Apes, including humans, are physiologically riders, meaning that they carry their babies with them as opposed to leaving them in nests or dens because their breast milk composition is not suited to leaving their infants alone for long periods of time (Ross, 2001). Unlike humans, apes don’t need a tool to carry their infants in part because they have body hair for their infants to cling to but it’s more complicated than simply having body hair and a baby that can grasp it. Hair strength, density, infant weight, carrying position, adult posture, and even humidity play a part in successful infant carrying without tool use.
“Animal hair is composed of three parts: an external thing cuticle (protective covering with a scale structure from root to tip), a thicker cortex with fibrous proteins, and a central porous medulla, which may be absent in finer fine. Ape hair viewed under an optical microscope are similar in structure to wool and human head hair […]” Amaral, 2008.
In a study by Lia Q. Amaral, preserved skin and hair samples from Gibbon, Gorilla, and Orangutan cadavers were tested for their properties with regards to infant carrying. Gibbons have smooth, silky hair about 4 cm in length, Orangutans have hard, rough hair around 10cm in length, and Gorilla hair texture is somewhere in between at 6 cm in length.
The strength of the hairs was tested with a Tensile Tester, which pulls on the hairs to measure their resistance to breakage both within the hair shaft and pulling from the skin. You can get an idea of your own hair’s tensile strength by getting a single strand and pulling on it– if your hair is strong and well conditioned you will see it stretch before breaking, if it’s damaged and brittle it will just snap. Wet (or well conditioned) human hair will stretch up to 50% while dry hair will only stretch 10%. Not only does wet or humid conditions increase hair strength it increases its friction due to the hydrogen bonds connecting the hairs in contact and the structure of the open cuticle ratcheting against other hairs.
To test the friction of the hair, samples were attached to wood with various weights at different angles to see how long the static friction coefficient held out before the clinging sample fell. In repeated tests of friction, the humidity and temperature of the room were changed to see what the effect would be. In a dorsal (back) carry, gorilla infants are able to rely on the friction between their hairs and the weight of the infant’s body to stay attached while at rest or asleep if the mother maintains a quadrupedal posture (knuckle walking).
“It is clear that to withstand the clinging infant weight, a large number of hairs held together is required. Results obtained evidence that bunches of about 100 hairs are necessary to carry infants weighing a few kilogram force. Infant hands (and feet) can possibly grasp a bunch of hairs available in some square centimeters of skin so that safety in the usual pattern of nonhuman primate infant carrying critically depend on the density of hairs.” Amaral, 2008
The Great Apes have the lowest hair density of the primates, which range from 1,000 hairs/cm sq for gibbons to 100 hairs/cm sq, making the properties of their hair that much more important for safely carrying their heavier infants. Infant grasping size (the size of their hands and feet) determines how many hairs they can grasp at a given time as well as the density and length the adult’s body hair needs to be in any given area used for clinging. Amaral determined that the grasping area per infant limb is 3 cm sq for gibbons, 10 cm sq for gorillas, and 50 cm sq for orangutans. However, depending on how the infant is clinging, their entire body weight may be supported by one limb grasping one bunch of hair.
The hair density affects the lifestyle of ape mothers, the Gibbon, with 1000 hairs/cm sq, has more than enough body hair for her small infant to cling securely to while she swings high in the branches, whereas Gorillas tend to stay on the ground. As gorillas develop their positioning on their mother changes to accommodate their increased weight on a part of the body where hair density and friction will aide in safe carrying. For the first couple months of a gorilla’s life, they are supported manually (the same is true too for infants who cannot cling with all four limbs, including disabled or even dead infants), once they are able to cling until around 4–6 months of age they ride in a ventral position (front, chest) before switching to a dorsal (back) position where hairs are denser and gravity and friction can work together to keep the infant attached. Though, in dangerous situations, the infant, even an older heavier infant, may cling to their mother’s chest, especially if she is trying to flee in an arboreal (tree branches) environment.
For chimpanzees, our nearest surviving evolutionary cousins, Jane Goodall described two primary causes of death for chimp infants are poor attachment (emotionally) to their mother and injuries from falling from the mother. Clinging is part of the attachment process, in Mother Nurture, Sarah Blaffer Hrdy describes the low incidence of primates’ maternal failure to attach to their offspring when compared to other mammals:
“In fairness, however, it must be noted that primate neonates themselves deserve some credit for this, since they often clinch the deal by grasping hold of the mother’s fur right after birth. The neonate quite literally attaches to his mother.” (Hrdy, 178).
She went on to say that maternal ape attachment was as simple as “If it clings, I will carry.” So if an ape offspring is born unable to cling shortly after birth, for whatever reason, they are doubly at risk for death. Attachment to their mothers and their mother’s attachment to them is necessary for survival.
Our evolutionary ancestors began losing their body hair around four-million-years ago– in the time of australopithecus afarensis, commonly referred to as Lucy for the famous specimen. The hair loss occurred in conjunction with birth size doubling to 6% of maternal body mass and bipedal feet reducing the infant’s number of grasping, and bipedal posture meant that they could not use gravity and friction to assist with carrying.
“Birthing larger infants… also introduces the energetic and biomechanical challenge of transporting a relatively large, helpless newborn. This is particularly the case for pretechnological, upright walking hominids, some of which had reduced pedal grasping abilities.” DeSilva, 2011
What a mess, right? Add to this the fact that Lucy likely still climbed into trees for food, shelter, or to escape predation (based on the long curving fingers similar to modern chimpanzees). It seems an impossible struggle, juggling a large poorly clinging infant in-arms without a tool to assist with carrying; a tool that would allow such a species to survive and pass on its traits. And that tool is of course, the infant carrier. The infant carrier took the place of body hair helping poorly grasping, big babies to cling to their bipedal mothers.
“We suspect that the energetic drain of carrying an infant would be such that some sort of carrying device would have been required soon after the development of bipedalism and definitely to allow long-distance travel, especially that out of Africa and across Asia.” (Wall-Scheffler, et al.)
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[This post was originally published Oct 2015, Revised Oct 2018]
Blaffer-Hrdy, Sarah. 1999. Mother Nature: Maternal Instincts and How They Shape the Human Species. New York: Ballantine Books.
DeSilva, Jeremy M. 2011. “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: 1022-027.
Ross, Caroline. 2001.“Park or Ride? Evolution of Infant Carrying in Primates.” International Journal of Primatology 22.5: 749-71.
Wall-Scheffler, C.M., K. Geiger, and K.l. Steudel-Numbers. 2007. “Infant Carrying: The Role of Increased Locomotory Costs in Early Tool Development.” American Journal of Physical Anthropology 133.2: 841-46.