As billions of people can attest, giving birth is hard for humans. Our infants have an exceptionally large head for their body size and yet have to squeeze through a very narrow pelvis. Appendages can get stuck; bones can fracture. At worst, the consequences can be lethal for mothers or babies. Until recently, many researchers believed that our species weathered that particular hardship alone: Other primates, they supposed, didn’t need to strike the same compromise between super-brains and walking upright, and so could birth babies with relative ease. But new evidence has started to challenge the notion that human childbirth is uniquely dangerous.
A new paper published today in Nature Ecology & Evolution offers one of the most compelling cases to date against that assumption—showing that other primates, too, must push their babies through some seriously restricted spaces, contributing to infant-death rates that can exceed 34 percent. Humans have long put ourselves on an evolutionary pedestal—“We always think we are special,” Nicole Webb, an evolutionary biologist at the Senckenberg Research Institute and Natural History Museum, in Germany, who wasn’t involved in the new research, told me. But the more scientists have looked across the animal kingdom, the more the biological realities of other animals have upended this narrative.
The assumption that people have it especially rough during childbirth can largely be traced back to a scientist named Adolph Schultz. Schultz’s research was revolutionary: Nearly a century ago, he was the first to gather evidence on the pelvic proportions of several primate species as a proxy for how easily their babies would fit. But his approach also had serious flaws, Nicole Torres-Tamayo, an anthropologist at the Miquel Crusafont Catalan Institute of Paleontology, in Spain, and one of the new study’s authors, told me. Schultz was wrong about the orientation in which the fetal head of different primates moved through the birth canal. He also incorrectly assumed that the measurements that scientists had taken of the human birth canal would be the most relevant ones to take for other species.
Generally, the most limited section of the human birth canal is the section between the upper part of the sacrum and the front of the pelvis. But in other primates, the birth canal narrows farther down than that. Those errors led Schultz to overestimate just how much room other primates had to shepherd their babies through the birth canal.
So a few years ago, Torres-Tamayo and her colleagues decided to take a new set of measurements with a more open-minded approach. After surveying more than two dozen primate species, they found that humans were far from alone in having to squish babies through an unaccommodating pelvis. According to their research, we’re not even the most disadvantaged among primates. Human babies have heads almost exactly as big as the mother’s pelvis—a squeeze by any standard and a tighter fit than other great apes. But some other primates, including tamarins and bush babies, must birth infants whose head is almost twice as large as what their mom’s pelvis seems to accommodate.
Squirrel monkeys—a petite, chirruping, tree-climbing species whose dark muzzle gives the impression of permanent 5 o’clock shadow—have to deal with that proportional dilemma, too. Although they’re one of the smallest primates around, they can give birth to babies that, overall, weigh as much as 15 percent of what the mother does. (For a 150-pound human, that’d be the equivalent of vaginally delivering a 22.5-pound infant—a feat that, historically, does not end well.) Some data suggest that, at least in captivity, more than a third of squirrel-monkey babies may die, Lia Betti, an anthropologist at University College London and one of the study’s authors, told me. In one study from the 1990s, a researcher observing seven squirrel-monkey births watched two of the babies get stuck; neither survived.
And yet humans, squirrel monkeys, and all other still-living primates have somehow made it work, in large part by evolving anatomical work-arounds. Many primates, for instance, emerge from the birth canal not with the crown of the baby’s head popping out first, as humans do, but face-first—a position that seems to put the head in the least obtrusive position as it transits through the pelvis. Squirrel monkeys are also able to successfully birth babies by completely dislocating their pelvis during delivery—and their infants have been documented pulling themselves out of the birth canal once their shoulders are in the clear.
These particular options aren’t available to humans, though. Our erect posture and the position of the spine makes it very risky—and uncomfortable—to deliver babies face-first. For us, a baby who is delivered head down and is facing the mother’s spine, with their chin tucked toward their chest, has the easiest path out. And for us, a strong, solid pelvis is thought to be key to supporting our weight as we walk upright, nixing the option of keeping those bones loose and fully dislocatable, Betti told me.
Making these kinds of comparisons, though, can tell researchers only so much. Each species has such a unique anatomy, physiology, and evolutionary history that no single metric or measurement can fully capture the complexity of birth. Anna Warrener, an anthropologist at the University of Colorado at Denver who wasn’t involved in the new paper, also pointed out that most serious human-birth complications actually don’t involve babies getting stuck: They instead get dangerous when a mother starts to bleed uncontrollably, or suffers complications from an infection. Birth is probably treacherous for many, many animals in ways that humans don’t yet appreciate.
Ultimately, what sets humans apart the most may be our ability to cope with birth in creative ways. Humans monitor pregnancies closely and attend one another’s births; we can intervene to minimize bleeding and infection, and extract babies surgically. Many experts have cautioned against the dangers of over-intervening during delivery; at the same time, the availability of clinical care, when properly deployed, has clearly saved countless lives. And even outside of that clinical assistance, “from a grand evolutionary perspective, our species has done a pretty good job,” Warrener said. Difficult births aren’t necessarily unsuccessful ones—and certainly, she added, “there wouldn’t be 8 billion of us if we hadn’t cracked the code.”
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