Giving birth permanently alters your bones — fossil records reveal

Evidence of giving birth and lactation are visible in a fossil record.

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Microscope images of the cross section of the seven femora (thigh bones) included in this study, ide...
Image courtesy of Paola Cerrito and Timothy Bromage.

Depending on whom you ask, giving birth can be a miraculous, horrifying, incredible, painful, or primal experience. In most cases, it transforms one’s life — and body — at the very least.

One of these transformative changes occurs in the skeleton — not merely in the shape of the pelvis but in the mineral composition of the bones themselves. A team of researchers from New York University, Brown University, and Texas State University performed an exploratory investigation of adult macaque bone growth and composition to see what life story the skeleton told. They found a surprising indicator of gestation and parenthood.

The researchers published their work this week in the journal PLOS ONE. The study details how evidence of giving birth and lactation are visible in a fossil record.

What’s new — Researchers analyzed a one-centimeter sample of the thigh bones from seven macaques that died of natural causes. Three came from males and four from females. Two of the females had given birth in their lifetime, and two had not.

Using powerful scanning electron microscopes and energy-dispersive X-ray analysis, the team measured concentrations of calcium, phosphorous, oxygen, sodium, and magnesium in these bone samples throughout the specimen’s lifespan. They compared all of this to a detailed record of each monkey’s life events, such as when they were pregnant, when they gave birth, and when they were breastfeeding.

The researchers found that, compared to both males and females that hadn’t given birth, those that had gone through pregnancy had lower concentrations of calcium and phosphorus during pregnancy, and while they were breastfeeding, they had lower levels of magnesium in their bones.

Where are these elements going? Into babies. The team says the elements from our bones contribute to fetal and infant growth. We’ve known that post-partum and menopausal bodies undergo dramatic changes, but the idea that the record of giving birth and breastfeeding exists in fossils is brand new.

“This was really fascinating for me,” lead author Paola Cerrito tells Inverse. “We know how much reproduction has a toll on the female organism, but even with time, something that’s written in your skeleton doesn't go away.” Cerrito conducted this research as an anthropology doctoral student at NYU, and now serves as a postdoctoral fellow at the Swiss Federal Institute of Technology in Zurich.

Microscope images of the cross-section of the seven femora (thigh bones) included in this study, identified by age and sex.

Image courtesy of Paola Cerrito and Timothy Bromage.

Why it matters — Children perennially puzzle over where babies come from, but even experts are still in the dark about how the birth process evolved.

“Human reproduction is completely, mind-bogglingly different from anything we’d expect from other primates,” Cerrito says. “How our way of reproducing evolved is a complete mystery.”

For instance, we don’t know why human birth is far more complex than any other animal's birth. In 1960, physical anthropologist Sherwood Larned Washburn coined the term “obstetrical dilemma,” which refers to the physiological catch-22 of narrow pelvic dimensions for walking upright with a large birth canal that can safely pass a whole baby.

What’s more, this finding reveals a little more about how taxing birth is on the body. “Mothers feed babies everything they need until they’re weaned,” Cerrito tells Inverse. “The skeleton serves as a reservoir also to build the skeleton of a fetus.” Studies in humans and rodents show that the body undergoes dramatic changes that often revert post-partum, such as body weight.

She emphasizes that this permanent change in the bones isn’t bad or dangerous. Further, their conclusion doesn’t inform best practices for pregnancy right now. Rather, it illustrates how birthing and lactation show up in the fossil record.

But this insight could also teach us more about infant development and the timing of weaning, Cerrito says. The rate at which magnesium declines in the bones of females who have given birth may indicate how long breastfeeding lasted, and how long it took for an infant to be weaned.

What’s next — More subjects — both in number and species type — are necessary for further research, according to Cerrito. She also notes that she’s an animal rights advocate and promoted conducting further research on ethically obtained animal samples.

While humans are an obvious specimen to include, Cerrito wants to extend the research to our other primate brethren. Seeing how we differ from other primates may give hints about the evolution of human birthing mechanisms, and also the formation of human communities. Understanding the way bodies evolved to give birth, she says, indicates how societies evolved to cooperate to raise offspring.

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