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Birth & Eggs & Evolution

We can’t get enough of this article in Aeon Media (purpose: “to explore and communicate knowledge that addresses our shared need to make sense of the world.”)

Headline: “How like the kiwi we are.”

The article was written by Antone Martinho-Truswell, an evolutionary biologist and Dean of Graduate House at St Paul’s College, University of Sydney. He writes on human culture and society, evolution, and animal behavior. His latest book is The Parrot in the Mirror: How Evolving to Be Like Birds Made Us Human.

Read the article (we haven’t explored the book) and you will ponder human pregnancy and birds laying eggs in a whole new, comparative way.

How are we similar to kiwis? And how come we are not more like elephants when it comes to childbirth? And is evolution to blame for the pain mothers endure when they have a baby?

Why are some babies born more ready to take on the world than others? Ponder how much care, feeding, nursing, protecting a human baby requires, for instance, compared to that of a foal?

Photo by Lance Reis on Unsplash

Okay, a quick vocabulary lesson. Animals that are born nearly independent on the spot—the ones that can walk or feed themselves shortly after they are born—are called precocial.

The word sounds like precocious. That’s because the words are related—referring to young animals (or children) who do things a lot earlier than you might expect.

Most precocial animals have hooves. Why? Because they are prey animals—and they need to be able to move, immediately in some cases, with the pack. Early walking as survival!

In the vast world of “babies,” there is a sliding scale of care. Lizards deposit their eggs, cover them, and forget them. According to National Geographic, one species of chameleon lays their eggs before winter. The eggs hatch in the summer and the adults, meanwhile, will have died. In other words, some baby chameleons are instant orphans. They don’t even know the concept of mom and dad. They have to figure out everything on their own.

Humans? Well, us humans can’t even feed ourselves or walk for about a year.

And this brings us back to the kiwis.

As Martinho-Truswell points out, pregnancy and childbirth have driven us humans to be very unlike other mammals.

Thanks to evolution.

“Instead of functioning as a refining, perfecting tool, evolution in the real world is all about trade-offs: life has limitations, and big changes in one area often mean sacrifices in others,” he writes. “We humans are the smartest, most complex animals on the planet, but we do not have the best or most optimized biology by any stretch, especially not when it comes to reproduction.”

Indeed, other mammals give birth, he notes, with near “nonchalance.” While for human mothers the experience is painful and dangerous.

Kiwis also take an extreme approach to birth. They lay the largest egg, relative to their body size, of any bird. They are about the same size as a domestic chicken but their eggs are six times larger. And kiwis will sit on that egg for about 85 days!

It’s interesting to ponder—mammals such as elephants need only take care of themselves (with food) in order to develop and nurture a baby until the day it is born. A kiwi needs to make sure all the nutrients are inside the egg when the egg is laid.

“By laying such enormous eggs, and incubating them so long, the kiwi is giving its babies the maximum amount of development time it can before the babies hatch out and have to face the world. Achieving that maximum is not without its costs. Laying eggs is fairly draining for any bird, and the kiwi more than most. She must eat as much as three times her normal intake for the full month in which the egg develops, and is left significantly weakened by the experience,” writes Martinho-Truswell.

Longer gestation for humans might mean less onerous childrearing. For kiwis, this works. Their babies are much more well developed than other birds.

“Evolution is always a game of compromises,” he notes, “and in pursuing our own key evolutionary advantage—our brain power—we have inadvertently given up the advantages of pregnancy and ended up back with the constraints of eggs.”

Not surprisingly, the issue of human brain size plays a role. So does our “fantastically precise” and manipulable hands because agile hands required a larger brain. But in the process of learning to walk upright the size and angle of our hips changed, narrowing the pelvis. It might be preferable to have human babies that gestate longer and need less care once they emerge from the womb, but the size of the baby would indeed be even more problematic when it came to childbirth.

“Our pregnancies are fundamentally limited in length by our physiology, just like birds’ incubations. In birds, the ultimate limit is egg size through hips, and in humans it is head size through hips. And so, our babies are helpless, unlike other mammals and very much like birds.”

Fascinating piece! We didn’t want to poach (no egg-related pun intended) too much. But as Mother’s Day approaches, another reason to be thankful for all the nurturing and care we received when we were truly helpless.







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