Study Finds Other Primates Also Face Tight Birth Canals

A study published today in Nature Ecology & Evolution measured pelvic dimensions across more than two dozen primate species and found that several face birth-canal constraints comparable to those in humans. The research challenges a long-standing view that only humans must balance large infant heads against narrow pelvises.

Earlier measurements by Adolph Schultz nearly a century ago had suggested other primates gave birth with relative ease.

The new team used updated techniques that corrected earlier errors in the orientation of the fetal head through the birth canal. In many primates the narrowest section lies farther down the pelvis than the upper sacral region identified in humans. Squirrel monkeys emerged as one of the most constrained species.

Their newborns can weigh up to 15 percent of the mother’s body mass, and data from captive populations indicate infant mortality can exceed one-third.

Different primate species have evolved distinct delivery positions. Many give birth face-first, placing the head in the least obstructed orientation as it passes through the pelvis. Humans cannot use the same position because of upright posture and spinal alignment.

Instead, human infants typically emerge head-down and facing the mother’s spine. Researchers noted that human birth complications often involve bleeding or infection rather than mechanical obstruction alone. Clinical monitoring and surgical options have reduced mortality in modern settings.

"From a grand evolutionary perspective, our species has done a pretty good job," an anthropologist not involved in the study said.