Curious Parents

Compared to Other Baby Animals, Human Infants Are Late Bloomers

When a horse gives birth, her foal can walk within hours. Sea turtle hatchlings emerge from their shells looking like carbon copies of their parents, able to scuttle toward the ocean within days. Compared to these A+ students of the animal kingdom, our own species is at the back of the class — it takes us months to even raise our heads, and a year to begin learning how to walk. What gives, Mother Nature?

Hips Don't Lie

Here's a vocab word for you: altricial, as in, newborns can't move around on their own, which makes humans an altricial species. The common reason most people give for why human babies are born altricial (undeveloped) is the size of our brains. If babies were born with their brains at full size, the story goes, they'd get stuck in women's narrow, bipedal hips. Instead, they stop gestating before their noggins cause a problem for Mom. That theory is so popular that it even has a name: the obstetrical dilemma. But a 2012 study found that it's not Mom's pelvis that can't handle a bigger baby; it's her metabolism.

University of Rhode Island anthropologist Holly Dunsworth and her team argue that humans don't actually cut gestation all that short. If you control for body size, our pregnancy period is longer than all primate species except orangutans. At the same time, human babies are huge. Their brains are 47 percent larger than the brains of gorilla newborns (the second largest primate infant) and their bodies are twice as large. We're growin' 'em big, and we're taking plenty of time to do it. At the same time, they found that women could have a wide enough pelvis to accommodate a more fully developed baby head without many repercussions. But they don't. Why?

I'm Givin' Her All She's Got, Captain!

Being pregnant takes a whole lot of energy. At six months, a pregnant woman is using twice her usual energy to maintain her basic metabolic processes. As the baby develops, that just goes up. It's believed that humans max out at 2.5 times their basal metabolic rate, and by nine months, that fetus is getting dangerously close to that threshold. Your species can't make its babies twice the size of gorilla newborns without paying the price.

So in the end, it's true that our brains are why we're born a bit early. But the human pelvis has nothing to do with it. Growing our big brains in the womb takes energy, which means that Mom reaches her metabolic cutoff before we're fully developed. The pelvis, then, evolved to accommodate our big melons, not the other way around. Once we're born, our brains just keep on growing, leading to cognitive abilities advanced enough to use language, create art, and read this article. We might have been at the back of the class when we were born, but don't worry — we're just late bloomers.

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Written by Ashley Hamer September 13, 2017

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