not elicit the transport response, it may help in early diagnosis of perinatal brain disorders, such as cerebral palsy. It can also help caregivers determine whether the cause for crying was acute or chronic as infants who are distressed will resume crying behaviors when they stop being carried. “Because the calming effect is only limited to the period of ambulatory carrying, the infant may resume crying if the underlying aggravation remains after the carrying ends, like hunger or chronic pain.” (Esposito)
In part two, I will discuss how the transport response fits into the wider evolutionary framework: when transport response may have developed in our ancestors and why, as well as how its associated behaviors may be maladaptive today.
Bjorklund, David F. “The Role of Immaturity in Human Development.” American Psychological Association Psychological Bulletin 122.2 (1997):153-169.
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.
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.
MacKinnon, Joanne. “Effects of Two Rates of Horizontal Rocking on Infant Respiration.” Simon Fraser University (1985).
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.
Rosas A, et. al. “The growth pattern of Neandertals, reconstructed from a juvenile skeleton from El Sidrón (Spain).” Science 357.6357 (2017): 1282-1287. Web. 20 Oct. 2017.
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.
St. James-Roberts, Ian, et. al. “Supplementary Carrying Compared With Advice to Increase Responsive Parenting as Interventions to Prevent Persistent Infant Crying.” Pediatrics 95.3 (1995): 381-388.
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.