When a chimpanzee is born, they are able to grasp their mother’s fur with their hands and feet and cling on effectively within weeks. A human newborn is not as strong or directed in their efforts though they still retain some of the so-called primitive reflexes from our common primate ancestor which fade as a human baby gets older. Newborn humans can strongly grasp with their hands (palmar grasp reflex) and they can flex their toes (plantar reflex) but due to their foot morphology, which lacks an opposable hallux (a big toe that looks more like a thumb) they cannot grasp with their feet as they do with their hands.
This is a problem for a species of “riders“, mammals whose breastmilk composition requires them to carry their baby with them for frequent feedings, unlike “parkers” whose offspring can be left for long stretches while their mother hunts or forages (or goes to the bathroom by herself for crying out loud!) If a baby cannot cling to it’s unlikely to survive, most rider mothers cannot afford the extra energy (literally in the form of calories) to carry their infant and it would certainly slow her down in the face of danger. Even if the mother did everything in her power to carry and protect her baby, the odds are against their survival and so that babies trait, the trait for not being able to cling on, wouldn’t get passed on. Unless of course, the mother was clever enough to make a technological adaptation for easier carrying… (you see where I am going here). Continue reading
Humans, like the other Great Apes, are physiologically riders; breast milk composition is not suited to parking infants for long periods between feeding (Ross). For this reason among many others, our species has had to carry our babies with us wherever we go. Unfortunately, humans find carrying infants more difficult than our evolutionary cousins for three reasons: lack non-grasping feet and body hair, physically helpless infants and, most importantly for this post, the relative size of our infants.
Non-human apes in the taxonomic family Hominidae (literally means “Great Ape” and makes me think of Charlotte’s Web’s “Some Pig” but I digress) have smaller, more precocious babies than humans. Adult gorillas, for example, are significantly larger than adult humans, yet their newborns are about half the size of human newborns. Chimpanzees, which are our closest extant evolutionary relative and have a similar adult body weight to us, give birth to newborns that are around 3% of their adult size, while humans have newborns that are around 6% of adult size (DeSilva). At what point since our common ancestor with chimpanzees, did hominins start having bigger babies? Continue reading
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 all the time. I tried to say that “the weight seems to disappear in a carrier because of the distribution…” but she wasn’t listening. Fully glazed expression as she was strapping him into a 20 lbs car seat, in order to swing it from arm to arm, for the. next. four. hours.
But back to that bit about infant carriers making baby’s weight seem to disappear. Those of 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. But is there a way to scientifically quantify how an infant carrier reduces the energetic drain of carrying? Why yes, yes there is– by measuring the differences in calorie expenditure between different ways of carrying.
Have you ever wondered why some animals create a nest or den for their offspring, while others carry their babies everywhere they go? These represent two reproductive strategies, or ways of producing and caring for offspring that survive to sexual maturity, called parking and riding respectively. Continue reading