As billions of people can attest, giving birth is hard on humans. Our babies have an unusually large head for their body size and still have to squeeze through a very narrow pelvis. attachments can get stuck; bones can be broken. Worse, the results can be bad for the mother or the children. Until recently, many researchers believed that our species was only able to cope with this difficulty: other primates, they thought, did not need to find the same compromise between high intelligence and upright walking, and thus could easily give birth to children. But new evidence has begun to challenge the notion that human birth is uniquely dangerous.
A new paper published today in Natural Ecology and Evolution offers one of the most compelling cases to date against that notion—showing that other primates, too, must push their young into highly restricted areas, contributing to infant mortality rates that can exceed 34 percent. Humans have long placed themselves at the base of evolution—”We’ve always thought we were special,” said Nicole Webb, an evolutionary biologist at the Senckenberg Research Institute and Museum of Natural History, in Germany, who was not involved in the new study. he told me. But the more scientists look at the animal world, the more the biological reality of other animals reinforces this narrative.
The concept that people have a very bad situation during childbirth can be largely traced to a scientist named Adolph Schultz. Schultz’s research was revolutionary: Nearly a century ago, he was the first to collect evidence on the pelvic proportions of several primate species as a proxy for how easily their offspring fit. But his method was also seriously flawed, Nicole Torres-Tamayo, an anthropologist at the Miquel Crusafont Catalan Institute of Paleontology, in Spain, and one of the study’s new authors, told me. Schultz was wrong about the direction in which the fetal head of different primates passed through the birth canal. He also mistakenly assumed that the measurements scientists had taken on the human birth canal would be the most important to take on other species.
In general, the smallest part of the human birth canal is the part between the top of the sacrum and the front of the pelvis. But in some primates, the birth canal narrows much lower than that. These errors caused Schultz to overestimate how well other primates had the opportunity to take care of their young through the reproductive system.
So a few years ago, Torres-Tamayo and his colleagues decided to take a new set of measurements in a more transparent way. After examining more than two dozen species of primates, they discovered that humans were not alone in conceiving children through an improper pelvis. According to their research, we are not even the least capable of the apes. Babies have heads that are about the same size as the mother’s pelvis—tighter by any measure and tighter than other great apes. But other primates, including tamarins and jungle babies, must give birth to babies whose heads are almost twice what their mother’s pelvis appears to carry.
The squirrel monkey—a small, screeching, tree-climbing species whose black beak casts a permanent 5 o’clock shadow—must deal with the same problem, too. Although they are one of the smallest primates, they can give birth to babies that, in general, weigh 15 percent of what the mother does. (For a 150-pound human, that would be the equivalent of delivering a 22.5-pound infant via vaginal delivery—which historically, it doesn’t end well.) Some data show that, at least in captivity, more than a third of squirrel-monkey babies can die, Lia Betti, an anthropologist at the University of London and one of the study’s authors, told me. In one study from the 1990s, a researcher studying the birth of seven squirrel-monkeys watched two of the babies get stuck; neither did he survive.
And yet humans, chimpanzees, and all other living primates still make it work, in large part by evolving anatomical functions. Many primates, for example, exit the birth canal not with the baby’s head crown-first, as humans do, but face-first—a situation that seems to put the head in a more unrestricted position as it passes through the pelvis. Squirrel monkeys can also successfully give birth by separating their pelvis during childbirth—and their babies have been recorded. to pull themselves outside the reproductive tract once their shoulders are exposed.
These specific options are not available to humans, though. Our upright posture and spine position make it extremely dangerous—and uncomfortable—to deliver babies face to face. For us, a baby who is born upside down and facing the mother’s spine, with the chin pointed to the chest, has the easiest way out. And for us, a strong and stable pelvis is thought to be the key to supporting our weight when we walk upright, and choosing the option to make those bones loose and separate completely, Betti told me.
Making these kinds of comparisons, though, can only tell researchers so much. Each species has such a unique history of anatomy, physiology and evolution that no single measure or measure can fully capture the complexity of innateness. Anna Warrener, an anthropologist at the University of Colorado in Denver who was not involved in the new paper, also said that the most serious problems in human births do not involve babies getting stuck: Instead, they become dangerous when the mother begins to bleed uncontrollably, or suffers complications from an infection. Birth is probably tricky for many animals, many in ways that humans still don’t appreciate.
Ultimately, what distinguishes humans most may be our ability to cope with birth in creative ways. Humans closely monitor pregnancy and attend another person’s birth; we can intervene to reduce bleeding and infection, and surgically remove children. Many experts have warned of the dangers of over-intervention during childbirth; at the same time, access to clinical care, when properly implemented, has saved countless lives. And even outside of that medical support, “from a good evolutionary perspective, our species has done a pretty good job,” Warrener said. Being born with problems doesn’t necessarily mean failure—and indeed, he added, “we wouldn’t have 8 billion people if we didn’t break the rules.”
