Study suggests many primates face tighter childbirth tradeoffs

primate childbirth – A reassessment of primate pelvic anatomy finds that several species likely struggle to deliver large-headed newborns through pelvises that may be too narrow—an issue that may stretch back more than 50 million years. The work also challenges a long-held view sh

Golden lion tamarins. tiny primates that live fast and move lightly. have a grim trick for arriving into the world: during childbirth. they dislocate bones of the pelvis. temporarily doubling the size of the birth canal. For humans, that kind of solution isn’t on the table. Walking would be unbearably painful.

Now, a comprehensive analysis of primate anatomy suggests this isn’t a special case limited to a few species. Many primates—especially the small ones—may face a childbirth problem that looks worse than what we usually assume for our own species. The reason is straightforward, and brutal: newborn heads can be too large for the passage provided by the pelvis.

For decades, it has been assumed evolution left humans with unique childbirth difficulties. The conventional explanation traced the trouble to two major evolutionary shifts. First, our ancestors began walking on two legs, which required a pelvis narrow enough to support bipedal movement. Then. a few million years later. hominin brains evolved to be larger. and infant heads became bigger—yet the pelvis didn’t expand to make delivery easier.

Other primates were thought to have a different story. largely because of an influential study published by anthropologist Adolph Schultz in the 1940s. Schultz examined a range of primate species and concluded that. in the vast majority. an infant’s head could fit comfortably through a female pelvis.

But Nicole Torres-Tamayo, at University College London, says that analysis was flawed. One of her main critiques is that Schultz applied measurements developed for the human pelvis to all primates.

Schultz identified landmark points on the human pelvis that define the maximum width and depth of a horizontal plane at the top of the birth canal. He then assumed those same landmarks would define the maximum width and depth of any primate birth canal. They don’t, Torres-Tamayo argues. The human pelvis has a very unusual shape. and when Schultz’s landmarks are mapped onto other primate pelvises. they typically land on an inclined plane that sits slightly above the birth canal.

That mapping overestimates the size of the birth canal. It effectively creates an oblique, oval-shaped slice through a cylinder representing the birth canal—making it look as though the infant has more space than it actually does.

Torres-Tamayo and her colleagues recalculated birth canal shape in 29 primate species. They also brought in data on newborn-skull size and shape for each species. The picture they produced is uncomfortable for many animals: their results indicate that several primates have a pelvis that seems too narrow to give birth.

Small primates have the most severe conflict. For bush babies and tamarins, the newborn’s head is almost twice the size of the birth canal. Lia Betti, also at University College London, said she was not expecting to find such a large mismatch across the number of primates in the study.

Betti also suggests the mismatch could be the ancestral condition in primates, particularly because early primates were small. The work therefore points to childbirth difficulty as something that may have begun more than 50 million years ago, with the first primates.

Not all primates respond the same way. Bush babies and tamarins dislocate bones of the pelvis, temporarily doubling the size of the birth canal. Betti contrasts this with humans: the same approach would make walking unbearably painful for a large, bipedal species.

The study also reports that birth difficulties are much less likely to arise in the great apes. One explanation offered is scale: great apes are so much larger than the tiny tree-dwelling primates. In this framing, humans remain distinctive because they are the only large ape with the problem, Betti says.

But the story doesn’t end there. Nicole Webb, at the University of Zurich, Switzerland, is not convinced by that particular conclusion. In a study Webb and her colleagues published in 2024. they reported that even chimpanzees show an uncomfortably close match between the size of the birth canal and the infant’s head. “That discrepancy is strange. It’s probably a reflection of the methods used. ” Webb said. adding that the new paper gives “a really nice incentive for us to revisit our own hypothesis.”.

In one sweep, the new findings reshape a long-running debate. Whether the mismatch is unique to humans or shared across primates hinges on how anatomy is measured and how pelvis shapes are interpreted. But across 29 species. the study lands on a clear. unsettling message for the smallest primates: for many of them. the passage through the pelvis may not be generous enough—so nature has had to improvise.

primate anatomy childbirth pelvis newborn head size bush babies tamarins Adolph Schultz evolution great apes chimpanzees