Scientists have long thought humans have a uniquely difficult birth compared to other primates.

And it’s true that from an evolutionary perspective, we face an obstetrical dilemma. As we evolved to walk on two feet, our pelvis changed shape and size and our birth canal became smaller and oddly twisted. Meanwhile, the evolution of our enormous brain meant that a large-headed baby needed to fit through it.

But our new study found that some primates have to give birth to babies that are squashed even more tightly in the birth canal.

Primate births usually happen at night which makes them difficult to study, especially in the wild. Because of this, we do not know much about them. Rare observations suggest a more solitary, shorter and less noisy labour compared to humans.

To get around this lack of direct observations, in 1949, the Swiss anthropologist Adolph H. Schultz looked for clues in anatomy instead. He measured the size of the pelvic birth canal and of the neonatal head in humans and seven other primates. He wanted to find out how tightly the baby’s head fitted through the birth canal. His results suggested that humans were unique in having a rounder birth canal, one in which the baby’s head only just fitted through. Other great apes seemed to have an oval canal with plenty of space for the baby’s head.........

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