Anyone who has cared for a newborn has been there. That moment when you feel like you have tried everything to soothe your crying baby and you just want to sit down and cry yourself. But what if there was some magic trick that would calm your baby and brighten your own mood? According to science there just might be some magic in taking a walk with your baby. There’s one caveat: you’ll need carry them.

In Non-Humans

In altricial mammals, those that are born blind and helpless, like puppies or kittens, being picked up and carried by their mother elicits sudden relaxation which makes them easier to move. If you’ve ever had a pet dog or cat, you’ve probably seen it in action. The mother picks up her baby and the baby’s hind legs pull up, the tail tucks, and they passively dangle while mom moves, but that’s just the outside. Internally the puppy’s or kitten’s breathing and heart rate slows as though they were asleep. This deep relaxation while being carried is called Transport response and it helps to ensure the survival of both mother and 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

Mammals with altricial young are typically parkers, they have litters of offspring which are kept in a den while their mother goes out to hunt or forage (Ross). Ideally, they are not supposed to have to leave that den until they are developed enough to get up and wander out on their own paws. However, sometimes a mama needs to find a new den to avoid predators or other threats. When this happens she needs to move them one by one, in her mouth, to the new location. Having a noisy, failing newborn while she’s trying to walk, run, climb, or jump just won’t do. Fortunately, transport response helps keeps infants passive and quiet.

Mammals with precocious young, that is infants who are born with muscle strength and coordination to move like an adult, like our nearest relations chimpanzees and bonobos, tend to have singleton births and carry their babies with them all the time (Ross). Or rather, their babies actively cling on to their mother while their mother’s arms are free. If a baby chimp suddenly elicited the transport response when its mother started moving, it would fall off, which is quite dangerous if its mama is getting groceries way up in the canopy.

And Humans

Then there are humans, evolution’s hot mess. Our species are the newest member of a family of biological riders with precocious babies, but our newborns are born with secondary altriciality. All of their senses are ready to go but they can’t cling to a caregiver, and without out a tool to help parents carry them, infants must be held in arms.

It doesn’t help matters that human infants are born proportionally large compared to other primates, with an Infant-Mother-Mass-Ratio (IMMR) of around 5-6% whereas gorillas and chimps have newborns of 2-3% IMMR (Desilva, 2015). Plus, even if our newborns could hold their body weight (for more than a few seconds, newborn reflexes don’t count), our infants are born without clinging feet to parents without enough body hair to cling to (no doubt there are individual exceptions).

With all of this in mind, if humans did elicit a transport response it would make them easier to carry…

How do we test for transport response?

One researcher, Esposito, compared mice and human infants. Mice are an altricial-parker mammal species that elicit the transport response in response to being carried by their mother. This maternal grasp is important to the research because they found that they couldn’t fake the proprioceptive stimulation, or the sensation of being carried by a walking caregiver, who in Esposito’s studies is the infant’s mother.

“Indeed animal studies find that the tactile sensation from maternal grasp and proprioception are required to elicit the carrying-induced calming responses”

Esposito

It’s the same for human infants: stationary maternal holding or being rocked in a bassinet (or mechanical swing) will not produce the same effect on the infant as being carried during a walk. No doubt someday instead of mechanical, vibrating baby swings we’ll have walking baby-minder robots, just send them out for a stroll around the block when baby is fussy.

“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

Carrying and Crying

Not all researchers agree that humans have a transport response and much of it comes down to the mixed results of carrying-crying studies that have failed to show a correlation.

In the 1980s, Hunziker found that six-week-old infants who were carried three hours a day in a soft carrier or in arms* cried 43% less overall and 51% less during the evening hours than infants who were not given “supplemental” carrying, that is carrying beyond what is necessary for feeding and responding to crying.

_{*the majority of the participants didn’t use the carriers, therefore this study, so often pointed out as evidence for babywearing reducing crying, this is not the study to cite.}

However, subsequent studies did not show a reduction in crying with supplemental carrying. Why the discrepancy? Similar to what Esposito found in his study of transport response in infants, Korner and Thoman’s research showed that vestibular stimulation, or carrying while walking of an infant in an upright position, was more effective at soothing than stationary holding alone. Future research needs to clearly distinguish between stationary holding and carrying while walking or during other movements.

In another study of the effect of supplemental carrying on crying rates, 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.).

There is also a question of whether participants in the supplemental carrying groups were taught how to use the infant carriers that were provided by the researchers and whether those carriers could be safely and comfortably used hands-free while walking. More and better-designed and better-defined research needs to be done in this area.

Beyond Soothing

Though the studies emphasize maternal carrying, all parents, caregivers, and infants benefit from understanding the role of transport response in the reduction of crying. For example, it can help caregivers determine whether the cause of crying is acute or ongoing. If the trigger for crying remains, infants will resume crying once 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

Esposito adds that if an infant does not elicit the transport response at all, it may help in the early diagnosis of perinatal brain disorders, such as cerebral palsy, which will help in early treatment (I have reservations about this claim).

Evolutionary Pressures

Secondary altriciality and transport response may have emerged in tandem with bipedalism and proportionally large infants, two traits that emerged by the time of Australopithecus Afarensis, around 4 million years ago. In terms of natural selection, infants who, for whatever reason, work against their mother’s attempts to carry them are, sadly, less likely to survive. That kind of selection pressure would help to conserve transport response to the present day.

“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 altricial mammalian species supports the adaptive value of this behavior in mother-infant relationships, and, as a consequence, infant survival.”

Esposito

Take Home

Though today, many human infants will survive just fine without the transport response, understanding it can help caregivers reduce crying as well as determine the seriousness of the crying. Combining this knowledge with babywearing can help make carrying your baby easier while keeping your hands and arms free for other activities, such as nursing that chai latte, winning that internet debate, or fending off local wildlife on the local trails.

When you can’t walk

And for parents and caregivers who cannot walk but can babywear (i.e. arms, shoulders, neck, and spine are healthy enough to carry a baby), I have developed a method that seems to work. (Designed for wheelchair users but can work for any situation where walking is not possible.)

With the caregiver sitting and the infant in a carrier (upright and facing the caregiver), firmly pat the baby’s bottom at around 60-90 beats per minute to simulate the movement they would experience if you were taking steps. The pats should mildly jostle the baby and 60-90 beats a minute is the average pace of a briskly walking adult. A slight side-to-side sway between pats is the icing on the cake. Obviously, for wheelchair users, this works much better for those with a power wheelchair that can be operated with one hand.

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Sources:

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.

DeSilva, Jeremy. “Brains, Birth, Bipedalism, and the Mosaic Evolution of the Helpless Human Infant” Costly and Cute: Helpless Infants and Human Evolution. Ed. Wenda Travathan and Karen Rosenberg. Santa Fe: School for Advanced Research Press (2016): 67-86.

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.

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.

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.