The Cost of Carrying

Many moons ago at a family gathering, a relative was complaining about her baby’s fussing, “He won’t let me put him down and he’s so @#%!ing heavy!” I offered to get one of my baby carriers out of my car and she refused, claiming that he was too heavy to be carried. I tried to say that “the weight seems to disappear in a carrier because of the distribution…” but she wasn’t listening. She had a fully glazed expression as she strapped him into a 20 lb car seat, in order to swing it from arm to arm, for the. next. four. hours. to keep him calm.

You must really hate babywearing.

… but back to that bit about infant carriers making baby’s weight disappear. Us babywearing nerds can give a good schpeel about how (ergonomic) infant carriers distribute the weight through the pelvis instead of pulling on the shoulders and upper back. But is there a way to scientifically quantify how an infant carrier reduces the energetic drain of carrying?

Why, yes. Yes there is.

The Research

An experiment was done at the University of Wisconsin to determine the energetic costs (literally, the calories required to the do the job) of carrying an infant in-arms vs. carrying in a sling. They discovered that carrying in arms was 16% more costly than carrying in a sling, or to put it another way: carrying in-arms was more costly than lactation. It had to do with kinematic changes, the difference in the way the participants moved, whether walking unencumbered, carrying a baby in-arms or when using a sling.

Using a sling freed the arms, improved posture compared carrying in-arms and allowed for more hip movement. The movements of walking using a sling were more similar to the movement of an unencumbered walker. Carrying in-arms forced the walker to take smaller, uneven strides, by reducing the movement in the hips.

For a nomadic, hunter-gatherer group carrying in-arms while producing breastmilk would have been a recipe for malnutrition or even starvation. Of course, other members of the group could help carry the baby and/or help bring food to the nursing mother. But in the case of active travel, whoever is carrying the baby will be expending more calories while tying up their arms. It’s far from ideal. Let alone the risks of trying to escape predators by climbing into a tree with a baby without a tool to help carry them.

 “The cost of carrying an infant in one’s arms would have been meaningful enough to reward the development of carrying tools rapidly following the advent of bipedalism,” (Wall-Scheffler, et al, 845).

The invention of the infant sling would have support other tool production, for example, transportation of raw material for the Oldowan and Acheulean stone tool industries which existed during the time of late Australopithecus and early homo. The fossil record shows that the range of the stone tool industries widely expanded in range during these early stone tool industries, from less than a mile to an average 12-24 miles, with one example of obsidian being transported over 62 miles around 1.5 mya (Wang and Crompton). 

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Wall-Scheffler, C.m., K. Geiger, and K.l. Steudel-Numbers. “Infant Carrying: The Role of Increased Locomotory Costs in Early Tool Development.” American Journal of Physical Anthropology 133.2 (2007): 841-46. Web.

Wang, W.-J., and R. H. Crompton. “The Role of Load-carrying in the Evolution of Modern Body Proportions.” Journal of Anatomy 204.5 (2004): 417-30. NCBI. Web. 25 Jan. 2015.

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