Why Are Newborn Humans Useless? |Squished Pt3|

On a recent episode of the Dear Hank and John podcast (in which two brothers give “dubious advice”, I recommend it!) a listener sent in a question about why human infants were so useless. And even though both of them have children of their own, they didn’t really know what the answer was so they gave it their best shot: (paraphrasing here) because of bipedalism, women’s hips are too narrow and the baby needs to come out prematurely otherwise they won’t fit.

If you’ve read the earlier parts in this series debunking the Obstetrical Dilemma, you can imagine my epic head shaking and eye-rolling. I had already had this post waiting in the wings but I decided to rework the intro and title when I realized that lay people who have never heard the phrase “obstetrical dilemma” will forward it without even being aware of it. So, why are newborn humans useless?

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Body Hair

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). But 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. Continue reading

The Myth of Childbearing Hips | Squished Pt 2 |

The phrase “childbearing hips”, besides being extremely cringey in any context, is a misnomer. It’s a byproduct of the kind of thinking that went into the Obstetrical Dilemma: in order to give birth women traded in bipedal efficiency for wider hips. It’s a hypothesis that was and is widely assumed to be true. But you know what they say about making assumptions? … As it turns out, pelvic width has nothing to do with bipedal efficiency, nor is it a constraint for fetal head growth or childbirth. Continue reading

Baby Feet

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

Big Babies

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.

giphy

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, yenewborn and adult weightst 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

The Costs 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 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.

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