Golden lion tamarins dislocate the bones of the pelvis during childbirth
Edwin Giesbers / naturepl.com
Childbirth can be extremely challenging for humans – but some other primates may have it even worse. A comprehensive analysis of primate anatomy concludes that many species must squeeze large-headed infants through too-narrow pelvises. The problem may have begun with the very first primates, which lived more than 50 million years ago.
It has been assumed for decades that evolution has left humans with unique childbirth difficulties. The conventional view is that the trouble began when our ancestors first walked on two legs, which required the pelvis to be narrow. A few million years later, hominin brains evolved to be larger and infant heads became bigger – but the pelvis was unable to expand to allow for their easy delivery.
Other primates were thought to have things easier, largely because that was the conclusion of an influential study published by anthropologist Adolph Schultz in the 1940s. Schultz looked at a range of primate species and concluded that in the vast majority, the infant head could fit comfortably through the female pelvis.
But his analysis was flawed, says Nicole Torres-Tamayo at University College London. “One of the main problems was that it applied measurements that were originally developed for the human pelvis to all primates,” she says.
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. The human pelvis has a very unusual shape, and when Schultz’s landmarks are mapped onto other primate pelvises, they typically define an inclined plane that sits slightly above the birth canal. This plane overestimates the size of the birth canal, because it is effectively an oblique, oval-shaped slice through a cylinder representing the birth canal.
Torres-Tamayo and her colleagues reassessed birth canal shape in 29 primate species, while also looking at data on newborn-skull size and shape in each species. They concluded that several primates have a pelvis that seems too narrow to give birth. Small primates including bush babies and tamarins have the most severe conflict. In these primates, the newborn’s head is almost twice the size of the birth canal.
“I was not expecting to have a mismatch in quite such a large number of primates,” says research team member Lia Betti, also at University College London.
Birth difficulties may even be the ancestral condition in primates, says Betti, particularly considering that early primates were small.
“It’s super cool to have such a big sample,” says Nicole Webb at the University of Zurich, Switzerland. “These species are doing very different things, living in different niches and they do tend to be quite anatomically diverse.”
Different primates have also found their own solutions to the problem. For instance, the bush babies and tamarins dislocate the bones of the pelvis, temporarily doubling the size of the birth canal. Humans can’t do this, says Betti: it would make walking unbearably painful for a large, bipedal species.
Torres-Tamayo and Betti and their colleagues also found that birth difficulties are much less likely to arise in the great apes, maybe because they are so much larger than the tiny tree-dwelling primates. In this sense, humans are still unique in having birth difficulties, because we are the only large ape with the problem, says Betti.
But Webb isn’t so sure about this point; in a study she and her colleagues published in 2024, they concluded that even chimpanzees have 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,” says Webb. “This new paper is providing a really nice incentive for us to revisit our own hypothesis.”
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